U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 99-4224
Dover Air Force Base in Kent County, Delaware, has many contaminated sites that are in active remediation. To assist in this remediation, a steady-state model of ground-water flow was developed to aid in understanding the hydrology of the system, and for use as a ground-watermanagement tool. This report describes the hydrology on which the model is based, a description of the model itself, and some applications of the model.
Dover Air Force Base is underlain by unconsolidated sediments of the Atlantic Coastal Plain. The primary units that were investigated include the upper Calvert Formation and the overlying Columbia Formation. The uppermost sand unit in the Calvert Formation at Dover Air Force Base is the Frederica aquifer, which is the deepest unit investigated in this report. A confining unit of clayey silt in the upper Calvert Formation separates the Frederica aquifer from the lower surficial aquifer, which is the basal Columbia Formation. North and northwest of Dover Air Force Base, the Frederica aquifer subcrops beneath the Columbia Formation and the upper Calvert Formation confining unit is absent. The Calvert Formation dips to the southeast. The Columbia Formation consists predominately of sands, silts, and gravels, although in places there are clay layers that separate the surficial aquifer into an upper and lower surficial aquifer. The areal extent of these clay layers has been mapped by use of gamma logs.
Long-term hydrographs reveal substantial changes in both seasonal and annual ground-water recharge. These variations in recharge are related to temporal changes in evaporation, transpiration, and precipitation. The hydrographs show areas where extensive silts and clays are present in the surficial aquifer. In these areas, the vertical gradient between water levels in wells screened above and below the clays can be as large as several feet, and local ground-water highs typically form during normal recharge conditions. When drought conditions persist, water drains off these highs and the vertical gradients decrease. At the south end of Dover Air Force Base, hydrographs of water levels in the Frederica aquifer show that off-Base pumping can cause the water levels to decline below sea level during part of the year.
A 4-layer, steady-state numerical model of ground-water flow was developed for Dover Air Force Base and the surrounding area. The upper two layers represent the upper and lower surficial aquifers, which are in the Columbia Formation. In some areas of the model, a semi-confining unit is used to represent an intermittent clay layer between the upper and lower surficial aquifer. This semi-confining unit causes the local groundwater highs in the surficial aquifer. The third model layer represents the upper part of the Calvert Formation, a confining unit. The fourth model layer represents the Frederica aquifer. The model was calibrated to hydraulic heads and to ground-water discharge in Pipe Elm Branch, both of which were measured in September 1997. For the calibrated model, the root-mean-squared errors for the hydraulic heads and the ground-water discharge in the Pipe Elm Branch were 9 percent of the range of head and 3 percent of discharge, respectively. Heads simulated by use of the model were consistent with a map showing average water levels in the region.
The U.S. Geological Survey's MODPATH program was used to simulate ground-water-flow directions for several areas on the Base. This analysis showed the effects of the local groundwater highs. In these areas, ground water can flow Hydrogeology and Simulation of Ground-Water Flow at Dover Air Force Base, Delaware 2 from the highs and then dramatically change flow direction as it enters the lower surficial aquifer.
The steady-state model has several limitations. The entire ground-water system is under transient hydraulic conditions, due mainly to seasonal and yearly changes in recharge and to withdrawal from irrigation wells. Yet this steady-state model is still considered to be an effective tool for understanding the ground-water-flow system underlying the Base for average conditions. If the ground-water system undergoes changes, such as an increase in pumping from existing or new wells in the surficial aquifer or in the Frederica aquifer at or near the Base, then the model may need to be verified for these conditions and, if necessary, recalibrated. Nevertheless, the model can be used to determine ground-water-flow pathlines in areas of the Base where flow directions are constant. In addition, the steady-state model is a necessary step in the development of transient models and solutetransport models, which are planned for future ground-water monitoring on the Base.
Purpose and scope
Description of investigation area
Hydraulic-head distribution and fluctuations
Upper confining unit of Calvert Formation
Hydraulic-head distribution, fluctuation, and gradient
Simulation of ground-water flow
Model design and boundary conditions
Calibration of the model
Simulated water budget
Sensitivity of the model
Comparison of simulated flow paths and ground-water recharge dates
Results of selected particle-tracking analyses
Natural Attenuation Project area
Long-term monitoring at OT-40
Contaminant plume at Area 6
Selected limitations of the ground-water-flow model
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