Analysis of the spatial variability of groundwater NO{sub}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{sub}3-N concentration in shallow (6.0 m) groundwater in a 12-ha. glacial till aquitard and to estimate NO{sub}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{sub}3-N data. Results indicated a weak spatial structure of NO{sub}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{sub}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{sub}3-N concentrations in shallow wells. Geostatistical estimation using ordinary kriging indicated relatively higher N03-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 N03-N loading rates in the glacial till aquitard.
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