Paper No. 7
Presentation Time: 9:35 AM
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.
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