A multi-tracer study of saltwater origin, cross -formational flow, and the geochemical evolution of groundwater in the Southern High Plains aquifer along the Western Caprock Escarpment, east-central New Mexico

Sustainable management of groundwater resources requires sufficient knowledge of the distribution of fresh and saline groundwater and the processes affecting saltwater intrusion that may influence the beneficial use of groundwater. A hydrogeologic investigation that coupled various chemical and isotopic tracers, including 3H/3He, 14C, δD, δ18O, 87Sr/ 86Sr, and δ11B, with the physical characteristics of the aquifer was conducted to determine source waters, the origin of saltwater and its influence through cross-formational flow, and water-rock interactions in the Southern High Plains aquifer along the Western Caprock Escarpment. Sub-aquifers or local flow systems are present along the Western Caprock Escarpment, and the study site\u27s local flow system 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 or regional flow system that is more oxygenated and a mixed cation-HCO3- water type with low dissolved solids (390 to 520 mg/L). The local flow system contains old water (5,500 to 21,000 years) that is similar in age and composition to the underlying, upper Dockum aquifer (Na-Cl water type, 970 to 13,000 mg/L dissolved solids, 12,000 to 27,000 years). The δD and δ18O values for the local flow system (-71.74 to -47.96‰ and -9.95 to -6.52‰, respectively) and upper Dockum aquifer (-67.20 to -51.70‰ and -9.11 to -6.93‰) were lower and more variable compared to the regional flow system (-45.97 to -43.29‰ and -6.30 to -6.09‰). Groundwater δD and δ 18O values in the mixing zone (-45.19 to -43.90‰ and -6.14 to -5.85‰) indicated an additional water source or further evaporation. To resolve the groundwater evolution along the Western Caprock Escarpment, 87Sr/86Sr and δ11B values were coupled with major ion, trace element, age, and δD and δ 18O values. 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 contributions from chemical weathering through inverse calculations. Boron concentrations (59 to 1,740 mg/L) and δ 11B 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 B and δ11B values in the mixing zone indicated the loss of B and decrease in δ11B values likely from plant uptake, adsorption, and weathering contributions in the soil/vadose zone prior to recharge beneath or near agricultural fields. With confirmation of this additional influence in the mixing zone, results from the Sr inverse calculations were used to reinterpret δD and δ 18O values to account for agricultural recharge. Geochemical tracer analysis allowed the formation of a conceptual flow model. Groundwater interaction with Permian bedded salts and Dockum Group shales produces a high dissolved-solids groundwater with a strong halite signal that can strongly influence groundwater composition in the Southern High Plains aquifer through cross-formational flow. Cross-formational flow from the Permian bedded salts into the Dockum Group provides a water source where none was expected because of the hydrologic divide of the escarpment, and this water likely originates in the Pecos River Basin and crosses beneath the hydrologic divide through the Permian bedded salts. The mixing of young (less than 100 years), local recharge from surface pathways at the Western Caprock Escarpment and much older (greater than 20,000 years) saltwater from the Permian bedded salts and Dockum Group is spatially variable and dependent on available flowpaths created by fracturing of the Dockum Group shales from Permian bedded-salt subsidence. Groundwater flow in local systems of the Southern High Plains aquifer along the Western Caprock Escarpment mixes with regional flow systems of larger saturated thickness where the geochemical signal of the halite-influenced saltwater is substantially reduced but visible in a thin mixing zone. Alteration of geochemical signals from groundwater flow through Dockum Group shales and the effect of agricultural recharge limited the effectiveness of certain tracers for identifying source waters, mixing patterns, and water-rock interactions