Geology topics

Gregory S. Cherry

Publications and source records attributed to Gregory S. Cherry.

16 recordsLinked to original sources

USGS2019

Simulation of groundwater flow in the Brunswick Area, Georgia, for 2004 and 2015, and selected groundwater-management scenarios

The Upper Floridan aquifer (UFA) is the principal water source for industrial and public supply in Glynn County, Georgia. Wells in active pumping centers that tap the UFA for industries near the city of Brunswick have created an upward hydraulic-head gradient in the Floridan aquifer system, which has allowed high chloride (saline) groundwater from the Fernandina permeable zone of the Lower Floridan aquifer (LFA) to migrate upward into freshwater zones. Chloride concentrations of more than 250 milligrams per liter—the State and Federal secondary drinking-water standard—have been measured in a 2-square-mile area near downtown Brunswick. An existing regional U.S. Geological Survey modular finite-difference groundwater-flow model (MODFLOW-2000) was modified using greater horizontal and vertical resolution to enable more detailed simulation of the effects of pumping in the vicinity of chloride contamination. Modifications to the regional model consisted of (1) limiting grid size to a maximum of 500 feet (ft) per side in the vicinity of the chloride plume; (2) representing the upper and lower Brunswick aquifers with distinct model layers; (3) similarly, representing upper and lower water-bearing zones of the UFA with distinct model layers in Glynn and Camden Counties, Ga.; and (4) establishing new hydraulic-property geographic zones in the UFA within Glynn County. The revised groundwater-flow model was calibrated to steady-state conditions that were assumed to exist during 2000 and 2004. The calibration and framework of the revised groundwater-flow model were documented in a separate report. For the current study, steady-state conditions were calibrated using October 2015 pumping rates in the Brunswick/Glynn County area as a 2015 Base Case. The 2015 Base Case simulation was used as the basis to evaluate seven groundwater-management scenarios in the Brunswick/Glynn County area. Seven groundwater management-scenarios were developed on the basis of short- and long-term groundwater-use projections for the UFA in the Brunswick/Glynn County area. Scenarios A and B simulated additional pumping in the upper water-bearing zone (UWBZ) of the UFA at existing public-supply wells located near a chloride plume and planned public-supply wells to be constructed north of downtown Brunswick. Scenario C simulated a shutdown at Brunswick Cellulose Inc. and Pinova Inc. and the resulting deactivation of nine production wells, with a combined total pumping of 31.3 million gallons per day (Mgal/d) for the 2015 Base Case simulation. Scenario D (three scenarios) simulated 12.5, 25, and 50 percent (designated Scenarios D1, D2, and D3) of the total pumping of 31.3 Mgal/d at Brunswick Cellulose and Pinova. The objective of Scenario D was to determine pumping rates that may reverse groundwater-flow directions toward the Brunswick Cellulose well field and potentially allow groundwater with higher chloride concentration to migrate toward nearby public-supply wells. Scenario E simulated an additional pumping of 5 Mgal/d from the UWBZ of the UFA at a recently constructed production well within the Brunswick Cellulose well field. Backward particle-tracking (MODPATH) analysis in public-supply wells located just outside the chloride plume to the north shows that predominant groundwater-flow directions are from the northeast toward the Brunswick Cellulose well field. The analysis covered 20- and 50-year periods for the 2015 Base Case and Scenario C simulations with 100 percent of backtracked particles remaining in the UWBZ and lower water-bearing zone of the UFA. Groundwater-flow directions are characterized by some vertical movement and dominant horizontal movement away from the chloride plume in the northern Brunswick area. For the 2015 Base Case simulation, the mean rate of particle movement ranged from 268 to 413 feet per year. For the Scenario C simulation, the mean rate of particle movement ranged from 89 to 182 feet per year with 50 percent of particles migrating from the chloride plume area. The rate of particle movement is influenced most by the horizontal hydraulic-head gradient in the UWBZ of the UFA. The revised groundwater-flow model is subject to the limitations documented in the original model. In addition, the values used for the specified-head boundaries in the Floridan aquifer system for the 2004 calibrated model were based on the sparse data available and were not changed for the 2015 update to the model. These model boundaries control 80 percent of the inflows and about 60 percent of the outflows. Composite-scaled sensitivities of the model parameters indicate the revised model is most sensitive to pumping rates, followed by the horizontal hydraulic conductivity in the UFA for zones along coastal Georgia.

Georgia↗

USGS2017

Simulated effects of Lower Floridan aquifer pumping on the Upper Floridan aquifer at Barbour Pointe, Chatham County, Georgia

