Enrichment of Manganese at Low Background Level Groundwater Systems: A Study of Groundwater from Quaternary Porous Aquifers in Changping Region, Beijing, China

Manganese (Mn) could adversely affect water quality and is a concern for drinking water safety. In contrast to Mn-contaminated groundwater systems, Mn concentrations can increase up to 2 orders of magnitude in low background level groundwater systems (Mn < 100 mu g/L). It is interesting to explore related hydrogeological and hydrochemical processes and controlling mechanisms. Our data showed that Mn of groundwater from Quaternary porous aquifers in the Changping region of Beijing in China varied from 0.02 to 522 mu g/L, with an average of 45 mu g/L. The high Mn concentration (>100 mu g/L) in groundwater occurred in the lower plain, while the low Mn concentration was distributed in the upper plain. Association was not obvious between the Mn concentration and the depth of the groundwater. The water-rock interaction (e.g., dissolution of Mn-bearing minerals) and redox reactions dominated the distribution of Mn in groundwater. Particularly, the reduction of Mn4+ was a major cause of increasing Mn2+ concentration in groundwater. This is further evidenced by declining dissolved oxygen (DO) and NO3 in groundwater from the upper plain to the lower plain part. This study contributes to further understandings of hydrochemical processes for the enrichment of Mn in groundwater and offers a reference for the management and oversight of groundwater containing an excessive Mn level.