Factors affecting nitrate distribution in shallow groundwater under a beef farm in South Eastern Ireland

Groundwater contamination was characterised using a methodology which combines shallow groundwater geochemistry data from 17 piezometers over a 2 yr period in a statistical framework and hydro-geological techniques. Nitrate-N (NO(3)-N) contaminant mass flux was calculated across three control planes (rows of piezometers) in six isolated plots. Results showed natural attenuation occurs on site although the method does not directly differentiate between dilution and denitrification. It was further investigated whether NO(3)-N concentration in shallow groundwater (<5 m below ground level) generated from an agricultural point source on a 4.2 ha site on a beef farm in SE Ireland could be predicted from saturated hydraulic conductivity (K(sat)) measurements, ground elevation (m Above Ordnance Datum), elevation of groundwater sampling (screen opening interval) (m AOD) and distance from a dirty water point pollution source. Tobit regression, using a background concentration threshold of 2.6 mg NO(3)-N L(-1) showed, when assessed individually in a step wise procedure, K(sat) was significantly related to groundwater NO(3)-N concentration. Distance of the point dirty water pollution source becomes significant when included with K(sat) in the model. The model relationships show areas with higher K(sat) values have less time for denitrification to occur, whereas lower K(sat) values allow denitrification to occur. Areas with higher permeability transport greater NO(3)-N fluxes to ground and surface waters. When the distribution of Cl(-) was examined by the model, K(sat) and ground elevation had the most explanatory power but K(sat) was not significant pointing to dilution having an effect. Areas with low NO(3) concentration and unaffected Cl- concentration points to denitrification, low NO(3) concentration and low Cl(-) chloride concentration points to dilution and combining these findings allows areas of denitrification and dilution to be inferred. The effect of denitrification is further supported as mean groundwater NO(3)-N was significantly (P < 0.05) related to groundwater N(2)/Ar ratio, redox potential (Eh), dissolved 02 and N(2) and was close to being significant with N(2)O (P = 0.08). Calculating contaminant mass flux across more than one control plane is a useful tool to monitor natural attenuation. This tool allows the identification of hot spot areas where intervention other than natural attenuation may be needed to protect receptors. (C) 2009 Elsevier Ltd. All rights reserved