Geology topics
Source-linked reports with geographic coverage including South Carolina.
This map, designed to accompany the geologic map of the crystalline rocks of South Carolina (Overstreet and Bell, 1965b), shows Mesozoic diabase dikes recognized in this area since that map was compiled in 1960.
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Lakes Marion and Moultrie are two large, interconnected reservoirs in the Coastal Plain of South Carolina. Bathymetric surveys during 1984-85 determined that, for the full-pool water surface elevation of 76.8 ft, the volume of Lake Marion is 1,425 ,000 acre-ft, and for the full-pool water surface elevation of 75.0 ft, the volume of Lake Moultrie is 1,060,000 acre-ft. These volumes are considered accurate to within 5%. Comparison with volumes determined in 1941, when the reservoirs were filled, shows decreased in volume of 73,000 acre-ft in Lake Marion and 30,000 acre-ft in Lake Moultrie. These changes in volume, however, are subject to relatively large possible error because they represent relatively small differences between relatively large volumes, and because the accuracy of the 1941 volumes is not known. Measurements of sediment inflow and outflow and of deposition rate suggest the volume losses were more likely about 29,000 acre-ft in Lake Marion and about 1,600 acre-ft in Lake Moultrie. The discrepancy in the estimate of volume changes does not affect the accuracy of the volume computation or the maps. (USGS)
A study was conducted to determine the low-flow frequency of freshwater flow in the Atlantic Intracoastal Waterway (AICW) near Myrtle Beach, South Carolina and to determine the effects of proposed freshwater withdrawals of 45 cu ft/sec at the location of the saltwater-freshwater interface. Discharges simulated in the AICW for 1982-86 using BRANCH one-dimensional flow model were used to establish a relation of 7-day average flows in the AICW to summed 7-day average flows of four tributary streams. This relation was used with the tributary records for 1954-86 climatic years to generate 7-day minimum flows of the AICW, which were then used to develop a low-flow frequency relation. The relation indicated that the 7-day, 10-year flow of the Atlantic Intracoastal Waterway is 192 ct ft/s. A relation of the mile position of the saltwater-freshwater interface to recorded specific conductances at Vereen 's Marina was established. The 1982-85 period of record of specific conductance was used to simulate interface positions which were then used to establish a relation of 7-day average interface position to 7-day average discharge of the AICW. This relation indicated that the 7-day average interface position would be at mile 355.5 for the 7Q10 and at mile 356.2 if 45 cu ft were withdrawn during the 7Q10. The analysis indicates that the AICW can provide a reliable supply of freshwater at the proposed withdrawal location at mi 363.3 in the vicinity of Myrtle Beach, even during the 7Q10 low-flow conditions. (USGS)
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In 1941 a Coastal Plain reach of the Santee River was impounded to form Lake Marion and diverted into a diked-off part of the Cooper River basin to form Lake Moultrie. Rates of sediment inflow and outflow of the lakes were determined by the U.S. Geological Survey for the periods July 1966 - June 1968 and October 1983 - March 1985. Total sediment discharge was estimated for two inflow stations and continuous streamflow monitors and automatic suspended-sediment samplers were used for computation of suspended-sediment discharge. Bedload discharge was computed by the modified Einstein procedure. Suspended-sediment discharge was monitored at three outflow stations, with the suspended-sediment concentration measured on a weekly basis. During the 1983-1985 study, mean annual suspended-sediment inflow to Lakes Marion and Moultrie was estimated to be 722,000 tons, and the outflow was estimated at 175,000 tons, for a trap efficiency of 76% and a deposition rate of about 547,000 tons/year. This is about 33% less than the deposition rate determined during the 1966-68 study. The deposition rate for suspended and bedload sediment during the 1983 - 1985 study was about 650,000 tons/year. (USGS)
The Black Creek aquifer contains dilute seawater near the North Carolina State line, probably the result of incomplete flushing of ancient seawater. Data do not indicate that the dilute seawater has migrated toward areas of fresh ground-water withdrawals. The concentration of chloride in ground-water samples ranges from 5 to 720 milligrams per liter and that of sodium from 160 to 690 milligrams per liter. Ion-exchange reactions (sodium for calcium and fluoride for hydroxyl) occur with the calcium carbonate dissolution reaction which produces calcium, bicarbonate, and hydroxyl ions. The reaction sequence and stoichiometry result in an aqueous solution in which the sum of bicarbonate and chloride equivalents per liter is equal to the equivalents per liter of sodium. Calcium ions are exchanged for sodium ions derived from sodium-rich clays upgradient of the dilute seawater. The cation-exchange reaction equilibrates at a sodium concentration of 280 milligrams per liter. Amounts of sodium greater than 280 milligrams per liter are contributed from dilute seawater. The cation-exchange reaction approaches an equilibrium which represents a mass-action limit in terms of the ratio of sodium to calcium in solution versus the ratio of exchangeable sodium to calcium on clay surfaces. Where the limit of calcium carbonate solubility is approached and dissolution ceases, some precipitation of calcite probably takes place. The dissolution of calcite exposes fossil shark teeth which release fluoride ions to the ground water through anion exchange with aqueous hydroxyl ions.
