Six aquifer systems underlie the ACE Basin. They encompass rocks of the Late Cretaceous-age Cape Fear, Middendorf, and Black Creek formations, the Tertiary-age Black Mingo Formation, Santee and Ocala limestones, Cooper Formation, and a veneer of Quaternary-age deposits mainly associated with barrier-island formation. Review the Geologic Time Scale for geologic ages. Each aquifer system has unique and diagnostic combinations of lithology, hydraulics, and water chemistry, and the aquifer systems are separated by confining units 18 to 76 meters (60 to 250 feet) thick. The six aquifer systems also share the characteristic condition of localized saltwater intrusion. Ground water supplies almost all water demand. The public water supply systems for Edisto Beach and Walterboro are the largest users. Private domestic wells are used elsewhere and account for most of the water pumped.
Information provided in this summary is derived principally from the publications listed at the end of the section. Files of the South Carolina Department of Natural Resources (SCDNR) Land, Water, and Conservation Division provided supplemental information on well depths and yields, aquifer hydraulics, and water chemistry. (See hydrologic sections and selected wells )
There are no reports devoted specifically to the hydrogeology of the ACE Basin, but a number of ground-water study areas include, overlap, or lie near the basin, and they provide insight into local conditions. Siple (1965 and 1967) summarized saltwater encroachment along the South Carolina coast and presented geologic sections, an Eocene-limestone structure map, and a chloride-distribution map, all of which included the ACE Basin. The ACE Framework Study (South Carolina Water Resources Commission 1972) was a planning study that summarized geologic, water-resource, and climate conditions in the basin and surrounding counties. Ground-water conditions in the South Carolina Low Country were investigated by Hayes (1979), and his geologic sections, structure maps, and water-level maps extend into the basin. Potential contamination sites and lithologic and background water-quality data for shallow aquifers of the lower coastal plain were described in a nine-volume report (Glowacz et al. 1980). A Trident-area ground-water study included a geologic section; and structure, water-level, and water-quality data for the Santee Limestone at Edisto Island and Edisto Beach (Park 1985) . A hydrologic framework of the South Carolina coastal plain was presented by Aucott et al. (1987), and their extensive atlas defines six principal aquifer systems with hydrologic sections and depth-to-aquifer maps that encompass the ACE Basin. Potentiometric-data for the basin are included in water-level atlases for the Floridian aquifer (Crouch et al. 1987; DNR file data), the Black Creek aquifer (Hockensmith 1998), and the Middendorf aquifer (Hockensmith and Waters 1998). Pre-development water level maps for the South Carolina coastal plain were published by Aucott and Speiran (1984) and Aucott (1988). Aucott and Speiran (1985) and Stringfield and Campbell (1993) published isodecline maps and potentiometric maps for 1982 and 1989, respectively.
Useful reports on areas near the ACE Basin include Hassen (1985), Speiran (1985), Dale (1995), and Hockensmith (1997). Hassen presented detailed water-level and water-quality data for the Floridian aquifer in northeastern Beaufort County, and he mapped ground-water movement from Ladies and St. Helena islands into the St. Helena Sound estuary, Speiran (1985), Dale (1995), and Hockensmith (1997) made water-table aquifer studies and constructed ground-water flow models for parts of Wadmalaw and Hilton Head Islands. The reports included information on geology, ground-water behavior, and chemical quality that are representative of the shallow aquifer in much of the ACE Basin.
The SCDNR Hydrology Section, using recent hydro-stratigraphic nomenclature proposed by Aadland et al. (1995) and core-drilling data, is revising its definitions for coastal plain aquifers. Until the reclassification process is completed, the hydrogeologic framework defined by Aucott et al. (1987) is being used for regional projects such as potentiometric mapping (Hockensmith 1997; Hockensmith and Waters 1998), and it is used for this report.
