Effect of flash flood on trace metal pollution in the groundwater - Wadi Baysh Basin, western Saudi Arabia

This study focused on the impact of flash flood (FF) on trace metal pollution in the groundwater, Wadi Baysh Basin, Western Saudi Arabia using spatial interpolations, integrating pollution indices and statistical analysis. Groundwater (n = 182) is neutral to alkaline in nature and the order of mean metals before and after FF are Sr > Ba > B > Fe > Al > V > Mn > Cr > Cu = Pb > Mo > Zn and Sr > Al > B > Fe > V > Ba > Cr > Cu > Pb > Mn > Mo > Zn > Ni > Cd, respectively. Enhancement (Al, Cr, Cu, Mn, Mo, Pb and V) and depletion (B, Ba, Fe, Sr) of trace metals are noticed in the groundwater after FF, which is due to dilution, dissolution/desorption, flushing of weathered layer, saline sources and surface input. Before and after FF, 57-63%(Pb), 55-56%(V), 43-16%(B), 27-0%(Ba), 15-5%(Sr), 6-3%(Fe), 0-16%(Al), 0-9%(Cr) and 1-2%(Mn) of groundwater samples, respectively, exceeded the drinking water standards. Coastal wells have elevated B, Sr and Mn concentration due to saline water intrusion. Integrated pollution evaluation indices (Heavy metal pollution index (HPI), Contamination index (C-d) and Heavy metal evaluation index (HEI)) reveal that number of samples in the unsuitable classes are increased after FF event (HPI: 13%-55%; Cd: 11%-38%; HEI: 5%-19%) and groundwater quality is degraded due to the trace metals enhancement after FF especially in the downstream wells. Wells in the upstream is flushed by FF recharge and groundwater flow accumulated the metals in the downstream and coastal region. This study demonstrates that metal accumulation and mobility are regulated by the groundwater flow in the study region. Pearson correlation analysis and Principle component analysis also justify that water chemistry is influenced by the saline sources, flushing of weathered layer, natural processes and anthropogenic inputs. Continuous groundwater monitoring is an important task to protect and manage valuable groundwater resources in the arid region.