Spatial and temporal variations in the geochemistry of shallow groundwater contaminated with nitrate at a residential site

Abstract

The concentrations of nitrate (NO₃⁻), major ions, and dissolved inorganic carbon (DIC) and the stable carbon isotopes of DIC (δ¹³C_DIC) in shallow groundwater below a 45 × 60 m residential property was investigated over a period of 38 months. Our aim was to identify the processes which control the spatial and temporal distribution of NO₃⁻ in the shallow groundwater and assess water-rock interactions linked to denitrification. Groundwater sampled quarterly from eight locations showed an average NO₃⁻ concentration of 36.8 mg/L and a range between 0.1 and 214.9 mg/L compared to the US EPA maximum contaminant level of 10 mg/L. Heterogeneity in nitrate distribution was from residential application of N-based fertilizers offsite and from onsite application on flower beds and for lawn care. The temporal behavior of nitrate at all eight groundwater locations was markedly different and independent of seasonal hydrologic variations. Nitrate attenuation was spatially controlled by heterogeneous denitrification and rain dilution near roof drains. Groundwater locations with active denitrification were characterized by higher DIC concentrations and lower δ¹³C_DIC from organic carbon mineralization and by higher ionic concentrations from weathering of aquifer minerals. The variation in the relative standard deviations (RSD) of the measured parameters over space (RSD-s) and time (RSD-t) was highest for NO₃⁻ associated with variable spatiotemporal input and lowest for pH, pCO₂, and δ¹³C_DIC indirectly controlled by denitrification. Denitrification induced mineral weathering products such as DIC, Ca²⁺, Mg²⁺, and HCO₃⁻ showed medium to high RSD-s and RSD-t. The RSD-s and RSD-t were positively correlated (R² = 0.85) with the RSD-s showing approximately twofold higher magnitude than RSD-t due to greater variability between monitoring wells locations than variability at each groundwater location over time. Nitrate contamination and denitrification represent important long-term driver of aquifer weathering and changes in groundwater geochemistry below residential communities.