Abstarct: This research presents a hydrogeochemical and environmental assessment of groundwater in the Khomein plain which is located in the southern part of the Markazi province. baxsed on the study carried out by the Regional Water Organization of the Markazi province، the aquifer in this region is considered as a unit aquifer. To investigate the hydrogeochemical properties of the aquifer، the physical and chemical characteristics of groundwater were measured twice (October 2012 and June 2013). The electrical conductivity and pH were directly measured in the filed. Besides، the chemical analysis of water samples was performed for major cations and anions in the water laboratory of the Regional Water Organization of the Markazi province. In addition، for a better understanding of groundwater contamination، a few samples were analyzed for heavy mextals by ZarAzma laboratory. The results show that the quality of water is affected by rising of conductivity، TDS، hardness and other hydrogeochemical parameters from the west of the plain (recharge zone) to the east (discharge area). Furthermore، the groundwater exhibits a bicarbonate type in west and sulfate type in east of the study plain. Among the heavy mextals، copper، iron and arsenic were dominant. Despite being a correlation، the concentration of iron and copper was bellower their permissible limits. However، arsenic with no correspondence with any of elements and the hydrogeochemical parameters was above its allowable level. The presence of arsenic in groundwater in the plain wherein the samples were taken reflects an anthropogenic origin