How Bacteria Appear To Affect Arsenic Concentrations In Groundwater

To see if bacteria affect arsenic concentration, groundwater samples were taken from two high-arsenic areas in Maine, and water chemistry and two bacterial populations were measured. Wells with high total arsenic concentrations had a higher proportion of iron-reducing bacteria than wells with lower arsenic.

Iron reducing bacteria, such as members of the genus Geobacter, grow in the absence of oxygen and can transform solid phase iron (Fe(III)) into Fe(II), which is soluble in water. Solid phase Fe(III) can bind to arsenic, immobilizing it on the surface of the solid. When bedrock or soil Fe(III) is transformed into Fe(II), any bound arsenic would also be released into the groundwater. Arsenic (As) also has two commonly occurring forms in groundwater: As(III) and As(V).

The prevalence of NP4, a microbe from the genus Sulfurospirillum that can transform As(V) to As(III), which is the more toxic form of As, was also measured. NP4 was more abundant in water samples with higher As(III) concentrations. Thus it appears that iron reducing bacteria affect the overall arsenic concentration, and that arsenic reducing bacteria (NP4) control its form, and thus toxicity, in these regions of Maine.

This work was completed at the University of Maine by Ph.D. candidate Jennifer Weldon under the supervision of Dr. Jean MacRae. The project, funded by the National Science Foundation (NSF) Career program (Bioengineering and Environmental Systems), is being presented at the General Meeting of the American Society of Microbiology (ASM), June 8th 2005 in Atlanta, GA.

The 15 groundwater samples used in the study were examined using fluorescence in-situ hybridization (FISH). This technique allowed the target populations, Geobacter and NP4, to be distinguished from other bacteria. The number of each type of microorganism was compared to the total number of microorganisms in the sample. Geobacter ranged from 1-35% and NP4 from 0-17% of the total suspended bacterial population. Metals were measured by inductively coupled plasma atomic emission absorption (ICP-AES), and arsenic speciation was obtained by passing a sample through an ion exchange resin in the field to obtain As(III). Total arsenic concentrations ranged from less than 2 parts per billion (ppb) to 2000 ppb, which is 200 times higher than the EPA's water quality limit of 10 ppb. As(III) ranged from <2 ppb to 1100 ppb. We hope that by learning more about the arsenic release mechanisms, we will be able to target high-risk areas for testing and develop management options to minimize arsenic concentrations.