14.5: Applying microbiology to remediate contaminated water

In this final section, we consider how we can use knowledge from this book to design strategies for remediating contaminants in aqueous systems. We focus our attention on the use of sulfate reduction as a strategy for remediating arsenic contamination. This example is just one of a virtually endless number of possibilities. However, it allows us to illustrate the basic approach to developing a microbial bioremediation strategy, which readers may then apply to other contaminants.

The basic premise of bioremediation is centered around the two-way interactions between microorganisms and their environments. We must first consider microbial impacts on their environments by asking: what change(s) in microbial activity would decrease the concentration of the contaminant? Then, we consider the ways environments control microbial activity by asking: what can we alter about the environment that can bring about those change(s) in microbial activity?

Regarding the first question, there may be multiple answers. Some microorganisms in a community may increase contaminant levels, in which case the desired change might involve decreasing their activities. Other microorganisms in a community may decrease contaminant levels, in which case the goal may be do increase their activities. A bioremediation strategy might leverage one of both of these possibilities, depending on the extents to which they are attainable.

Regarding the second question, it may be possible to do nothing, and simply let an indigenous microbial community clean up the contaminant over time, as is occurring at the Bemidji site. This approach is referred to as natural attenuation. Alternatively, we may want to speed up contaminant attenuation by increasing the activity of select groups. We can refer to this approach as enhanced bioremediation, which we may be able to achieve via biostimulation and/or bioaugmentation. Biostimulation enhances growth of select microbial populations by altering conditions, such as the availability of nutrients or energy resources. Bioaugmentation occurs when we add microorganisms to the environment to give added functional capabilities to a community. For example, bioaugmentation is used at some chlorinated solvent sites to increase the abundance and activity of Dehalococcoides, the only group of bacteria known to be able to complete the final steps of dechlorination (Section 14.4.4).

Applying these questions to arsenic contamination, we can see that multiple approaches may be possible. Iron-reducing microorganisms are the main group responsible for mobilizing arsenic. Thus, our strategy might involve decreasing their activity. Alternatively, iron oxidizers and sulfate reducers can reduce arsenic mobility by forming mineral phases that can sequester arsenic, so we might seek to increase their activities. Of these possibilities, stimulating sulfate reducers may be simplest and most cost effective. In many contaminated aquifers, arsenic levels are only elevated in zones where the microbial community has exhausted the supply of sulfate (Section 14.4.2). Thus, we may be able stimulate growth of sulfate reducers within these zones simply by dissolving a sulfate salt such as gypsum in water and then injecting it into the contaminated zones of an aquifer (Kirk et al., 2004).

Researchers recently tested this approach at an industrial site contaminated with arsenic-based herbicides in Florida, USA (Lee et al., 2019; Saunders et al., 2018). Previous attempts with conventional techniques stabilized the arsenic plume at the site but arsenic concentrations remained high, ranging from about 300 to 1,000 µg/L. Alongside these elevated arsenic concentrations, sulfate concentrations were low. Thus, the researchers aimed to stimulate sulfate-reducing activity by injecting sulfate into the aquifer. In addition to sulfate, the researchers also injected organic electron donors and iron to ensure the availability of key ingredients needed to rapidly generate sulfide biominerals. Within one week of the injection, significant growth of sulfate reducers began to occur, which persisted for at least two months following injection. In response to their growth, pyrite crystals began forming that contained 0.05 to 0.4 weight percent arsenic. Precipitation of these minerals lowered dissolved arsenic concentrations to below 50 µg/L for about six months, until untreated upgradient water displaced the treated groundwater being sampled at their monitoring wells. Overall, however, more than 90% of the dissolved arsenic was removed during the treatment period, demonstrating a strong potential of this strategy as an arsenic bioremediation technique.

Microbial bioremediation strategies such as that described here for arsenic are often more cost effective and sustainable than conventional treatment strategies. For example, a common approach for cleaning up a groundwater contaminant is known as pump-and-treat, in which groundwater is pumped out of a contamination plume, treated, and then discharged (Mackay and Cherry, 1989). This strategy can lower contamination levels, but it may require decades and enormous costs to reach safe levels. Alternatively, in situ microbial solutions may require less energy inputs and infrastructure development. They may even just take over on their own, as is the case with natural attenuation. By considering the two-way interactions between microorganisms and their environments, many such opportunities will be identified and used to achieve sustainable positive goals in the years to come.

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