Use of environmental isotopes to assess groundwater pollution caused by agricultural activities

ABSTRACT

This work applied environmental isotope techniques to validate the results of previous studies on recharge sources in a rural area in central Chile (34.3° S and 71.3° W) and discern the origin of nitrate contamination in wells. Stream water and groundwater samples were taken during three surveys, two during spring snowmelt and one in low-water conditions. Chemical analyses included major cations and anions, isotope analyses included ¹⁸O-H₂O; ²H-H₂0; ³H-H₂0; ¹⁸O-NO₃⁻ and ¹⁵N-NO₃⁻. The stable isotope data show that surface water and deep groundwater present depleted isotope values associated with recharge from the Andes Mountains and that shallow groundwater has more enriched isotope values that reflect the contribution of local recharge from rainwater infiltration. Depleted isotope values observed in shallow groundwater show the effect of recirculated river water used for irrigation. These results are consistent with the conceptual groundwater model developed in previous studies. Some wells have nitrate concentrations above the allowable limit for drinking water. The stable nitrogen isotopes indicate that nitrate is associated mainly with urea and ammoniacal fertilizers, and nitrate is attenuated by denitrification. The results of this study are relevant to improving management of groundwater resources used for drinking water.

KEYWORDS:

• Agriculture
• Chile
• groundwater
• hydrogen-2
• hydrogen-3
• irrigation
• isotope hydrology
• nitrates
• nitrogen-15
• oxygen-18
• pollution

Acknowledgements

The authors wish to express their gratitude to Proyecto RLA/7/021 ARCAL CXLIX. They would also like to express their gratitude to the Comisión Chilena de Energía Nuclear (CCHEN), la Dirección General de Aguas (DGA), the Water Center (CHRIAM ANID/Fondap/15130015) and the University of Waterloo for the support given to some of the previously described works. Finally, the authors wish to thank the editor of this journal and the anonymous reviewers, who helped them improve this article.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by International Atomic Energy Agency: [Grant Number Proyecto RLA/7/021 ARCAL CXLIX]; Agencia Nacional de Investigación y Desarrollo: [Grant Number ANID/Fondap/15130015].