Hydrology - Water Resources Report 14

Contributions to the Hydrology of South Carolina

Short papers on techniques, procedures, and transferable findings that can be used to better understand the hydrology of South Carolina.

ABSTRACTS

Correction for tidal effects on water-level measurements in Floridan aquifer wells, southern coast of South Carolina

Radiocarbon isotopes in the interpretation of ground-water flow regimes at Hilton Head Island, South Carolina

Chemical evolution of ground water in Cretaceous aquifers of the Coastal Plain in South Carolina

Well efficiency – its importance and its calculation

Analysis of interference-type pumping tests of the Coastal Plain aquifers in South Carolina

Hydraulic conductivity of the principal Cretaceous aquifers in South Carolina


Correction for Tidal Effects on Water-Level Measurements in Floridan Aquifer Wells, Southern Coast of South Carolina

By
Constance E. Gawne
1997

Water levels in coastal wells fluctuate in response to tides in the adjacent ocean and estuaries. Unless corrected for, these tidal effects obscure static water levels, hydrologic gradients, and longer-term water-level changes. Several methods of tide correction are described, and examples are used to evaluate the relative accuracy of each method under different conditions.

Corrections may be made by calculations using measured tide levels; by averaging over a 24-hour, 50-minute diurnal tidal cycle; and by calculations using predicted tide levels. Under optimal tide and weather conditions, corrections made by any of these methods will reduce error in water-level measurements to 25 percent or less of the departures from static level observed in the well. As average tide levels depart from annual mean tide level, for astronomical reasons or because of weather disturbances, corrected levels calculated by using measured estuarine water levels appear to remain near static level. However, systematic errors appear in corrected water levels calculated by averaging or form predictions: Calculations from predictions do not remove weather-related effects, and averaging retains effects of both long-term tidal variations and weather.


Radiocarbon Isotopes in the Interpretation of Ground-Water Flow Regimes at Hilton Head Island, South Carolina

By
Joffre E. Castro
1997

Hilton Head Island, a barrier island at the south end of South Carolina, obtains virtually all its water supply from the upper Floridan aquifer. Changes in the ground-water gradient of this aquifer during the past 50 years, owing to intensive pumping in Georgia, have reversed the flow direction underneath the island. Moreover, lower hydraulic heads and increased rates of vertical infiltration have produced a serious threat of contamination of the aquifer with saltwater. To better understand and quantify the ground water regime in Hilton Head Island, a series of hydrologic and geologic studies has been made.

This study uses radiocarbon and chemical data to define the ground water flow regime of the island following earlier work by Back and others (1970). The radiocarbon analysis demonstrates that the island’s ground water is a mixture of water of various chemical compositions and ages. The ground water at the north end of the island is characterized by freshwater, in the middle by a mixture of freshwater and old native ground water, and at the south end by old native ground water and probably leakage from overlying aquifers. Large fractions of modern seawater were identified north of the island and underneath Port Royal Sound. Finally, stable and isotopic carbon data can provide valuable information about the ground water system when used in combination with chemical data. Geochemical and isotopic models, thus, can be used to correct 14C activities and compute meaningful ground water ages.


Chemical Evolution of Ground Water in Cretaceous Aquifers of the Coastal Plain in South Carolina

By
Joffre E. Castro
1997

Three water samples from the Black Creek Formation and the chemical composition of rainwater and seawater were used to characterize the makeup of the ground water in northeastern South Carolina. Specific chemical reactions were identified from calculations of equilibrium speciation and mineral saturation, using WATEQ4F, and by mass-balance calculations using NETPATH. Both WATEQ4F and NETPATH are computer programs developed by the U.S. Geological Survey.

It was found that ground water near the recharge area in Marlboro County is noticeably affected by oxidation of pyrite and organic matter that renders the ground water soft and aggressive, with low pH, almost no buffering capacity, and little dissolved solids. Farther downgradient, around Marion and Dillon Counties, ground water is principally affected by calcite dissolution and calcium-sodium exchange. The water has a neutral pH, strong buffering capacity, and often a high concentration of hydrogen sulfide. Close to the coast in Horry County, the ground water is significantly affected by calcite dissolution. Because this water has been in contact with the aquifer matrix for a longer period of time, it has dissolved many of the mineral phases present in the aquifer. The ground water, thus, has a high concentration of constituents in solution. Specifically, the water is characterized by high pH, bicarbonate, chloride, and sodium. Generally the ground water was found to be of only marginal drinking-water quality.


Well Efficiency – Its Importance and Its Calculation

By
Roy Newcome, Jr.
1997

Efficient water wells cost less to operate than inefficient wells. Well efficiency can be calculated from a pumping test in which the well is pumped at a constant rate and the resulting water-level drawdown is measured. The same data that are used to calculate the aquifer transmissivity are used to calculate the well’s specific capacity. The ideal specific capacity of a well producing from a confined aquifer bears a linear relation to the transmissivity. Well efficiency is the ratio of actual specific capacity to ideal specific capacity. One-third of the nearly 500 wells tested in South Carolina’s Coastal Plain have well efficiencies greater than 90 percent.


Analysis of Interference-Type Pumping Tests of the Coastal Plain Aquifers in South Carolina

By
Roy Newcome, Jr.
1997

In 76 interference-type pumping tests made in the South Carolina Coastal Plain, drawdown was measured in 61 of the pumping wells. The pumped-well and observation-well data provided corroborating transmissivity values in 50 tests. Examination of the 11 tests in which the transmissivity indicated by pumped and observation wells differed by 23 percent or more provided a definite or likely explanation in all cases. Usually it was a matter of pumped well and observation well not producing from exactly the same aquifer(s). Well efficiency is not relevant in calculating transmissivity from pumped-well data, and storage coefficient probably can be as reliably estimated as calculated from data. Therefore, single-well pumping tests provide results that are likely to be as reliable and useful as those obtained from interference tests, and at much less cost.


Hydraulic Conductivity of the Principal Cretaceous Aquifers in South Carolina

By
Roy Newcome, Jr.
1997

Hydraulic conductivity, calculated from transmissivity revealed by pumping tests and thickness shown by electric logs, is generally greater for aquifers in the Middendorf Formation than for the Black Creek Formation in the Coastal Plain of South Carolina. Both formations contain confined aquifers of Upper Cretaceous age that are extensively tapped for water supplies. In both formations the hydraulic conductivity declines seaward.


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