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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.

Assessing aquifer vulnerability is crucial for preventing groundwater pollution. In this study, aquifer vulnerability to fecal coliform (FC) pollution was assessed using auxiliary environmental data in the Pingtung Plain, Taiwan. Moreover, key environmental factors inducing different fecal pollution levels were determined. First, 23 explanatory variables on land uses, population density, livestock and poultry densities, sanitary condition, antecedent precipitation, groundwater quality, aquifer characteristics, and subsurface hydrology were obtained using geographic information systems in 2014. As dependent variables, groundwater FCs were also simultaneously obtained. Then, multi-threshold logistic regression (LR) was adopted to model aquifer vulnerability assessment after cross validation. The thresholds of aquifer vulnerability causing risks of incidental ingestion were analyzed by risk assessment. Risks to human health were acceptable for a low-level threshold and exceeded the acceptable level for medium- and high-level thresholds when residents incidentally ingested FC-polluted groundwater. Finally, key environmental factors inducing low, medium, and high levels of groundwater FC pollution were characterized. The key environmental factors for the LR with low- and medium-level thresholds were sand and gravel soil textures of unsaturated aquifers and antecedent 3-day cumulative precipitation, and those for the LR with high-level thresholds were chicken farming, urban land use, and ratio of tap water use. Thus, the multi-threshold LR indicated that environmental factors must be ranked for assessing aquifer vulnerability.