Temporal and spatial assessment of groundwater contamination with nitrate by nitrate pollution index (NPI) and GIS (case study: Fasarud Plain, southern Iran)

Groundwater resources in arid and semi-arid regions are the most and sometimes the only water resource used for agricultural, industrial, and urban water supply. Irregular and immense application of nitrogen fertilizers in the lands under cultivation and nitrate leakage from livestock farming have affected the groundwater quality. In such areas, nitrate is one of the main pollutants in the groundwater. In this study, the temporal and spatial trend of nitrate contamination in 31 wells in Fasarud Plain, southern Iran, from April 2017 to March 2018 were assessed. To survey the geochemical quality of the plain, a geographic information system to expand geographic location maps and spatial distribution maps of nitrate concentration and nitrate pollution index (NPI) was applied. Nitrate concentrations ranged between 2.43 and 96 mg L-1. Results indicated that nitrate temporal trend was increased significantly in most of the wells, and the spatial trend of area percentage of nitrate class 3 (not permissible limit of more than 50 mg L-1) was positive. The greatest quantities of this variable in groundwater samples detected in northern, western, and eastern areas of the plain have a direct relation with the fertilization of agricultural lands. Generally, by ending the irrigation season, nitrate concentration and NPI reduced temporally in the samples and the percentage area of nitrate class 3 decreased gradually, again beginning the agricultural season, the NPI, nitrate concentration, and percentage area of nitrate class 3 began to increase. Overall, the change of nitrate concentration and distribution of agricultural regions have illustrated that nitrate originated from nitrogenous inorganic fertilizers applied within irrigation periods.