Environmental tracers of groundwater source, age, and geochemical evolution

First posted April 25, 2023 For additional information, contact: Director,California Water Science CenterU.S. Geological Survey6000 J Street, Placer HallSacramento, California 95819 Hexavalent chromium, Cr(VI), was discharged in cooling wastewater to unlined surface ponds from 1952 1964 and reached the underlying unconsolidated aquifer at Pacific Gas Electric Company (PG&E) Hinkley compressor station Mojave Desert, 80 miles northeast of Los Angeles, California. A suite environmental tracers analyzed water samples collected more than 100 wells characterize source, age, geochemical evolution groundwater within near Cr(VI) plume Valleys. This information used help determine extent associated with releases identify natural sources.The source most wells, indicated by stable oxygen hydrogen isotope values for water, delta oxygen-18 deuterium, recharge infiltration intermittent flows River. With exception small 1958, River largely dry between 1969. period spans station; 1952–69 also high tritium levels precipitation resulting atmospheric testing nuclear weapons and, as a consequence, concentrations Valley were comparatively low. Groundwater ages (time since recharge) increased downgradient depth. Tritium, measured helium ingrowth study reporting level 0.05 unit, detected 51 percent detectable far 7 Tritium higher, tritium/helium-3 younger, shallower downgradient. Agricultural pumping has decreased much 60 feet 1952. As result this pumping, some containing tritium, presumably anthropogenic been removed aquifer. The distribution having carbon-14 activities or greater 100-percent modern carbon, consistent post-1952 similar tritium. Carbon-14 low 8.9-percent (unadjusted reactions materials) almost 20,000 years before present (ybp), sampled deep wells. chromium older 11 micrograms per liter but did not exceed 3.6 completed “Mojave-type” deposits (composed felsic stream near-shore lake sourced River); value may represent an upper limit on Mojave-type that likely approximates background area. Chlorofluorocarbons released atmosphere hydrologic cycle industrial activity beginning 1930s. Chlorofluorocarbon data generally suitable groundwater-age dating because nonatmospheric contributions local sources.Strontium-87/86 ratios isotopes, chromium-53, provide Highly radiogenic strontium-87/86 0.71000 coarse-textured diagnostic these materials. Nonradiogenic less 0.70950 weathered materials northern subarea eroded Miocene (23–5 million ybp) east Values chromium-53 ranged 0 2.8 parts thousand (‰) difference. reductive fractionation, mixing native groundwater, longitudinal dispersion October–December 2015 (Q4 2015) regulatory can be estimated basis composition plume. Reduction trivalent Cr(III), occur presence reductants oxic groundwater. Although interest Q4 margin, indicate is conservative reacted materials; have groundwater.Environmental tracers, (post-1952) premodern (pre-1952) inform understanding naturally occurring processes margins compositions data, mineralogy chemistry part summative-scale analysis later professional paper (chapter G).