Evaluation of intrinsic vulnerability to nitrate contamination of groundwater: appropriate fertilizer application management

In agricultural areas, fertilizer application is the main source of nitrate contamination of groundwater. To develop fertilizer management strategies to combat this problem, arable land in Hokkaido, Japan was evaluated using geographic information system techniques for intrinsic groundwater vulnerability to nitrate contamination. The DRASTIC method was modified to adapt it to the Hokkaido environment and used for the evaluation. Of the seven original DRASTIC factors, the depth to water (D), net recharge (R), soil media (S), topography (T), and impact of vadose zone media (I) were selected and used to explain the vertical movement of contaminants to the aquifer. The rating for the net recharge factor was also modified to a dilution factor for contaminants, rather than as a transporter. The frequency of wells with nitrate concentrations exceeding the Japanese environmental standard (10 mg/L) was reasonably explained by vulnerability evaluation results (GLM: logit-link, quasi-binomial distribution, Y = [1 + exp(6.873765 − 0.045988 × X)]⁻¹, p < 0.001). However, in the paddy fields and pastures, vulnerability did not exhibit a clear relationship with the frequency of wells exceeding the standard. This suggests that the modified DRASTIC method is applicable for fertilizer application management in upland fields. In addition, under the ongoing policy for acreage allotment for rice production, this method will be useful for deciding the arrangement of arable land and crop rotation taking into consideration the potential risk of fertilizer-induced nitrate contamination of groundwater.