Cape Fear Aquifer
Pilot holes at Fripp, Parris, and Hilton Head islands penetrated the Cape Fear, and about 30 m (100 ft) of screen is set in the aquifer at the Hilton Head Island test well. Poor sorting, the prevalence of silt and clay, and interbeded clay within the sand units limit the transmissivity of screened intervals to less than 186 m2/day (2,000 ft2/day) in spite of the section's thickness. Transmissivity estimates based on aquifer tests range from 288 to 400 m2/day (3,100 to 4,300 ft2/day) for 58 m (190 ft) of screen open to the Middendorf and Cape Fear systems (Anonymous 1993). Sidewall cores in the thin sand and gravel layers had hydraulic conductivities of 3 to 10 m/day (10 to 20 ft/day) in the upper half of the system and 1 to 3 m/day (3 to 10 ft/day) in the lower half (DNR file data; Anonymous 1993). Cape Fear wells can be expected to produce more than 1,900 L/min (500 gpm), owing to the depth available for drawdown.
Chemical analyses of water samples from -844 m (2,786 ft) and -862 m (2,831 ft) were reported at two Parris Island wells (Siple 1960). The water chemistry is typical of Cretaceous aquifers along the South Carolina coast: soft, moderately basic, sodium bicarbonate type water with total dissolved-solids concentrations of about 1,000 mg/L (milligrams per liter) and fluoride concentrations of 4.0 to 5.0 mg/L. Dissolved iron concentrations are less than 100 mg/L and chloride concentrations are about 60 mg/L. U. S. Geological Survey (USGS ) analyses of samples squeezed from sidewall cores at Hilton Head reported chloride concentrations of about 1,500 mg/L near the top of the Cape Fear (-965 m or -3,164 ft) and 260 mg/L at the base (-1,108 m or -3,634 ft). The ground-water temperature is about 110°F near the base of the system at Hilton Head. Review the analyses of Cape Fear aquifer water samples .
The Middendorf system is tapped by wells at Walterboro and at Kiawah, Seabrook, Fripp, Parris, and Hilton Head islands. A transmissivity of 325 m²/day (3,500 ft²/day) and a specific capacity of 23 L/min/m (1.7 gpm/ft) was calculated at Kiawah Island well 20FF-v1 (Aucott and Newcome, 1986). They reported specific capacities of 220 to 290 L/min/m (16 and 22 gpm/ft) for two 488 to 537-m (1,600 to 1,760-ft) wells having 18 m (60 ft) of screen at Walterboro. Transmissivity at the Walterboro site probably exceeds 558 m²/day (6,000 ft²/day). Tests of sidewall cores taken between -854 and -915 m (-2,800 and -3,000 ft) at Hilton Head measured hydraulic conductivities of 1.4 to 6.4 m/day (4.7 to 6.4 ft/day) (DNR files; Anonymous 1993). The reported well yield at Kiawah Island was 1,600 L/min (430 gpm); 4,500 to 5,300 L/min (1,200 to 1,400 gpm) is reported for Middendorf aquifer wells at Walterboro (Newcome 1989).
Analyses of composite samples from wells at Walterboro and Fripp and Parris islands indicate that the water is soft, basic and high in fluoride, sodium, and bicarbonate. Salinity ranges from fresh in the northwestern part of the basin to brackish along the coast. Total dissolved solids increase coastward from about 200 mg/L to more than 1,000 mg/L. Fluoride concentration increases coastward from less than 1.0 to more than 4.5 mg/L.
Total dissolved solids and chloride concentrations typically increase with depth in coastal, brackish-water aquifers. Discrete-interval samples from Middendorf wells near the eastern ACE Basin do not display this tendency everywhere, however--probably owing to relatively effective bed separation, to differences in hydraulic conductivity, and to variations in the continuity of individual sand beds. Total dissolved solids concentrations of 1,660 to 2,577 mg/L and chloride concentrations of 60 to 464 mg/L were reported at Kiawah Island between -619 and -677 m (-2,030 and -2,220 ft) (Park 1985). The highest concentrations occur at the top of the aquifer and the lowest occur in the middle. Discrete samples at Hilton Head Island show chloride increasing from about 150 to 1,500 mg/L between the middle Middendorf and the upper Cape Fear and decreasing with depth to 260 mg/L about 61 m (200 ft) from the base of the Cape Fear. Ground-water temperature ranges between 80 and 104° F.