Freshwater to supply Hilton Head Island, S.C., is obtained from the upper permeable zone of the Upper Floridan aquifer. Long-term pumping at Savannah, Ga., and the steadily increasing pumping on Hilton Head Island, have lowered Upper Floridan heads near the center of the island from about 10 feet above sea level to about 6 to 7 feet below sea level. The seaward hydraulic gradient that existed before pumping began has been reversed, thus increasing the potential for saltwater intrusion. Simulations of predevelopment, recent, and future ground-water flow in the Floridan aquifer system beneath the north end of Hilton Head Island and Port Royal Sound are presented. A finite-element model for fluid-density-dependent ground-water flow and solute transport was used in cross section. The general configuration of the simulated predevelopment flowfield is typical of a coastal aquifer having a seaward gradient in the freshwater. The freshwater flows toward Port Royal Sound over an intruding wedge of saltwater. The simulated flowfield at the end of 1983 shows that ground water in the Floridan aquifer system beneath most of Hilton Head Island has reversed its predevelopment direction and is moving toward Savannah. The distribution of chloride concentrations, based on simulation at the end of 1983, is about the same as the predevelopment distribution of chloride concentrations obtained from simulation. Results of two 50-year simulations from 1983 to 2034 suggest that there will be no significant threat of saltwater intrusion into the upper permeable zone of the Upper Floridan aquifer if heads on Hilton Head Island remain at current levels for the next 45 to 50 years. However, if head decline continues at the historical rate, any flow that presently occurs from the north end of the island toward Port Royal Sound will cease, allowing lateral intrusion of saltwater to proceed. Even under these conditions, chloride concentrations in the upper permeable zone of the Upper Floridan aquifer beneath Hilton Head Island should remain below 250 milligrams per liter for the next 45 to 50 years. Aquifer properties and selected boundary conditions were tested with several 1,000-year simulations which show that lateral permeability, transverse dispersivity, and landward boundary flow have the most influence on saltwater movement in the Upper Floridan aquifer.
The influence of thermal disturbance and site characteristics on distribution of herbs was studied in portions of a 3020 ha wetland in the southeastern USA. Presence-absence of 52 species in 130 0.25 m 2 plots was determined from four sites with different disturbance histories and from an undisturbed site. Data from the four disturbed sites were ordinated by detrended correspondence analysis. Differences in species composition among sites (coarse scale) were associated with water depth, light, and substrate type. Within a site (at a fine scale), correlation of environmental variables with ordination scores at a chronically disturbed site was weakly correlated with light ( r =0.50). At two sites with episodic disturbance, species composition correlated significantly and positively with substrate and water depth. At a recovering site, vegetation patterns were moderately correlated with water depth ( r =−0.52). Species richness was correlated with substrate type along the disturbance gradient. Our results are consistent the intermediate disturbance hypothesis and the subsidy-stress gradient concept.
Continuous hourly measurements of twelve meteorologic variables recorded during 1983 and 1984 were used to examine the microclimate and actual evapotranspiration at a low-level radioactive-waste burial site near Barnwell, South Carolina. The study area is in the Atlantic Coastal Plain of southwestern South Carolina. Monthly, daily, and hourly trends in net radiation, incoming and reflected short-wave radiation, incoming and emitted long-wave radiation, soil-heat flux, dry- and wet-bulb temperatures, soil temperatures, wind direction and speed, and precipitation were used to characterize the microclimate. Average daily air temperatures ranged from −9 to 32° Celsius during the period of study. Net radiation varied from about −27 to 251 watts m −2 and was dominated by incoming short-wave radiation throughout the year. The peak net radiation during a summer day generally occurred 2–3h before the peak vapor pressure deficit. In the winter, these peaks occurred at about the same time of day. Monthly precipitation varied from 15 to 241 mm. The Bowen ratio method was used to estimate hourly evapotranspiration, which was summed to also give daily and monthly evapotranspiration. Actual evapotranspiration varied from 0.0 to 0.7 mm h −1 , 0.8−5 mm d −1 , and 20–140 mm month −1 during 1983 and 1984. The maximum rate of evapotranspiration generally occurred at the same time of day as maximum net radiation, suggesting net radiation was the main driving force for evapotranspiration. Precipitation exceeded evapotranspiration during 14 months of the 2yr study period. Late fall, winter, and early spring contained the majority of these months. The maximum excess precipitation was 115 mm in February 1983.
