IDENTIFICATION OF GROUNDWATER SOLUTE SOURCES USING BORON ISOTOPIC COMPOSITION

In a long-term evaluation of the injection of treated wastewater into an alluvial aquifer near El Paso, TX, the boron (B) concentration and B stable isotopic values were investigated as potential intrinsic tracers of recharge-water source. The B stable isotopic values (delta(11)B) of background groundwater (14 parts per thousand) and water from anthropogenic sources such as treated municipal wastewater (6-10 parts per thousand) and irrigation-affected water (> 40 parts per thousand) were distinctly different. The B concentrations in combination with the B stable isotope values distinguished these water types and natural saline water as sources of recharge to groundwater. Boron is suited for use as a conservative tracer in groundwater because of its high solubility in aqueous solution (predominantly as boric acid), its natural presence in nearly all water, and the lack of effects by evaporation or volatilization, by oxidation-reduction reactions, or by mineral precipitation or dissolution in all but extremely saline waters. The variability of any observed isotopic ratio in groundwater can be from several processes, such as variability in the B source, mixing, and partitioning reactions, including sorption, which cause isotopic fractionation. For rocks and minerals, a range in delta(11)B values of more than 70 parts per thousand has been reported. Water sources sampled to date have a similarly broad range. In this injection test, B concentrations in all samples were below 1 mg/l, and differences between individual sources of salinity were indistinct based on analytical concentration alone. In contrast, a hypothetical, nonlinear mixture between solutions with different delta(11)B values and different B concentrations demonstrated a detectable isotopic shift for smalt volume percent contributions of the source water with the largest boron concentration. The Hueco Bolson aquifer at the injection site is geologically heterogeneous, and the cost to establish a sufficient density of monitoring wells to describe the flow of injected water in groundwater is prohibitive. For this and other sites, chemical and isotopic tracers such as B and delta(11)B may be used to identify the influence of various water sources with potentially adverse environmental consequences.