Black Creek Aquifer
Few wells are known to be completed in the aquifer system near the ACE Basin. They probably would produce sodium bicarbonate type water with high fluoride concentrations throughout most of the basin. High chloride concentrations are likely to extend farther inland than in the underlying systems, owing to low hydraulic conductivity and consequently poor circulation. Well yields will be impracticably small except in the western extent of the basin because of poor hydraulic characteristics and system depth.
Tertiary Sand Aquifer
Wells open only to the Tertiary sand aquifer system are rare, for the system is tapped by open-hole wells that also obtain water from the overlying Floridian aquifer system. Tertiary sand/Floridian wells are ubiquitous in Charleston, Berkeley, and Dorchester counties (Park 1985), and they are common in the northeastern half of the ACE Basin. The Tertiary sand aquifer yields water to wells more consistently than the Floridian, and drillers use it routinely to assure the success of their wells. Caliper logs show relatively smooth, bit-diameter boreholes through the Floridian section of the well and wider diameter washouts in the sandy section of the Tertiary sand. Sand pumping is seldom a problem in open-hole Floridian/Tertiary sand wells, even where pumping rates exceed 1,000 L/min (300 gpm). At least one well in the basin has a recorded yield of 2,500 L/min (660 gpm); several screened wells near the western reach of the basin have reported yields of 280 to 760 L/min (75 to 200 gpm); and yields adequate for domestic supply are found everywhere in the basin. Specific capacities typically are 55 to 81 L/min/m (4 to 6 gpm per foot of drawdown).
Water in the Tertiary sand aquifer is of the sodium bicarbonate type and grades into the sodium chloride type coastward. The highest known chloride concentration, about 6,000 mg/L, was measured in a well just north of Edisto Beach. Hardness and high iron concentrations rarely cause problems, but fluoride concentrations increase coastward and range from 2.0 to 4.0 mg/L at and southeast of Edisto Island. Dissolved-silica concentrations exceeding 25 mg/L occur in the Black Mingo Formation section of the Tertiary sand aquifer, and the silica is attributed to the presence of silica-cemented sandstone, cristobalite, and clinoptilite (Park 1985). Water temperatures are about 68° F throughout the ACE Basin.
The Floridian aquifer system is composed principally of fine-grained and impure limestone in which permeability is poorly developed, and wells open to the Floridian commonly are also completed in the top of the Tertiary sand system. Locally, thin water-yielding zones may be associated with geologic contacts, but such zones are less continuous than the contacts. A relatively clean, permeable section occurs at about 150 m (500 ft) msl beneath eastern Edisto Island and produces as much as 1,900 L/min (500 gpm) to wells. Poor yields from wells of similar depth can occur locally, although yields of at least 380 L/min (100 gpm) probably can be obtained in most of the basin. Yields of 380 to 760 L/min (100 to 200 gpm) also can be obtained from wells less than 30 m (100 ft) deep along the southeastern boundary of the basin: they tap the upper permeable zone of the Ocala Limestone, which thickens southward to more than 30 m (100 ft) and becomes the most productive aquifer in South Carolina.
Specific capacities through most of the basin probably range from 15 to 80 L/min/m (1 to 6 gpm/ft), but are 135 to 270 L/min/m (10 to 20 gpm/ft) at Edisto Island and in the northwestern reaches of the basin. Aucott and Newcome (1986) reported 230 L/min/m (17 gpm/ft) for a Floridian (and Tertiary sand aquifer) well at Walterboro. Hayes (1979) reported 14 to 54 L/min/m (1 to 4 gpm/ft) and as much as 2,000 L/min (530 gpm) from eight Floridian wells in Colleton County. The hydraulic conductivity of the Ocala upper permeable zone ranges from 15 to 45 m/day (50 to 150 ft/day) on northern Port Royal Island; transmissivity is less than 46 m²/day (500 ft²/day) (Hughes et al. 1989). Similar values of hydraulic conductivities probably occur at geologic contacts within the Santee Limestone section of the Floridian aquifer, but the thickness of the permeable sections is small.
