Hydrogeology in the Vicinity of Test Holes and Wells on St. Croix, St. Thomas, and St. John, U.S Virgin Islands By Robert P. Graves U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 96-4004 Prepared in cooperation with the U.S. OFFICE OF MANAGEMENT AND BUDGET San Juan, Puerto Rico 1996 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Gordon P. Eaton, Director For additional information write to: District Chief U.S. Geological Survey GSA Center, Suite 400-15 651 Federal Drive San Juan, Puerto Rico 00965 Copies of this report can be purchased from: U.S. Geological Survey Branch of Information Services Box 25286 Denver, CO 80225-0286 CONTENTS Abstract.................................................................................................................................................................................. 1 Introduction ................................................................................................................................................................^ 1 Test-hole drilling and well completion.................................................................................................................................. 3 Description of study areas ..................................................................................................................................................... 3 Geology................................................................................................................................................................................. 23 St. Croix....................................................................................................................................................................... 23 St. Thomas and St. John .............................................................................................................................................. 27 Ground-water hydrology....................................................................................................................................................... 27 Ground-water occurrence and movement.................................................................................................................... 27 Hydraulic characteristics of the aquifers ..................................................................................................................... 34 Water quality.......................................................................^ 35 Summary.......................................................................................................................................................^ 46 References............................................................................................................................................................................. 47 ILLUSTRATIONS 1-10. Maps showing: 1. Location of the U.S. Virgin Islands....................................................................................................................... 2 2. Location of wells 1 through 5 and well 27, St. Croix, U.S. Virgin Islands........................................................... 13 3. Location of wells 6 through 20, wells 24 through 26, and wells 28 through 36, St. Croix, U.S. Virgin Islands............................................................................................................................................ 14 4. Location of wells 21 through 23, St. Croix, U.S. Virgin Islands .......................................................................... 16 5. Location of wells 37 through 58, St. Thomas, U.S. Virgin Islands ...................................................................... 17 6. Location of wells 59 through 66, St. John, U.S. Virgin Islands............................................................................ 18 7. Location of wells 67 through 77, St. John, U.S. Virgin Islands............................................................................ 19 8. General drainage features of St. Croix, U.S. Virgin Islands ................................................................................. 22 9. General drainage features of St. Thomas, U.S. Virgin Islands ............................................................................. 24 10. General drainage features of St. John, U.S. Virgin Islands................................................................................... 25 11. Bar graphs showing mean monthly rainfall for St. Croix, St. Thomas, and St. John, U.S. Virgin Islands, 1982-92........................................................................................................................................ 26 Contents III 12 - 17. Graphs showing: 12. Daily mean ground-water levels in wells VIWAPA 02, VIWAPA 03, and VIWAPA 17, Adventure well field, and monthly rainfall, St. Croix, U.S. Virgin Islands, May 1990 through September 1992......................................................................................... 28 13. Daily mean ground-water levels in wells VIWAPA 06/DPW 06, Golden Grove, VIWAPA 02, Fairplains, and VIWAPA 23A, Barren Spot, and monthly rainfall, St. Croix, U.S. Virgin Islands, May 1990 through September 1992................................................................ 29 14. Daily mean ground-water levels in wells STT-VIEO-06, STT-VIEO-11, and Grade School 03, St. Thomas, and monthly rainfall, St. Thomas, U.S. Virgin Islands, May 1990 through September 1992................................................................................................................. 30 15. Daily mean ground-water levels in wells STT-VIEO-01, STT-VIEO-10, and STT-VIEO-09, St. Thomas, and monthly rainfall, St. Thomas, U.S. Virgin Islands, May 1991 through September 1992................................................................................................................. 31 16. Daily mean ground-water levels in Guinea Gut well, DPW-06 Sussanaberg, and NPS-06 Cinnamon Bay, St. John, and monthly rainfall, St. John, U.S. Virgin Islands, May 1990 through September of 1992................................................................................................ 32 17. Daily mean ground-water levels in STJ-VIEO- 02, 04, and 03, St. John, and monthly rainfall, St. John, U.S. Virgin Islands, May 1991 through September 1992.................................................... 33 18 - 20. Diagrams showing: 18. Major constituents in ground water in wells on St. Croix, U.S. Virgin Islands ................................................... 45 19. Major constituents in ground water in wells on St. Thomas, U.S. Virgin Islands................................................ 45 20. Major constituents in ground water in wells on St. John, U.S. Virgin Islands..................................................... 46 TABLES 1. Description of test holes drilled on St. Croix, U.S. Virgin Islands......................................................................... 4 2. Description of test holes drilled on St. Thomas, U.S. Virgin Islands..................................................................... 8 3. Description of test holes drilled on St. John, U.S. Virgin Islands .......................................................................... 11 4. Well name and site identification number of wells drilled on St. Croix, St. Thomas, and St. John, U.S. Virgin Islands....................................................................................................................... 20 5. Transmissivity estimated from specific capacity at selected wells on St. Thomas and St. John, U.S. Virgin Islands....................................................................................................................... 34 6. Chemical analyses of ground water on St. Croix, U.S. Virgin Islands................................................................... 36 7. Chemical analyses of ground water on St. Thomas, U.S. Virgin Islands............................................................... 39 8. Chemical analyses of ground water on St. John, U.S. Virgin Islands .................................................................... 40 IV Contents CONVERSION FACTORS, ABBREVIATED WATER-QUALITY UNITS, AND ACRONYMS Multiply inch foot mile acre By 25.4 0.3048 1.609 0.4047 To obtain millimeter meter kilometer hectare square mile 250.0 hectare cubic foot per second 0.02832 cubic meter per second gallon per minute 0.06308 liter per second gallon per minute per foot 0.2070 liter per second per meter foot per day 1 0.3048 meter per day square foot per day 0.09290 square meter per day The standard unit for transmissivity is cubic foot per day per square foot times foot of an aquifer thickness [(ft3/d)/ft2]ft. In this report, the mathematically reduced form, foot squared per day (ft /d), is used for convenience. Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C= 5/9 (°F - 32) Abbreviated water-quality units used in report: microgram per liter (|ig/L) milligram per liter (mg/L) microsiemens per centimeter at 25 °C (fiS/cm) Acronyms used in report: National Oceanic and Atmospheric Administration (NOAA) U.S. Geological Survey (USGS) U.S. Virgin Islands (USVI) U.S. Virgin Islands Energy Office (VIEO) U.S. Virgin Islands Water and Power Authority (VIWAPA) Contents Hydrogeology in the Vicinity of Test Holes and Wells on St. Croix, St. Thomas, and St. John, U.S. Virgin Islands By Robert P. Graves ABSTRACT The islands of St. Croix, St. Thomas, and St. John, U.S. Virgin Islands, were devastated by Hurricane Hugo on September 17 and 18, 1989. Damage to the islands water-supply infrastructure prevented rapid restoration of this basic service. In St. Croix, several well fields, which had limited use in recent years, were used to provide emergency water supplies to the island. Recognizing the need for a better understanding of the hydrogeology of the U.S. Virgin Islands, the U.S. Geological Survey conducted an investigation from May 1990 through September 1992, to determine the occurrence and quality of ground water so emergency ground-water supplies could be developed to mitigate natural or man induced disasters. The geology of the U.S. Virgin Islands is varied. The geology of St. Croix includes alluvial deposits and carbonate and volcanic rocks. The geology of St. Thomas and St. John is dominated by volcanic rocks with local alluvial deposits in small coastal embayments. Carbonate deposits are present on St. Thomas and St. John but only in minor isolated outcrops. Ground water occurs under water-table conditions on St. Croix, St. Thomas, and St. John. On St. Croix, most ground-water development occurs in the carbonate rock and overlying alluvial deposits. On St. Thomas and St. John the principal source of ground water is from the fractured volcanic rock and overlying alluvial deposits. Ground water depth below land surface ranges from approximately 4 to 62 feet on St. Croix, 3 to 74 feet on St. Thomas, and 8 to 60 feet on St. John. Aquifer transmissivities range from about 200 to 4,000 feet squared per day on St. Croix and from 200 to 2,500 feet squared per day on St. Thomas and St. John. Well yields for emergency supply wells on St. Croix, St. Thomas, and St. John range from about 14 to 80 gallons per minute. Ground water in the U.S. Virgin Islands is predominantly of the sodium chloride type. Ground-water samples collected from selected wells had chloride concentrations ranging from 116 to 3,870 milligrams per liter and dissolved solids concentrations ranging from 542 to 2,470 milligrams per liter. On St. Croix, depending on location, the sodium chloride is due to connate water or to seawater intrusion. Data were not available to determine the source of sodium chloride on St. Thomas or St. John. INTRODUCTION The islands of St. Croix, St. Thomas, and St. John, U.S. Virgin Islands (USVI) (fig. 1), were devastated by Hurricane Hugo on September 17 and 18, 1989. Rapid restoration of basic services such as drinking water was not possible because of damage to the infrastructure that provides water to these islands. For municipal water supplies, the USVI had been supplied primarily by desalinization plants and, only minimally, by isolated ground-water supplies. Introduction I 5 (Q I O 55' o (D W 0) ai w o 66W 45' 30' I I T ATLANTIC OCEAN 15' 65°00' ~1 I ST. THOMAS Culebra EXPLANATION U.S VIRGIN ISLANDS a w Figure 1. Location of the U.S. Virgin Islands. Following Hurricane Hugo the desalination plant in St. Croix was so heavily damaged that full recovery of the plant was not possible for weeks following the storm. Fortunately, in St. Croix, several well fields, which had limited use in recent years, were connected to the water distribution system for emergency water supply. The availability of water from these well fields prevented a serious water crisis in St. Croix. Recognizing the need for a better understanding of the hydrogeology of the USVI, the U.S. Geological Survey (USGS), in cooperation with the U.S. Office of Management and Budget, conducted an investigation from May 1990 through September 1992, to determine the occurrence and quality of ground water so emergency ground-water supplies could be developed to mitigate natural or man induced disasters. This report describes the hydrogeology of selected lands on the islands of St. Croix, St. Thomas, and St. John, USVI. Data used in this report are based largely on information obtained through test-hole drilling and well completion. TEST-HOLE DRILLING AND WELL COMPLETION During this investigation, 61 test holes were drilled (tables 1, 2, and 3). Of these test holes, 12 were completed as observation wells, 27 were completed as emergency water supply wells, and 22 were back- filled and abandoned. Of the 12 observation wells completed, 8 were instrumented by the USGS with continuous water-level recorders to monitor ground- water level fluctuations. Of the 27 emergency water supply wells completed, specific capacity tests to determine aquifer transmissivity were conducted on 12 wells and all of the wells were sampled for water- quality analysis. The test-hole drilling for this study was completed using the reverse-air circulation drilling method with an open-center reverse-air drill bit (Graves, 1992). Information used in this report not only includes data from the test holes drilled and wells completed, but also from 16 previously existing wells. Ten of the existing wells were instrumented with continuous water-level recorders. Water-quality samples were collected at six of the existing wells. The locations of all wells used in this study are shown in figures 2 through 7. Well numbers, names, and site identification numbers for these wells are given in table 4. The well numbers in table 4 apply only to this report. However, the site-identification numbers conform with the established USGS Ground- Water Site Inventory numbering system (Mathey, 1990). The site-identification number does not change and can be used to reference a specific well in other USGS publications. DESCRIPTION OF STUDY AREAS St. Croix, St. Thomas, and St. John are the largest of more than 50 islands and cays that make up the USVI. The USVI are located from 50 to 100 miles east and southeast of Puerto Rico and form part of the Antilles island arch, which separates the Caribbean Sea from the Atlantic Ocean (fig.l) (Jordan and Cosner, 1973). The island of St. Croix is approximately 22 miles long, ranges from 1 to 6 miles in width, and has an area of approximately 84 square miles. The island is characterized by a mountainous area in the north, west, and east, with the northern mountainous area flanked by a rolling plain to the south (fig. 8) (Rivera and others, 1970). The highest elevation on St. Croix is 1,165 feet above mean sea level. The northern and western mountains are marked by many narrow, steep- sided valleys through which deeply cut intermittent streams discharge. The stream valleys of the eastern mountainous area are also intermittent but are not as sharply incised as the northern and western areas. Drainage on St. Croix can occur in all directions; however, the principal direction of drainage is either south or southeast. The island of St. Thomas is approximately 14 miles long, ranges from 2 to 3 miles in width, and has an approximate area of 32 square miles. The land surface of St. Thomas is almost entirely sloping and extends seaward from a central ridge, which attains an elevation of 800 to 1,200 feet above mean sea level and runs the length of the island (Jordan and Cosner, 1973). Coastal plains are almost completely absent and flat land is confined to only a few small embay ments. Test-Hole Drilling and Well Completion Table 1. Description of test holes drilled on St. Croix, U.S. Virgin Islands [mm-dd-yy, month-day-year; ft-blsd, feet below land surface datum; gal/min, gallons per minute; mg/L, milligrams per liter; jiS/cm, microsiemens per centimeter at 25 degrees Celsius; Q. 5 (Q (DOo 3Y in the Vici 1a51 Xo (D V> to a. Z. 5To3 O3 H O3 to y> to a. y> c_ O3" 3 C m <3 3 5T 0) (0 STXT, St. Croix-test hole that was backfilled; Date Well number and . ... . 1 drilled name , . . . (mm-dd-yy) , 1 STXT-01 07-15-92 2 STX-EGWS-01 07-15-92 3 STX-EGWS-02 07-17-92 4 STXT-02 07-20-92 5 STX-EGWS-03 07-21-92 6 STXT-03 07-22-92 7 STX-EGWS-04 07-23-92 8 STXT-04 07-28-92 STX, St. Croix; EGWS, emergency ground- water supply Estimate Depth of ... . t. sustained , ;. . Well construction M . .. test hole ,ft .. .. well yield (ft-blsd) in-Disaj (gal/min) 80 test hole back filled 70 6-inch casing 45 0-30 6-inch screen 30-70 69 6-inch casing 30 0-30 6-inch screen 30-69 60 test hole back filled 80 6-inch casing 20 0-40 6-inch screen 40-80 120 test hole back filled 160 6-inch casing 15 0-120 6-inch screen 120 - 160 120 test hole back filled ; , data not available] Chloride, dissolved (mg/L asCI) 440 210 225 715 200 1,400 465 - Specific conductance (lab value US/cm) 2,230 1,470 1,400 2,750 1,440 5,650 2,680 - Subsurface material Alluvial deposits Volcanic rock Alluvial deposits Volcanic rock Alluvial deposits Volcanic rock Topsoil Alluvial deposits Alluvial deposits Volcanic rock Kinghill Limestone Topsoil Kingshill Limestone Topsoil Alluvial deposits Kingshill Limestone (ft-blsd) 0-46 46-80 0-70 70 0-60 60-69 0-1 1 -60 0-70 70-80 0- 120 0- 1 1 -160 0-2 2-19 19- 120 T3 CD D _C "c o O CO -o c 03 _co c "E» > CO Z>x" "2 o CO c0 ~o _CD 'C T3 CO CD O .C *- COo **ro c0' s. l_ 0 CO CD Q 0)a£ ^ CO JD H_» ^ "CO l_ .2 03 E 0)o g 3 CO -Q3 C/) 0)O 0) 0 C 3 _ ^ 3 "co E 0 0 > 0 Q) * -^L *iaa o ~ ' o « !-.<=> ~ .> -~ o o o p u co E w JZ CO O- CO 0 =5 ^"^1^ CO c .® 'F E re >^-i S = i s> LiJ CO > -^ c..0 "o E =0 to JS c .a 8d . _ "55 ^ o 5 «? £ °^ S.f s a> co .