INTERACTION BETWEEN QUICK AND DIFFUSE FLOW IN A KARST AQUIFER REVEALED THROUGH DYE TRACING TO FAY AND SEMPELES SPRINGS IN WINCHESTER, VIRGINIA

Fay and Sempeles Springs are located in the City of Winchester in the northern Virginia portion of the Appalachian Great Valley. The discharge of Fay Spring has been gaged by the USGS since May, 2007; the spring responds rapidly to intense storm events and has a mean discharge of 1.1 cfs (0.3 m³/s). The discharge of Sempeles Spring is not continuously gaged but is estimated to be 2-3 times that of Fay Spring, based on periodic flow measurements. Fay and Sempeles springs are 300 m apart and lie along a common fault, thus it was hypothesized that these two artesian, perennial springs receive groundwater sourced along a common flow path. In order to determine the effect of sinking stormwater runoff on these springs a quantitative dye trace was conducted from a terminal sink point along Sunnyside Run, an ephemeral stream located approximately 1 km due west of Fay and Sempeles Springs.

Two kg of Rhodamine WT dye was injected with 3500 gal of treated municipal water on June 30, 2009. The dye breakthrough occurred at both springs 48 hr after injection, indicating the velocity of initial dye transit was at least 500 m/day. Dye concentration reached a peak value of 0.35 ppb at Sempeles Spring compared to 0.24 ppb at Fay. Approximately 34 g of the dye was recovered at the two springs within the first two weeks, or 1.7% of the amount injected. Subsequent rainfall events caused peaks in dye concentrations at Fay and Sempeles springs more than two weeks after injection, indicating that dye was retained within more stagnant zones of the groundwater system. Major ion data indicate that the chemical compositions of Fay and Sempeles Springs were very similar and did not vary appreciably during the six-week monitoring period, except for the calcium and silica concentrations; calcium decreased in response to rainfall events while silica increased. The Na/Cl molar ratio was nearly 1.0 for all samples and the Cl/Br ratios were between 600-700, indicating what appears to be a persistent presence of road salt in the groundwater, with a background chloride concentration of 35-40 mg/L.

Flow from these springs has both quick and diffuse sources. Under the moderately wet conditions of this trace, quick flow from the point of dye injection was determined to be highly diluted in the spring discharge, yet persisted for the 6-week monitoring duration.