Four unsaturated zone monitoring sites and a meteorologic station were installed at the low level radioactive waste burial site near Barnwell, South Carolina, to investigate the geohydrologic and climatologic factors affecting water movement in the unsaturated zone. The study site is located in the Atlantic Coastal Plain. The unsaturated zone consists of a few centimeters to > 1 m of surface sand, underlain by up to 15 m of clayey sand. Two monitoring sites were installed in experimental trenches and two were installed in radioactive waste trenches. Two different trench designs were evaluated at the monitoring sites. A meteorologic station was used to measure precipitation and to calculate actual evapotranspiration using the Bowen ratio method. Soil-moisture tensiometers, soil-moisture conductance probes, and temperature sensors were used to monitor soil-water movement in and adjacent to the trenches. Tracer tests using sodium chloride were conducted at each monitoring site. Data collection at the monitoring sites began in January 1982 and continued until early May 1984. Tensiometer data show that the unsaturated materials had their highest percent saturations in the winter and spring. Saturations in the backfill sand varied from 20 to 100%. They varied from about 75 to 100% in the adjacent undisturbed and overlying compacted clayey sand. Additionally, because tensiometer data indicate negligible water storage changes in the unsaturated zone, it is estimated that approximately 43 cm of recharge reached the water table. During 1984, the rise and fall of ponded water in an experimental trench was continuously monitored with a digital recorder. A cross-sectional finite element model of variably saturated flow was used to test the conceptual model of water movement in the unsaturated zone and to illustrate the effect of trench design on water movement into the experimental trenches. Monitoring and model results show that precipitation on trenches infiltrated the trench cap and moved vertically into the trench backfill material. The trench construction practice of placing a compacted clayey-sand barrier around the trench greatly inhibits soil water from entering the trench. (Author 's abstract)
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Water resources data for the 1986 water year for South Carolina consist of records of stage, discharge, and water quality of streams; stage, contents, and water quality of lakes and reservoirs; and water levels of ground-water wells. This volume contains records for water discharge at 100 gaging stations, stage only at 7 gaging stations, stage and contents at 13 lakes and reservoirs, water quality at 47 gaging stations, and water levels at 40 observation wells. Also included are data for 38 crest-stage partial-record stations. Locations of these sites are shown on figures 3,4,5, and 6. Additional water data were collected at various sites not involved in the systematic data-collection program. These data represent that part of the National Water Data System collected by the U.S. Geological Survey and cooperating State and Federal agencies in South Carolina.
The U.S. Geological Survey is conducting a series of investigations of aquifers throughout the United States as a part of the RASA (Regional Aquifer System Analysis) program. These investigations provide a comprehensive regional understanding of groundwater resources throughout the Nation. The Coastal Plain aquifers in South Carolina are being studied as a part of this program. An important part of a description of the groundwater resources is the development of a geohydrologic framework. Such a framework delineates the aquifers through which groundwater flows and the confining units which retard the flow of groundwater between aquifers. The Coastal Plain of South Carolina is underlain by a wedge of sediments that thickens from its inner margin, the Fall Line, to the coast and consists of sand, silt, clay, and limestone of Late Cretaceous to Holocene age. These sediments are underlain by pre-Cretaceous rocks consisting of consolidated sedimentary rocks of Triassic age and a complex of metamorphic and igneous rocks similar to those found near the surface in the Piedmont province of the State. The geohydrologic framework that divides the sediments of the South Carolina Coastal Plain into the Coastal Plain aquifer system is delineated by eleven geohydrologic sections and four maps showing the configuration of the top or base of individual aquifers. Although flow within the Coastal Plain aquifer system is three dimensional, simplifying the system by dividing it into a framework of discrete hydrologic units can aid significantly in understanding the hydrology of the system. This framework is the basis for the aquifers used in potentiometric mapping, transmissivity mapping, geochemical analysis, and groundwater flow modeling for the South Carolina RASA program. (Lantz-PTT)