Hydrological and Geochemical Controls on Episodic Acidification of Streams in Shenandoah National Park, Virginia:  Development and Testing of a Predictive Model

Karen C. Rice, Jeff P. Raffensperger, U.S. Geological Survey
Rick Webb, Department of Environmental Sciences, University of Virginia

Episodic acidification of a stream results in a temporary decrease in stream acid-neutralizing capacity (ANC) and pH during stormflow, with a return to normal ANC and pH as streamflow returns to baseflow conditions. These acidic pulses, which are well documented in the streams of Shenandoah National Park (SNP), are the direct result of inputs of acid rain and can have deleterious effects on aquatic biota.  Subsurface stormflow often is a significant contributor to storm runoff in forested catchments that are underlain by thin, permeable soils above less-permeable bedrock.  It is not well understood, however, how the paths of subsurface stormflow and the chemical reactions that occur along these paths contribute to these episodic stream acidification events.  As such, existing hydrologic models generally do not explicitly incorporate components of subsurface stormflow. The objective of this study is to further the understanding of subsurface flowpaths and chemical reactions along these flowpaths in three forested catchments in SNP through the development of a linked hydrological and geochemical model

TOPMODEL is a semi-distributed water-balance model that simulates the relation between rainfall and stormflow; the model is based on the concept of a topographic index, which can be determined for all points in a catchment on the basis of the land-surface topography.  Inputs to the model include potential evapotranspiration and precipitation.  The model produces (1) predicted discharge, which can be compared with measured discharge; (2) values of actual evapotranspiration as a function of time; and (3) saturation deficit, a measure of the relative depth to the water table. 
Researchers at the University of Virginia (UVA) have extended the TOPMODEL concept to explicitly include subsurface stormflow.  TOPMODEL with the subsurface stormflow component is being calibrated for the three catchments in SNP, using existing hydrological data from a joint water-quality monitoring effort between UVA and SNP.  One goal of the present study is to calibrate and test the TOPMODEL framework in the three catchments at time scales ranging from episodic to annual to evaluate whether the approach can be generalized to other forested catchments.  

Once the hydrological portion of TOPMODEL is calibrated for the three catchments, TOPMODEL will be coupled with an ANC-transport component, using existing data collected by the joint UVA-SNP effort.  The present study will further the understanding of soil-water contribution to stormflow by installing and sampling soil-water lysimeters at various depths in the three catchments and incorporating the results into the modeling effort.  The results of coupling geochemistry with the hydrological model for predicting the severity of future episodic acidification events will be valuable to managers of SNP.

Rice, K.C., Raffensperger, J.P. and Webb, R.J., 1999, Hydrological and geochemical controls on episodic acidification of streams in Shenandoah National Park, Virginia--Development and testing of a predictive model [abs.]:  Virginia Water Research Symposium, November 15, 1999, Richmond, Va.

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