£ Q 3 & S o -o 75 = "o Q:§ E £ a c CO O>T- ^i a> E i i 2 1 0 22 o « S s o o .0 CC *J CO ti > 45 .S 45 S3 U5 C U5 P bO Z 00 0 S a S ^ « < 2 ^ O 0 ON O °i ^ CN *-*" O in TT ON m CN in ' m bo C cu -a '3 S ^ u > r3 >> TO j3 co r3 r-| co 'C co 3 O co 3 5 BO O co BO O =5 s is g- c -a < ^ ^ £ 2 3 8 8 ON ON ' ^-< 0 0 ^H ON m CN ! "^ CN BO C « ^ S ^ £ £ 3 _ 2 O a 03 CN Q J3 y^ cj in « -w j^ 45 ' 45 i TO g ° g 42 "71 'V M3 M3 O M3 ON ON CN CN ON ON 0 O rn m r~ t^- o o 9 00 M3 pi g s is S 00 00 ^H CN ' ' ' ' _ nS O° - 11 U hJa "3 -3 u J "O rz3 ^ 45 ' ^ 45 ' ><*> 5M Tfo X H 00 X H 00 X H 00 X H 00 Description of Study Areas Table 1. Description of test holes drilled on St. Croix, U.S. Virgin Islands-Continued 3 (Q I£ Well number and f. name1 3 a 2? c. 22 STXT-10 o31 to 23 STX-EGWS-13 < <3 3 (0 5T g. 24 STX-EGWS-14 0) Date Depth of drilled test hole (mm-dd-yy) (ft-blsd) 08-07-92 - 120 08-10-92 08-11-92- 120 08-12-92 08-13-92- 190 08-14-92 08-18-92- 140 08-19-92 08-19-92- 240 08-20-92 08-24-92 120 08-25-92 130 08-31-92 120 Estimate Well construction sus|ain^ /fi u.i«*j\ WGII ymiu (Tt-DlSQ) (gal/rmn) 6-inch casing 80 0-100 6-inch screen 100-120 6-inch casing 35 0-80 6-inch screen 80-120 6-inch casing 35 0-140 6-inch screen 140-190 test hole back filled 6-inch casing 80 0- 150 6-inch screen 150 - 230 test hole back filled 6-inch casing 14 0-70 6-inch screen 70-130 6-inch casing 35 0-70 6-inch screen 70-120 Chloride, dissolved (mg/L asCI) 720 740 290 380 1,000 705 375 285 Specific conductance (lab value US/cm) 3,670 3,640 2,520 2,360 4,290 4,270 2,210 1,980 Subsurface material Alluvial deposits Kingshill Limestone Topsoil Alluvial deposits Kingshill Limestone Alluvial deposits Kingshill Limestone Alluvial deposits Kingshill Limestone Topsoil Alluvial deposits Kingshill Limestone Kingshill Limestone Alluvial deposits Volcanic rock Alluvial deposits Kinghill Limestone (ft-blsd) 0-64 64-120 0-1 1-60 60-120 0-8 8-190 0-54 54 - 140 0- 1 1-23 23 - 240 0- 120 0-60 60-130 0-70 70-120 Table 1. Description of test holes drilled on St. Croix, U.S. Virgin Islands-Continued O (D (0 O f O 1 Q. (S D) (0 ... . . Date Depth of Well number and , ... . . ,, . 1 drilled test hole name (mm-dd-yy) (ft-blsd) 25 STXT-11 09-01-92- 160 09-02-92 26 STXT-12 09-02-92 70 27 STXO-01 09-03-92 70 28 STXO-02 09-04-92 130 29 STX-EGWS-15 09-10-92 130 30 STXO-03 09-08-92 120 Estimate ... .. , ,. sustained Well construction .. . . . /« ui _i\ well yield (ft-blsd) . ./ . . v ' (gal/mm) test hole back filled test hole back filled 4-inch casing 25 0-30 4-inch screen 30-70 4-inch casing 45 0-90 4-inch screen 90-130 6-inch casing 20 0-85 6-inch screen 85 - 125 4-inch casing 30 0-80 4-inch screen 80- 120 Chloride, Specific dissolved conductance (mg/L (lab value as Cl) (is/cm) 340 2,200 well dry well dry 250 1 ,650 425 2,580 350 2,270 370 2,310 Subsurface material Topsoil Alluvial deposits Kingshill Limestone Alluvial deposits Volcanic rock Alluvial deposits Volcanic rock Topsoil Kingshill Limestone Topsoil Alluvial deposits Kingshill Limestone Jealousy Formation Topsoil Alluvial deposits Kingshill Limestone (ft-blsd) 0- 1 1-77 77-160 0-62 62-70 0-65 65-70 0-2 2- 130 0-1 1-52 52 - 127 127- 130 0-1 1 -40 40 - 120 1 Well number refers to well location as shown on figures 2, 3, or 4. 00 Table 2. Description of test holes drilled on St. Thomas, U.S. Virgin Islands [mm-dd-yy, month-day-year; ft-blsd, feet below land surface datum; gal/min, gallons per minute; mg/L, milligrams per liter; (iS/cm, microsiemens per centimeter at 25 degrees Celsius; a o 3a 1 (0 o3 CO po. *' CO H O 0>J" 0>3a CO o3" _3 C CO < (Q5' 5T 0>3a in -, data not available; STT, St. Thomas; EGWS emergency ground- water supply; VIEO, Virgin Islands Energy Office] Well number and name1 37 STT-EGWS-01 38 STT-EGWS-02 39 STT-EGWS-03 40 STT-VIEO-01 (observation well) 41 STT-EGWS-04 42 STT-EGWS-05 43 STT-VIEO-05 Date drilled (mm-dd-yy) 11-19-90- 12-04-90 01-28-91 - 01-31-91 01-31-91 - 02-04-91 02-05-91 06-24-91 - 06-25-91 06-26-91 - 06-27-91 07-01-91 Estimate Depth of ... .. . .. sustained * iu i Well construction .. . .. test hole ,, .. well yield (ft-blsd) in-Disa) (gal/min) 160 6-inch casing 17 0-90 6-inch screen 90- 160 158 6-inch casing 18 0-100 6-inch screen 100- 158 140 6- inch casing 45 0- 100 6-inch screen 100 - 140 145 4-inch casing 35 0- 100 4-inch screen 100 - 145 70 6-inch casing 30 0-50 6-inch screen 50-70 95 6-inch casing 40 0-62 6-inch screen 62-94 20 test hole back filled Chloride, Specific dissolved conductance 0 . . / /i /i u i Subsurface (mg/L (lab value as Cl) [is/cm) 400 2,150 Topsoil Alluvial deposits Volcanic rock 212 1,646 Alluvial deposits Volcanic rock 274 1,740 Topsoil Alluvial deposits Volcanic rock 180 1,300 Topsoil Alluvial deposits Volcanic rock 116 1,090 Topsoil Alluvial deposits Volcanic rock 140 1,180 Alluvial deposits Volcanic rock Alluvial deposits Volcanic rock material (ft-blsd) 0-3 3-65 65 - 160 0-45 45 - 158 0-1 1 -50 50 - 140 0-1 1 -54 54 - 145 0-1 1 -32 32-70 0-50 50-95 0- 12 12-20 Table 2. Description of test holes drilled on St. Thomas, U.S. Virgin Islands-Continued 1? CO qf 5' (0 Q. < > S CO Well number and name1 44 STT-VIEO-06 (observation well) 45 STT-EGWS-06 46 STT-VIEO-07 47 STT-VIEO-08 (observation well) 48 STT-VIEO-09 (observation well) 49 STT-EGWS-07 50 STT-VIEO-10 (observation well) 51 STT-EGWS-08 Date drilled (mm-dd-yy) 07-01-91 07-02-91 - 07-05-91 07-08-91 07-09-91 07-10-91 07-10-91 - 07-11-91 07-12-91 - 07-15-91 07-15-91 - 07-16-91 Estimate Depth of ... . .. sustained . VV . Well construction .. . . . test hole ., . . ., well yield (ft-blsd) (n-Disa) (ga|/min) 76 4-inch casing 14 0-56 4-inch screen 56-76 1 20 6- inch casing 50 0-70 6-inch screen 70-120 40 test hole back filled 57 4-inch casing 0-37 4-inch screen 37-57 35 4-inch casing 0-6 4-inch screen 6-35 58 6-inch casing 35 0-38 6-inch screen 38-58 53 4-inch casing 0-27 4-inch screen 27-53 35 6-inch casing 25 0-13 6-inch screen 13-33 Chloride, dissolved (mg/L asCI) 116 224 1,910 780 350 288 454 Specific conductance (lab value US/cm) 1,180 1,680 6,800 3,360 2,060 1,940 2,400 2,600 Subsurface material Topsoil Alluvial deposits Volcanic rock Fill material Alluvial deposits Volcanic rock Alluvial deposits Volcanic rock Topsoil Alluvial deposits Volcanic rock Alluvial deposits Volcanic rock Alluvial deposits Volcanic rock Limestone Volcanic rock Alluvial deposits (ft-blsd) 0-2 2-35 35-76 0- 12 12-63 63 - 120 0-33 33-40 0-1 1-24 24-57 1-28 28-35 0-32 32-58 0-44 44-53 0-35 Table 2. Description of test holes drilled on St. Thomas, U.S. Virgin Islands-Continued X 1 ^geology in th< w o' 3 «< O II O w D>3 Q. I W O r O 5 X w o D> W D>3 Q. ~ C_ O p c (Q5' 5T3 Q. W Well number and Date drilled name1 (mm-dd-yy) 52 STT-EGWS-09 07-17-91- 07-18-91 53 STT-EGWS- 10 07-1 9-9 1 - 07-22-91 54 STT-VIEO- 1 1 07-23-9 1 - (observation well) 07-24-91 55 STT-VIEO- 12 07-24-91 - 07-25-91 56 STT-EGWS- 1 1 07-29-9 1 - 07-30-91 57 STT-VIEO- 13 07-30-91- 07-31-91 1 Well number refers to well location Depth of ... j. Well construction /.U. ui j\ (ft-blsd) (ft-blsd) 1 40 6-inch casing 0-100 6-inch screen 100-140 1 20 6-inch casing 0-80 6-inch screen 80-120 110 6-inch casing 0-90 6-inch screen 90-110 115 test hole back filled 110 6-inch casing 0-70 6-inch screen 70-110 105 test hole back filled as shown in figure 5. Fctimato C-oiii i idle? *-\i i i s-t 'f . . . Chloride, Specific sustained .. . ' ^ ± .. . .. dissolved conductance 0 . , , . . ... . . .. well yie d , .. ,. . . Subsurface materia (ft-blsd) / ,, ^ (mg/L (abvaue v ' <9al/mln' as Cl) MS/cm) 40 222 1,290 Fill material Alluvial deposits Volcanic rock 40 182 1,475 Alluvial deposits Volcanic rock 14 426 2,200 Topsoil Alluvial deposits Volcanic rock 426 1,600 Alluvial deposits Volcanic rock 30 212 1,550 Alluvial deposits Volcanic rock 676 3,475 Alluvial deposits Volcanic rock 0 2 38 0- 100 0 1 87 0 12 0 73 0 12 -2 -38 - 140 100 -120 -1 -87 -110 -12 -115 -72 - 110 - 12 -105 Table 3. Description of test holes drilled on St. John, U.S. Virgin Islands [mm-dd-yy, month-day-year; ft-blsd, feet below land surface datum; gal/min, gallons per minute; mg/L, milligrams per liter; jxS/cm, microsiemens per centimeter at 25 degrees Celsius; STJ, St. John; SJWS, St. John water supply; VIEO, Virgin Islands Energy Office; -, data not available] o COo 1 O3a s Q. ! Well number and name1 68 STJ-SJWS-01 69 STJ-VIEO-02 (observation well) 70 STJ-SJWS-02 71 STJ-SJWS-03 72 STJ-SJWS-04 73 STJ-VIEO-03 (observation well) 74 STJ-SJWS-05 75 STJ-VIEO-04 (observation well) Date drilled (mm-dd-yy) 02-14-91 - 02-15-91 02-15-91 - 02-19-91 02-19-91 - 02-20-91 02-21-91 02-22-91 02-25-91 - 02-26-91 02-26-91 02-26-91 - 02-27-91 Depth of XA/|| t t. Est;mate, i Well construction sustained ° e (ft-blsd) well yield (ft"blsd) (gal/min) 100 test hole back filled 70 4-inch casing 7 0-26 4-inch screen 26-66 47 6-inch casing 20 0-26 6-inch screen 26-46 46 test hole back filled 47 test hole back filled 110 4-inch casing 4 0-50 4-inch screen 54-110 50 test hole back filled 50 6-inch casing 17 0-20 6-inch screen 20-50 Chloride, dissolved (mg/L asCI) 1,256 1,048 526 3,870 550 1,178 1,828 788 Specific conductance (lab value (is/cm) 4,680 4,180 2,160 11,870 3,100 4,750 6,610 3,010 Subsurface Alluvial deposits Volcanic rock Topsoil Alluvial deposits Volcanic rock Topsoil Alluvial deposits Volcanic rock Alluvial deposits Volcanic rock Topsoil Alluvial deposits Alluvial deposits Volcanic rock Alluvial deposits Topsoil Alluvial deposits material (ft-blsd) 0-24 24-100 0-1 1-32 32-70 0-1 1-20 20-47 0-30 30-46 0-1 1-47 0-54 50-110 0-50 0-1 1-50 Table 3. Description of test holes drilled on St. John, U.S. Virgin Islands-Continued 8 os s. Xo 0) Q. I (0 o3 W O 5 Well number and name 1 76 STJ-SJWS-06 77 STJ-SJWS-07 Date drilled (mm-dd-yy) 02-28-91 02-28-91 Depth of test hole (ft-blsd) 29 10 Well construction (ft-blsd) test hole back filled test hole back filled Estimate sustained well yield (gal/min) Chloride, dissolved (mg/L asCI) 894 Specific conductance (lab value US/cm) 2,600 Subsurface material Topsoil Alluvial deposits Volcanic rock Alluvial deposits Volcanic rock (ft-blsd) 0-1 1 -21 21-29 0-6 6-10 1 Well number refers to well locations as shown on figures 6 or 7. o 0) JO 0) Q. (0 O c 64'52' 17°44' 17°42' , --' "~ '/* "l 'E "v- X^^" " "£'" " -- ::: :~.