Geostatistical estimates of groundwater nitrate-N with spatial auxiliary information on DRASTIC-LU-based aquifer vulnerability

Groundwater nitrate-N contamination typically involves several natural and anthropogenic factors, such as hydrology, hydrogeology, topography, and land uses. DRASTIC-LU-based aquifer vulnerability can be adopted to explore pollution potentials of groundwater nitrate-N. Furthermore, groundwater nitrate-N pollution frequently has high levels of spatial variability because of various pollution sources and hydrogeological and hydrochemical conditions. Estimates are typically uncertain owing to limited in-situ data. Geostatistics is a commonly used technique of spatial estimates with limited data. To reduce the underestimation and overestimation of ordinary kriging (OK), this study employed regression kriging (RK) with environmental auxiliary information on DRASTIC-LU-based aquifer vulnerability to characterize groundwater nitrate-N pollution in the Pingtung Plain, Taiwan. First, the relationship between groundwater nitrate-N pollution and aquifer vulnerability assessment was determined using stepwise multivariate linear regression (MLR). Then, simple kriging was adopted to estimate residuals acquired from gaps between nitrate-N observations and MLR predictions. The sum of the estimated residuals and MLR predictions was the RK estimates for groundwater nitrate-N. Finally, groundwater nitrate-N distributions were spatially analyzed using RK, OK, and MLR. To reduce groundwater nitrate-N pollution, feasible environmental management strategies were discussed according to the study results. The study results revealed that the orchard land-use types and medium- and coarse-sand fractions of vadose zone were related to groundwater nitrate-N. Moreover, the fertilizer application in orchards was the major source of groundwater nitrate-N pollution. The RK estimates could characterize the characteristics of the pollution source of the orchard land uses, and exhibited higher spatial variability and accuracy via the correction of the residuals than MLR predictions and OK estimates. In addition, feasible management strategies of orchards at the eastern and western regions with high areal ratios of orchard land uses should be implemented to reduce nitrate-N leaching, such as organic fertilizer uses, ground covers, and irrigation with low intensity and high frequency.