AN 11-YEAR RECORD OF GROUNDWATER AGE, CHEMISTRY AND STABLE ISOTOPIC COMPOSITION OF DISCHARGE FROM FURNACE SPRING, SHENANDOAH NATIONAL PARK, VIRGINIA

Variations in climatic conditions have complex relationships with groundwater properties, quality, and availability. In order to better understand these processes, discharge from Furnace Spring (FS) (elev. 1045 m) in Shenandoah National Park (SNP), Virginia was sampled approximately monthly for 11 years (1999-2010) for age tracers (CFC-11, CFC-12, CFC-113, and SF₆), dissolved gases (N₂, O₂, Ar, CO₂, and CH₄), stable isotopes of water, and major and trace inorganic constituents. Temperature, discharge, pH, specific conductance, and dissolved oxygen were recorded at 30-minute intervals for the period of record at FS. Atmospheric mixing ratios of CFCs and SF₆ were measured monthly in air from the Air Monitoring Station at Big Meadows, SNP.

Seasonal and long term trends were evident in the spring water data. The apparent (piston flow) SF₆ age of the water varied seasonally between about modern in January to 2.5 years in June, and returned to near modern by December. The apparent SF₆ age was inversely correlated with discharge. δ²H and δ¹⁸O showed variations that were correlated to the annual Standardized Precipitation Index (SPI), with δ²H and δ¹⁸O trending more positive with below-normal precipitation, and more negative in periods with elevated amounts of precipitation. Recharge temperature from dissolved gasses (N₂-Ar) was found to vary with water discharge temperature on a seasonal basis. Large precipitation events caused high excess air in spring discharge and abrupt changes in water discharge temperatures; initial cooling in summer and initial warming in winter, and return to ambient, indicating a relatively short interval between recharge and discharge. The water discharge rate correlated inversely with concentrations of dissolved SiO₂, Ca²⁺, Mg²⁺, Na⁺, Cl^-, and HCO₃^-, that were at a maximum during low discharge in late summer (August-September). Levels of SO₄^2-, and NO₃^- were invariant in comparison.

Over the entire FS dataset, the average pH of Furnace Spring water decreased from 6.64 to 5.90, the specific conductance increased from 48 to 52 , and the mean annual discharge temperature increased from 8.3 in 1999 to 8.6ºC in 2007. During this time period, the concentrations of Ca²⁺, Mg²⁺, Na⁺, Cl^-, HCO₃^-, and SO₄^-2 increased, while NO₃^- remained constant.