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

Abstract

We investigated whether nitrate-N (NO 3 −-N) concentrations of shallow groundwater (<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 NO 3 −-N concentrations were collected biannually for 3 years at 88 sites within the Waikato Region of New Zealand. The land use was classed as either the predominant land use of the 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 ability 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 NO 3 −-N in shallow groundwater in the Waikato Region varied considerably, both temporally and spatially. Nine percent of sites surveyed had groundwater NO 3 −-N concentrations exceeding maximum allowable concentrations of 11.3 ppm recommended by the World Health Organisation for potable drinking water which is accepted as a public health standard in New Zealand. Over half (56%) of the sites had concentrations that exceeded 3 ppm, indicating effects of human activities (commonly referred to as a human activity value). Very few trends in NO 3 −-N concentration that could be attributed to land use were identified, although market garden sites had higher concentrations of NO 3 −-N in underlying 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, NO 3 −-N concentrations in groundwater increased as the proportion of area used for dairy farming increased. Compared to pastoral land, market gardens had lower total C and N, potentially mineralisable N and denitrifying enzyme assay. However, none of these soil properties were directly related to groundwater NO 3 −-N concentrations. Instead, the DRASTIC index (which ranks sites according to their risk of solute leaching) gave the best correlation with groundwater NO 3 −-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 parameters measured was able to account for nearly half the variance in groundwater NO 3 −-N concentrations. We suggest that non-point source groundwater NO 3 −-N contamination in the region reflects the intensive agricultural practices, and that localised, site-specific, factors may affect NO 3 −-N concentrations in shallow groundwaters as much as the general land use in the surrounding area.