DEVELOPING A QUANTITATIVE MODEL FOR UNDERSTANDING WATER QUALITY THAT INTEGRATES SURFACE AND SUBSURFACE CHEMICAL INPUTS IN THE MAURY RIVER WATERSHED, VIRGINIA, USA

The mostly forested and pastured surface of the Maury River watershed in central Virginia is underlain by a thin, if existent, veneer of soil and a variety of carbonate and crystalline bedrock types. Assessing the effects of human (urban and agricultural) land use activities on surface water quality in such an environment using water quality proxies like conductivity is complicated both by variable flow conditions and the potential for diversity in the bedrock composition of tributary watersheds, a factor that is exacerbated by the role that bedrock type can play in land use patterns. In an attempt to decouple the effects of surface (land use) and subsurface (bedrock composition and weathering rate) inputs on water quality, this study considers 38 (major, minor, and trace) elements in 51 bulk rock samples from the 25 rock units that comprise the bedrock of the Maury River watershed (mapped at 1:500K scale) analyzed using ICP-OES coupled with experimentally-determined dissolution rates and surface water from the Maury River and its tributaries sampled from 10 different watersheds with varieties of size, topography, land use, and bedrock composition during a range of flow conditions throughout the summer (and is ongoing into the fall) and analyzed in order to quantitatively assess potential contributions from bedrock to water quality at the watershed-scale for specific dissolved ions and ionic complexes.

For elements (such as Ca, Mg, Si, Na, K, and Cl) that tend to be abundant at relatively high concentrations in both the lithosphere and dissolved in fresh surface water, spatial variation and variability in concentration in surface water can be attributed in most instances to contributions from the bedrock and to flow conditions respectively. Spatial variation in many trace elements (such as Ba, Sr, and As) observed between streams from different watersheds can also be explained at least in part by contributions from the respective bedrock while others require substantial inputs from the surface for some of the watersheds investigated. Integrating potential bedrock contributions into models that examine surface inputs to water quality at a variety of flow conditions will help to better inform less involved proxy studies of water quality and ecosystem health that may be applicable to other similar surface water systems.