MICROBIAL CONTROL ON ARSENIC CONCENTRATION IN GROUNDWATER FROM A GLACIAL AQUIFER SYSTEM

Chemical and microbiological analyses from the Mahomet glacial aquifer system in east-central Illinois demonstrate the importance of microbial processes in controlling arsenic concentration in groundwater there. Wells we have sampled fall into two categories: (1) those with high arsenic concentrations occur in zones where methanogenesis dominates, and (2) those with little arsenic are found where sulfate reduction dominates. Iron reduction also appears to be occurring to varying degrees within both zones.

In order to evaluate the dominate microbial metabolisms, we augmented existing Illinois State Water Survey data from 21 domestic wells completed within the Mahomet with analyses for hydrogen, methane, and sulfide. We also examined biomass sampled from four wells using the most probable number (MPN) culturing method, and with terminal restriction fragment length polymorphism (TRFLP). The MPN results verify the presence of these metabolic groups, and TRFLP results confirm that the microbial community differs significantly between the low and high arsenic zones.

These findings reflect the role that sulfate reducing bacteria play in controlling arsenic concentration. Where these bacteria are active, the sulfide produced by the oxidation of organic matter reacts with metals in solution, including ferrous iron produced by iron reducing bacteria, causing the metals to precipitate. Arsenic may precipitate or coprecipitate with other metals as a sulfide mineral. Where the sulfate content of the groundwater has been exhausted, on the other hand, methanogenesis takes over as the dominant microbial metabolism consuming organic matter. Arsenic, in the absence of a precipitation pathway here, accumulates in the groundwater.

Well records indicate the arsenic-rich, methanogenic zones commonly occur in finer grained portions of the aquifers. These areas are at risk of developing high arsenic concentrations for two reasons. An abundant supply of dissolved organic compounds from fermentation within fine-grained sediments coupled with restricted recharge of sulfate in the slowly flowing groundwater allows sulfate to become depleted. Groundwater flow in these sediments is more restricted, furthermore, limiting the rate at which arsenic can be flushed from the aquifer.