-rll-*^ ^ "^'"^ ^- ^*^ '"* *\./^> '*f-_\ a Granger]" ^' ^ |||| ^^-wmnBe^.^ | ^ ^ ^Mgi ^ ^^tT-~* -t- ^ ^'s^iS/,44^ ^vT2? fc\> 1 St fetriekiJU J^SV V ' -^ k^ e < F^^,ak ^S^ <- J' ISS^^l^'^tf " -i<^c^ FreSenck^Wg^S-;:.,^^^ \ . >5c^ ^ v^.^^fev^.:"i BASE FROM USGS TOPOGRAPHIC MAP FREDERIKSTED, V.I. QUADRANGLES, 1982 CONTOUR INTERVAL 20 FEET, DOTTED LINES REPRESENT 10-FOOT CONTOURS 1 KILOMETERS EXPLANATION WELL AND NUMBER Figure 2. Location of wells 1 through 5 and well 27, St. Croix, U.S. Virgin Islands. Description of Study Areas 13 17°44' 64°50' 64°48' Wf^frf , m* l *\? \ ]) >S/ K^i'-'T v-'^.^-^ ^-^LJ ^\*3J*H%fi\\ ^--vJ^55^^ [V^IP^^JP . £v- ; ^^ i^:^^^r-E^^Wi\ -^- :- ^A-5feSr %*^^i-^r_...s% v 17°42' <=fcs^-7'" '^^,-,'.-:-/ I ?K 04. 7AV'~ U '* i\C*;:^(^' X v, .x^^X! ''"^ "s ^Mo^lam '*' y| |^^ ) / ] \e|^%<^4*~*" ^ *A ^ ^\ \ *\-.' *#^\\ ^^\LF^^J$£ _ V x-^:="^T \ \5 -i Xi -\/\BH i^.^^U^^4aS^/'"X K ^ \ ,<£&& ".-- x -^*i«^- v. i "O"? ££:p&*^ j \u-Vx %' ^ ^^- ^ -» _ N. « \ * ,'' ^ .<-' ^'x^^C?^ a.\. -*> v/ -----,/*« -,,^ .^-^-^ ,(g| jsteoi. ^->A Vs >. ^;-^\ s 29© "/Wl^rj^ri M *>^.-K^' - , -j .1 - '" *" "* ^\ - / b' I X± "' / f] *' -,.>' ' i V" 17-46 BASE FROM USGS TOPOGRAPHIC MAP FREDERIKSTED, V.I. QUADRANGLES, 1982 CONTOUR INTERVAL 20 FEET, DOTTED LINES REPRESENT 10-FOOT CONTOURS 1 MILES 1 KILOMETERS EXPLANATION 8 0 WELL AND NUMBER Figure 3. Location of wells 6 through 20, wells 24 through 26, and wells 28 through 36, St. Croix, U.S. Virgin Islands. 14 Hydrogeology In the Vicinity of Test Holes and Wells on St. Croix, St. Thomas, and St. John, U.S. Virgin Islands 64°47' 64°46' 17"44' 17°42' » ^ % - ; " '#%£n^, mip^w^^fi" '^-^.^wi . " \ X-^M ; - ; j ^fe^^vv'X ^^&!f^ N^fel l-~! > Vl|%j - " '"*""" ->^ - - .........................~.-*Yk. s'*w'""""^'/^?:J^;v^^~^"*t-- p." v. "" ' ^ -in *-.*fe5f 3J. - ^L t^^.. - .. «. -^ /" -x - ' . '.- j ^fi-_ ,i^-«fcx' -:/; m^W^M. -. Bssata^ .-i ,' to:s VVx l^W S?^ -'.,,, -^<^z::~ ^...............-. - .-^^......j, .. "^^*^«^;TTr ^^w^^^r:,^?*v* ,"" ^St^^^^^fc^1?:|.| L. ,S' .x'^fcr-^v^t ;.:.. .'*jmM.-4®~ -' ;-r. ,"--f " ^v^>^^* ^. ^.^ 5 «?*C" ^ ' ^ii^^I "-x --. %!$ '%( - H^-.....^"^^.,^":^^* ^,j - %)"'-^ / -X ^' vs ^Stfi . ' -, '-i$&'f S^~$'*'*%!*iii&$ "" --'ii! '^"^"^^iV-^P9" ^\"":" i%v^D^ier 'B^s.'^Ri* %^^fe^\^^ 5r "i1* -J" ^^^^M.V ^wiH '*s%:i^s'j-' ir; ''~\J "." r 5%y;^ ;; * fe -aj?ri/^.^; --- -JB,-^' C^*" M'//' = l- ' .^TMtatHJi ^- -rr . i /.^.tL'.,f r.--; ..^. yy,1* - - . "< " '. V --"::1Mtt1|%V»i--:-^:--^--C..".X\^^ -> V ^;~* \-f. ' ' "V*-" 'VTfTTPKh vl-^;;;-^J._.^ ^ " ' ' | Jj \ _s -^ BASE FROM USGS TOPOGRAPHIC MAP CHRISTIANSTED, V.I., QUADRANGLES, 1982 CONTOUR INTERVAL 20 FEET, DOTTED LINES REPRESENT 10-FOOT CONTOURS KILOMETERS EXPLANATION 29 © WELL AND NUMBER Figure 3. Location of wells 6 through 20, wells 24 through 26, and wells 28 through 36, St. Croix, U.S. Virgin Islands-Continued. Description of Study Areas 15 64°44' 64°42' I 5 CO (D O O CO o B) W B) Q. O p c 0) < (55' 17°45' 17'44' BASE FROM USGS TOPOGRAPHIC MAP CHRISTIANSTED, V.I., QUADRANGLES, 1982 CONTOUR INTERVAL 20 FEET, DOTTED LINES REPRESENT 10-FOOT CONTOURS 1 MILES 1 KILOMETERS EXPLANATION 22 WELL AND NUMBER Figure 4. Location of wells 21 through 23, St. Croix, U.S. Virgin Islands. 64°53' 64°52' 18°19'30 STUDY AREA 1 64'58'20" 64°58' 64°54' 18°20'50" r=r 18°20'30" STUDY AREA 2 BASE FROM USGS TOPOGRAPHIC MAPS CENTRAL ST. THOMAS, V.I., AND EASTERN ST. THOMAS, V.I., QUADRANGLES, 1982 CONTOUR INTERVAL 40 FEET, DOTTED LINES REPRESENT 20-FOOT CONTOURS NO/?~'X.._ - w^;\';\^c^--x-^ili!f:.- - -.-,,. . X- >$s i , -»" 45 ' " .7Y^^fc^USMii/s (' ^- ~&-^t >S2 © © -k ^^51 i j !K^///// J S^rt^'N ^ K, ^SS^^^^^M^^^iN^ 18°20'30 STUDY AREA 3 65'pO' 64'55' 64-50' CARIBBEAN EXPLANATION WELL AND NUMBER 0.5 1 KILOMETERS Figure 5. Location of wells 37 through 58, St. Thomas, U.S. Virgin Islands. Description of Study Areas 17 64'48' 64°46' I 8 o (Q<5' 3 (Q5' ^^iiiiihyiii^^ ' A Ir^V/N^^ "^ if® j &T'\ A ;'! 4 ! j /'/. ' '&/,/! ' ! i'i. . : »,» V l^S-. S^M^sS ,/ /^- :'f *"??%"' " ; ;. \^ > \*^, -..»,.. i .. , ss ; ,-,'y. J ^ .:. -"^ ' f ^/7^^> ' :^.'V'^y?'- '3'^ '? ...O/'/ji,1..i; ;--^' ^ r~ ;- * . < > ?/-^-^' ^v'%n ? ^r-.^^§p^ftitm^JS^^ DOTTED LINES REPRESENT 20-FOOT CONTOURS 18°20' 1 KILOMETERS .60 EXPLANATION WELL AND NUMBER VIRGIN ISLAND NATIONAL PARK BOUNDARY Figure 6. Location of wells 59 through 66, St. John, U.S. Virgin Islands. 64°44' 64°42' 18°21'30" 18°20'30" BASE FROM USGS TOPOGRAPHIC MAP WESTERN ST. JOHN, V.I., QUADRANGLES, 1982 CONTOUR INTERVAL 40 FEET, DOTTED LINES REPRESENT 20-FOOT CONTOURS 1 MILES 1 KILOMETERS .73 EXPLANATION 0 " WELL AND NUMBER VIRGIN ISLAND NATIONAL PARK BOUNDARY Figure 7. Location of wells 67 through 77, St. John, U.S. Virgin Islands. Description of Study Areas 19 Table 4. Well name and site identification number of wells drilled on St. Croix, St. Thomas, and St. John, U.S. Virgin Islands [STXT, St. Croix-test hole that was backfilled; STX, St. Croix; EGWS, emergency ground-water supply; STXO, St. Croix, observation well; VIWAPA, Virgin Islands Water and Power Authority; Adv, Adventure; DPW; Department of Public Works; GG, Golden Grove; FP, Fairplains; BS, Barren Spot; STT, St. Thomas; VIEO Virgin Islands Energy Office; NFS, National Park Service; STJ, St. John; SJWS, St. John water supply; WAPA, Water and Power Authority] Well number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Test hole name STXT-01 STX-EGWS-01 STX-EGWS-02 STXT-02 STX-EGWS-03 STXT-03 STX-EGWS-04 STXT-04 STX-EGWS-05 STXT-05 STXT-06 STX-EGWS-06 STX-EGWS-07 STX-EGWS-08 STXT-07 STXT-08 STX-EGWS-09 STX-EGWS-10 STX-EGWS-11 STXT-09 STX-EGWS-12 STXT- 10 STX-EGWS-13 STX-EGWS-14 STXT- 11 USGS Site identification 174303064523200 174309064523000 174307064523000 174303064523300 174312064523000 174222064474800 174222064475300 174231064484900 174223064480300 174324064481600 174318064481300 174251064480200 174251064480500 174252064480900 174257064481600 174253064481200 174237064471800 174247064471900 174309064464600 174240064473800 174419064444500 174344064433000 174438064413800 174240064474300 174238064480700 Well number 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 Test hole name STXT- 12 STXO-01 STXO-02 STX-EGWS-15 STXO-03 VIWAPA 17, Adv VIWAPA 03, Adv VIWAPA 02, Adv VIWAPA 06 / DPW 06. GG VIWAPA 02, FP VIWAPA 23A, BS STT-EGWS-01 STT-EGWS-02 STT-EGWS-03 STT-VIEO-01 STT-EGWS-04 STT-EGWS-05 STT-VIEO-05 STT-VIEO-06 STT-EGWS-06 STT-VIEO-07 STT-VIEO-08 STT-VIEO-09 STT-EGWS-07 STT- VIEO- 10 USGS Site identification 174341064492600 174310064523000 174222064480200 174243064480800 174251064480600 174316064480800 174308064482800 174303064481100 174243064475100 174225064472000 174319064454401 182037064545100 182033064545000 182038064545300 182036064545200 182037064580000 182042064580600 182037064580200 182038064580000 182039064544900 181928064512400 181927064513600 181917064524600 181936064524500 182131064541000 20 Hydrogeology in the Vicinity of Test Holes and Wells on St. Croix, St. Thomas, and St. John, U.S. Virgin Islands Table 4. Well name and site identification number of wells drilled on St. Croix, St. Thomas, and St. John, U.S. Virgin Islands-Continued Well number 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 Test hole name STT-EGWS-08 STT-EGWS-09 STT-EGWS-10 STT-VIEO-11 STT-VIEO-12 STT-EGWS-11 STT-VIEO-13 Grade School 03, St. Thomas NPS-03, Cruz Bay Guinea Gut Well, St. John WAPA/DPW-02, Susannaberg WAPA/DPW-05, Susannaberg DPW-06, Sussanaberg, St. John NPS-05, Trunk Bay Cinnamon Bay Campground NPS-06, Cinnamon Bay, St. John VI Government WAPA well (USGS 14A) STJ-SJWS-01 STJ-VIEO-02 STJ-SJWS-02 STJ-SJWS-03 STJ-SJWS-04 STJ-VIEO-03 STJ-SJWS-05 STJ-VIEO-04 STJ-SJWS-06 STJ-SJWS-07 USGS Site identification 181917064524502 182037064545700 182031064545800 182035064550200 182039064550401 182043064580300 181914064532300 182038064550300 182008064473000 181956064464500 182044064455000 182044064454600 182042064454500 182109064460300 182113064451900 182116064451000 182058064432300 182107064425900 182110064430000 182048064430500 182052064430500 182052064430400 181950064422300 181952064421700 182048064430400 182054064430600 182054064430800 Description of Study Areas 21 I 5 to (D O O O 0) Q. 17°45' 17°40' 64°50' I 45' 40' 64°35' ST. CROIX 0 1 2 KILOMETERS I " r-1 i 0 1 2 MILES CARIBBEAN SEA EXPLANATION ;> Topographic contours. Contour interval variable \ Surface drainage Figure 8. General drainage features of St. Croix, U.S. Virgin Islands. The only variation in the generally rugged topography is in the upper valley of the Turpentine Run drainage basin (fig. 