Steady-state simulations using a revised regional groundwater-flow model based on MODFLOW were run to assess the potential long-term effects on the Upper Floridan aquifer (UFA) of pumping the Lower Floridan aquifer (LFA) at well 36Q398, located at Barbour Pointe in coastal Georgia near Savannah. Simulated pumping of well 36Q398 at a rate of 750 gallons per minute (gal/min; or 1.08 million gallons per day [Mgal/d]) indicated a maximum drawdown of about 2.19 feet (ft) in the UFA directly above the pumped well and at least 1 ft of drawdown within a nearly 190-square-mile area (scenario A). Induced vertical leakage from the UFA provided about 98 percent of the water to the pumped well. Simulated pumping of well 36Q398 caused increased downward leakage in all layers above the LFA, decreased upward leakage in all layers above the LFA, increased inflow to and decreased outflow from lateral specified-head boundaries in the UFA and LFA, and an increase in the volume of induced inflow from the general-head boundary representing outcrop units. Water budgets for scenario A indicated that changes in inflows and outflows through general-head boundaries would compose about 45 percent of the simulated pumpage from well 36Q398, with the remaining 55 percent of the pumped water derived from flow across lateral specified-head boundaries. Additional steady-state simulations were run to evaluate a pumping rate in the UFA of 240 gal/min (0.346 Mgal/d), which would produce an equivalent maximum drawdown in the UFA as pumping from well 36Q398 in the LFA at a rate of 750 gal/min (called the “drawdown offset”; scenario B). Simulated pumping in the UFA for the drawdown offset produced about 2.18 ft of drawdown, comparable to 2.19 ft of drawdown in the UFA simulated in scenario A. Water budgets for scenario B also provided favorable comparisons with scenario A, indicating that 42 percent of the drawdown-offset pumpage (0.346 Mgal/d) in the UFA originates as increased inflow and decreased outflow across general-head boundaries from overlying units in the surficial and Brunswick aquifer systems and that the remaining simulated pumpage originates as flow across general- and specified-head boundaries within the UFA and LFA. The revised model was evaluated for sensitivity by first altering horizontal and vertical hydraulic conductivity in the Lower Floridan semiconfining unit and then adjusting horizontal and vertical hydraulic conductivity in the LFA to match the 35.6 ft of drawdown at pumping well 36Q398. These adjustments also affected the maximum simulated drawdown in the UFA and the equivalent offset pumping in the UFA that would produce the same amount of drawdown. The maximum drawdown in the UFA ranged from 1.82 to 2.57 ft and the equivalent offset pumping in the UFA ranged from 199 to 278 gal/min. The revised model reasonably depicts changes in groundwater levels resulting from pumping the LFA at Barbour Pointe at a rate of 750 gal/min. Results are limited, however, by the same model assumptions and design as the original model, and placement of boundaries and type of boundary used exert the greatest control on overall groundwater flow and interaquifer leakage in the system. Simulation results have improved regional characterization of the Floridan aquifer system, which could be used by State officials in evaluating requests for groundwater withdrawal from the LFA.

Georgia↗

USGS2017

Saltwater intrusion in the Floridan aquifer system near downtown Brunswick, Georgia, 1957–2015

Introduction The Floridan aquifer system (FAS) consists of the Upper Floridan aquifer (UFA), an intervening confining unit of highly variable properties, and the Lower Floridan aquifer (LFA). The UFA and LFA are primarily composed of Paleocene- to Oligocene-age carbonate rocks that include, locally, Upper Cretaceous rocks. The FAS extends from coastal areas in southeastern South Carolina and continues southward and westward across the coastal plain of Georgia and Alabama, and underlies all of Florida. The thickness of the FAS varies from less than 100 feet (ft) in aquifer outcrop areas of South Carolina to about 1,700 ft near the city of Brunswick, Georgia. Locally, in southeastern Georgia and the Brunswick– Glynn County area, the UFA consists of an upper water-bearing zone (UWBZ) and a lower water-bearing zone (LWBZ), as identified by Wait and Gregg (1973), with aquifer test data indicating the upper zone has higher productivity than the lower zone. Near the city of Brunswick, the LFA is composed of two permeable zones: an early middle Eocene-age upper permeable zone (UPZ) and a highly permeable lower zone of limestone (LPZ) of Paleocene and Late Cretaceous age that includes a deeply buried, cavernous, saline water-bearing unit known as the Fernandina permeable zone. Maslia and Prowell (1990) inferred the presence of major northeast–southwest trending faults through the downtown Brunswick area based on structural analysis of geophysical data, northeastward elongation of the potentiometric surface of the UFA, and breaches in the local confining unit that influence the area of chloride contamination. Pronounced horizontal and vertical hydraulic head gradients, caused by pumping in the UFA, allow saline water from the FPZ to migrate upward into the UFA through this system of faults and conduits. Saltwater was first detected in the FAS in wells completed in the UFA near the southern part of the city of Brunswick in late 1957. By the 1970s, a plume of groundwater with high chloride concentrations had migrated northward toward two major industrial pumping centers, and since 1965, chloride concentrations have steadily increased in the northern part of the city. In 1978, data obtained from a 2,720-ft-deep test well (33H188) drilled south of the city showed water with a chloride concentration of 33,000 milligrams per liter (mg/L), suggesting the saltwater source was located below the UFA in the Fernandina permeable zone (FPZ) of the LFA. All U.S. Geological Survey (USGS) data collected for this study, including groundwater levels in wells and water-chemistry data, are available in the USGS National Water Information System.

