The use of geochemical mass-balance and mixing models to determine groundwater sources

The groundwater recharge system in the vicinity of the Idaho National Engineering Laboratory is complex, with potential recharge from horizontal regional flow from the NE, horizontal flow originating in drainage basins N and NW of the site, streambed infiltration and infiltration of direct precipitation. Groundwater chemistry, tritium,δD/δ18O, δ34S, δ13C and14C were measured at 9 wells along 2 groundwater flow paths at the Idaho National Engineering Laboratory. Geochemical mass balance and mixing models were used to determine groundwater sources along the flow paths. Groundwater from the most distal well, NPR-WO I, was found to be a mixture of groundwater flow from the NW, groundwater flow from the NE and water from the Big Lost River. The difference between measured and calculated14C concentrations at this well indicate that the source waters are modern, which is consistent with the rapid horizontal flow velocities of groundwater in the Eastern Snake River Plain aquifer.