VARIATION IN THE WATER QUALITY OF A SMALL, SEMI-URBAN STREAM: UNDERSTANDING THE OFF-SEASON LEGACY OF DE-ICING CHEMICALS APPLIED WITHIN A WATERSHE

Woods Creek drains a 19 km² watershed in central Virginia as it travels through rolling forested and pastured uplands in its first 5.7 km before entering a town of 7,000 and two small university campuses over its final 3.5 km. The stream is listed (as of 2012) as impaired by the VADEQ for its entire 9.2 km length. This study considers conductivity and the concentrations of 29 dissolved ions and ionic complexes (measured using ICP-OES and IC) in ~200 water samples collected over the past year from several locations along Woods Creek and from its tributaries during a range of flow and weather conditions in order to quantitatively assess potential contributions from the application of de-icing chemicals, other potential urban and agricultural inputs, and leaching of the underlying bedrock to water quality in the downstream reaches of the stream.

Under baseflow conditions, Woods Creek tends to reflect the composition of its watershed which is underlain by a thin, if existent, veneer of soil and bedrock of mostly limestone with some dolostone. Despite upstream pastures, nutrient pollution is relatively minor and baseflow conductivity tends to be around 350 µS/cm on the upstream side of town creeping slowly upward to around 550 µS/cm at the mouth of the stream. Following two winter storms, the quality of the water upstream of town changed only marginally while conductivity downstream at the mouth of the creek increase substantially (to 1310 µS/cm) due entirely to increases in the concentration of Na and Cl in the water. While temporary increases in Na and Cl in Woods Creek are most notable shortly following intensive application of de-icing chemicals, high discharge events during the spring, summer, and fall are also marked by increases in the concentration of Na, Cl, N, and P accompanying notable dilutions in all other measured dissolved constituents. Tributaries springs, and culverts (particularly those downstream from areas of intensive runoff from paved surfaces) remain high in Na and Cl concentration throughout the year contributing a considerable portion of the baseflow dissolved solids to the stream. High stormflow Na and Cl in the main stem and high baseflow Na and Cl in some contributing springs and tributaries suggest lingering effects of de-icing chemicals throughout the year for Woods Creek and similar small, semi-urban streams.