Specific vulnerability assessment of nitrate in shallow groundwater with an improved DRSTIC-LE model

Datong basin is one of the most important agriculture zone of Shanxi Province, China, where intensive fertilizer and pesticide applications cause the groundwater contamination. Hence, it is necessary to carry out groundwater specific vulnerability assessment of nitrate. The prediction accuracy of conventional DRASTIC model for groundwater vulnerability assessment is severely limited by the inherent subjectivity in determining main parameters, rating scales and weighting coefficients. This paper attempts to overcome these problems by changing the evaluation parameters, parameters rating and weight calculation method. Based on the hydrogeological conditions and nitrate pollution characteristics of Datong Basin, the traditional groundwater vulnerability model DRASTIC was improved, called DRSTIC-LE model to assess the specific vulnerability of nitrate, which involves Depth of water table (D), Net recharge (R), Soil media (S), Topography (T), Impact of the vadose zone (I), hydraulic Conductivity (C), land use type (L), and groundwater exploitation (E) as evaluation parameters. And the theoretical weight of each parameter were determined with the aid of the improved weights determination method by the effective combination of the entropy weight method and analytic hierarchy process. Moreover, single-parameter sensitivity analysis was performed to evaluate the effect of each parameter on the groundwater specific vulnerability. According to the results of groundwater specific vulnerability map, very high and high vulnerability mainly situated in central, northern and northeastern portion of study area, with 6.25%, 17.93% of total area, respectively. Also, single-parameter sensitivity analysis represented that I and D are the main parameter which impacts groundwater to pollution while C contributes least to pollution in the Datong basin. The model is validated with the measured nitrate concentration and results have shown better correlation between SVI and nitrate concentration. Additionally, by comparing the correlation between the effective weights and the theoretical weights calculated by the three methods (EW, AHP and EW-AHP method), we can see that prediction accuracy of the EW-AHP method is higher than other methods. The research established the efficacy of EW-AHP method as a method of determining weights and provided a basis for regional control of groundwater nitrate pollution.