9) The valley has relatively gentle topography consisting of rolling hills in a basin surrounded by steep slopes and sharp ridges (Jordan and Cosner, 1973). Because of the central ridge running through St. Thomas, drainage can occur in all directions; however the principal direction of drainage is to the north and to the south. The island of St. John is approximately 9 miles long, ranges 2 to 4 miles in width, and has an approximate area of 19 square miles. The island is composed of a main eastward trending ridge with steep slopes to the north descending to the sea. In contrast, the south side of the ridge has several prominent spur ridges that extend southward (fig. 10) (Cosner, 1972). The highest elevation on St. John is 1,277 feet above mean sea level. The prominent direction of surface drainage on St. John is to the north and to the south. As with St. Thomas, coastal plains are almost completely absent. Rat land is confined to only a few small embayments. About two-thirds of the island lies within the boundaries of the Virgin Islands National Park, which is administered by the U.S. National Park Service. The mean annual air temperature recorded by the National Oceanic and Atmospheric Administration (NOAA) for St. Croix, St. Thomas, and St. John from 1982 to 1992 was 79.6, 80.5, and 78.4 degrees Fahrenheit respectively (U.S. Department of Commerce, 1982 to 1992). Mean annual rainfall recorded by NOAA from 1982 to 1992 for St. Croix, St. Thomas, and St. John was 40.26, 45.03, and 44.96 inches, respectively. Mean monthly rainfall for 1982 through 1992 ranged from 1.48 inches in February to 5.75 inches in November for St. Croix, ranged from 1.54 inches in March to 6.28 inches in November for St. Thomas, and ranged from 1.46 inches in March to 6.9 inches in November for St. John (fig. 11). GEOLOGY Much has been written about the geology of the U.S. Virgin Islands (Cederstrom, 1950; Donnelly 1959; Gerhard and others, 1978; Gill and Hubbard, 1986 and 1987; Hubbard, 1989; and Whetten 1966), as well as about the water resources of the islands (Cederstrom 1950; Cosner, 1972; Graves, 1995; Jordan, 1975; Jordan and Cosner, 1973; and Robison, 1972). Although the geology of the U.S. Virgin Islands is quite complex, in the literature of the water resources of the islands, the geology has been simplified and discussed in generalized terms of volcanic rock or rock of volcanic origin, localized igneous plutonic rock, and carbonate and alluvial deposits. For the purpose of this report, in terms of ground-water development, the geology of the U.S. Virgin Islands will be discussed only in terms of alluvial deposits and carbonate and volcanic rock. For a more detailed discussion of the geology of the U.S. Virgin Islands, the reader is referred to the sited references. St. Croix The geology of St. Croix includes alluvial deposits and carbonate and volcanic rocks. Volcanic rock underlies St. Croix and predominates in the mountainous areas around the island (Robison, 1972). The carbonate rock and alluvial deposits are principally in the southerly plains. The carbonate rock is comprised of the Jealousy Formation of Eocene to middle Miocene age, the Kingshill Limestone of Miocene to Pliocene age (Gerhard and others, 1978), and post-Kingshill Carbonates of Pliocene and younger age (Gill and Hubbard, 1986). The results of the test-hole drilling on St. Croix revealed the following lithologies (table 1). ALLUVIAL DEPOSITS-gravel, sand, silt, and clay, with the dominating grain size varying over the study area. The alluvial deposits are localized and not laterally extensive. A thin 1- to 2-foot topsoil generally overlies the alluvial deposits. The alluvial deposits range in thickness from 8 to 76 feet. Geology 23 I o (D (D < o| Ia £io (D in 0) Q. z. in o i O 5 ^x w 65°00' 65°00' 18°22' 18'19* 65°00' ST; THOMAS EXPLANATION ^ Turpentine Run drainage basin Topographic contours. Contour interval variable r r*~~ ( Surface drainage 0) I Figure 9. General drainage features of St. Thomas, U.S. Virgin Islands. ff) (D Os 18°22' 64°47' ATLANTIC OCEAN 64°47' ST. JOHN 0 1 2 KILOMETERS 2 MILES CARIBBEAN SEA EXPLANATION Topographic contours. Contour interval variable W Surface drainage Figure 10. General drainage features of St. John, U.S. Virgin Islands. ST. JOHN O ST. THOMAS iiilliilllh JFMAMJJASOND Figure 11. Mean monthly rainfall for St. Croix, St. Thomas, and St. John, U.S. Virgin Islands, 1982-92 (Data from U.S. Department of Commerce, National Oceanic and Atmospheric Administration, 1982 and 1992). KINGSHILL LIMESTONE-generally a white to buff marl which is locally fractured and commonly has iron and manganese oxide stains in the fracture zones. The top of the Kingshill Limestone is at land surface or underlies the thin topsoil or alluvial deposits. Where the bottom of the Kingshill Limestone was penetrated by test holes, the limestone had a thickness ranging from 50 to greater than 217 feet. JEALOUSY FORMATION-Generally a bluish-gray marl. The Jealousy Formation underlies the Kingshill Limestone. The top of the Jealousy Formation, where penetrated by test holes, ranged from 88 to 127 feet below land surface. The thickness of the Jealousy Formation has been estimated to exceed 1,400 feet (Gill and Hubbard, 1986). VOLCANIC ROCK-blue to olive gray in color, dense with minor fractures, locally contains zones of highly fractured or shattered rock. The drilling of most test holes on St. Croix was halted when the volcanic rock was encountered, so a thickness was not determined for this unit. St. Thomas and St. John The geology of St. Thomas is similar to that of St. John. The islands were formed primarily of volcanic rocks (Donnelly, 1959). The alluvial deposits on each island are present only in the small coastal embayments surrounding the islands and in the valley of Turpentine Run on St. Thomas (Cosner, 1972; Jordan and Cosner, 1973). Carbonate rock is found only rarely on St. Thomas and St. John and is comprised primarily of the Outer Brass Limestone (Donnelly, 1959). The results of the test-hole drilling on St. Thomas and St. John revealed the following lithologies (tables 2 and 3). ALLUVIAL DEPOSITS-sand and gravel, with some cobbles and boulders, locally contains thin lenses of silt and clay. Locally a 1- to 2-foot top soil overlies the alluvial deposits. Also present locally overlying the alluvial deposits is fill material deposited for land surface build-up for local construction. VOLCANIC ROCK-greenish to gray in color, dense, indurated, with horizontal and vertical fractures. Locally the volcanic rock is a highly weathered volcanic material (regolith) that occurs as saprolite. Localized zones of highly fractured to shattered volcanic rock are also present. 26 Hydrogeology in the Vicinity of Test Holes and Wells on St. Croix, St. Thomas, and St. John, U.S. Virgin Islands GROUND-WATER HYDROLOGY Ground water on St. Croix, St. Thomas, and St. John occurs in the alluvial deposits and carbonate and volcanic rocks. On St. Croix, the principal sources of ground water are the carbonate rock and the overlying alluvial deposits; however, on St. Thomas and St. John, the principal sources of ground water are the volcanic rock and overlying alluvial deposits (Gomez- Gomez and others, 1985). Ground-Water Occurrence and Movement Ground-water occurs under water-table conditions on St. Croix and is present principally in the alluvial deposits and the Kingshill Limestone deposits in the southern plains of the island. Because the alluvial deposits are hydraulically connected to the Kingshill Limestone, the alluvial deposits and Kingshill Limestone are considered to be a single hydrologic unit in which the alluvial deposits are saturated. The bottom of this hydrologic unit is the top of the Jealousy Formation, which underlies all of south St. Croix. The Jealousy Formation has been described by Jordan (1975) as a unit of low permeability. Clay facies in the Jealousy Formation are believed to act as a boundary for the overlying Kingshill Limestone and alluvial deposits. Only 7 of 30 wells were drilled into the volcanic rocks on St. Croix (table 1). Of these 7 wells, 2 were back filled and 1 was used as an observation well. Consequently, because of sparse data, a detailed discussion of the occurrence and movement of ground water in the volcanic rocks of St. Croix is not possible. However, when the results of the test-hole drilling into the volcanic rock of St. Thomas and St. John are discussed, it may be assumed that this information could be transferable to the island of St. Croix in terms of ground-water availability. Ground-water occurs under water-table conditions on St. Thomas and St. John and is present primarily in the regolith and zones of highly fractured to shattered volcanic rock. As with the Kingshill Limestone on St. Croix, the alluvial deposits on St. Thomas and St. John are hydraulically connected to the underlying volcanic rock and, subsequently, a single hydrologic unit is assumed to exist in these two units where the alluvial deposits are saturated. The bottom of the water-table aquifer on St. Thomas and St. John is the dense indurated volcanic rock underlying the regolith and zones of highly fractured rock. The low permeability of this indurated rock limits vertical ground-water movement and subsequently acts as the boundary of the overlying rock and alluvial deposits. Water-table