Characterization of groundwater dynamics and contamination in an unconfined aquifer using isotope techniques to evaluate domestic supply in an urban area

In the urban area of Rio Cuarto groundwater is used for domestic supply. The objective of the current research was to investigate the water dynamics and nitrate contamination in an unconfined aquifer system of Rio Cuarto River basin. Stable isotopes of water (delta O-18 and delta H-2) and nitrate (delta N-15-NO3 and delta O-18-NO3) were used and combined with conventional chemical techniques and mixing modelling approaches to determine the recharge areas and identify the main origin of nitrate pollution. More enriched water isotope values were recorded in Rio Cuarto city than in the piedmont and mountains indicating local recharge for well batteries 2 and 3. The well battery 1 and the infiltration gallery showed more negative isotopic values, demonstrating a strong influence by the recharge from the piedmont sector (impoverished groundwater). The delta N-15-NO3 values and the Bayesian modeling showed that the dominant nitrate contamination source in the urban area is the on-site sanitation systems whereas in the peri-urban area nitrate contamination originates mostly from animal wastes. Both sources supply the aquifer with anthropogenic organic matter (>50%). The highest delta N-15-NO3 values were correlated with low dissolved oxygen values, indicating the occurrence of denitrification processes in some places. High NO3 ions in the rural sector were attributed to the application of fertilizers and consequent nitrification processes. The samples from Rio Cuarto river located upstream of the Rio Cuarto City and close to the infiltration gallery showed the highest input from fertilizers (similar to 40%), due to surrounding agricultural fields. The municipal well batteries showed good water quality (freshwater of low to extremely low nitrate concentration) which is linked to the high hydraulic conductivity of the aquifer that increases its capacity to dilute the contaminants. These results will be useful for local water administrators to improve water management.