Dangerously high concentrations of arsenic can be found in groundwater drawn from unconsolidated sediments around the world. Previous studies have shown that bacteria, particularly those that reduce arsenate, can release arsenic from sediments and, in essence, add it to the groundwater.
Kirk et al. have studied the Mahomet glacial aquifer in central Illinois and found that high arsenic concentrations correlate with low sulfate concentrations. The authors suggest that in regions where sulfate-reducing bacteria are active, they produce sulfides that precipitate arsenic and remove it from the water. In contrast, where methanogenic bacteria are active, little sulfide is produced and arsenic is not precipitated. If arsenic concentrations are indeed affected by bacteria in this fashion, then a low sulfate concentration, which is much easier to measure, can be used as a sign of potentially unsafe water. Furthermore, adding sulfate to arsenic-rich aquifers may stimulate sulfate-reducing bacteria and thus reduce arsenic concentrations. -- LR
Geology 32, 953 (2004).
