Time-Dependency of Nitrate and Phosphate Concentrations with Aquifer Vulnerability Indexes: A Case Study in Shahrekord Aquifer, Iran

Shahrekord aquifer is located in Chahrmah-Va-Bakhtiyari province, Iran, and is a source for drinking water. The main objectives of this study were 1) to examine the seasonal fluctuations of nitrate and phosphate concentrations and patterns in the aquifer and 2) to evaluate the effect of sampling time of nitrate and phosphate on the accuracy of the prediction DRASTIC and SINTACS models. Almost 100 agricultural wells were sampled four times in 2010 and 2011, and nitrate and phosphate concentrations were measured in the samples. Geostatistical methods were used for mapping of nitrate and phosphate concentrations within the aquifer. The results show that the aquifer nitrate concentration varied between nil to 55mg/L and that of phosphate concentration between 0.07 to 0.15 mg/L. Mean concentrations of nitrate and phosphate during three seasons (spring, summer, and fall) significantly changed. In the fall, almost two-thirds of the aquifer (especially in northern parts) had a lower nitrate concentration (between 0 to 20 mg/L). However, the southern part of the aquifer is relatively polluted, and nitrate concentrations exceed the Iran water quality standard limit (45 mg/L). Groundwater vulnerability maps show areas with the relative potential for contamination. Comparison of nitrate or phosphate concentrations with an aquifer vulnerability map during spring, summer, and fall showed that the results of DRASTIC and SINTACS models can be somewhat sampling time dependent for both constituents (nitrate or phosphate). There is a significant difference between the correlation of nitrate or phosphate with SINTACS or DRASTIC maps with sampling time during the year. The largest correlation was seen in the spring and the lowest correlation occurred at the end of summer. Thus, to verify DRASTIC and SINTACS models, the sampling time of pollutant must be considered. Concentration of both nitrate and phosphate increased, i.e., nitrate concentration from 18 to 27 mg /L, and phosphate concentration from 0.05 to 0.15 mg/L during the period 2006 to 2011. Comparison of maps shows that the contamination of both nitrate and phosphate has increased in the area, during the years 2006 to 2011. During this period, the zone of low contamination has shrunk and the zone of high contamination has expanded.