摘要：Impact of heavy metals on the environment is a series issue worldwide. The objective of this study is to determine the groundwater quality variations of the unconsolidated shallow aquifer system in the Midyan Basin of northwestern Saudi Arabia. A total of 72 groundwater samples have been collected in January 2012 and analyzed for physicochemical parameters (total alkalinity, total solids, suspended solids and chemical oxygen demand). The level of heavy metals (lead, cadmium, chromium and zinc) and metalloid (arsenic) have also been assessed among the entire collected samples. The concentration of physicochemical parameters , heavy metals and metalloid do not exceed a maximum permissible level as per the World Health Organization standards. The observed total alkalinity, total solids, suspended solids and chemical oxygen demand concentrations have a mean value of 175.49, 2432.3, 662.7 and 1.178 mg L-1 for the studied water samples, respectively. The results for heavy metals lead, cadmium, chromium and zinc concentrations in this study show location based variability, ranging between 0.00015 and 0.0038 mg L-1, 0.0004 and 0.0009 mg L-1, 0.0008 and 0.0114 mg L-1 and 0.00035 and 0.1597 mg L-1 with a mean of 0.00112, 0.00059, 0.002133 and 0.00779 mg L-1, respectively. The range of metalloid arsenic concentration in the studied groundwater is 0.0008-0.0051 mg L-1 with a mean of 0.00185 mg L-1. The Hydrogen ion concentration values of the groundwater samples indicates an alkaline nature with an average value of 7.80, where the maximum value of 8.29 and a minimum value of 7.38 have been recorded. Calculated average daily dose and noncarcinogenic risk indicate that the range in all the studied boreholes is below the Reference Dose. In addition, the noncarcinogenic risk values are less than 1, indicating that they are within the tolerable range. Chemical contents in the studied groundwater are mainly contributed by the surrounding rocks, agriculture and other anthropogenic sources. In addition, surface and subsurface flow direction, wells locations and surface geological structures (including faults and fractures) appeared to be significantly causing the elevated chemical contents. The results also show that boreholes located in the southeast are more vulnerable to metals contamination.
