SPATI AL ANALYSIS OF NO3 –N CONCENTRATION IN GLACIAL TILL

Abstract

Analysis of the spatial variability of groundwater NO3 –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 NO3 –N concentration in shallow ( 6.0 m) groundwater in a 12–ha. glacial till aquitard and to estimate NO3 –N concentration in unsampled locations. Omnidirectional and directional semivariogram analysis, statistical anisotropy analysis, and model fitting were performed for average and extreme monthly groundwater NO3 –N data. Results indicated a weak spatial structure of NO3 –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 NO3 –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 NO3 –N concentrations in shallow wells. Geostatistical estimation using ordinary kriging indicated relatively higher NO3 –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 NO3 –N loading rates in the glacial till aquitard.