Arsenic is a toxic, naturally occurring element that is sometimes found at high concentrations in well water. High arsenic concentrations can occur even in areas without a pollution source when the bedrock or soil that is in contact with the ground water releases arsenic as the water flows over the surfaces. This is the case in West Bengal (India) and Bangladesh, where the contamination of well water is creating serious health problems for the population (the "biggest calamity of the world").
Arsenic contamination of drinking water is also of concern in the US and other industrial nations. For example, in Maine (USA), approximately 70% percent of state residents rely on groundwater sources for their drinking water, with about 56% using private wells. Unfortunately many of these wells contain arsenic concentrations in excess of the USEPA drinking water limit of 10 ppb, and since there is no requirement to test private wells, many people may unknowingly be exposed to high arsenic concentrations.
The concentration of arsenic in groundwater can be affected by the activity of certain bacteria according to the results of a project that 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), has been presented at the General Meeting of the American Society of Microbiology (ASM)
, June 8th 2005 in Atlanta, GA.
Microorganisms can affect arsenic mobility either directly or indirectly. By acting directly on arsenate, they may produce arsenite, which is more toxic and more mobile in groundwater. Indirect means of altering arsenic mobility include the reduction of solid phase bedrock materials or coatings such as iron hydroxides that normally bind arsenic.
To see how 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.
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. The researchers hope that by learning more about the arsenic release mechanisms, they will be able to target high-risk areas for testing and develop management options to minimize arsenic concentrations.
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