Georgia↗

USGS2015

Groundwater flow in the Brunswick/Glynn County area, Georgia, 2000-04

An existing regional steady-state model for coastal Georgia, and parts of South Carolina and Florida, was revised to evaluate the local effects of pumping on the migration of high chloride (saline) water in the Upper Floridan aquifer located in the Brunswick/Glynn County, Georgia (Ga.) area. Revisions were focused on enhancing the horizontal and vertical resolution of the regional model grid in the vicinity of saline water. Modifications to the regional model consisted of (1) limiting grid size to a maximum of 500 feet (ft) per side in the vicinity of chloride contamination; (2) representing the upper and lower Brunswick aquifers with distinct model layers; (3) similarly, representing upper and lower water-bearing zones of the Upper Floridan aquifer with distinct model layers in Glynn and Camden Counties, Ga.; and (4) establishing new hydraulic-property zones in the Upper Floridan aquifer. The revised model simulated steady-state conditions that were assumed to exist during 2000 and 2004. Calibration of the revised steady-state model using pumping rates from 2000 indicates a "good" match (±10 ft) based on 181 observations, with median residuals (simulated minus observed water levels) in each of the active model layers ranging from -8.62 to 4.67 ft, and root mean square error (RMSE) ranging from 10.9 to 11.4 ft. In the Brunswick/Glynn County area, groundwater-level residuals in the upper water-bearing zone of the Upper Floridan aquifer (layer 7) indicate an "excellent" match (±5 ft) based on 41 observations with a median residual of -0.35 ft and RMSE of 4.32 ft. Calibration of the revised steady-state model using 2004 pumping rates and adjusted specified-head input values in the Floridan aquifer system indicates a "good" match (-10 ft) based on 88 observations, with median residuals in each of the active model layers ranging from -6.31 to -2.05 ft, and RMSE ranging from -6.95 to 14.5 ft. In the Brunswick/Glynn County area, groundwater-level residuals in the upper water-bearing zone of the Upper Floridan aquifer (layer 7) indicate an "excellent" match (±5 ft) based on 32 observations with a median residual of -1.50 ft and RMSE of 5.34 ft. Simulated potentiometric surfaces for 2000 and 2004 indicate coastward groundwater flow in the Upper and Lower Floridan aquifers influenced by pumping centers at Savannah, Jesup, and Brunswick, Ga., and indicate steep potentiometric gradients to the west and north of the Gulf Trough. In the Brunswick/Glynn County area, simulated industrial production wells located north of downtown Brunswick intercept local groundwater flow in the upper and lower water-bearing zones of the Upper Floridan aquifer and have created a cone of depression that locally alters the regional coastward flow direction. Maps of simulated water-level change during the 2000-04 period show differences in groundwater levels in the Upper Floridan aquifer that range from -2.5 ft to more than 5 ft in areas of coastal Georgia, and more than 20 ft near the Georgia-Florida State Line. Positive values indicate higher simulated water levels during 2004 than during 2000, which were caused by reduced pumping in the Upper Floridan aquifer prompted by the shutdown of a paper mill near the southern model boundary in 2002 and increased recharge following a prolonged drought during 1998-2002. Simulated potentiometric profiles for 2000 and 2004 were used to evaluate the potentiometric gradients in the upper water-bearing zone of the Upper Floridan aquifer (layer 7) near the chloride plume in the downtown Brunswick area. Four potentiometric profiles were constructed for 2000 to compare the simulated and observed water levels in 13 wells and were oriented outward from a primary well field. The simulated potentiometric gradients from the four profiles for 2000 ranged from 3.6 to 5.2 feet per mile (ft/mi) compared to observed values ranging from 4.1 to 5.6 ft/mi. The five potentiometric profiles constructed for 2004 allowed for a similar comparison using simulated and observed water levels in 18 wells. The simulated potentiometric gradients from the five profiles for 2000 ranged from 3.6 to 11.1 ft/mi compared to observed values ranging from 3.8 to 10.2 ft/mi. Simulated potentiometric gradients were higher for 2004 than for 2000 because of the inclusion of a well located within the cone of depression near downtown Brunswick. Composite-scaled sensitivities of the model parameters indicate the revised model is most sensitive to pumping rates, followed by the horizontal hydraulic conductivity in the Upper Floridan aquifer for zones along coastal Georgia. The revised model is least sensitive to the horizontal hydraulic conductivity of the confining units and vertical hydraulic conductivity of the aquifers. For parameters defined by hydraulic-property zones in the upper and lower water-bearing zones of the Upper Floridan aquifer, such as horizontal hydraulic conductivity, model sensitivity was not as great in the Brunswick/Glynn County area as other areas along coastal Georgia. The model exhibited more sensitivity to these parameters however, than to parameters representing the majority of zones defining the vertical hydraulic conductivity of the confining units, which originally were assumed to govern upward migration of chloride contamination into this aquifer. Analysis of simulated water-budget components for 2000 and 2004 indicate that specified-head boundaries in the Floridan aquifer system to the south and southwest of the regional model area control about 70 percent of inflows and nearly 50 percent of outflows to the model region. Other water-budget components indicate an 80-million-gallon-per-day decrease in pumping from the Floridan aquifer system during this period.

