Spatial analysis of NO3-N concentration in glacial till

Analysis of the spatial variability of groundwater NO(3)-N concentration is a logical step for a meaningful groundwater quality assessment, for mapping out areas of environmental concern, and for developing appropriate management schemes in a glacial till aquitard. This study was conducted to characterize the spatial variability of NO(3)-N concentration in shallow (<6.0 m) and deep (>6.0 m) groundwater in a 12-ha. glacial till aquitard and to estimate NO(3)-N concentration in unsampled locations. Omnidirectional and directional semivariogram analysis, statistical anisotropy analysis, and model fitting were performed for average and extreme monthly groundwater NO(3)-N data. Results indicated a weak spatial structure of NO(3)-N concentration for both shallow and deep well data. However, the best-fitted variogram models generally performed satisfactorily during cross validation, yielding a mean reduced error of -0.01 to -0.074 and reduced variance of 0.6 to 2.18. Untransformed shallow-well NO(3)-N exhibited a lower range of correlation than deep-well data. Statistical anisotropy was found to coincide with the general groundwater flow directions for the average and maximum observed NO(3)-N concentrations in shallow wells. Geostatistical estimation using ordinary kriging indicated relatively higher NO(3)-N concentrations at the down-gradient areas for shallow wells and at regions close to nitrogen fertilizer application sites for the deep wells. With satisfactory cross-validation performance of the variogram models, the geostatistical results of this study may be used as basis for estimating spatially variable NO(3)-N loading rates in the glacial till aquitard.