Patuxent River Naval Air Station Groundwater

Objectives

The objectives of this study are (1) to develop a groundwater flow model that simulates flow in the Piney Point-Nanjemoy, Aquia, and Upper Patapsco aquifers underlying Pax River, and (2) to develop a management model that is designed to minimize Pax River's contribution to the large drawdowns in the aquifers underlying Southern Maryland.

Statement of Problem

Water levels throughout Southern Maryland in the Piney Point-Nanjemoy, Aquia, and Upper Patapsco aquifers have declined during the last half of the 20th century. In the vicinity of Lexington Park, Maryland, water levels in the Aquia aquifer are now as much as 170 feet below sea level. At the present rate the water levels will soon decline to a state mandated maximum. The effect of the substantial withdrawal by Patuxent River Naval Air Station from these aquifers on this declining potentiometric surface has caused concern among the base managers.

Strategy and Approach

Inventory of existing data from USGS databases; QWDATA, GWSI, Surface WaterUD and Navy databases will be completed to establish the groundwork of known hydrogeologic data near the air station.

Design a long-term, water-level monitoring network for the aquifers that will integrate with the USGS/MGS Southern Maryland Network. Several wells (either newly constructed or existing) will be instrumented with continuous recorders to monitor water-level changes over time. Synoptic water level measurements will be made in conjunction with Southern Maryland Network during April and October of each year.

Determine the hydrogeological framework of the confined aquifers from existing and new data. Geologic cross-sections, existing and new well lithology logs, fence diagrams, surface and borehole geophysical data, and structure contour maps of aquifer surfaces, thickness, and potentiometric surfaces will be prepared to conceptualize the hydrogeologic framework controlling ground water flow and geochemistry of the Piney Point-Nanjemoy and Aquia aquifers.

Hydraulic testing - A multi-aquifer, multi-well test will be conducted at NAS Patuxent River to determine hydraulic properties of the upper sands of the Patapsco Formation to evaluate water-supply potential of the aquifer. The multiaquifer test will also provide information on possible leakage between the Aquia aquifer and the Upper Patapsco aquifer. A single-aquifer, multi-well test of the Upper Patapsco aquifer will be conducted at WOLF in FY2001. Further investigation of the hydraulic properties, including storage values, using numerical methods will continue into FY2002.

Investigate water-quality trends and determine the potential for salt-water intrusion in the Piney Point-Nanjemoy and Aquia aquifers. Spatial and temporal trends in existing water quality data will be analyzed and additional samples where needed will be sampled to improve the spatial analysis of water quality data. Trends in water-quality will be used to determine if potable water-quality is improving, staying the same, or degrading with time. Salt-water intrusion will be analyzed using Piper Diagrams, sodium concentrations, chloride concentrations, ratios of bromide to chloride, etc., and isochrons (lines of equal concentration) to determine the extent of salt-water intrusion (if any). Water-quality samples will be obtained from the permeable sands of the Patapsco aquifer and analyzed at the USGS National Water Quality Lab for EPA drinking water parameters that currently have MCL and SMCL standards. These parameters will include, but are not limited to major ions, nutrients, and radionuclides.

Develop detailed ground-water flow model and optimization algorithms for all three aquifers at the NAS Patuxent River. Water managers will be able to use the model to develop management scenarios to minimize the air station's contribution to the large cone-of-depression in the potentiometric surface of the aquifers in Southern Maryland. The ground-water flow model will be based on the current Maryland Geological Survey model of St. Marys and Calvert County, but will improve the simulation of ground water flow at the air station by increasing model cells to 1200 (40 times greater than the current density of 30 cells). This task will begin in FY2002, and will be completed in FY2003.

Visualization of geologic data, ground-water flow and optimization of management scenarios will be conducted using three-dimensional visualization techniques. This display of the final results of the project will aid planners, water managers and regulators in understanding the complex patterns of ground-water flow. It is anticipated it will be possible at the end of this project to do time animations of ground-water flow, water-level changes and possible salt-water intrusion using the various management scenarios of pumping determined by the air station water managers.