Understanding Sources and Fate of Nitrate in a Highly Developed Aquifer System
Abstract
Understanding the processes affecting the transport and fate of nitrate in groundwater has become of great interest in recent years due to increased concern of wide spread nitrate contaminations. Nutrient loading through submarine groundwater discharge (SGD) can contribute to eutrophication and hypoxia ("dead zones") in coastal surface waters (e.g., Gulf of Mexico). Novel dual isotopic methods have shown promise for identifying sources and fate of nitrate in shallow groundwater. However, in relatively deep dynamic aquifer systems, the isotopic signatures may be overprinted by mixing of different end members and biogeochemical processes. In this study, δ15N and δ18O of groundwater nitrate (δ15NNO3- and δ18ONO3-) are coupled with other forensic geochemistry methods such as Cl-/ Br- , SO42-/Cl-, and NO3-/Cl- mass ratios and land use analysis in order to constrain the isotope correlations and better understand contaminant sources and biogeochemical processes. Most δ15NNO3- values were not higher than expected for nitrate formed by ammonia nitrification in soil. Furthermore, the persistent presence of nitrate in concentrations >0.19 mg/L (average: 4.2 mg/L (n=34); maximum: 18.4 mg/L) and the relatively low δ15NNO3- and δ18ONO3- (average: 4.41±0.2‰AIR and 5.12±0.5‰VSMOW, n=34) indicate no direct evidence of denitrification. However, denitrification was inferred for a few samples whereby elevated δ15NNO3- and δ18ONO3- values coupled with increase in SO42- /Cl- and decrease in NO3- /Cl- ratios were observed. Finally, mixing trends were identified for a few of the samples as indicated by δ15NNO3- and δ18ONO3- mass ratios and consistent with the study area's land use/land cover distribution. The combination of methods utilized in this study revealed that in some cases mass ratios were better diagnostics in elucidating the impact of denitrification, mixing processes, and source identification within dynamics aquifer systems than the dual isotope technique.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.H34B..08M
- Keywords:
-
- 1041 GEOCHEMISTRY / Stable isotope geochemistry;
- 1090 GEOCHEMISTRY / Field relationships;
- 1829 HYDROLOGY / Groundwater hydrology;
- 1834 HYDROLOGY / Human impacts