Hydrogeologic and water-quality data were collected at the Explosive Experimental Area, Naval Surface Warfare Center, Dahlgren Site at Dahlgren, Virginia, as part of a hydrogeologic assessment of the shallow aquifer system begun in 1993. The U.S. Geological Survey conducted this study to provide the U.S. Navy with hydrogeologic data to aid in the evaluation of the effects from remediation of contaminated sites and to protect against additional contamination. This report describes the groundwater observation-well network, hydrogeologic, and water-quality data collected between October 1993 and April 1995. The report includes a description of the locations and construction of 28 observation wells on the Explosive Experimental Area. Hydrogeologic data include lithologic logs, geophysical logs, and vertical hydraulic conductivity measurements of selected core intervals. Hydrologic data include synoptic and hourly measurements of groundwater levels, and observation-well slug tests to determine horizontal hydraulic conductivity. Water-quality data include analyses of major dissolved constituents in ground water and surface water. DESCRIPTION OF OBSERVATION-WELL NETWORK Observation wells were sited to obtain maximum areal coverage of the EEA study area, to have drill sites accessible by drilling equipment, and to avoid areas of known or suspected contamination. Observation well locations are shown on plate 1. Well Construction All 28 observation wells installed at the NSWCDL for the hydrogeologic assessment were constructed of 4-in. inside-diameter polyvinyl chloride (PVC) casing. Twenty wells were screened in the Columbia aquifer, of which 17 wells were augered and 3 wells (wells EEA-S6,

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PLATE 1. Map showing locations of observation wells, tide gage, and surface-water sampling sites in study area In pocket FIGURES Contents III Water temperature is expressed in degrees Celsius (°C), which can he converted to degrees Fahrenheit (°F) by the following equation:°F = 18(°C) + 32 Sea level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929-a geodetic datum derived from a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929. Abbreviated water-quality units: Chemical concentration and specific electrical conductance in this report are expressed in metric units. Chemical concentration is expressed in milligrams per liter (mg/L) or in mocrograms per liter (|J.g/L). Specific electrical conductance of water is expressed in microsiemens per centimeter at 25 degrees Celsius (u,S/cm). Values of the stable isotope ratios of hydrogen and oxygen (8D and 518O) refer to the relative difference between the ratios 2H/] H and 18O/16O, respectively, in the sample and the ratios in standard mean ocean water.

Efforts to derive scientifically-sound nutrient criteria and/or allowable loads must recognize that receiving waters' response to nutrient loads depends on many site-specific characteristics. A recent project supported by the Water Environment Research Foundation developed a nutrient Modeling Toolbox that links surface waters nutrient loads to ecological response indicators. Although many nutrient load-response models exist, these models should evolve in response to increases in the knowledge base. In an effort to guide this evolution, this project identified model improvement and associated data and research needs. The capabilities of models for dissolved oxygen, phytoplankton, water clarity, attached algae and pH are more limited by the availability of site-specific data than by inherent limitations in model frameworks. Scientific understanding of fundamental ecosystem processes is currently insufficient to allow mechanistic representation of some of the higher level response indicators, such as harmful algal blooms, submerged aquatic vegetation, and fish and benthic macroinvertebrate community integrity.