Georgia↗

USGS2015

Simulated effects of Lower Floridan aquifer pumping on the Upper Floridan aquifer at Rincon, Effingham County, Georgia

Steady-state simulations using a revised regional groundwater-flow model based on MODFLOW were run to assess the potential long-term effects on the Upper Floridan aquifer (UFA) of pumping the Lower Floridan aquifer (LFA) at well (36S048) near the City of Rincon in coastal Georgia near Savannah. Simulated pumping of well 36S048 at a rate of 1,000 gallons per minute (gal/min; or 1.44 million gallons per day [Mgal/d]) indicated a maximum drawdown of about 6.8 feet (ft) in the UFA directly above the pumped well and at least 1 ft of drawdown within a nearly 400-square-mile area (scenario A). Induced vertical leakage from the UFA provided about 99 percent of the water to the pumped well. Simulated pumping of well 36S048 indicated increased downward leakage in all layers above the LFA, decreased upward leakage in all layers above the LFA, increased inflow to and decreased outflow from lateral specified-head boundaries in the UFA and LFA, and an increase in the volume of induced inflow from the general-head boundary representing outcrop units. Water budgets for scenario A indicated that changes in inflows and outflows through general-head boundaries would compose about 72 percent of the simulated pumpage from well 36S048, with the remaining 28 percent of the pumped water derived from flow across lateral specified-head boundaries. Additional steady-state simulations were run to evaluate a pumping rate in the UFA of 292 gal/min (0.42 Mgal/d), which would produce the equivalent maximum drawdown in the UFA as pumping from well 36S048 in the LFA at a rate of 1,000 gal/min (called the drawdown offset; scenario B). Simulated pumping in the UFA for the drawdown offset produced about 6.7 ft of drawdown, comparable to 6.8 ft of drawdown in the UFA simulated in scenario A. Water budgets for scenario B also provided favorable comparisons with scenario A, indicating that 69 percent of the drawdown-offset pumpage (0.42 Mgal/d) in the UFA originates as increased inflow and decreased outflow across general-head boundaries from overlying units in the surficial and Brunswick aquifer systems and that the remaining simulated pumpage originates as flow across general- and specified-head boundaries within the UFA. A steady-state simulation representing implementation of drawdown-offset-pumping reductions totaling 292 gal/min at Rincon UFA production wells 36S034 and 36S035 and pumping from the new LFA well 36S048 at 1,000 gal/min (scenario C) resulted in decreased magnitude and areal extent of drawdown in the UFA compared with scenario A. In the latter scenario, the LFA well was pumped without UFA drawdown-offset-pumping reductions. Water budgets for scenario C yielded percentage contributions from flow components that were consistent with those from scenario B. Specifically, 69 percent of the increased pumping in scenario C originated from general-head boundaries from overlying units of the surficial and Brunswick aquifer systems and the balance of flow was derived from general- and specified-head boundaries in the UFA. In all scenarios, the placement of model boundaries and type of boundary exerted the greatest control on overall groundwater flow and interaquifer leakage in the system.

Georgia↗

USGS2013

Simulated effects of Lower Floridan aquifer pumping on the Upper Floridan aquifer at Pooler, Chatham County, Georgia

A revised regional groundwater-flow model was used to assess the potential effects on the Upper Floridan aquifer (UFA) of pumping the Lower Floridan aquifer (LFA) from a new well (35Q069) located at the City of Pooler in coastal Georgia near Savannah. The spatial resolution of the original regional, steady-state, groundwater-flow model was increased to incorporate detailed hydrogeologic information resulting from field investigations at Pooler and existing wells in the area. Simulation results using the U.S. Geological Survey finite-difference code MODFLOW indicated that long-term pumping at a rate of 780 gallons per minute (gal/min) from the LFA well 35Q069 would cause a maximum drawdown of about 2.52 feet (ft) in the UFA (scenario A). This maximum drawdown in the UFA was greater than the observed draw-down of 0.9 ft in the 72-hour aquifer test, but this is expected because the steady-state simulated drawdown represents long-term pumping conditions. Model results for scenario A indicate that drawdown in the UFA exceeded 1 ft over a 163-square-mile (mi 2 ) area. Induced vertical leakage from the UFA provided about 98 percent of the water to the LFA; the area within 1 mile of the pumped well contributed about 81 percent of the water pumped. Simulated pumping changed regional water-budget components slightly and redistributed flow among model layers, namely increasing downward leakage in all layers, decreasing upward leakage in all layers above the LFA, increasing inflow to and decreasing outflow from lateral specified-head boundaries in the UA and LFA, and increasing the volume of induced recharge from the general head boundary to outcrop units. An additional two groundwater-pumping scenarios were run to establish that a linear relation exists between pumping rates of the LFA well 35Q069 (varied from 390 to 1,042 gal/min) and amount of drawdown in the UFA and LFA. Three groundwater-pumping scenarios were run to evaluate the amount of UFA pumping (128 to 340 gal/min) that would produce maximum drawdown in the UFA equivalent to that induced by pumping the LFA well 35Q069 at rates specified in scenarios A, B, and C (390 to 1,042 gal/min). Scenarios in which the LFA well 35Q069 was pumped produced a larger drawdown area in the UFA than scenarios in which the UFA well was pumped to offset the maximum UFA drawdown simulated by scenarios A, B, and C. Three additional groundwater-pumping scenarios were run to evaluate the combination of pumping reductions at existing Pooler UFA public-supply wells with the addition of pumping from the new LFA well. For each scenario, LFA well 35Q069 was pumped at different rates, and pumping at existing Pooler supply wells, located about 3.7 miles northward, was reduced according to UFA drawdown offsets (128 to 340 gal/min) established by scenarios D, E, and F. Decreases in the magnitude and areal extent of drawdown in the UFA in response to pumping the LFA well were realized for scenarios that simulated drawdown offsets (reductions) for the existing UFA wells at Pooler when compared with the magnitude and extent of drawdown resulting from scenarios that did not simulate drawdown offsets for the existing UFA wells at Pooler (scenarios A, B, and C). The revised model was evaluated for sensitivity by altering horizontal and vertical hydraulic conductivity in layers 5 through 7 (Floridan aquifer system) for newly established hydraulic-property zones by factors of 0.1, 0.5, 2.0, and 10.0. Results of the sensitivity analysis indicate that horizontal and vertical hydraulic conductivity of the UFA and LFA are the most important parameters in model simulations. The least sensitive parameters were the horizontal and vertical hydraulic conductivity of the Lower Floridan confining unit; changes to these parameters had little effect on simulated leakage and groundwater levels. The revised model reasonably depicts changes in groundwater levels resulting from pumping the LFA at Pooler at a rate of 780 gal/min. However, results are limited by the same model assumptions and design as the original model and placement of boundaries and type of boundary used exert the greatest control on overall groundwater flow and interaquifer leakage in the system. Simulation results have improved regional characterization of the Floridan aquifer system, which could be used by State officials in evaluating requests for groundwater withdrawal from the LFA.

