MNA of Groundwater Nitrate, Presented by Brad Esser, PhD at AFCEE Conf. San Antonio, TX April 2010

1. MONITORED NATURAL ATTENUATION OF GROUNDWATER NITRATE Characterization using nitrate isotopic composition and excess nitrogen Dr. Bradley K. Esser Lawrence Livermore National Laboratory Robert A. Ferry Brown and Caldwell Victor Madrid Lawrence Livermore National Laboratory Mike Singleton Lawrence Livermore National Laboratory 08 April 2010 San Antonio, TX

8. Nitrate isotopic composition Source attribution and process identification The “ dual isotope ” approach refers to the use of both nitrogen (nitrate-  15 N) and oxygen (nitrate-  18 O) isotopic composition to attribute nitrate source and to constrain nitrate cycling Ranges based on data compiled in Kendall (1998).

9. Identifying denitrification isotopically Nitrate isotopic composition dual isotope plot Denitrification enriches both 15 N and 18 O (the “heavy” isotopes) in residual nitrate. In most natural terrestrial systems, relative  18 O/  15 N enrichment is ~1/2, and distinguishes denitrification from ammonia volatilization+nitrification (which only enrich 15 N). Characteristic slope on dual isotope plot California dairies (Singleton, 2007, EST) Note : While excess nitrogen is only useful in the saturated zone, the dual isotope method can be used for both groundwater and sediment samples. California dairy sediment samples

10. Identifying denitrification isotopically Correlation between isotopics and concentration Inverse correlation between 15 N enrichment and residual nitrate concentration A strong case can be made for denitrification if changes in nitrate isotopic composition correlate to changes in nitrate concentration along a groundwater flow path; i.e., downgradient waters are low in nitrate and enriched in  15 N. California dairy sediment samples Note : The largest isotopic effects are often observed in samples with low concentrations of residual nitrate.

15. Extent of denitrification Excess nitrogen allows estimation of initial nitrate Excess N 2 allows determination of the amount and extent of denitrification, and can be used with groundwater age or velocity to constrain rate. Extent of denitrification = 1 – f, Where f = fraction initial nitrate remaining Stratified aquifer underlying a California dairy operation Caveat: With extensive denitrification or methanogenesis, nitrogen gas can be lost through gas ebullition.

18. Excess nitrogen indicates that denitrification is occurring in the lower anoxic aquifer Excess nitrogen and denitrification Case study at a California dairy operation

19. Bacterial population profiles show that denitrification occurs at the oxic-anoxic interface PCR and denitrification Case study at a California dairy operation

20. Groundwater age and denitrification Case study at a California dairy operation Tritium-helium groundwater age dating provides constraints on the timing of nitrate contamination and the rate of denitrification

22. LLNL Site 300 A DOE HE testing facility in the California Coast Range

23. Site 300 case study Nitrate contamination threat to drinking water wells Off-site water supply wells

24. Site 300 hydrogeology Complex marine sedimentary sequence

26. Identification of denitrification Nitrate isotopic composition

27. Confirmation of denitrification Excess nitrogen and dissolved oxygen Dissolved N 2 detected Dissolved N 2 detected > 15 mg/L equivalent NO 3 Dissolved N 2 not detected

28. Confirmation of denitrification Excess nitrogen and nitrate-  15 N

31. Identification of electron donor Changes in sulfate isotopic composition 5.3 4.2 3.4 2.5 0.3 Observed changes in sulfate-S isotopic composition along flow path are consistent with autotrophic denitrification Oxidation of sulfide to sulfate favors the lighter isotope, and produces sulfate-S isotopically lighter than pyrite-S. The observed trend is consistent with addition of isotopically light sulfate through pyrite oxidation x.x  34 S data

32. Denitrification at Site 300 CERCLA Monitored Natural Attenuation remedy approved Consistent set of geochemical indicators demonstrating autotrophic denitrification driven by oxidation of naturally occurring pyrite