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 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 NO3(-)-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 NO3(-)-N leaching to groundwater at each site. The concentration of NO3(-)-N in shallow groundwater in the Waikato Region varied considerably, both temporally and spatially. Nine percent of sites surveyed had groundwater NO3(-)-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 NO3(-)-N concentration that could be attributed to land use were identified, although market garden sites had higher concentrations of NO3(-)-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, NO3(-)-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 NO3(-)-N concentrations. Instead, 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 parameters measured was able to account for nearly half the variance in groundwater NO3(-)-N concentrations. We suggest that non-point source groundwater NO3(-)-N contamination in the region reflects the intensive agricultural practices, and that localised, site-specific, factors may affect NO3(-)-N concentrations in shallow groundwaters as much as the general land use in the surrounding area.

[1]  D. Kolpin.  Agricultural Chemicals in Groundwater of the Midwestern United States: Relations to Land Use , 1997 .

[2]  D. Kolpin,et al.  Hydrologic and land-use factors associated with herbicides and nitrate in near-surface aquifers , 1993 .

[3]  N. Selvarajah,et al.  DETERMINATION OF SUSTAINABLE NITROGEN LOADING RATES FOR LAND TREATMENT SYSTEMS WITHOUT ADEQUATE SOIL AND GROUND WATER INFORMATION: DAIRY FARM EFFLUENT APPLICATION ONTO GRAZED PASTURE IN THE WAIKATO REGION , 1996 .

[4]  R. Shedlock,et al.  Pesticides in Shallow Groundwater in the Delmarva Peninsula , 1993 .

[5]  J. Tiedje,et al.  Phases of denitrification following oxygen depletion in soil , 1979 .

[6]  James M. Tiedje,et al.  Denitrification in north temperate forest soils: Relationships between denitrification and environmental factors at the landscape scale , 1989 .

[7]  Roy F. Spalding,et al.  Occurrence of nitrate in groundwater-a review , 1993 .

[8]  K. Senger,et al.  Fatal outcome of methemoglobinemia in an infant. , 1987, JAMA.

[9]  Robert K. Hubbard,et al.  Nutrient concentrations in the soil solution and shallow groundwater of a liquid dairy manure land application site , 1996 .

[10]  A. S. Dylla,et al.  Nitrogen Leaching as Influenced by Nitrogen Management and Supplemental Irrigation Level 1 , 1981 .

[11]  Erik Lichtenberg,et al.  Agriculture and Nitrate Concentrations in Maryland Community Water System Wells , 1997 .

[12]  D. Baker,et al.  Well water quality, well vulnerability, and agricultural contamination in the midwestern United States , 1996 .

[13]  G. Sparling,et al.  Relationship between mineralisable nitrogen and microbial biomass in a range of plant litters, peats, and soils of moderate to low pH , 1986 .

[14]  C. Helling,et al.  Effect of conventional vs. No-tillage on pesticide leaching to shallow groundwater , 1990 .

[15]  J. M. Bremner,et al.  Comparison and Evaluation of Laboratory Methods of Obtaining an Index of Soil Nitrogen Availability1 , 1966 .

[16]  F. Rich,et al.  Survey of Nitrate Contamination in Shallow Domestic Drinking Water Wells of the Inner Coastal Plain of Georgia , 1995 .

[17]  E. Hansen,et al.  Nitrate leaching as affected by long-term N fertilization on a coarse sand , 1996 .

[18]  J. M. Bremner,et al.  A simple chemical method of assessing potentially available organic nitrogen in soil , 1986 .

[19]  D. Arp,et al.  Quantification and Removal of Some Contaminating Gases from Acetylene Used to Study Gas-Utilizing Enzymes and Microorganisms , 1987, Applied and environmental microbiology.

[20]  Pixie A. Hamilton,et al.  Effects of Agriculture on Ground‐Water Quality in Five Regions of the United States , 1995 .

[21]  Tom Addiscott,et al.  Concepts of solute leaching in soils: a review of modelling approaches , 1985 .

[22]  D. Glotfelty,et al.  Pesticide Contamination of Groundwaters in the Mahantango Creek Watershed , 1988 .

[23]  P. Stackelberg,et al.  Relation of Ground-Water Quality to Land Use on Long Island, New York , 1995 .

[24]  P. Brookes,et al.  Seasonal changes of soil microbial biomass in an arable and a grassland soil which have been under uniform management for many years , 1990 .