Georgia↗

USGS2011

Groundwater conditions in the Brunswick-Glynn County area, Georgia, 2009

The Upper Floridan aquifer is contaminated with saltwater in a 2-square-mile area of downtown Brunswick, Georgia. The presence of this saltwater has limited the development of the groundwater supply in the Glynn County area. Hydrologic, geologic, and water-quality data are needed to effectively manage water resources. Since 1959, the U.S. Geological Survey (USGS) has conducted a cooperative water program with the City of Brunswick and Glynn County to monitor and assess the effect of groundwater development on saltwater intrusion within the Floridan aquifer system. The potential development of alternative sources of water in the Brunswick and surficial aquifer systems also is an important consideration in coastal areas. During calendar year 2009, the cooperative water program included continuous water-level recording of 13 wells completed in the Floridan, Brunswick, and surficial aquifer systems; collecting water levels from 46 wells to map the potentiometric surface of the Upper Floridan aquifer in Glynn County during August 2009; and collecting and analyzing water samples from 55 wells completed in the Floridan aquifer system, of which 27 wells were used to map chloride concentrations in the upper water-bearing zone of the Upper Floridan aquifer in the Brunswick area during August 2009. Periodic water-level measurements also were collected from two wells completed in the Upper Floridan aquifer and four wells completed in the Brunswick aquifer system on Jekyll Island. Equipment was installed on one well to enable real-time specific conductance monitoring in the area surrounding the chloride plume. During 2008-2009, water levels in 30 of the 32 wells monitored in the Brunswick-Glynn County area rose at a rate of 0.24 to 7.58 feet per year (ft/yr). The largest rise of 7.58 ft/yr was in the Upper Floridan aquifer. These rises corresponded to a period of above normal precipitation and decreased pumping. Declines during 2008-2009 were recorded in wells completed in the Brunswick aquifer system (0.37 ft/yr) and Lower Floridan aquifer (0.83 ft/yr). Chloride data collected by two local industrial groundwater users at their well fields since 1958 were compiled and compared with data collected by the USGS during the same period. The results indicate that chloride concentrations at the two well fields have continued to rise despite modification of production wells to eliminate deep saline zones and decreases in pumpage at both facilities. One of the industrial users, Pinova Inc., plugged the lower portions of nine production wells in the mid to late 1960s, which generally decreased chloride concentrations to less than 100 milligrams per liter (mg/L) for a period of 10 to 20 years. However, chloride concentrations eventually returned to previous levels despite decreases in pumpage. During 1990-2009, chloride concentrations at the other industrial user's well field (Georgia-Pacific Cellulose LLC) generally increased despite a 16 million gallon per day decrease in pumpage during this period. Data from the Georgia-Pacific Cellulose well field and additional chloride data from USGS observation wells located to the east indicate continued movement of chloride from the source area located southeast of the site toward the well field.

Georgia↗

USGS2011

Groundwater modeling to evaluate interaquifer leakage in the Floridan aquifer system near Hunter Army Airfield and Fort Stewart

Simulations using a modified regional groundwater- flow model were used to determine the amount of leakage from the Upper Floridan aquifer (UFA) through the Lower Floridan confining unit (LFC) into the Lower Floridan aquifer (LFA) resulting from pumping about 1 million gallons per day at newly constructed LFA production wells at Hunter Army Airfield and Fort Stewart in coastal Georgia. Simulated steadystate drawdown at each of the LFA production wells closely matched observed drawdown during a 72-hour aquifer test with the observed water levels reaching steady-state by the end of the test period. However, simulated drawdown was greater than observed drawdown in the UFA because of the short duration of the aquifer test and the time required for groundwater movement through the LFC into the LFA. Steadystate simulations provide an estimate of leakage based on the long-term continuous operation of each production well. Results of model simulations indicate that interaquifer leakage accounts for 48 percent of the flow to the well at Hunter Army Airfield, and 98 percent of the flow to the well at Fort Stewart. Simulated results near the Hunter Army Airfield production well indicated that 65 percent of the leakage from the UFA to the LFA occurs within a 1-mile radius, whereas simulated results near the Fort Stewart production well indicated 80-percent leakage from the UFA to the LFA within the same radius. The greater amount of leakage to the production well near Fort Stewart can be attributed to the higher transmissivity of the UFA and higher vertical hydraulic conductivity in the LFC near the well.

