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Dorothy F. Payne

Publications and source records attributed to Dorothy F. Payne.

6 recordsLinked to original sources
USGS2010

Effects of sea-level rise and pumpage elimination on saltwater intrusion in the Hilton Head Island area, South Carolina, 2004-2104

Saltwater intrusion of the Upper Floridan aquifer has been observed in the Hilton Head area, South Carolina since the late 1970s and currently affects freshwater supply. Rising sea level in the Hilton Head Island area may contribute to the occurrence of and affect the rate of saltwater intrusion into the Upper Floridan aquifer by increasing the hydraulic gradient and by inundating an increasing area with saltwater, which may then migrate downward into geologic units that presently contain freshwater. Rising sea level may offset any beneficial results from reductions in groundwater pumpage, and thus needs to be considered in groundwater-management decisions. A variable-density groundwater flow and transport model was modified from a previously existing model to simulate the effects of sea-level rise in the Hilton Head Island area. Specifically, the model was used to (1) simulate trends of saltwater intrusion from predevelopment to the present day (1885-2004) and evaluate the conceptual model, (2) project these trends from the present day into the future based on different potential rates of sea-level change, and (3) evaluate the relative influences of pumpage and sea-level rise on saltwater intrusion. Four scenarios were simulated for 2004-2104: (1) continuation of the estimated sea-level rise rate over the last century, (2) a doubling of the sea-level rise, (3) a cessation of sea-level rise, and (4) continuation of the rate over the last century coupled with an elimination of all pumpage. Results show that, if present-day (year 2004) pumping conditions are maintained, the extent of saltwater in the Upper Floridan aquifer will increase, whether or not sea level continues to rise. Furthermore, if all pumpage is eliminated and sea level continues to rise, the simulated saltwater extent in the Upper Floridan aquifer is reduced. These results indicate that pumpage is a strong driving force for simulated saltwater intrusion, more so than sea-level rise at current rates. However, results must be considered in light of limitations in the model, including, but not limited to uncertainty in field data, the conceptual model, the physical properties and representation of the hydrogeologic framework, and boundary and initial conditions, as well as uncertainty in future conditions, such as the rate of sea-level rise.

South Carolina↗
USGS2010

Effects of climate change on saltwater intrusion at Hilton Head Island, SC. U.S.A.

Sea‐level rise and changes in precipitation patterns may contribute to the occurrence and affect the rate of saltwater contamination in the Hilton Head Island, South Carolina area. To address the effects of climate change on saltwater intrusion, a threedimensional, finite‐element, variable‐density, solute‐transport model was developed to simulate different rates of sea‐level rise and variation in onshore freshwater recharge. Model simulation showed that the greatest effect on the existing saltwater plume occurred from reducing recharge, suggesting recharge may be a more important consideration in saltwater intrusion management than estimated rates of sea‐level rise. Saltwater intrusion management would benefit from improved constraints on recharge rates by using model‐independent, local precipitation and evapotranspiration data, and improving estimates of confining unit hydraulic properties.

South Carolina↗
USGS2006

Simulation of saltwater movement in the Upper Floridan aquifer in the Savannah, Georgia-Hilton Head Island, South Carolina, area, predevelopment-2004, and projected movement for 2000 pumping conditions

