Nitrogen loading of shallow groundwater aquifers in varying soil and topographic settings of southwestern Indiana

Abstract

Numerous sources of nitrogen capable of impacting groundwater exist in rural areas of the midwestern United States. These sources include commercial and non-commercial fertilizers as well as on-site septic distribution systems. Over the past three years, we have undertaken detailed monitoring studies aimed at quantifying nitrate loading of shallow groundwater aquifers resulting from natural recharge at seven sites in southwestern Indiana. The sites occur in a variety of topographic settings and are associated with both well drained and poorly drained soils. Measured changes in soil-moisture profiles were used along with continuous measurements of precipitation and potential evapotranspiration to calculate the storage and movement of groundwater in the unsaturated zone. Nitrate loading of the shallow aquifers was then calculated by combining the flow rate with analytical data on solute chemistry from multiple depths within the unsaturated zone. The results of these calculations show that the highest loading rates occur at the study sites adjacent to agricultural fields treated with commercial and non-commercial (manure) fertilizers. The calculated nitrogen loading at these three sites ranged from 21 to as high as 136 kg of N per hectare (the highest loading rate occurred at the site where the manure was applied). In contrast, much lower loading rates were calculated using data collected from four sites associated with residential on-site septic distribution systems. In these cases, the calculated nitrogen loading values were an order of magnitude lower and ranged from 1.3 to 7.4 kg of N per hectare. These findings have implications for land-use management and have been used to guide the compilation of GIS-based maps that identify high- and low-risk areas throughout Indiana. This was accomplished by evaluating areas on the basis of soil characteristics and unsaturated zone thicknesses.