Problem:  The largest resource-management problem that the Shenandoah National Park faces is acid rain and its effects on water quality in the Park. The acid-neutralizing capacity (ANC) of a stream is a measure of acid that water can neutralize; stream-water quality and ANC are strongly related to the type of bedrock underlying the catchment. In general, streams with low ANC have a correspondingly low pH, and streams with a high ANC have near-neutral pH. The more acidic the stream, the less likely the stream will support aquatic life. The ability to predict future changes in water quality in relation to ANC and in response to changes in external factors is limited by the complexity of hydrobiogeochemical processes in catchments.

Objective: To develop, calibrate, and apply a coupled water and solute transport model in three catchments in the Shenandoah National Park. The result will be a predictive model of stream discharge and ANC, which can be used to assess potential future changes in acid inputs and/or soil-rockbuffering capacity, as well as to test hypotheses concerning important hydrological controls on stream acid-base status.

Relevance and Benefits: This project meets the following broad goals for WRD work:
1) advancing analytical methodology (i.e., linking a surface-water model with a water-quality component);
2) advancing understanding of hydrologic processes (i.e., identifying causes of episodic acidification and linking episodic acidification to hydrologic flow paths);
3) providing water-resources information that will be used by multiple parties for planning and operational purposes (i.e., the National Park Service could use the developed model for resource-management decisions);
4) contributing data to national data bases that will be used to advance the understanding of regional and temporal variations in hydrologic conditions (i.e., the Surface Water Acidification Study of the University of Virginia will incorporate data and results from this study to interpret regional and temporal variations in hydrologic and water-quality conditions).

Approach: Develop a theoretical framework for quantifying transient, topographically controlled water movement and reactive mass transport in the subsurface using TOPMODEL and a depth-dependent ANC production model. Calibrate and test this theoretical framework at three different catchments, at time scales ranging from episodic to annual. Examine model sensitivity to both hydrological and geochemical critical controls on stream chemistry.