fluctuation occurs seasonally in response to rainfall variation and can fluctuate daily in wells located near pumping wells. Depth below land surface to the water table can range from approximately 4 to 62 feet on St. Croix (figs. 12 and 13), 3 to 74 feet on St. Thomas (figs. 14 and 15), and 8 to 60 feet on St. John (figs. 16 and 17). Because of the complexity of the aquifers on St. Croix, St. Thomas, and St. John it is impossible to complete an islandwide potentiometric map of any of the islands. Isolated potentiometric maps have been completed by Jordan and Cosner, (1973), Graves and Gonzalez, (1988), and Torres-Gonzalez and Rodriquez del Rio, (1990). In general the water-table surface on St. Croix, St. Thomas, and St. John parallels the topography and ground-water movement is down gradient, perpendicular to the topographic contour lines, towards the sea. The quantity of water recharging the Kingshill Limestone aquifer on St. Croix has been estimated to be 3 percent of annual rainfall (Robison, 1972). On St. Thomas, aquifer recharge is infrequent and probably occurs only after periods of heavy rainfall or a series of lesser rains (Jordan and Cosner, 1973). Jordan and Cosner postulated that because of relatively low annual rainfall and high average annual temperatures, most recharge occurred after major storms, which produced more than 2 inches of rainfall. On St. John, Cosner (1972), indicated that because of similarities between the soils of St. John and St. Thomas, recharge on St. John would be similar to that which occurs on St. Thomas. Large water-table rises corresponding with large amounts of rainfall (figs. 12 - 17) are strong evidence of the occurrence of aquifer recharge following a major rainstorm. Ground-Water Hydrology 27 < Q. O (Q (D O O (Q <5' ^ 3! CD (5" = c 5? 3 P-ro 03 Q. fa §3 (D W X g_ (D Q)2 Q. I (Q5' SfL Q)3 Q. => 2 ^§ = Q. W ^ r* ^ O CD 5 > CO O O> O =^' CD ! ? -5 03 < 5 > Q- "D CD < CD < O > ^ "D i" 51" o o C CO CQ - zr 03 CD < If CD Zj CD- > Q. CD CO CO C_ > O) O z. a O) o 33 O X 33 co > COo a> O VIWAPA 23A, BARREN SPOT USGS OBSERVATION WELL 174319064454401 VIWAPA 02, FAIRPLAINS USGS OBSERVATION WELL 174225064472000 cc 10 P I 20 30 40 VIWAPA 06/DPW 06, GOLDEN GROVE USGS OBSERVATION WELL 174243064475100 MJJ ASOND 1990 JFMAMJJ ASOND 1991 JFMAMJJ AS 1992 15 CO UJ I 10 ST. CROIX RAINFALL MJJ ASOND 1990 JFMAMJJ ASOND 1991 JFMAMJJ AS 1992 Figure 13. Daily mean ground-water levels in wells VIWAPA 06/DPW 06, Golden Grove, VIWAPA 02, Fairplains, and VIWAPA 23A, Barren Spot, and monthly rainfall, St. Croix, U.S. Virgin Islands, May 1990 through September 1992. Ground-Water Hydrology 29 21 25 30 33 30 32 GRADE SCHOOL 03, ST. THOMAS USGS OBSERVATION WELL 182038064550300 STT-VIEO-11, ST. THOMAS USGS OBSERVATION WELL 182035064550200 Hi CD w 34 LLJ > 3g i i i i i i i i i i i i i i i i i i i i i i i i i i i i n i IT 22 24 26 28 30 4-7l STT-VIEO-06, ST. THOMAS USGS OBSERVATION WELL 182038064580000 MJJ ASOND 1990 JFMAMJJ ASOND 1991 JFMAMJJ AS 1992 10 CO HI Xo ST. THOMAS RAINFALL MJJ ASOND 1990 J FMAMJ J ASOND 1991 JFMAMJJ AS 1992 Figure 14. Daily mean ground-water levels in wells STT-VIEO-06, STT-VIEO-11, and Grade School 03, St. Thomas, and monthly rainfall, St. Thomas, U.S. Virgin Islands, May 1990 through September 1992. 30 Hydrogeology in the Vicinity of Test Holes and Wells on St. Croix, St. Thomas, and St. John, U.S. Virgin Islands cc O 16 CO STT-VIEO-09, ST. THOMAS USGS OBSERVATION WELL 181917064524600 . STT-VIEO-10, ST. THOMAS USGS OBSERVATION WELL 182131064541000 18 20 - 22 a 24 < 68 70 72 74 76 STT-VIEO-01, ST. THOMAS USGS OBSERVATION WELL 182036064545200 MJJASOND 1991 MJJASONDS 1992 co 6 LLJI O £ 4 ST. THOMAS RAINFALL MJJASOND 1991 MJJASONDS 1992 Figure 15. Daily mean ground-water levels in wells STT-VIEO-01, STT-VIEO-10, and STT-VIEO-09, St. Thomas, and monthly rainfall, St. Thomas, U.S. Virgin Islands, May 1991 through September 1992. Ground-Water Hydrology 31 OZ! o (Q O <§ «< w 0) a. I w o o U) a a. o li o p Ig 03 < 3 CQ Zo &T 0) = CD c^^- r* CD&i ^- CO p 5- CO 'Q. O ^ c 0) CD. Q.-~ cn O CO °c^ =± CO ^ 05 O ^ CQ O" 0) a w CD o I? ^ O) II ro RAINFALL, IN INCHES WATER LEVEL, IN FEET BELOW LAND-SURFACE DATUM e_ e_ 0> O n c_ Tl S > S C_ c_ > O) O Z 0 c_ Tl S > S C_ e_ > O) - - - - - - - - w c_ U z TJ Z Tl P - - - 1 - - - - - I.I.I - - - - 52.0 52.5 Q LU LL53.0 cc 10 1" LJJ CD LJJ LJJ LLZ 12 _f LJJ cc 13 [^ < 21 22 23 24 25 STJ-VIEO-03, ST. JOHN USGS OBSERVATION WELL 181950064422300 STJ-VIEO-04, ST. JOHN USGS OBSERVATION WELL 182048064430400 STJ-VIEO-02, ST. JOHN USGS OBSERVATION WELL 182110064430000 MJJASOND 1991 MJJASONDS 1992 C/D 6 LJJI O ? 4 I 2 Ql I ill l l l __L ST. JOHN RAINFALL MJJASOND 1991 MJJASONDS 1992 Figure 17. Daily mean ground-water levels in wells STJ-VIEO-02, 04, and 03, St. John, and monthly rainfall, St. John, U.S. Virgin Islands, May 1991 through September 1992. Ground-Water Hydrology 33 Hydraulic Characteristics of the Aquifers The ground-water development potential of any aquifer is dependent upon hydraulic characteristics of transmissivity and storage coefficient. Transmissivity is a measure of the ability of the aquifer to transmit water and is defined as the product of the hydraulic conductivity and the aquifer thickness. The storage coefficient is the volume of water an aquifer releases from or takes into storage per unit surface area of the aquifer per unit change in head. During this study, estimated aquifer transmissivities ranged from 200 to 2,500 feet squared per day in the fractured volcanic rock aquifers of St. Thomas and St. John (table 5). Transmissivity values based on data for selected wells were determined from specific capacity tests. The specific capacity tests were conducted for a period of 8 hours and values determined from these tests ranged from 1 to 11 gallons per minute per foot of drawdown. The specific capacity values were converted to transmissivity using a method described by Meyer (1963), that relates well diameter, specific capacity, and the aquifer storage coefficient to aquifer transmissivity. Aquifer storage coefficients were not determined from field tests for this report, but rather are assumed to be typical water- table storage coefficients, which range from 0.1 to 0.3 (Lohman, 1979). The specific capacity/aquifer transmissivity relation developed by Meyer (1963) assumes a water-table storage coefficient of 0.1. Field tests to determine specific capacity and subsequently aquifer transmissivity were conducted only on St. Thomas and St. John. However, Graves (1995) and Torres-Gonzalez (1991) reported transmissivity values in the Kingshill Limestone ranging from 200 to 4,000 feet squared per day for St. Croix. Table 5. Transmissivity estimated from specific capacity at selected wells on St. Thomas and St. John, U.S. Virgin Islands (Estimates based on method described by Meyer, 1963) [(gal/min)/ft, gallons per minute per foot of drawdown; ft /d, feet squared per day; STT, St. Thomas; EGWS, emergency ground-water supply; STJ, St. John; SJWS, St. John water supply] Well 37 38 39 41 42 45 49 51 52 53 56 70 number and name1 STT-EGWS-01 STT-EGWS-02 STT-EGWS-03 STT-EGWS-04 STT-EGWS-05 STT-EGWS-06 STT-EGWS-07 STT-EGWS-08 STT-EGWS-09 STT-EGWS-10 STT-EGWS-11 STJ-SJWS-02 Specific capacity [(gal/min)/fl] 1 1 3 2 3 4 2 11 2 2 2 9 Transmissivity (ft2/d) 200 200 600 400 600 800 400 2,500 400 400 400 2,000 Length of test (hours) 8 8 8 8 8 8 8 8 8 8 8 8 1 Well number refers to well location as shown on figures 5, 6, or 7. 34 Hydrogeology in the Vicinity of Test Holes and Wells on St. Croix, St. Thomas, and St. John, U.S. Virgin Islands The relatively large range of transmissivity values is due to the heterogeneity of the aquifer material. On St. Croix the higher values of transmissivity in the Kingshill Limestone are common primarily in areas where there is a high density of intersecting fracture zones. On St. Thomas and St. John areas of high transmissivities will also coincide with areas of high fracture density within the volcanic rock aquifer. The large range of transmissivity values on St. Croix may be due to localized areas of dissolution along the fracture zones resulting in large openings and, subsequently, the high transmissivity values. These values of transmissivity may be larger than they would have been because of the presence of the overlying alluvial deposits where the deposits are sufficiently thick and saturated. In the alluvial deposits, where the percentage of sand or gravel is high the transmissivity will be high; conversely, where the percentage of silt or clay is high the transmissivity will be low. The development of ground-water supplies from the aquifers of the US VI is complicated due to the nature of the aquifer materials (limestone, marl, and dense volcanic rock) and the localized extent of fracture zones within the aquifers. However, according to the literature, the chances of producing a well that might yield sufficient quantities of water for emergency use are greatly improved if the wells are drilled in the draws or valleys of principal drainage areas (Heath, 1980; Daniels and Sharpless, 1983; and Daniels, 1987). This concept was applied in choosing the location of most test holes drilled with reasonably acceptable results. If the number of test holes drilled for observation wells (12) and test holes back filled (22) are subtracted from the total test holes