Multi-isotopic study (15N, 34S, 18O, 13C) to identify processes affecting nitrate and sulfate in response to local and regional groundwater mixing in a large-scale flow system

Abstract

The integrated use of hydrogeologic and multi-isotopic approaches (δ15N, δ18ONO3, δ34S, δ18OSO4 and δ13CHCO3) was applied in the Selva basin area (NE Spain) to characterize NO3- and SO42- sources and to evaluate which geochemical processes affect NO3- in groundwater. The studied basin is within a basin-and-range physiographic province where natural hydrodynamics have been modified and different scale flow systems converge as a consequence of recent groundwater development and exploitation rates. As a result, groundwaters related to the local recharge flow system (affected by anthropogenic activities) and to the generally deeper regional flow system (recharged from the surrounding ranges) undergo mixing processes. The δ15N, δ18ONO3 and δ34S indicated that the predominant sources of contamination in the basin are pig manure and synthetic fertilizers. Hydrochemical data along with δ15N, δ18ONO3, δ34S, δ18OSO4 and δ13CHCO3 of some wells confirmed mixing between regional and local flow systems. Apart from dilution processes that can contribute to the decrease of NO3- concentrations, the positive correlation between δ15N and δ18ONO3 agreed with the occurrence of denitrification processes. The δ34S and δ18OSO4 indicated that pyrite oxidation is not linked to denitrification, and δ13CHCO3 did not clearly point to a role of organic matter as an electron donor. Therefore, it is proposed that the mixing processes between deeper regional and local surface groundwater allow denitrification to occur due to the reducing conditions of the regional groundwater. Thus, isotopic data add useful complementary information to hydrochemical studies, especially in those areas where hydrochemical data is not conclusive.