Georgia↗

USGS2010

Groundwater conditions and studies in the Brunswick–Glynn County area, Georgia, 2008

The Upper Floridan aquifer is contaminated with saltwater in a 2-square-mile area of downtown Brunswick, Georgia. This contamination has limited development of the groundwater supply in the Glynn County area. Hydrologic, geologic, and water-quality data are needed to effectively manage water resources. Since 1959, the U.S. Geological Survey has conducted a cooperative water program with the City of Brunswick to monitor and assess the effect of groundwater development on saltwater contamination of the Floridan aquifer system. During calendar year 2008, the cooperative water program included continuous water-level recording of 12 wells completed in the Floridan, Brunswick, and surficial aquifer systems; collecting water levels from 21 wells to map the potentiometric surface of the Upper Floridan aquifer during July 2008; and collecting and analyzing water samples from 26 wells to map chloride concentrations in the Upper Floridan aquifer during July 2008. Equipment was installed on 3 wells for real-time water level and specific conductance monitoring. In addition, work was continued to refine an existing groundwater-flow model for evaluation of water-management scenarios.

Georgia↗

USGS2008

Ground-Water Conditions and Studies in the Brunswick-Glynn County Area, Georgia, 2007

The Upper Floridan aquifer is contaminated with saltwater in a 2-square-mile area of downtown Brunswick, Georgia. This contamination has limited the development of the ground-water supply in the Glynn County area. Hydrologic, geologic, and water-quality data are needed to effectively manage water resources. Since 1959, the U.S. Geological Survey has conducted a cooperative water-resources program with the City of Brunswick to monitor and assess the effect of ground-water development on saltwater contamination of the Floridan aquifer system. The potential development of alternative sources of water in the Brunswick and surficial aquifer systems also is an important consideration in coastal areas. During calendar year 2007, the cooperative water-resources monitoring program included continuous water-level recording of 13 wells completed in the Floridan, Brunswick, and surficial aquifer systems; collecting water levels from 22 wells to map the potentiometric surface of the Upper Floridan aquifer during July and August 2007; and collecting and analyzing water samples from 76 wells to map chloride concentrations in the Upper Floridan aquifer during July and August 2007. In addition, work was initiated to refine an existing ground-water flow model for evaluation of water-management scenarios.

Georgia↗

USGS2007

U.S. Geological Survey Georgia Water Science Center and City of Brunswick–Glynn County Cooperative Water Program— Summary of activities, July 2005 through June 2006

Since 1959, the U.S. Geological Survey has conducted a cooperative water resources program (CWP) with the City of Brunswick and Glynn County in the Brunswick, Georgia, area. Since the late 1950s, the salinity of ground water in the Upper Floridan aquifer near downtown Brunswick, Georgia, has been increasing, and its occurrence has been detected across an area of increasing size. Pumping of the Upper Floridan aquifer near downtown Brunswick has lowered water levels in the aquifer and resulted in an upward hydraulic gradient between the highly saline parts of the Lower Floridan aquifer and the normally fresh Upper Floridan aquifer. Saltwater likely enters the Upper Floridan aquifer through localized, vertically oriented conduits of relatively high permeability and moves laterally in response to the distribution of stresses within the aquifer. The Brunswick-Glynn County CWP for fiscal year 2006 includes the operation and maintenance of 12 continuous water-level recorders. In addition, water-level data were collected from 52 wells and water from 70 wells was analyzed for chloride concentration during June 2005. Geophysical logs were obtained from one well to assess whether the cause of elevated chloride concentration could be due to leaky well casing. A summary of the Georgia Department of Natural Resources, Environmental Protection Division (GaEPD) Georgia Coastal Sound Science Initiative (CSSI) activities that directly benefit the CWP-Brunswick-Glynn County is included in this report. The GaEPD CSSI is a program of scientific and feasibility studies to support development of a final strategy to protect the Upper Floridan aquifer from saltwater contamination. These data presented in this report are needed by State and local authorities to manage water resources effectively in the coastal area of Georgia.

Georgia↗

USGS2007

Simulation and Particle-Tracking Analysis of Selected Ground-Water Pumping Scenarios at Vogtle Electric Generation Plant, Burke County, Georgia

The source of ground water to production wells at Vogtle Electric Generation Plant (VEGP), a nuclear power plant in Burke County, Georgia, was simulated under existing (2002) and potential future pumping conditions using an existing U.S. Geological Survey (USGS) MODFLOW ground-water flow model of a 4,455-square-mile area in the Coastal Plain of Georgia and South Carolina. Simulation results for three steady-state pumping scenarios were compared to each other and to a 2002 Base Case condition. The pumping scenarios focused on pumping increases at VEGP resulting from projected future demands and the addition of two electrical-generating reactor units. Scenarios simulated pumping increases at VEGP ranging from 1.09 to 3.42 million gallons per day (Mgal/d), with one of the scenarios simulating the elimination of 5.3 Mgal/d of pumping at the Savannah River Site (SRS), a U.S. Department of Energy facility located across the Savannah River from VEGP. The largest simulated water-level changes at VEGP were for the scenario whereby pumping at the facility was more than tripled, resulting in drawdown exceeding 4-8 feet (ft) in the aquifers screened in the production wells. For the scenario that eliminated pumping at SRS, water-level rises of as much as 4-8 ft were simulated in the same aquifers at SRS. Results of MODFLOW simulations were analyzed using the USGS particle-tracking code MODPATH to determine the source of water and associated time of travel to VEGP production wells. For each of the scenarios, most of the recharge to VEGP wells originated in an upland area near the county line between Burke and Jefferson Counties, Georgia, with none of the recharge originating on SRS or elsewhere in South Carolina. An exception occurs for the scenario whereby pumping at VEGP was more than tripled. For this scenario, some of the recharge originates in an upland area in eastern Barnwell County, South Carolina. Simulated mean time of travel from recharge areas to VEGP wells for the Base Case and the three other pumping scenarios was between about 2,700 and 3,800 years, with some variation related to changes in head gradients because of pumping changes.