drilled (61), then by island, the percentage of emergency water-supply wells completed to test holes drilled was 50 percent for St. Croix (15 wells completed of 30 test holes drilled), 52 percent for St. Thomas (11 wells completed of 21 test holes drilled), and 10 percent for St. John (1 well completed of 8 test holes drilled) (tables 1, 2, and 3). Well yields for the 27 emergency supply wells ranged from about 14 to 80 gallons per minute (tables 1, 2, and 3). WATER QUALITY In order to determine the quality of ground water on St. Croix, St. Thomas, and St. John, 33 wells were sampled for common cations and anions and trace elements (tables 6, 7, and 8). Using the Piper diagram (Piper, 1953), results of these analyses indicate that ground water in the U.S. Virgin Islands is predominantly a sodium-chloride type water (figs. 18, 19, and 20). However, on St. Thomas, data for water samples from a majority of the wells sampled indicated that the ground water is a mixed type; no one cation or anion exceeded 50 percent of the total milliequivalents of cations or anions in water from 9 of 11 wells sampled. In an island setting, ground water that is a sodium chloride type water commonly is considered to contain a mixture of freshwater and seawater (Gomez- Gomez, 1984). The source of the seawater could be lateral migration of seawater from the ocean through the aquifer, upconing of saline water as a result of excessive pumping, or connate seawater that has not been completely flushed out of the aquifer. To determine the source of the seawater, long-term water- quality data must be collected and seasonal changes in chloride concentrations compared. If the seasonal variations in the chloride concentrations are small, connate water is the most likely source of the sodium chloride, assuming that areal ground-water pumping is relatively constant. If the variations in chloride concentration are large, the source of the sodium chloride is most likely upconing or lateral migration (intrusion) of seawater that coincides with seasonal water-level fluctuations in the aquifer, and the related movement of the saltwater-freshwater interface. The limited timeframe of this study did not allow for long- term monitoring of seasonal water-quality changes. However, recent data available for the island of St. Croix (Graves, 1995) indicate that the source of the sodium chloride in wells STX-EGWS- 06, 07, 08, 14, and 15 could be connate water, and that the source in wells STX-EGWS- 09 and 10 is possibly seawater intrusion. To adequately discuss the source of seawater in the aquifers on St. Thomas and St. John, more information is needed. Water Quality 35 Table 6. Chemical analyses of ground water on St. Croix, U.S. Virgin Islands [[iS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; STX, St. Croix; EGWS, emergency ground-water supply; --, data not available; <, less than detection limit which is number following symbol] 0 If* jeology in the Vicinity o a o les and > 2. eo" o (A o 3 -i 'homas, ar 0. (A O C (A el5' eo" D) 0. CO Well number and name1 2 STX-EGWS-01 3 STX-EGWS-02 5 STX-EGWS-03 7 STX-EGWS-04 9 STX-EGWS-05 12 STX-EGWS-06 13 STX-EGWS-07 15 STX-EGWS-08 18 STX-EGWS-09 19 STX-EGWS-10 20 STX-EGWS-11 22 STX-EGWS-12 24 STX-EGWS-13 25 STX-EGWS-14 30 STX-EGWS-15 Date 10-05-92 10-06-92 10-05-92 10-27-92 10-27-92 10-06-92 10-06-92 10-07-92 10-26-92 10-26-92 10-28-92 11-02-92 10-27-92 10-27-92 10-07-92 Specific conductance (lab value, US/cm) 1,470 1,500 1,460 2,840 2,600 2,040 2,220 2,080 3,650 3,750 2,430 4,380 1,990 2,190 2,150 pH (field value, standard units) 7.0 7.0 7.1 7.5 7.3 7.1 7.0 7.2 7.2 7.1 7.6 7.1 7.3 6.9 7.2 Hardness (mg/L as CaCO3 440 460 470 150 290 400 470 390 430 450 110 480 380 540 420 Alkalinity, lab (mg/L as CaCO3) 361 313 424 519 565 572 541 548 402 396 479 401 438 370 525 Calcium dissolved (mg/L as Ca) 120 130 120 31 50 72 85 65 84 88 31 130 67 110 66 Magnesium, dissolved (mg/L as Mg) 35 32 41 17 41 54 62 56 53 57 7 38 51 64 63 Sodium, dissolved (mg/L as Na) 120 120 120 560 460 300 310 310 650 650 500 750 280 260 320 Potassium, dissolved (mg/L as K) 3.5 1.3 1.5 2.4 1.2 1.1 1.1 0.8 3.3 3.1 6.2 7.5 1.1 1.8 0.9 Sulfate, dissolved (mg/L as S04) 37 39 40 180 130 120 130 120 230 230 130 300 95 100 100 Table 6. Chemical analyses of ground water on St. Croix, U.S. Virgin Islands-Continued i D SL Well number and name1 2 STX-EGWS-01 3 STX-EGWS-02 5 STX-EGWS-03 7 STX-EGWS-04 9 STX-EGWS-05 12 STX-EGWS-06 13 STX-EGWS-07 15 STX-EGWS-08 18 STX-EGWS-09 19 STX-EGWS-10 20 STX-EGWS-11 22 STX-EGWS-12 23 STX-EGWS-13 25 STX-EGWS-14 30 STX-EGWS-15 Date 10-05-92 10-06-92 10-05-92 10-27-92 10-27-92 10-06-92 10-06-92 10-07-92 10-26-92 10-26-92 10-28-92 11-02-92 10-27-92 10-27-92 10-07-92 Chloride, dissolved (mg/L as Cl) 220 260 200 480 410 230 360 300 750 770 360 980 290 350 340 Fluoride dissolved (mg/L as F) 0.5 0.5 0.5 0.8 0.9 0.4 0.4 0.5 0.8 0.7 0.6 0.4 0.5 0.5 0.7 Silica, dissolved (mg/L as SiO2) 35 37 34 30 42 51 45 46 39 41 16 26 42 31 46 Dissolved solids (mg/L) 787 808 811 1,610 1,470 1,170 1,320 1,230 2,050 2,080 1,340 2,470 1,090 1,140 1,250 Nitrogen, NO2+NO3, dissolved (mg/L as N) - - - 5.2 3.3 - - - 3.8 3.5 10.0 3.5 3.4 3.2 - Iron, ,. Barium . ' Manganese, .. . . dissolved .. , ' dissolved . ,. dissolved , .. (ug/L as . .. ... (UQ/L as Fe) WLasMn) ^ 327 7 1 8 4 6 10 20 <10 <100 <10 <10 <100 10 10 <100 <10 10 <100 <10 <10 <100 <10 <10 <100 <10 <10 <100 <10 <10 <100 10 <10 <100 3 1 44 <10 40 <100 <10 <10 <100 Aluminum, dissolved (\ig/L as Al) 20 20 10 10 <10 <10 <10 10 <10 <10 20 30 :: <10 Table 6. Chemical analyses of ground water on St. Croix, U.S. Virgin Islands-Continued ogeology in the Vici fo o 3.x £ 5! o3 0) 0) Q. c_o3- C (Q5' to" 0)3 Q. (0 [|j.S/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; NPS, National Park Service; VIWAPA, Virgin Islands Water and Power Authority; Islands; USGS, U.S. Geological Survey; STJ, St. John; SJWS, St. John Water Supply; <, less than detection limit which is number following Well number and ... , 1 Date name 1 59 NPS-03, Cruz Bay 05-20-9 1 61 VIWAPA DPW-02, 06-04-91 Susannaberg 62 VIWAPA DPW-05, 06-04-9 1 Susannaberg 64 NPS-05, Trunk Bay 05-20-91 65 Cinnamon Bay 05-21-91 campground 67 VI Government/ 03-09-90 VIWAPA well (USGS-14A) 70 STJ-SJWS-02 05-20-91 Specific conductance (lab value, |j.S/cm) 2,280 1,770 1,520 1,720 2,020 825 2,220 pH (field value, standard units) 7.1 7.1 7.1 7.2 7.2 7.1 6.7 Hardness (mg/L as CaCO3 490 570 390 470 510 150 370 Alkalinity, lab (mg/L as CaCO3) 462 615 563 463 519 113 236 Calcium dissolved (mg/L as Ca) 86 100 69 100 86 24 49 Magnesium, dissolved (mg/L as Mg) 68 77 53 54 72 21 60 symbol; , Sodium, dissolved (mg/L as Na) 330 150 180 180 250 120 310 VI, Virgin indicates data not available] Potassium, dissolved (mg/L as K) 2.2 2.6 3.4 3.1 2.8 4.8 6.9 Sulfate, dissolved (mg/L as S04) 54 24 24 52 24 21 93 0) (D oc Table 8. Chemical analyses of ground-water on St. John, U.S. Virgin Islands-Continued Well number and name1 59 NPS-03 Cruz Bay 61 VIWAPA DPW-02, Susannaberg 62 VIWAPA DPW-05, Susannaberg 64 NPS-05, Trunk Bay 65 Cinnamon Bay campground 67 VI Government/ VIWAPA well (USGS-14A) 70 STJ-SJWS-02 Date 05-20-91 06-04-91 06-04-91 05-20-91 05-21-91 03-09-90 05-20-91 Chloride, dissolved (mg/L as Cl) 400 230 170 270 360 250 540 Fluoride dissolved (mg/L as F) 0.4 0.4 0.6 0.5 0.9 0.2 0.2 Silica, dissolved 36 43 37 38 40 27 34 Dissolved solids (mg/L) 1,250 996 875 975 1,150 542 1,230 Nitrogen, NO2+NO3, dissolved (mg/L as <0.05 0.57 1.10 <0.05 <0.05 0.10 1.40 Iron, ., Barium ,. . ' Manganese, ,. . , dissolved ,. , , dissolved dissolved (UQ/L as , . , > (UQ/L as Vf jle) (^ig/L as Mn) vp|a) 10 <10 <100 6 <1 19 8 <1 5 44 <1 66 <10 <10 <100 5,600 310 49 270 70 <100 Aluminum, dissolved - - - - - <10 - X a 3 (O I o (O «< o_ (D (0 0) a I w o O 3 o Oi y> a3a Table 8. Chemical analyses of ground-water on St. John, U.S. Virgin Islands-Continued Well number and name 1 Date Arsenic, dissolved (jig/L as As) Cadmium, dissolved (jig/L as Cd) Chromium, dissolved (jig/L as Cr) Mercury, dissolved (jig/L as Hg) Selenium, dissolved (jig/L as Se) Silver, dissolved (jig/L as Ag) Lead, dissolved (jig/L as Pb) 59 NPS-03, Cruz Bay 05-20-91 61 VIWAPA DPW-02, 06-04-91 Susannaberg 62 VIWAPA DPW-05, 06-04-91 Susannaberg 64 NPS-05, Trunk Bay 05-20-91 05-21-91 1 65 Cinnamon Bay campground 67 VI Government/ VIWAPA well (USGS-14A) 70 STJ-SJWS-02 0.4 03-09-90 05-20-91 <1 <1 <1 <1 1 Well number refers to well location as shown on figure 6 or 7. (5' ^ <§> ^ EXPLANATION ^ 2 « x .25 ' Ca Calsium 5 » 22 K Potassium 19 » Mg Magnesium v 13* p4 18V Na Sodium _jr"i4 Cl Chloride CO3 Carbonate HCO3 Bicarbonate v SO4 Sulfate ' O O Figure 18. Diagram showing major constituents in ground water in wells on St. Croix, U.S. Virgin Islands. Values shown are percent of total milliequivalents per liter. Numbers refer to wells in table 6. * 9> X* * EXPLANATION Ca Calsium K Potassium Mg Magnesium Na Sodium Cl Chloride CO3 Carbonate HCO3 Bicarbonate S04 Sulfate s/ Figure 19. Diagram showing major constituents in ground water in wells on St. Thomas, U.S. Virgin Islands. Values shown are percent of total milliequivalents per liter. Numbers refer to wells in table 7. Water Quality 45 X»* .70 61" 6.4 59 67 65 62 EXPLANATION Ca K Mg Na Cl CO3 HCO3 SO. Calsium Potassium Magnesium Sodium Chloride Carbonate Bicarbonate Sulfate Figure 20. Diagram showing major constituents