Recharge Conditions and Flow Velocities of Contaminated and Uncontaminated Ground Waters at Cape Cod, Massachussetts: Evaluation of ²H/¹H, ¹⁸O/¹⁶O, and Dissolved Gases

John-Karl Böhlke, Richard L. Smith, Tyler B. Coplen, Eurybiades Busenberg, and Denis R. LeBlanc

ABSTRACT

The isotopes of H and O in H₂O, and the concentrations of dissolved Ar, N₂, and O₂, were used to investigate recharge conditions and flow velocities in parts of a ground-water contaminant plume originating from disposal of treated domestic waste water and in surrounding uncontaminated ground water at the Massachusetts Military Reservation on Cape Cod. Contrasting patterns of isotope and gas abundances in the two water types are attributed to their different recharge sources and conditions: a low rate of areally distributed recharge of precipitation producing closely spaced, horizontal isochrons above the plume resulted in significant vertical variation in the H₂O isotope ratios but relatively little variation in atmospheric gases; conversely, a high rate of local recharge of recycled ground water producing more widely spaced, tilted isochrons in the plume resulted in significant horizontal variation in atmospheric gases but almost no variation in the H₂O isotope ratios. In addition, waters enriched in heavy isotopes by evaporation were detected in a pond and in the aquifer downgradient from the pond.

Concentrations of Ar and N₂ were consistent with recharge at temperatures ranging from about 8-24°C, followed by varying amounts of denitrification within the plume. Plume waters were anoxic, but O₂ concentrations above and below the plume averaged about 80-90 % of air saturation values at the temperatures implied by corresponding Ar concentrations, indicating relatively little O₂ reduction at time scales of up to at least 25 years. Vertical variations in ground-water isotope ratios above the plume at a site 140 m from the source in November 1996 were consistent with a precipitation recharge rate of 0.6-1.0 m/yr, an age of 1.0-1.5 yrs at the top of the plume, and a horizontal flow velocity of 140-210 m/yr. Longitudinal variations in Ar concentrations and H₂O isotopes in March 1998 were consistent with the position and age of the trailing edge of the plume following cessation of waste-water disposal in December 1995, and with horizontal flow velocities between about 120 and 240 m/yr. The isotope and dissolved-gas results are consistent (to within about ± 50 %) with travel times indicated by ³H/³He dating, injected tracer tests, regional flow modeling, and passage of other conservative tracers representing the trailing edge of the plume.