Predicting groundwater nitrate concentrations in a region of mixed agricultural land use: a comparison of three approaches

We investigated whether nitrate-N (NO3--N) concentrations of shallow ground water ( < 30 m from the land surface) in a region of intensive agriculture could be predicted on the basis of land use information, topsoil properties that affect the ability of topsoil to generate nitrate at a site, or the ' leaching risk' at different sites. Groundwater NO3--N concentrations were c ollected biannually for 3 years at 88 sites within the Waikato Region of Ne w Zealand. The land use was classed as either the predominant land use of t he farm where the well or bore was located, or the dominant land use within a 500 m radius of the well or bore. Topsoil properties that affect the abi lity of soil to generate nitrate were also measured at all the sites, and a leaching risk assessment model 'DRASTIC' was used to assess the risk of NO 3--N leaching to groundwater at each site. The concentration of NO3--N in s hallow groundwater in the Waikato Region varied considerably, both temporal ly and spatially. Nine percent of sites surveyed had groundwater NO3--N con centrations exceeding maximum allowable concentrations of 11.3 ppm recommen ded by the World Health Organisation for potable drinking water which is ac cepted as a public health standard in New Zealand. Over half (56%) of the s ites had concentrations that exceeded 3 ppm, indicating effects of human ac tivities (commonly referred to as a human activity value). Very few trends in NO3--N concentration that could be attributed to land use were identifie d, although market garden sites had higher concentrations of NO3--N in unde rlying groundwater than drystock/sheep sites when the land use within 500 m radius of a sampling site was used to define the land use. There was also some evidence that within a district, NO3--N concentrations in groundwater increased as the proportion of area used for dairy farming increased. Compa red to pastoral land, market gardens had lower total C and N, potentially m ineralisable N and denitrifying enzyme assay. However, none of these soil p roperties were directly related to groundwater NO3--N concentrations. Inste ad, the DRASTIC index (which ranks sites according to their risk of solute leaching) gave the best correlation with groundwater NO3--N concentrations. The permeability of the vadose zone was the most important parameter. The three approaches used were all considered unsuitable for assessing nitrate concentrations of groundwater, although a best-fit combination of parameter s measured was able to account for nearly half the variance in groundwater NO3--N concentrations. We suggest that non-point source ground-water NO3--N contamination in the region reflects the intensive agricultural practices, and that localised, site-specific, factors may affect NO3--N concentration s in shallow groundwaters as much as the general land use in the surroundin g area. (C) 2001 Elsevier Science Ltd. All rights reserved.