Perchlorate as An Agricultural Contaminant in Groundwater- Isotopic Signatures of Sources and Biodegradation

Perchlorate (ClO₄^-) is a common groundwater constituent with the potential to affect human thyroid function. Synthetic ClO₄^- is enriched locally in areas affected by military and industrial activities, whereas natural ClO₄^- is widely distributed in arid environments, possibly as a result of atmospheric deposition and evapotranspirative concentration. In addition, ClO₄^- appears to be present in some areas as a result of agricultural application of natural nitate (NO₃^-) fertilizer containing ClO₄^- from the Atacama Desert in Chile. Because the ClO₄^-/NO₃^- ratio of Atacama NO₃^- deposits and historical fertilizer products is higher than the ClO₄^-/NO₃^- ratio of recommended drinking-water limits, ClO₄^- can exceed drinking-water limits even where Atacama NO₃^- was only a small fraction of the total fertilizer N applied. Atacama ClO₄^- is presumed to have accumulated with NO₃^- largely from atmospheric deposition and is recognized isotopically by low δ³⁷Cl and high Δ¹⁷O values. ClO₄^- with these isotopic characteristics is a sensitive indicator of Atacama NO₃^- fertilizer use and has been identified in groundwater from several areas of the USA. At one such site in New York beneath agricultural land, concentrations of ClO₄^- are relatively low near the water table and higher in deeper older groundwaters (20-30 yrs), possibly because application rates of Atacama NO₃^- fertilizer, and(or) ClO₄^- concentrations in the imported fertilizer products, decreased in recent years. Similarly, groundwater ClO₄^- in some former agricultural sites in southern California is plausibly related to past Atacama NO₃^- fertilizer use. After entering groundwater, ClO₄^- and NO₃^- both are persistent in oxic conditions and are subject to microbial reduction under suboxic conditions, although ClO₄^- degradation commonly is inhibited in the presence of NO₃^-. During biodegradation, ClO₄^- and NO₃^- exhibit coupled isotopic fractionation with relatively constant ratios of εO/εCl (≈2.5 ± 0.2) and εO/εN (≈0.8 ± 0.2), respectively.