SOURCES OF FLOW TO A SPRING IN A FRACTURED AND KARSTIC AQUIFER: THE COMBINED USE OF ARTIFICIAL AND GEOCHEMICAL TRACERS

Fay Spring is located at Winchester, Virginia, within the Appalachian Great Valley and is situated on a northwest-trending strike-slip fault cutting Lower Ordovician dolomite and limestone. Continuous stage record for this perennial spring shows rapid response in discharge following large rain events with a concomitant decrease in conductivity; however, snowmelt leads to a prolonged increase in conductivity, indicating recharge by surface water carrying road salt. Quantification of local, surface-water input to the spring was sought through a dye trace in July 2009. Rhodamine WT injected at the terminal sink point of an ephemeral stream ~1 km west of Fay Spring provided conflicting evidence of initial rapid dye transit (>500 m/day) and overall low mass recovery (~12% after 5 weeks). Water samples were collected by an automatic sampler every six hours to daily during the trace period, and also from January-April, 2010 during melt-off of the largest snowfall on record for the region. Chemographs revealed dilution in NO₃, Cl, SO₄, and Mg with most large rain events; however, Cl increased during peak snowmelt with little change in other ion concentrations. We used principal components analysis (PCA) on major ions (Mg, SO₄, NO₃, Cl, Sr, and Si) in an effort to distinguish components of deeper, regional flow and shallow, local flow within the spring discharge. The PCA results show that two components cumulatively explain 72% of the variance of the data. The first component (54%) is most positively weighted on Mg and NO₃; the second component (17%) is most positively weighted on SO₄, and negatively weighted on Sr and Cl. We interpret the spring to be dominated by a diffuse flow component with elevated background levels of NO₃ (10-12 mg/L as N), SO₄ (45-50 mg/L) and Cl (35-40 mg/L) that is noticeably diluted by surface runoff only after large rain events. The background concentrations of SO₄ and NO₃ are similar between summer and winter; however, Cl concentration is higher in the winter (up to 80 mg/L). Overall, the impacts of rapid surface runoff are muted at Fay Spring compared with karst springs in other regions. A combination of approaches including dye tracing and interpretation of natural geochemistry is necessary for elucidating the nature of discharge for karst springs in fractured carbonate regions.