Application of boron isotope ratios for identifying nitrate contamination sources in the groundwater of Avra Valley, Arizona.

Abstract

The stable isotopes of the conservative element boron, ¹¹B and ¹⁰B, have been employed as co-migrating isotopic tracers to determine the origin of nitrate observed in groundwater from a large capacity 0.167 m³/s (2500 gpm) irrigation well in the Avra Valley of southeastern Arizona. The isotopic ratios of the conservative element, boron, provided an identifying signature for various nitrate rich source waters. Additional chemical parameters were also examined to corroborate the isotopic indications. Findings of this investigation indicate that most of the nitrate observed in groundwater from well CMID 18 at the beginning of the 1993 irrigation season was due to municipal wastewater infiltration. As the irrigation season progressed, an increasing proportion of nitrate was contributed by irrigation return flow from neighboring agricultural fields.