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

Abstract

Over the past decades, fractured and karst groundwater systems have been studied intensively due to their high vulnerability to nitrate (NO 3 − ) 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 NO 3 − transfer and turnover by combining isotopic analysis of NO 3 − and nitrite (NO 2 − ) 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 NO 3 − 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 NO 3 − (δ 15 N and δ 18 O) confirm that NO 3 − derives mainly from manure or fertilizer applications. Fractional N elimination by denitrification is indicated by relatively high δ 15 N- and δ 18 O-NO 3 − values, elevated NO 2 − concentrations (0.05–0.14 mM), and δ 15 N-NO 2 − values that were systematically lower than the corresponding values of δ 15 N-NO 3 − . 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 O 2 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.

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