Georgia↗

USGS2006

Simulation of selected ground-water pumping scenarios at Fort Stewart and Hunter Army Airfield, Georgia

A regional MODFLOW ground-water flow model of parts of coastal Georgia, Florida, and South Carolina was used to evaluate the effects of current and hypothetical groundwater withdrawal, and the relative effects of pumping in specific areas on ground-water flow in the Upper Floridan aquifer near Fort Stewart and Hunter Army Airfield (HAAF), coastal Georgia. Simulation results for four steady-state pumping scenarios were compared to each other and to a Base Case condition. The Base Case represents year 2000 pumping rates throughout the model area, with the exception that permitted annual average pumping rates for the year 2005 were used for 26 production wells at Fort Stewart and HAAF. The four pumping scenarios focused on pumping increases at HAAF resulting from projected future demands and additional personnel stationed at the facility and on reductions in pumping at Fort Stewart. Scenarios A and B simulate 1- and 2-million-gallon-perday (Mgal/d) increases, respectively, at HAAF. Simulated water-level change maps for these scenarios indicate an area of influence that extends into parts of Bryan, Bulloch, Chatham, Effingham, and Liberty Counties, Ga., and Beaufort and Jasper Counties, S.C., with maximum drawdowns from 0.5 to 4 feet (ft) for scenario A and 1 to 8 ft for Scenario B. For scenarios C and D, increases in pumping at HAAF were offset by decreases in pumping at Fort Stewart. Scenario C represents a 1-Mgal/d increase at HAAF and a 1-Mgal/d decrease at Fort Stewart; simulated water-level changes range from 0.4 to -4 ft. Scenario D represents a 2-Mgal/d increase at HAAF and 2-Mgal/d decrease at Fort Stewart; simulated water-level changes range from 0.04 to -8 ft. The simulated water-level changes indicate an area of influence that extends into parts of Bryan, Bulloch, Chatham, Effingham, Liberty, and McIntosh Counties, Ga., and Jasper and Beaufort Counties, S.C. In general, decreasing pumping at Fort Stewart by an equivalent amount to pumping increases at HAAF reduced the magnitude and extent of drawdown resulting from the additional pumping. None of the scenarios resulted in large changes in the configuration of the simulated potentiometric surface and related ground-water flow directions. The scenarios simulated vary from the original model only by increasing pumpage less than 1 percent of the total calibrated model withdrawals. The changes in pumpage are located near the center of the original model area. Thus, the scenarios described in this report are considered to be reasonable with no less uncertainty than the original calibrated model.

Georgia↗

USGS2006

Application of ground-water flow and solute-transport models to simulate selected ground-water management scenarios in coastal Georgia and adjacent parts of South Carolina and Florida, 2000-2100

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Florida, Georgia, South Carolina↗

USGS2006

Simulation and particle-tracking analysis of ground-water flow near the Savannah River site, Georgia and South Carolina, 2002, and for selected ground-water management scenarios, 2002 and 2020

Ground-water flow under 2002 hydrologic conditions was evaluated in an eight-county area in Georgia and South Carolina near the Savannah River Site (SRS), by updating boundary conditions and pumping rates in an existing U.S. Geological Survey (USGS) ground-water model. The original ground-water model, developed to simulate hydrologic conditions during 1987-92, used the quasi-three-dimensional approach by dividing the Floridan, Dublin, and Midville aquifer systems into seven aquifers. The hydrogeologic system was modeled using six active layers (A2-A7) that were separated by confining units with an overlying source-sink layer to simulate the unconfined Upper Three Runs aquifer (layer A1). Potentiometric- surface maps depicting September 2002 for major aquifers were used to update, evaluate, and modify boundary conditions used by the earlier ground-water flow model. The model was updated using the USGS finite-difference code MODFLOW-2000 for mean-annual conditions during 1987-92 and 2002. The specified heads in the source-sink layer A1 were lowered to reflect observed water-level declines during the 1998-2002 drought. These declines resulted in a decrease of 12.1 million gallons per day (Mgal/d) in simulated recharge or vertical inflow to the uppermost confined aquifer (Gordon, layer A2). Although ground-water pumpage in the study area has increased by 32 Mgal/d since 1995, most of this increase (17.5 Mgal/d) was from the unconfined Upper Three Runs aquifer (source-sink layer A1) with the remaining 14.5 Mgal/d assigned to the active layers within the model (A2-A7). The simulated water budget for 2002 shows a decrease from the 1987-92 model from 1,040 Mgal/d to 1,035 Mgal/d. The decreased ground-water inflows and increased ground-water withdrawal rates reduced the simulated ground-water outflow to river cells in the active layers of the model by 43 Mgal/d. The calibration statistics for all layers of the 2002 simulation resulted in a decrease in the root mean square (RMS) of the residuals from 10.6 to 8.0 feet (ft). The residuals indicate 83.3 percent of the values for the 2002 simulation met the calibration error criteria established in the original model, whereas 88.8 percent was within the specified range for the 1987-92 simulation. Simulated ground-water outflow to the Savannah River and its tributaries during water year 2002 was 560 cubic feet per second (ft3/s), or 86 percent of the observed gain in mean-annual streamflow between streamflow gaging stations at the Millhaven, Ga., and Augusta, Ga. At Upper Three Runs Creek, simulated ground-water discharge during 2002 was 110 ft3/s, or 83 percent of the observed streamflow at two streamflow gaging stations near the SRS. These results indicate that the constructed model calibrated to 1987-92 conditions and modified for 2002 dry conditions is still representative of the hydrologic system. The USGS particle-tracking code MODPATH was used to generate advective water-particle pathlines and their associated time-of-travel based on MODFLOW simulations for 1987-92, 2002, and each of four hypothetical ground-water management scenarios. The four hypothetical ground-water management scenarios represent hydrologic conditions for (1) reported pumping for 2002 and boundary conditions for an average year; (2) reported pumping for 2002 with SRS pumping discontinued and boundary conditions for an average year; (3) projected 2020 pumping and boundary conditions for an average year; and (4) projected 2020 pumping and boundary conditions for a dry year. The MODPATH code was used in forward-tracking mode to evaluate flowpaths from areas on the SRS and in backtracking mode to evaluate further areas of previously documented trans-river flow on the Georgia side of the Savannah River. Trans-river flow is a condition in which the local head gradients might allow migration of contaminants from the SRS into the underlying aquifers and beneath the Savannah River into Georgia. More...