in ground water in wells on St. John, U.S. Virgin Islands. Values shown are percent of total milliequivalents per liter. Numbers refer to wells in table 8. Chloride concentrations in ground-water samples collected from all wells drilled (tables 1, 2, and 3) and from wells sampled for analysis of common cations and anions (tables 6,7, and 8) ranged from 116 to 3,870 milligrams per liter (mg/L). The dissolved solids concentrations in water sampled for analysis of common cations and anions (tables 6, 7, and 8) ranged from 542 to 2,470 mg/L. The U.S. Environmental Protection Agency's (EPA, 1992) Secondary Maximum Contaminant Level for chloride and dissolved solids concentrations is 250 and 500 mg/L, respectively. Of the water sampled for analysis of common cations and anions (tables 6, 7, and 8), these levels were exceeded in 61 percent of the wells for chloride concentrations and in 100 percent of the wells for dissolved solids concentrations. However, the recommended maximum level for chloride is based largely on taste; water having a chloride concentration as high as 500 mg/L can be potable. For dissolved solids the Secondary Maximum Contaminant Level is based on an increase in the mineral taste in the water and possible economic consequences due to deterioration of plumbing and pipes because of the high mineral content. SUMMARY Following the devastation by Hurricane Hugo on September 17 and 18, 1989, the islands of St. Croix, St. Thomas, and St. John, were without drinking water because of the damage to the infrastructure that provides water to the islands. However, on St. Croix, several well fields which had seen limited use in recent years were put on line and connected to the water distribution system. Recognizing the need for a better understanding of the hydrogeology of the USVI, the U.S. Geological Survey, in cooperation with the U.S. Office of Management and Budget, conducted an investigation from May 1990 through September 1992 to determine the occurrence and quality of ground water so emergency ground-water supplies could be developed to mitigate natural or man induced disasters. 46 Hydrogeology in the Vicinity of Test Holes and Wells on St. Croix, St. Thomas, and St. John, U.S. Virgin Islands The geology of the U.S. Virgin Islands is varied. The geology of St. Croix includes alluvial deposits and carbonate and volcanic rocks. The geology of St. Thomas and St. John is dominated by volcanic rock with local alluvial deposits in small coastal embayments. Carbonate deposits are present on St. Thomas and St. John but only in minor isolated outcrops. Ground water occurs under water-table conditions on St. Croix, St. Thomas, and St. John. On St. Croix, the principal sources of ground water are the carbonate rock and overlying alluvial deposits. On St. Thomas and St. John the principal sources of ground water are the regolith and zones of highly fractured to shattered volcanic rock and overlying alluvial deposits. Depth below land surface to the water table ranges from 4 to 62 feet on St. Croix, 3 to 74 feet on St. Thomas, and 8 to 60 feet on St. John. Aquifer transmissivities range from 200 to 4,000 feet squared per day on St. Croix and from 200 to 2,500 feet squared per day on St. Thomas and St. John. Well yields for the emergency-supply wells on St. Croix, St. Thomas, and St. John range from 14 to 80 gallons per minute. Ground water in the U.S. Virgin Islands is predominantly a sodium chloride type. Ground-water samples collected from selected wells had chloride concentrations ranging from 116 to 3,870 milligrams per liter and dissolved solids concentrations ranging from 542 to 2,470 milligrams per liter. On St. Croix, depending on location, the source of sodium chloride is connate water or seawater intrusion. Data were not available to determine the source of sodium chloride on St. Thomas or St. John. REFERENCES Cederstrom, D.J., 1950, Geology and ground-water resources of St. Croix, Virgin Islands: U.S. Geological Survey Water-Supply Paper 1067,117 p., 6 pis. Cosner, O.J., 1972, Water in St. John, U.S. Virgin Islands: U.S. Geological Survey Open-File Report 72-78, 46 p. Daniels III, C.D., 1987, Statistical analysis relating well yield to construction practices and sitting of wells in the Piedmont and Blue Ridge provinces of North Carolina: U.S. Geological Survey Water- Resources Investigations Report 86-4132, 54 p. Daniels III, C.D., and Sharpless, N.B., 1983, Ground- water supply potential and procedures for well- site selection in the upper Cape Fear river basin, North Carolina: Cape Fear Basin Study, North Carolina Department of Natural Resources and Community Development and U.S. Water Resources Council, 73 p. Donnelly, T.W, 1959, Geology of St. Thomas and St. John, Virgin Islands: A dissertation presented to the faculty of Princeton University in candidacy for the degree of Doctor of Philosophy, accepted by the Department of Geology, May 2, 1959, 179 P- Gerhard, L.C., Frost, S.H., and Curth, P.J., 1978, Stratigraphy and depositional setting, Kingshill Limestone, Miocene, St. Croix, U.S. Virgin Islands: Amer. Assoc. Petrol. Geol. Bull., v. 62, no. 3, p. 403-418. Gill, I.P., and Hubbard, O.K., 1986, Subsurface geology of the St. Croix carbonate rock system: Water Resources Research Institute, College of the Virgin Islands, Technical Report No. 26, 71 p. ___ 1987, Subsurface geology of the St. Croix carbonate rock system, phase II: Water Resources Research Institute, College of the Virgin Islands, Technical Report No. 28, 66 p. Gomez-Gomez, Fernando, 1984, Water resources of the lower Rio Grande de Manati valley, Puerto Rico: U.S. Geological Survey Water-Resources Investigations Report 83-4199, 42 p. Gomez-Gomez, Fernando, Quinones-Marquez, Ferdinand, and Zack, Alien, 1985, U.S. Virgin Islands, Ground-Water Resources: U.S. Geological Survey Water-Supply Paper 2275, p. 409-413. Graves, R.P., 1992, Geohydrology of the Aguirre and Pozo Hondo areas, southern Puerto Rico: U.S. Geological Survey Water-Resources Investigations Report 91-4124, 43 p. References 47 ___ 1995, Hydrogeology of south-central St. Croix, U.S. Virgin Islands: U.S. Geological Survey Water-Resources Investigations Report 93-4162, 33 p. Graves, R.P., and Gonzalez, Ralph, 1988, Potentiometric surface of the Turpentine Run basin aquifer in the Tutu area, eastern St. Thomas, September 11, 1987: U.S. Geological Survey Water-Resources Investigations Report 88-4131, Ipl. Heath, R.C., 1980, Basic elements of ground-water hydrology with reference to conditions in North Carolina: U.S. Geological Survey Open-File Report 80-44, 86 p. Hubbard, D.K., editor, 1989, Terrestrial and marine geology of St. Croix, U.S. Virgin Islands; Special Publication Number 8, West Indies Laboratory, Teague Bay, St. Croix, ??p. Jordan, D.G., 1975, A survey of the water resources of St. Croix, Virgin Islands: U.S. Geological Survey Open-File Report, 51 p. Jordan, D.G., and Cosner, O.J., 1973, A survey of the water resources of St. Thomas, Virgin Islands: U.S. Geological Survey Open-File Report 72-201, 55 p. Lohman, S.W., 1979, Ground-water hydraulics: U.S. Geological Survey Professional Paper 708, 70 p. Mathey, S.B., 1990, National water information system user's manual, volume 2, chapter 4. Ground-Water Site Inventory System: U.S. Geological Survey Open-File Report, 89-587,283 P- Meyer, R.R., 1963, A chart relating well diameter, specific capacity, and the coefficient of transmissibility and storage, in Bentall, Ray, compiler, Methods of determining permeability, transmissivity, and drawdown: U.S. Geological Survey Water- Supply Paper 1536-1, p. 338-340. Piper, A.M., 1953, A graphic procedure in the geochemical interpretation of water analyses: in U.S. Geological Survey Ground Water Notes, Geochemistry, no. 12, p. 1-14. Rivera, L.H., Fredrick, W.D., Farris, Cornelius, Jensen, E.H., Davis, Lyle, Palmer, C.D., Jackson, L.F., and McKinzie, W.E., 1970, Soil survey of the Virgin Islands of the United States: United States Department of Agriculture Soil Conservation Service, 78 p. Robison, T.M., 1972, Ground water in central St. Croix, U.S. Virgin Islands: U.S. Geological Survey Open-File Report, 18 p. Torres-Gonzalez, Sigfredo, and Rodriguez-del-Rio, Felix, 1990, Potentiometric surface of the Kingshill aquifer and hydrologic conditions, St. Croix, U.S. Virgin Islands, July 1987: U.S. Geological Survey Water-Resources Investigations Report 89-4085, 1 pi. Torres-Gonzalez, Sigfredo, 1991, Steady-state simulation of ground-water flow conditions in the Kingshill aquifer, St. Croix, U.S. Virgin Islands, July 1987, in American Water Resources Association International Symposium on Tropical Hydrology, July 23 through 27, 1990, San Juan, Puerto Rico: AWRA Monograph Series No. 15, p. 93-108. U.S. Department of Commerce, 1982 to 1992, Climatological data annual summary - Puerto Rico and the Virgin Islands: National Oceanic and Atmospheric Administration, v. 28-38. U.S. Environmental Protection Agency, 1992, Secondary maximum contaminant levels (part 143, National primary drinking water regulations): U.S. Code of Federal Regulations, Title 40, Parts 100 to 149, revised as of July 1, 1992, p. 772-776. Whetten, J.T., 1966, Geology of St. Croix, U.S. Virgin Islands: Geol. Soc. America Mem., 98, p. 177- 239. 48 Hydrogeology in the Vicinity of Test Holes and Wells on St. Croix, St. Thomas, and St. John, U.S. Virgin Islands