SYNOPTIC VARIATIONS IN BASEFLOW AND WATER CHEMISTRY IN A STRIKE-PARALLEL STREAM BETWEEN SILICLASTIC AND KARST SEQUENCES

Streams receiving baseflow discharge from karst aquifers can be a useful source of quantitative information regarding flow rates and aquifer source characteristics. This investigation describes a baseflow survey of a stream flowing near the contact between a Silurian karst sequence (Helderberg-Tonoloway-Wills Creek formations) and a siliclastic siltstones/shale sequence (Upper Silurian McKenzie Formation-Clinton Group) along the east flank of the Cacapon Mountain anticline, WV, in the folded Ridge and Valley province. The two sequences both contain springs of distinctive water chemistry that mix in Sir Johns Run, a 11-km long strike-parallel stream along the contact between the two sequences. Because of the small size of the catchment, the stream carries primarily baseflow at most times of the year. A downstream flow/chemical profile was performed in fall 2004 to quantify inflows into the stream from both sources. Stations were measured at about 300 meter intervals. The survey was performed on 2 separate days 5 days apart, with a minor precipitation event between; however, overlapping flow measurements at two stations for both dates agreed within 15% of each other and allowed data from the two dates to be integrated. Flow increases monotonically and nearly linearly in a downstream direction, with the most abrupt increases in flow associated with springs from the Helderberg, a limestone aquifer with known conduit characteristics. Water chemistry is bimodal in the upper reaches where flows are lowest, but reaches a stable level in the lower reaches reflecting an outgassed mixture of the two end-member chemistries. Spring discharge from limestone, as opposed to both diffuse discharge and to spring discharge from siliclastics, contributes the greatest portion of Sir Johns Run baseflow. Observations of flow and stage decline during and after the baseflow survey provide useful information for quantitative parameterization of flow. Downstream water chemical evolution in Sir Johns Run provides useful corroboration of survey results.