Evaluation of groundwater vulnerability to nitrate in shallow aquifer using multi-layer fuzzy inference system within GIS environment

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Groundwater aquifer vulnerability to nitrate has become a major concern to all stakeholders involved in managing the quality of water in relation to human health. This paper presents fuzzy inference system (FIS) as an alternative to the conventional overlay index methods for the evaluation of groundwater vulnerability to nitrate pollution in watershed scale within GIS environment. A multilayer FIS model is developed taking advantage of expert knowledge-based DRASTIC system in order to overcome the problem of rule explosion. A case study relating to groundwater vulnerability assessment in shallow aquifer of Kathmandu basin in Nepalis accomplished. The correlations between groundwater vulnerability indices and nitrate-N concentration values were carried out for the validation of vulnerability models. The result shows that the best combination is obtained from fuzzy vulnerability model (r = 0.52) followed by DRASTIC method (r = 0.38). We have also developed a groundwater nitrate-N risk map by integrating groundwater aquifer vulnerability, the nitrate-N distribution map (pollution severity) and the probability of occurrence map based on the limiting threshold value of nitrate-N. Spatial distribution map of nitrate-N shows that nearly 10% of the study area lies within high hazard class which exceeds limiting value of 10 mg/L nitrate-N value, and nearly 50% of the shallow aquifer fall within medium hazard class which has impacted level of nitrate-N, i.e., between 2 mg/L and 10 mg/L. These results also indicate that the northern part of the Kathmandu basin and highly permeable alluvial deposits are dominated by very high vulnerability level, and are also under a threat of high nitrate-N risk. Groundwater aquifer vulnerability and nitrate risk maps are expected to facilitate design and development strategies towards protecting environment and preventing further deterioration of the affected aquifers.