A digital model was developed to simulate ground-water flow and solute transport for the Upper Floridan aquifer in the Savannah, Georgia–Hilton Head Island, South Carolina, area. The model was used to (1) simulate trends of saltwater intrusion from predevelopment to the present day (1885–2004), (2) project these trends from the present day into the future, and (3) evaluate the relative influence of different assumptions regarding initial and boundary conditions and physical properties. The model is based on a regional, single-density ground-water flow model of coastal Georgia and adjacent parts of South Carolina and Florida. Variable-density ground-water flow and solute transport were simulated using the U.S. Geological Survey finite-element, variable-density solute-transport simulator SUTRA, 1885–2004. The model comprises seven layers: the surficial aquifer system, the Brunswick aquifer system, the Upper Floridan aquifer, the Lower Floridan aquifer, and the intervening confining units. The model was calibrated to September 1998 water levels, for single-density freshwater conditions, then refined using variable density and chloride concentration to give a reasonable match to the trend in the chloride distribution in the Upper Floridan aquifer inferred from field measurements of specific conductance made during 2000, 2002, 2003, and 2004. The model was modified to simulate solute transport by allowing saltwater to enter the system through localized areas near the northern end of Hilton Head Island, at Pinckney Island, and near the Colleton River, and was calibrated to match chloride concentrations inferred from field measurements of specific conductance. This simulation is called the "Base Case." Water-level residuals ranged from –5.3 to 23.4 feet for September 1998 conditions and single-density freshwater conditions. When chloride transport was simulated, water-level residuals ranged from –12.5 to 23.3 feet. The simulated chloride distribution captures the general trends in the field data. Chloride transport is sensitive to the permeabilities assigned to the confining units in the source areas and the porosity assigned to the Upper Floridan aquifer. Results of the study indicate that if present-day (year 2000) pumping conditions are maintained, plumes of saltwater in the Upper Floridan aquifer will continue to expand and move toward Savannah and across Hilton Head Island; the rate of movement of the 250-mg/L (milligram per liter) isochlor toward Savannah is between 144 feet per year and 190 feet per year and that the 250-mg/L isochlor could reach the pumping center at Savannah in 800 years; if effective porosities are lower than those used in the model, as is likely, higher rates of solute transport would result; and plumes may have occurred along the northern shore of Hilton Head Island before substantial development began in the mid-1960s, and lesser amounts of intrusion may have already occurred prior to the onset of pumping during 1885. Model limitations include uncertainty in (1) field data, (2) the conceptual model, (3) the physical properties and representation of the hydrogeologic framework, and (4) uncertainty in the boundary and initial conditions. Results of simulations projected far into the future must be interpreted with caution because they are based on an assumed future pumping distribution and fixed boundary conditions, and because these conditions may differ substantially from those for which the model is calibrated.

South Carolina↗
USGS2005

Simulation of ground-water flow in coastal Georgia and adjacent parts of South Carolina and Florida-predevelopment, 1980, and 2000

A digital model was developed to simulate steady-state ground-water flow in a 42,155-square-mile area of coastal Georgia and adjacent parts of South Carolina and Florida. The model was developed to (1) understand and refine the conceptual model of regional ground-water flow, (2) serve as a framework for the development of digital subregional ground-water flow and solute-transport models, and (3) serve as a tool for future evaluations of hypothetical pumping scenarios used to facilitate water management in the coastal area. Single-density ground-water flow was simulated using the U.S. Geological Survey finite-difference code MODFLOW-2000 for mean-annual conditions during predevelopment (pre?1900) and the years 1980 and 2000. The model comprises seven layers: the surficial aquifer system, the Brunswick aquifer system, the Upper Floridan aquifer, the Lower Floridan aquifer, and the intervening confining units. A combination of boundary conditions was applied, including a general-head boundary condition on the top active cells of the model and a time-variable fixed-head boundary condition along part of the southern lateral boundary. Simulated heads for 1980 and 2000 conditions indicate a good match to observed values, based on a plus-or-minus 10-foot (ft) calibration target and calibration statistics. The root-mean square of residual water levels for the Upper Floridan aquifer was 13.0 ft for the 1980 calibration and 9.94 ft for the 2000 calibration. Some spatial patterns of residuals were indicated for the 1980 and 2000 simulations, and are likely a result of model-grid cell size and insufficiently detailed hydraulic-property and pumpage data in some areas. Simulated potentiometric surfaces for predevelopment, 1980, and 2000 conditions all show major flow system features that are indicated by estimated peotentiometric maps. During 1980?2000, simulated water levels at the centers of pumping at Savannah and Brunswick rose more than 20 ft and 8 ft, respectively, in response to decreased pumping. Simulated drawdown exceeded 10 ft in the Upper Floridan aquifer across much of the western half of the model area, with drawdown exceeding 20 ft along parts of the western, northern, and southern boundaries where irrigation pumping increased during this period. From predevelopment to 2000 conditions, the simulated water budget showed an increase in inflow from, and decrease in outflow to, the general-head boundaries, and a reversal from net seaward flow to net landward flow across the coastline. Simulated changes in recharge and discharge distribution from predevelopment to 2000 conditions showed an increase in extent and magnitude of net recharge cells in the northern part of the model area, and a decrease in discharge or change to recharge in cells containing major streams and beneath major pumping centers. The model is relatively sensitive to pumping and the controlling head at the fixed-head boundary and less sensitive to the distribution of aquifer properties in general. Model limitations include: (1) its spatial scale and discretization, (2) the extent to which data are available to physically define the flow system, (3) the type of boundary conditions and controlling parameters used, (4) uncertainty in the distribution of pumping, and (5) uncertainty in field-scale hydraulic properties. The model could be improved with more accurate estimates of ground-water pumpage and better characterization of recharge and discharge.

