RESUMEN
Groundwater is a precious natural water resource which can be considered as an available and safe source of water for domestic uses. In some cities in northern Iran, groundwater is being polluted due to various human activities. In this regard, the located municipal solid waste landfills close to these areas without the requested controls regarding the landfill leachate and gas emission are among the major sources of environmental pollution, which are deteriorating groundwater quality around landfill sites. In this context, the current study was aimed to assess the quality of collected groundwater samples around a landfill site using the modified Water Quality Index for groundwater resources (WQIG). Also, the water quality map has been prepared by using WQIG in GIS environment. Therefore, thirty-three groundwater samples were collected and analyzed around 11 water wells close to the landfill site. Variety of physicochemical parameters including nitrate (NO3), fecal coliform, sodium absorption ratio (SAR), electrical conductivity, biological oxygen demand (BOD5), phosphate (PO4), total dissolved solids, dissolved oxygen, pH were assessed in groundwater samples based on recommended standard methods by American Public Health Association (APHA). Geostatistics analysis including Kriging, semi-variogram, and variogram methods also was used to evaluate the spatial variations in the variables and to provide the necessary data for further interpolation. According to the results of WQIG, most of the groundwater wells (98.85%) are polluted due to the existing of the landfill site in the investigated area. Generally speaking, based on the WQIG the water samples were defined as not suitable for drinking applications. Also, the high concentrations of nitrate and hardness in the downstream wells (W1-W11) of the landfill were demonstrated by the results of Kriging assay which can be correlated with the penetration of leachate into these wells. Considering the SAR results, all wells in the studied area are classified as (C3S1) that means the groundwater of this area is suitable for agricultural approaches. Moreover, the reduction in the water quality from the south to the north and northeast was demonstrated by the results of spatial dispersion. Evaluation of the changes in water quality near landfill sites showed that 2149.56 m2 of total area had a relatively poor potential for the region's groundwater recharge.