Arsenic in Groundwater
Elevated levels of arsenic in groundwater have been observed throughout the U.S. There are “hot spots” in the desert southwest, northern regions of the Midwest and the New England area, where there is a region of elevated arsenic that roughly follows the coastline. In Maine, nearly half of the population relies on private groundwater wells for their drinking water, many of which are at risk for containing elevated levels of arsenic.
There are currently no regulations requiring private wells to be tested for arsenic. As a result, many residents of Maine and other regions that fail to require rigorous private well testing may be consuming drinking water laden with arsenic at levels exceeding the U.S. Environmental Protection Agency’s maximum contaminant level of 10 parts per billion.
Microorganisms have the potential to affect the concentration of arsenic in groundwater by changing the redox state of arsenic directly or by altering surface sites that bind arsenic to the solid phase. When arsenate (As(V)) is reduced by microorganisms or abiotic means to arsenite (As(III)), it appears to become more mobile under most conditions and is more toxic.
Iron or manganese hydroxides are common surface groups that bind arsenic species and are also prone to reduction by microbial activity. At circumneutral pH, arsenate—which is negatively charged—tends to bind to adsorption sites more strongly than arsenite, which has a neutral charge in this pH range.
Arsenic in Maine
A study was recently conducted in Northport, Maine, which has a fractured bedrock aquifer and groundwater with elevated levels of arsenic. Arsenopyrite veins have been found to run through the bedrock, and as the water moves from the recharge region to the discharge region of the aquifer, the arsenic concentration increases.
A previous study was used to look for specific microorganisms with the potential to increase the amount of arsenic in the aquifer groundwater.
The study looked for a specific arsenate-reducing bacterium that had been isolated from the site, and a genus, Geobacter, that includes many bacteria that can reduce iron.
These microorganisms were selected because they would release arsenic into the groundwater via different mechanisms: the arsenate-reducing bacterium by altering the redox state of arsenic and the iron-reducing bacteria by decreasing the available sorption sites for arsenic.
That study found that the total arsenic concentration correlated with the percentage of the microbial population that belonged to the genus Geobacter. This indicates that microorganisms may be involved in the indirect release of arsenic through iron reduction where insoluble iron (III) is reduced to the more soluble iron (II), thereby decreasing the number of sorption sites available for arsenic.
In this study, clone libraries were generated to examine the composition of the microbial populations in the Northport aquifer. Two wells from the recharge (R1 and R2) and two from the discharge region (D1 and D2) of the aquifer were selected to determine if there was a shift in the microbial populations as the water moved from the lower arsenic wells in the recharge region to the high arsenic wells within the discharge region of the aquifer.
Groundwater samples (250 ml) were filtered and the DNA was extracted using a MoBio soil kit. A large fragment of the 16S ribosomal RNA gene was amplified by PCR using primers 8F and 1492R, which target all bacteria. The amplified DNA fragments were inserted into plasmids and transformed into E. coli. The genes from clones that exhibited unique restriction enzyme banding patterns were sequenced and compared to the genes of known microorganisms in order to classify the organisms.
Figure 1 demonstrates the results of the cloning analysis. The percentage of the clones that fell into various phylogenetic groups is plotted as stacked bars. Overall, the microbial populations in the R1 and R2 wells are more diverse than in the discharge wells D1 and D2. In all wells, more than half of the microbial population consists of proteo-bacteria; however, there is a shift in the subclasses of proteobacteria that dominate.
Dominance shifts from alpha, beta and gamma proteobacteria species in the more oxic recharge wells to beta and delta proteobacteria in the more reduced discharge wells. The delta subclass of proteobacteria includes the genus Geobacter, which correlated with arsenic in the previous study as described above, in addition to other iron-reducing bacteria with the potential to alter arsenic binding sites.
In addition to shifts in the microbial populations, there are also shifts in the water chemistry. There is a decrease in the dissolved organic carbon concentration and the nitrate concentration when comparing the recharge wells to the discharge wells. There also is an increase in the total arsenic concentration, the arsenite concentration and an increase in the soluble iron concentration in the discharge wells (see Figure 2).
Discussion & Conclusions
The gamma proteobacterial genus Shewanella contains species that are capable of both iron and arsenic reduction. None of the clones identified were Shewanella species and no gamma proteobacterial clones were found in the discharge wells. As a result, it is highly unlikely that Shewanella species are playing a significant role in arsenic or iron reduction in the aquifer.
In one of the discharge wells, there was the appearance of close relatives of Deferribacter species. These organisms are of interest because of their ability to reduce iron and in some instances arsenate.
The increased iron and arsenic concentrations in groundwater from the discharge region of the aquifer relative to the recharge region suggest that arsenic has been released as iron surface coatings were solubilized. The increase in the portion of the population from the delta proteobacteria group is significant because anaerobic delta proteobacteria members tend to be sulfur, sulfate or iron reducers. It is likely that a large portion of the delta proteobacterial community would be involved in iron reduction due to the relative abundance of iron when compared to that of sulfate.
In aquifers with arsenic in the subsurface materials, it appears that species from delta proteobacteria can play an important role in release of arsenic into the groundwater.