The potentiometric surface of the Floridian aquifer dips southeastward across the basin. The hydraulic gradient is 0.5 m/km (2.5 ft/mile) across the northwestern reach of the basin and abruptly diminishes to 0.1 m/km (0.4 ft/mile) across the southeastern three quarters. Comparison of November 1982 (Park 1985) and July 1986 (Crouch et al. 1987) potentiometric surface maps show 0.3- to 1.2-m (1- to 4-ft) declines across the eastern half of the basin--much of the difference is the result of seasonal differences in ground-water use, but part is the result of increased withdrawals. Water levels were measured in November 1990, and these suggest that levels in southern Charleston County are 0.7 to 1.4 m (2.3 to 4.6 ft) lower than measurements made during November 1982. The levels at Edisto Island declined to -1.2 to -2.4 m (-4 to -8 ft) below MSL by November 1990 (Dale, pers. comm.).
Floridian aquifer water typically is a hard, calcium bicarbonate type with low iron concentrations. On the southeastern side of the basin, high iron concentrations are prevalent in the upper permeable zone. Water at the base of the system in southern Charleston County is similar to that from the Tertiary sand aquifer. Chloride concentrations increase coastward and exceed 500 mg/L at Edisto Beach. Chloride concentrations there can be expected to increase with time owing to pumping-induced upconing and saltwater intrusion. Additional selected chemical analyses are available for the Floridian aquifer.
Holocene fluvial deposits predominate along the major streams of the ACE Basin and were described as "fine gravel at the base of a sequence, through coarse to fine, locally muddy sand...to overbank mud at the top" (McCartan et al. 1990). Aquifers, mainly channel-lag and point-bar deposits, can have relatively high hydraulic conductivities but are laterally discontinuous. Extensive backbarrier and beach facies, whose ages increase landward, occur within stream interfluves. Backbarrier deposits are muddy sand with clay, shell, and sand layers. Little or no yield will be typical of wells completed in backbarrier deposits, although good yields are likely where wells are screened in tidal-inlet and -channel deposits. Beach deposits include barrier island depositional environments ranging from dune to shelf (McCartan et al. 1990) . They encompass the most hydraulically consistent and laterally extensive aquifers owing to the well- to moderately well-sorted sand of dune and beach environments and to their typical history of progradation, (See related sections: Geology, Geomorphology.)
Data on the hydraulic characteristics of the system are scant within the basin, but data from nearby, geologically similar areas have been published in several reports. Smith (1987) reported hydraulic conductivity values for a site on northern Port Royal Island, apparently located over backbarrier deposits. Horizontal and vertical conductivities (K and Kv) of 0.1 to 0.3 m/day (0.3 to 0.9 ft/day) and 6 x 10-5 to 2 x 10-1 m/day (2 x 10-4to 7 x 10-1 ft/day), respectively, were measured through an alternating sequence of fine sand and clay. Shallow-aquifer test results are published for areas underlain by beach facies at Wadmalaw, Hilton Head, and Edisto islands. At Wadmalaw Island where 4-6 m (14-20 ft) of beach sand overlies backbarrier deposits, 18 tests indicated hydraulic conductivities of 1.2 to 6.7 m/day (4 to 22 ft/day) and averaging 2.7 m/day (9 ft/day) (Hockensmith 1997). M. W. Dale and A. D. Park (unpublished data) obtained nearly identical results at two sites on Hilton Head Island: 16 tests indicated hydraulic conductivities of 1.8 to 7.3 m/day (6 to 24 ft/day) and averaging 3.0 m/day (10 ft/day). The K/Kv was about 20. The U. S. Navy Department (1993) and Landmeyer et al. (1996) reported hydraulic conductivities of 2.7 to 5.2 m/day (9 to 17 ft/day) for sandy Holocene deposits on Port Royal Island. Saturated thicknesses of 9 m (30 ft) or more occur in many such areas, and transmissivities of 25 to 45 m2/day (250 to 500 ft2/day) should be common. An average hydraulic conductivity of 5.8 m/day (19 ft/day) and a transmissivity of 56 m2/day (600 ft2/day) was reported for a pumping-test at Edisto Island (Park 1985).
