An assessment of the effects of excessive groundwater abstraction on the quality of groundwater resources of the Zanjan Plain, Iran

In the present study, the quality of groundwater and the effects of pumping on the phreatic surface beneath the Zanjan Plain located in northwestern Iran were investigated by measuring water levels in 81 observation wells as well as various physicochemical parameters including electrical conductivity (EC), total dissolved solids (TDS), total hardness (TH) and pH. In addition, the results of the chemical analysis of major anions and cations from 53 boreholes that were sampled during the period 2004-2018 were evaluated. The water level measurements indicated that the elevation of the phreatic surface had declined by up to about 8 m during the studied period, indicating an average decrease of about 0.57 m per year. A comparison of the measured physicochemical parameters with WHO, ISIRI, and EPA standards for drinking water quality showed that some parameters including EC, TDS, TH, SO4, and Cl in the northwestern and central parts of the plain exceeded the allowable level. An assessment of water quality index (WQI) values showed that most parts of the Zanjan Plain lie in the "good" quality class, although in some parts, the water quality of "poor" quality class, most likely due to the effects of urban activities. A comparison of the results from a Schoeller diagram assessment with WQI values showed some differences between the results of the qualitative classification of some parts of the Zanjan Plain groundwater based on the two mentioned methods. The results of the present study showed that groundwater of some areas of the Zanjan Plain is not suitable to be used for irrigation of agricultural lands with limited drainage. The Piper diagram also indicated that the dominant groundwater quality facies on the Zanjan Plain were Ca-Na-HCO3 and Ca-HCO3 water types. The results of the Spearman correlation analysis also showed a high correlation of most anions with cations. Analysis of the volume changes versus qualitative changes in groundwater also showed a general decreasing trend of water quality with decreases in the volume of groundwater in storage in the aquifer, although in some conditions due to changes in solubility of water-soluble materials because of the changes in groundwater level, other trends were also observed.