Utilizing geochemical, hydrologic, and boron isotopic data to assess the success of a salinity and selenium remediation project, Upper Colorado River Basin, Utah

Stream discharge and geochemical data were collected at two sites along lower Ashley Creek, Utah, from 1999 to 2003, to assess the success of a site specific salinity and Se remediation project. The remediation project involved the replacement of a leaking sewage lagoon system that was interacting with Mancos Shale and increasing the dissolved salinity and Se load in Ashley Creek. Regression modeling successfully simulated the mean daily dissolved salinity and Se loads (R2 values ranging from 0.82 to 0.97) at both the upstream (AC1) and downstream (AC2/AC2A) sites during the study period. Prior to lagoon closure, net gain in dissolved-salinity load exceeded 2177 metric tons/month and decreased after remediation to less than 590 metric tons/month. The net gain in dissolved Se load during the same pre-closure period exceeded 120 kg/month and decreased to less than 18 kg/month. Senʹs slope estimator verified the statistical significance of the modeled reduction in monthly salinity and Se loads. Measured gain in dissolved constituent loads during seepage tests conducted during September and November 2003 ranged from 0.334 to 0.362 kg/day for dissolved Se and 16.9 to 26.1 metric tons/day for dissolved salinity. Stream discharge and changes in the isotopic values of delta boron-11 (δ11B) were used in a mixing model to differentiate between constituent loadings contributed by residual sewage effluent and naturally occurring ground-water seepage entering Ashley Creek. The majority of the modeled δ11B values of ground-water seepage were positive, indicative of minimal seepage contributions from sewage effluent. The stream reach between sites S3 and AC2A contained a modeled ground-water seepage δ11B value of −2.4‰, indicative of ground-water seepage composed of remnant water still draining from the abandoned sewage lagoons.