The maximum yield of individual wells is probably about 200 L/min (50 gpm). Public-supply wells 15-17 m (50-55 ft) deep on eastern Edisto Island produced 100 to 180 L/min (25 to 48 gpm): a four-well header system was used at one time and probably pumped 280 to 380 L/min (75 to 100 gpm). Lower yields will be more typical but should be adequate for domestic supply. Domestic irrigation wells having 3 to 6 m (10 to 20 ft) of screen at Hilton Head Island produce 40 to 150 (10 to 40 gpm); 5- to 6-m (15- to 20-ft) observation wells with 1.5 to 3 m (5 to 10 ft) of screen produced 7 to 20 L/min (2 to 5 gpm) there. However, wells screened in backbarrier deposits are likely to produce little water. Yields from fluvial facies will be variable but generally low.
Seasonal fluctuations usually are less than 1.8 m (6 ft), and the range of fluctuation decreases with increasing proximity to streams. The hydrograph shows water-table depths at Edisto Island from October 1989 through September 1991 and reflects the seasonal range typically expected in the shallow system.
Water chemistry data is available for the shallow-aquifer, mainly from Glowacz et al. (1980) and Park (1985). Shallow-aquifer water is soft with low total dissolved solids and high iron concentrations in most of the basin. The iron is ubiquitous owing to iron-bearing heavy minerals. Hard, basic water is common in the lower third to half of the basin where fossil-shell material is present: soft, slightly-acidic water occurs farther inland where deposits are older and leaching has removed shell. In beach facies, total dissolved solids, pH, and hardness increase with depth where water moves from dune sand into fossiliferous sections of beach and backbarrier facies.
The ACE Basin contains six aquifer systems, and three are used; the Tertiary sand, the Floridian, and the shallow. The Tertiary sand and Floridian aquifer systems are the principal sources of domestic, commercial, and public water supplies, and well yields as great as 1,900 L/min (500 gpm) are reported for most of the basin. The shallow aquifer system is the least consistent with respect to well yield. However, wells drilled in areas underlain by beach facies provide enough water for domestic supply and produce up to 190 L/min (50 gpm) locally. The upper part of the Cape Fear aquifer system and the Middendorf aquifer system should yield more than 3,800 (1,000 gpm) to individual wells screened in both systems. Wells screened in the Middendorf aquifer system should produce 1,900 to 3,800 (500 to 1,000 gpm).
Water quality generally is good in the northwestern half of the basin, where low dissolved-solids concentrations are prevalent. Treatment is likely to be required for hardness in water from the Tertiary sand and Floridian aquifer systems and for dissolved iron in water from the Floridian and shallow systems. Fluoride, sodium, bicarbonate, and chloride increase coastward in all but the shallow system. The shallow aquifer produces water with low dissolved solids concentrations except where it contacts saltwater marshes and streams. Saltwater intrusion occurs in the Floridian aquifer system at Edisto Beach owing to water-level declines. The saltwater wedge is diffuse, hydraulic conductivities and gradients are small, and intrusion consequently is slow.
D. Park, SCDNR Land, Water, and Conservation Division
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Anonymous. 1993. Well completion report on Cretaceous aquifer test well (BFT-2055), Hilton Head, South Carolina. Report 89-1601. Atlanta Testing and Engineering, Columbia, SC.
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