Geology topics

E. J. Wexler

Publications and source records attributed to E. J. Wexler.

7 recordsLinked to original sources

USGS1993

A coupled surface-water and ground-water flow model for simulation of stream-aquifer interaction

Ground-water and surface-water flow models traditionally have been developed separately, with interaction between subsurface flow and streamflow either not simulated at all or accounted for by simple formulations. In areas with dynamic and hydraulically well-connected ground-water and surface-water systems, stream-aquifer interaction should be simulated using deterministic responses of both systems coupled at the stream-aquifer interface. Accordingly, a new, coupled ground-water and surface-water model was developed by combining the U.S. Geological Survey models MODFLOW and BRANCH; the interfacing code is referred to as MODBRANCH. MODFLOW is the widely used modular three-dimensional, finite-difference, ground-water model, and BRANCH is a one-dimensional, numerical model commonly used to simulate unsteady flow in open-channel networks. MODFLOW was originally written with the River package that calculates leakage between the aquifer and stream, assuming that the stream's stage remains constant during one model stress period. A simple streamflow routing model has been added to MODFLOW, but it is limited to steady flow in rectangular, prismatic channels. To overcome these limitations, the BRANCH model, which simulates unsteady, nonuniform flow by solving the entire St. Venant equations, was restructured and incorporated into MODFLOW. Terms that describe leakage between stream and aquifer as a function of streambed conductance and differences in aquifer and stream stage were added to the continuity equation in BRANCH. Thus, leakage between the aquifer and stream can be calculated separately in each model, or leakages calculated in BRANCH can be used in MODFLOW. Total mass in the coupled models is accounted for and conserved. The BRANCH model calculates new stream stages for each time interval in a transient simulation based on upstream boundary conditions, stream properties, and initial estimates of aquifer heads. Next, aquifer heads are calculated in MODFLOW based on stream stages calculated by BRANCH, aquifer properties, and stresses. This process is repeated until convergence criteria are met for head and stage. Because time steps used in ground-water modeling can be much longer than time intervals used in surface-water simulations, provision has been made for handling multiple BRANCH time intervals within one MODFLOW time step. An ption was also added to BRANCH to allow the simulation of channel drying and rewetting. Testing of the coupled model was verified by using data from previous studies; by comparing results with output from a simpler, four-point implicit, open-channel flow model linked with MODFLOW; and by comparison to field studies of L-31N Canal in southern Florida.

Open-File Report↗

USGS1993

Hydrogeology and simulation of ground-water flow near the Lantana Landfill, Palm Beach County, Florida

The Lantana landfill in Palm Beach County has a surface that is 40 to 50 feet above original ground level and consists of about 250 acres of compacted garbage and trash. Parts of the landfill are below the water table. Surface-resistivity measurements and water-quality analyses indicate that leachate-enriched ground water along the eastern perimeter of the landfill has moved about 500 feet eastward toward an adjacent lake. Concentrations of chloride and nutrients within the leachate-enriched ground water were greater than background concentrations. The surficial aquifer system in the area of the landfill consists primarily of sand of moderate permeability, from land surface to a depth of about 68 feet deep, and consists of sand interbedded with sandstone and limestone of high permeability from a depth of about 68 feet to a depth of 200 feet. The potentiometric surface in the landfill is higher than that in adjacent areas to the east, indicating ground-water movement from the landfill toward a lake to the east. Steady-state simulation of ground-water flow was made using a telescoping-grid technique where a model covering a large area is used to determine boundaries and fluxes for a finer scale model. A regional flow model encompassing a 500-square mile area in southeastern Palm Beach County was used to calculate ground-water fluxes in a 126.5-square mile subregional area. Boundary fluxes calculated by the subregional model were then used to calculate boundary fluxes for a local model of the 3.75-square mile area representing the Lantana landfill site and vicinity. Input data required for simulating ground-water flow in the study area were obtained from the regional flow models, thus, effectively coupling the models. Additional simulations were made using the local flow model to predict effects of possible remedial actions on the movement of solutes in the ground-water system. Possible remedial actions simulated included capping the landfill with an impermeable layer and pumping five leachate recovery wells. Results of the flow analysis indicate that the telescoping grid modeling approach can be used to simulate ground-water flow in small areas such as the Lantana landfill site and to simulate the effects of possible remedial actions. Water-quality data indicate the leachate-enriched ground water is divided vertically into two parts by a fine sand layer at about 40 to 50 feet below land surface. Data also indicate the extent of the leachate-enriched ground-water contamination and concentrations of constituents seem to be decreasing over time.