Florida, Georgia, South Carolina↗
USGS1997

Relation of hydrogeologic characteristics to distribution of radioactivity in ground water, Newark Basin, New Jersey

The distribution of radioactivity in ground water in the Newark Basin is controlled by the lithology of the aquifer and the degree of contact between the water that flows through the fractured strata and the radioactive lithologic units. The primary water-bearing lithologic units of the Newark Basin that contain elevated levels of radioactivity are the arkosic sandstones of the Stockton Formation and the black mudstones of the lower part of the Passaic Formation. Lithologic and hydrogeologic characteristics of these rock aquifers that affect the spatial distribution of radionuclides in the water include the concentration of radioactive minerals in the strata; the continuity of radioactive strata; the orientation, depth, interconnectedness, and continuity of regional and local fracture patterns; and the geochemical environment, which affects radionuclide solubility. The stratigraphic zones of uranium enrichment and the continuity of the radioactive strata differ between the Stockton Formation and the lower part of the Passaic Formation. Uranium enrichment in both formations, however, is identified in permeable sections of strata that have sharp contacts with, or that are enclosed by, less permeable zones. In the Stockton Formation, basal conglomerates overlying fine-grained mudstones are the most radioactive lithology. In interbedded red and black mudstones of the lower part of the Passaic Formation, the uranium-rich zones are the black and white carbonate-rich siltstone and mudstone laminae overlying massive fine-grained mudstones. Most samples of the arkosic sandstones of the Stockton Formation tended to be slightly enriched in uranium relative to thorium, whereas all samples of the black mudstones of the lower part of the Passaic Formation were heavily enriched in uranium with respect to thorium, with a maximum concentration of uranium of 6,200 parts per million (ppm). Spatial orientation of radioactive zones is controlled by the depositional environment of the host lithology. Radioactive strata in the arkosic sandstones of the Stockton Formation, which is deltaic or fluvial/deltaic in origin, arc stratigraphically discontinuous and nonplanar. In contrast, the radioactive lacustrine black mudstones of the lower part of the Passaic Formation are laterally continuous and planar. Geophysical logs of boreholes indicate that the number and depths of probable waterbearing fractures are greater in the arkoses of the Stockton Formation and red mudstones of the lower part of the Passaic Formation than in the interbedded red and black mudstones of the lower part of the Passaic Formation, despite the fact that fracture-density counts in cores indicate that black mudstones are more highly fractured than the other rock types. The total number of fractures is greater and bedding-plane fractures are generally more common in the lower part of the Passaic Formation than in the Stockton Formation. Fractures in the Stockton Formation form a well-connected network consisting of both bedding-plane and several sets of high-angle fractures present on a regional scale. The large-scale continuity of interconnected bedding-plane fractures and high-angle regional fractures may be partly responsible for the higher yields observed for wells in the Stockton Formation than for wells in the Passaic Formation and may facilitate migration of dissolved radionuclides far from their source. The well-connected fracture network may increase the likelihood that circulating water comes in contact with radioactive strata. Water in the black mudstones of the lower part of the Passaic Formation may also contain high levels of radioactivity, primarily as a result of the high frequency of radioactive strata and of fracturing, rather than the presence of a well-connected regional fracture system. Ground-water chemistry is dominated by the calcium cation and the bicarbonate anion, both derived from abundant secondary calcite in the fractures. The concentrations of uranium and radium in water that comes into contact with radioactive rock are controlled by the water chemistry. Uranium is the dominant alpha emitter in the oxidizing waters in the red mudstones of the Passaic Formation, whereas radium-226 (226Ra) is the dominant alpha emitter in the suboxic (reducing) water in the black mudstones of the lower part of the Passaic Formation. Because the lithology of the Stockton Formation and, in places, the lower part of the Passaic Formation varies greatly, wells can intercept multiple fracture zones that can contain oxic or suboxic waters, thereby resulting in the presence of both uranium and radium in detectable quantities in the same water sample.

New Jersey↗