2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 7
Presentation Time: 9:35 AM

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

BĂ–HLKE, J.K.1, HATZINGER, Paul B.2, STURCHIO, Neil C.3, GU, Baohua4, ABBENE, Irene5 and MROCZKOWSKI, Stanley J.1, (1)U.S. Geological Survey, 431 National Center, 12201 Sunrise Valley Dr, Reston, VA 20192, (2)Shaw Environmental, Lawrenceville, NJ 08648, (3)Earth & Environmental Sciences, University of Illinois at Chicago, 845 West Taylor Street, MC-186, Chicago, IL 60607, (4)Environmental Sciences Division, Oak Ridge National Laboratory, P.O.Box 2008, MS 6036, Oak Ridge, TN 37831-6036, (5)U.S. Geological Survey, 2045 Route 112, Bldg 4, Coram, NY 11727, jkbohlke@usgs.gov

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.