Investigation of Ground-Water Discharge to the Dalecarlia Reservoir

Objectives

The objectives of this study are to describe the ground-water-flow system in the vicinity of the reservoir and evaluate the potential for ground-water contamination to enter the reservoir.

Statement of Problem

Evaluation of the interaction between the ground-water flow system and the Dalecarlia Reservoir approach to addressing the objectives is complicated by the geology of the area of the reservoir. The geology is primarily a thin layer of soil and saprolite above fractured bedrock. Unlike porous media systems, such as a sand, or gravel, ground-water flow is predominantly through the fractures within the rock. In the immediate vicinity of the reservoir, at least three different rock types are present. The understanding of local and regional orientations of fractures, especially those that control groundwater flow are extremely important.

The USGS will provide technical support to the Washington Aqueduct Division of the USACE related to ground-water and surface-water flow in the area of the Dalecarlia Reservoir. This support may include the review of documents, attendance of meetings, and other activities as directed and agreed upon by both the Washington Aqueduct and the USGS.

The unnamed tributary that flows around the reservoir to the east and north may interact directly with the reservoir or the local ground-water flow system. The nature of this interaction will be determined by installing and collecting data from several clusters of temporary well points that are to be installed on both sides of the unnamed tributary at two locations in the study area. These temporary well points will be measured to determine the elevation of the ground-water table, or perched flow system (if present), and any vertical gradients that may exist within the shallow flow system.

If time and funding permits and a determination is made that the reservoir is not lined, two well points would be driven into two different locations within the reservoir. These well points would have water levels measured both inside the riser pipe (representing the ground-water level beneath the reservoir) and outside the riser pipe (representing the reservoir level).

The completion of a water-balance evaluation will be completed by using standard stream-discharge measurements during base flow conditions at the streams east and northeast of the reservoir. These measurements may determine the relative contributions of ground water into the surface-water streams around the reservoir. The inherent inaccuracy of these measurements may be greater than the amount of ground-water flow into the streams, which may limit the usefulness of this method. However, repeating these measurements every month will increase the ability to determine trends, as well as determine the relative discharge from ground water to surface water changes during the seasonal cycle and under different hydrologic conditions. During this initial field effort, the relative hydraulic head differences between the streams and reservoir will be measured using stage gages installed at the stream-discharge measurement locations along with a gage installed in the reservoir. Each time the discharge measurements are taken, the absolute stage also will be recorded.

Water-quality samples will be collected twice at each of the stream-discharge-measurement locations for major ions. The USGS could collect the samples during the sampling for the major ions. Because the rock types vary somewhat within the different drainage areas, the ground-water types may also vary and may be identifiable by the relative differences in proportions of major ions. The perchlorate samples, if collected, could provide valuable information for the USACE because this is the contaminant of concern in the Spring Valley area, and since these streams drain various parts of that area.

Strategy and Approach

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