Florida↗

USGS1988

Ground-water flow and solute transport at a municipal landfill site on Long Island, New York – Part 1: Hydrogeology and water quality

Hydrogeology and water quality in a 4 sq mi area surrounding the Brookhaven landfill site in the Town of Brookhaven, New York, were studied in 1981-83. The 60-acre sanitary landfill at the site was excavated in highly permeable glacial outwash that forms the upper glacial aquifer and is lined with a polyvinyl chloride membrane. Groundwater beneath the site is under water table conditions and flows southeast at approximately 1.1 ft/day. Samples from wells downgradient indicate that leachate has entered the aquifer despite the liner. A plume 3,700 ft long , 2,400 ft wide, and at least 90 ft thick was delineated based on specific conductance data. Water quality in the Magothy aquifer and in Beaverdam Creek, a groundwater fed stream 2,000 ft southeast of the landfill site, does not appear to be affected. (USGS)

New York↗

USGS1988

Ground-water flow and solute transport at a municipal landfill site on Long Island, New York — Part 2: Simulation of ground-water flow

Data on the hydrogeology of a 26-sq-mi area surrounding the Brookhaven landfill site in central Suffolk County were collected as part of a hydrologic investigation of solute transport from the site. These data were used to develop a steady-state groundwater flow model of the upper glacial (water table) aquifer in the area. The model accounts for the leakage through confining units underlying the aquifer, seepage to streams, recharge from precipitation, and pumpage and redistribution of water. Refined estimates of aquifer and confining-unit properties were obtained through model calibrations. Water table altitudes generated by the calibrated model were used to determine groundwater velocities and probable flow paths in the vicinity of the site under long-term average hydrologic conditions. Groundwater velocities and probable flow paths in the study area were calculated from simulated water table altitudes generated by the calibrated flow model. Groundwater at the center of the site flows southeastward at a velocity of 1.1 ft/d. The report is the second in a three part series describing the hydrologic conditions and groundwater quality, groundwater flow, and solute transport in the vicinity of the Brookhaven landfill.

New York↗

USGS1988

Ground-water flow and solute transport at a municipal landfill site on Long Island, New York. Part 3, simulation of solute transport

A solute transport model representing a 2.3-sq mi area surrounding and downgradient from a municipal landfill site in the Town of Brookhaven, N.Y. was used to simulate migration of a conservative solute (chloride) in the upper glacial aquifer. Aquifer values used in the model were: hydraulic conductivity, 200 ft/day; effective porosity, 0.30; longitudinal dispersivity, 100 ft; transverse dispersivity, 20 ft. Average concentration of chloride was set at 875.0 mg/L in leachate and 10 mg/L in recharge and in ambient groundwater. Entry of leachate into the aquifer was assumed to have begun in 1977. Chloride concentrations in the simulated plume after 6 years of travel matched reasonably well the chloride data collected in October-December 1982. After 12 years of travel, the simulated plume extended 6,200 ft and was 2,600 ft wide. Maximum predicted concentration at the site boundary was 160 mg/L. Additional simulations were made to test the model 's ability to predict the effect of several remedial strategies on the movement of solutes. These included capping the landfill with an impermeable surface, removal of contaminated groundwater through four recovery wells, and a combination of the first two actions.

New York↗

USGS1986

Ground-water movement in the upper glacial aquifer in the Manorville area, Town of Brookhaven, Long Island, New York, in November 1983

Water levels in 52 wells near the Manorville scavenger-waste disposal facility in the Town of Brookhaven were measured in November 1983 to determine the direction and gradients of groundwater flow in the upper glacial aquifer. Groundwater moves south-southeastward (S22 degrees E) from the groundwater divide, about 6 miles north of the facility, to discharge points near East Moriches and beneath Moriches Bay. The hydraulic gradient beneath the disposal facility is 6.5 ft/mi (0.0012 foot/foot), and the rate of horizontal flow is 0.9 to 1.7 ft/day. Vertical movement of groundwater since closure of the disposal facility in 1982 is probably negligible because the vertical gradients are small and the upper glacial aquifer is anisotropic. During operation of the facility, however, groundwater mounding may have developed beneath the unlined settling basins, which could have induced downward movement of water in the upper glacial aquifer.

New York↗

USGS1986

Organic compounds in ground water near a sanitary landfill in the Town of Brookhaven, Long Island, New York

Landfill leachate and groundwater near the Brookhaven landfill site were analyzed for volatile, acid-extractable, and base/neutral-extractable organic compounds classified by EPA as ' priority pollutants, ' and for dissolved organic carbon (DOC) and inorganic constituents. Thirteen priority pollutants were detected, including benzene, chlorobenzene, ethyl benzene, and naphthalene. The most commonly detected priority pollutant, benzene, was found in the landfill leachate and in five groundwater samples. The concentration of ethyl benzene was the highest of all organic compounds detected--55 micrograms/L (ug/L) in the leachate. Three samples downgradient from the site contained detectable levels of priority pollutants; two of these from wells 2000 ft downgradient from the site, contained trace amounts of chloroform, and the other, from a well 500 ft downgradient, contained four priority pollutants in concentrations totaling less than 20 ug/L. DOC concentrations ranged from 410 mg/L in the leachate to 0.8 mg/L, which is considered background level for groundwater upgradient of the landfill site. Samples from downgradient wells contained less than 8 mg/L DOC except at two wells 500 ft from the site, which contained 21 and 16 mg/L. (Author 's abstract)

Water-Resources Investigations Report↗