Georgia, South Carolina↗

USGS2001

Simulation of flow in the upper North Coast Limestone Aquifer, Manati-Vega Baja area, Puerto Rico

A two-dimensional computer ground-water model was constructed of the Manati-Vega Baja area to improve the understanding of the unconfined upper aquifer within the North Coast Province of Puerto Rico. The modeled area covers approximately 79 square miles within the municipios of Manati and Vega Baja and small portions of Vega Alta and Barceloneta. Steady-state two-dimensional ground-water simulations were correlated to conditions prior to construction of the Laguna Tortuguero outlet channel in 1940 and calibrated to the observed potentiometric surface in March 1995. At the regional scale, the unconfined Upper North Coast Limestone aquifer is a diffuse ground-water flow system through the Aguada and Aymamon limestone units. The calibrated model input parameters for aquifer recharge varied from 2 inches per year in coastal areas to 18 inches per year in the upland areas south of Manati and Vega Baja. The calibrated transmissivity values ranged from less than 500 feet squared per day in the upland areas near the southern boundary to 70,000 feet squared per day in the areas west of Vega Baja. Increased ground-water withdrawals from 1.0 cubic foot per second for 1940 conditions to 26.3 cubic feet per second in 1995, has reduced the natural ground-water discharge to springs and wetland areas, and induced additional recharge from the rivers. The most important regional drainage feature is Laguna Tortuguero, which is the major ground-water discharge body for the upper aquifer, and has a drainage area of approximately 17 square miles. The discharge to the sea from Laguna Tortuguero through the outlet channel has been measured on a bi-monthly basis since 1974. The outflow represents a combination of ground- and surface-water discharge over the drainage area. Hydrologic conditions, prior to construction of the Laguna Tortuguero outlet channel in 1943, can be considered natural conditions with minimal ground-water pumpage (1.0 cubic foot per second), and heads in the lagoon were 2.4 feet higher. The model was calibrated to March 1995 conditions during a dry period of minimal aquifer recharge and relatively constant water levels in the upper aquifer. For the steady-state 1995 model simulation, however, ground-water pumpage had been increased to 26.3 cubic foot per second, due to increased demand for public water supply, the heads at 0.9 feet, and the outflow to the sea at Laguna Tortuguero had been lowered considerably. Simulated ground-water inflow for 1940 hydrologic conditions included 35.9 cubic feet per second from areal recharge, contributions from streamflow along the southern boundary of 1.6 cubic feet per second, and streamflow infiltration to the upper aquifer of 4.2 cubic feet per second. Simulated ground-water outflow for 1940 hydrologic conditions are discharge to springs of 17.4 cubic feet per second, total ground-water withdrawals of 1.0 cubic feet per second, and aquifer contribution to streamflow or wetland areas of 23.4 cubic feet per second. Simulated ground-water inflow for hydrologic conditions of March 1995 include d contributions from streamflow along the southern boundary of 1.6 cubic feet per second, areal recharge of 35.9 cubic feet per second, and streamflow infiltration to the upper aquifer of 11 cubic feet per second. Simulated ground-water outflow for hydrologic conditions of March 1995 are ground-water withdrawals of 26.3 cubic feet per second, discharge from springs of 7.3 cubic feet per second, and aquifer contribution to streamflow or wetland areas of 14 .9 cubic feet per second. The overall ground-water budget increased from 41.8 cubic feet per second for 1940 conditions to 48.6 cubic feet per second for the hydrologic conditions of March 1995. The increase in ground-water budget is a direct result of increased ground-water withdrawals, which induced greater streamflow infiltration. Simulated ground-water flux to Laguna Tortuguero for 1940 conditions was 11 cubic feet per second, which drop

Water-Resources Investigations Report↗