Saltwater Intrusion and Cross-Formational Flow in the Southern High Plains Aquifer along the Western Caprock Escarpment, New Mexico

Sustainable management of groundwater requires sufficient knowledge of the distribution of fresh and saline groundwater and the processes affecting saline-water intrusion. A hydrogeologic investigation using several chemical and isotopic tracers including ³H-He, ¹⁴C, δD, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, and δ¹¹B were examined to determine water sources, the influence of saltwater intrusion through cross-formational flow, and water-rock interactions in the Southern High Plains aquifer. At the study site, a sub-aquifer drains a Na-Cl, high dissolved solids (2,000 to 9,500 mg/L) groundwater from the escarpment until it mixes with a regional aquifer that is more oxygenated and a mixed cation-HCO₃^- water type with low dissolved solids (390 to 520 mg/L). The sub-aquifer contains old water (5,500 to 21,000 years) that is similar in age and composition to the underlying Dockum aquifer (Na-Cl water type, 970 to 13,000 mg/L dissolved solids, 12,000 to 27,000 years) whose water is derived from upwelling groundwater from lower salt beds. The ⁸⁷Sr/⁸⁶Sr range of 0.70845 to 0.70906 and Sr concentrations of 0.90 to 31 mg/L were sufficient to estimate source-water fractions and chemical weathering. Boron concentrations (59 to 1,740 mg/L) and δ¹¹B values (+6.0 to +46.0‰) were used to resolve the influence of agricultural recharge in the mixing zone that was ambiguously identified with other tracers. Alteration of geochemical signals from transport through shales and the effect of agricultural recharge limited the effectiveness of certain tracers.