UNDERSTANDING THE EFFECTS OF HIGH FLOW EVENTS ON SHORT-TERM LONGITUDINAL VARIABILITY IN WATER QUALITY

Models for understanding how water quality is affected by natural and anthropogenic influences from the surface and bedrock/groundwater inputs from below cannot easily be simplified to isolate the effects of dilution during high flow events. This work is part of a larger study by Low and Hewitt (this session) that takes into account potential contributions from both the surface and subsurface and bedrock dissolution rates and changes in relative flow in understanding water quality in the Maury River and its tributaries as well as in similar hydrologic systems. A point of particular focus for this study is an analysis of how water quality changes as a function of discharge during low and normal flow as well as during the rising and falling limbs of several different storm events.

The Maury River watershed is a tributary of the James River and is part of the Chesapeake Bay. With a watershed area of 1676 km², the Maury’s bedrock composition is a mixture of carbonates (48%) (limestone (31%) and dolostone (17%)) sandstones (21%) and shale (27%), and a small percentage of igneous and metamorphic rocks (3%). In terms of land use, the Maury River watershed is primarily forested (72%), with some croplands and pasture interspersed and small towns comprising the rest.

Within the Maury River watershed, ten watersheds with varying sizes, land use parameters, and underlying bedrock compositions were chosen for study. Both water and bedrock samples throughout these watersheds were analyzed using ICP-OES. Water from the ten selected watersheds was sampled through June and into August of 2013 in order to analyze water composition at a variety of stages of flow, which was calculated using a combination of information from the four USGS stream gauges in the watershed and flow accumulations maps for the study area.

Preliminary analysis of water samples indicates much less variation for many dissolved elements in water as a function of changes in discharge during high flow events than might be expected given the accompanying changes in conductivity. This suggests that the role of high flow events in both temporarily altering the relative total inputs of elements from the surface and subsurface and the dilution effect of additional surface water volume have a much greater effect upon the concentrations of some dissolved ions in the water than others.