Fate of nitrate during groundwater recharge in a fractured karst aquifer in Southwest Germany

Over the past decades, fractured and karst groundwater systems have been studied intensively due to their high vulnerability to nitrate (NO₃⁻) contamination, yet nitrogen (N) turnover processes within the recharge area are still poorly understood. This study investigated the role of the karstified recharge area in NO₃⁻ transfer and turnover by combining isotopic analysis of NO₃⁻ and nitrite (NO₂⁻) with time series data of hydraulic heads and specific electrical conductivity from groundwater monitoring wells and a karstic spring in Germany. A large spatial variability of groundwater NO₃⁻ concentrations (0.1–0.8 mM) was observed, which cannot be explained solely by agricultural land use. Natural-abundance N and O isotope measurements of NO₃⁻ (δ¹⁵N and δ¹⁸O) confirm that NO₃⁻ derives mainly from manure or fertilizer applications. Fractional N elimination by denitrification is indicated by relatively high δ¹⁵N- and δ¹⁸O-NO₃⁻ values, elevated NO₂⁻ concentrations (0.05–0.14 mM), and δ¹⁵N-NO₂⁻ values that were systematically lower than the corresponding values of δ¹⁵N-NO₃⁻. Hydraulic and chemical response patterns of groundwater wells suggest that rain events result in the displacement of water from transient storage compartments such as the epikarst or the fissure network of the phreatic zone. Although O₂ levels of the investigated groundwaters were close to saturation, local denitrification might be promoted in microoxic or anoxic niches formed in the ferrous iron-bearing carbonate rock formations. The results revealed that (temporarily) saturated fissure networks in the phreatic zone and the epikarst may play an important role in N turnover during the recharge of fractured aquifers.