THE RELATIONSHIP BETWEEN CONDUCTIVITY AND MAJOR IONS WITHIN THE DAVIS SPRING DRAINAGE BASIN AS A METHOD TO DETERMINE THE SOURCE OF SPRING DISCHARGE

The Davis Spring drainage basin is a 190 km² karst basin in Greenbrier County, West Virginia underlain by the 300+ m sequence of the Mississippian Greenbrier Limestone Group which rests on top of the Maccrady Shale. Davis Spring is the largest karst spring in West Virginia with average flows of 3.1 m ³/sec and is supplied by both allogenic and autogenic flow. Davis Spring was sampled from August 2008 to March 2009. Over 400 samples were analyzed for major cations and anions over that period, most commonly at 6 –12 hour intervals. Conductivity and stage (converted to discharge) were automatically measured throughout the sampling period.

Discharge is inversely correlated with conductivity, particularly during the winter and early spring months. Sulfate ranges from 3.6 to 13.9 mg/L and correlates inversely with discharge. Magnesium shows an inverse correlation with discharge during many large storm events but is difficult to correlate elsewhere. The inverse correlation between calcium and discharge is occasionally clear but frequently no correlation can be established.

Using the Pearson’s Correlation method sulfate correlated well with conductivity (0.927) whereas calcium showed no relationship with conductivity (0.005). The Davis Spring basin is geographically divisible into two regions. The eastern region of the basin is underlain by the lower Greenbrier sequence. Previous studies have identified significant amounts of gypsum and trace amounts of pyrite in the basal Greenbrier member (Hillsdale Limestone) and underlying Maccrady Shale. The western region is underlain by the upper portion of the Greenbrier series which contains no gypsum. Since sulfate originates from the eastern region of the basin, and is a conservative ion, it can be used as a tracer for eastern region water. Periods with low sulfate suggest the dominance of western region water at Davis Spring while periods of high sulfate suggest eastern water is dominant. Since low sulfate periods correspond to high discharge periods, it is concluded that periods of baseflow are dominated by discharge from the eastern region of the basin. This supports previous conclusions that the basal Greenbrier forms a semi-independent aquifer representing base flow within the Davis Spring basin.