Understanding the sources and fate of nitrate in a highly developed aquifer system

• The isotopic signatures in deeper and dynamic aquifers are more complex.
• Denitrification and groundwater end-member mixing alter the isotopic signature.
• End-member mixing limits the use of δ15N and δ18O isotopes in dynamic aquifers.
• Denitrification is not an important process in the investigated system.

Understanding the processes affecting the transport and fate of nitrate in coastal aquifers has become of great interest in recent years due to concerns of nutrient loading to coastal waters. 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-member waters and biogeochemical processes. In this study, δ15N and δ18O of groundwater nitrate are coupled with other forensic geochemistry methods such as Cl/Br, SO4/Cl, and Cl/NO3 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 within ranges expected for nitrate formed by ammonia nitrification in soil. Furthermore, the persistent presence of nitrate in concentrations above background levels (median 2.3 mg/L) and the relatively low δ15NNO3 and δ18ONO3 (median: 4.5 ± 0.2‰ AIR and 5.2 ± 0.5‰ VSMOW, respectively) indicate no direct evidence of denitrification. However, denitrification was inferred for a few samples whereby more enriched δ15NNO3 and δ18ONO3 values coupled with an increase in SO4/Cl and Cl/NO3 ratios were observed. Finally, mixing trends were identified for a few of the samples as indicated by δ15NO3 and δ18ONO3 mixing ratios and were 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 dynamic aquifer systems than the dual-isotope technique.