B I O R E M E D I AT I O N
Success in bioremediation DEMANDS close monitoring Using bioremediation to treat ground impacted by chlorinated hydrocarbons (CHCs) essentially requires building a bioreactor below the ground; therefore, direct obser vation of results is not possible. Rather, the process must be scientifically monitored through specialised tests to ser ve as the integral lines of evidence needed to assess the health and per formance of the biobarrier. By Michelle Roux and Sathisha Barath
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he monitoring of this biobarrier, which is a living system, is done using specific in situ and laborator y tests. These tests will give insight into the key prevailing conditions that are required to maintain the biobarrier’s health and achieve the expected results in terms of CHC degradation. The characterisation of the affected ground will determine which geochemical conditions are most relevant to be included in the ongoing project assessment.
In situ monitoring of redox and pH For instance, cer tain microbes responsible for the degradation of the dense non-aqueous phase liquids (DNAPLs) will only thrive in an anaerobic environment – in which case a vital aspect of monitoring will be focused on checking for oxidation-reduction (redox) conditions. Acidity in the groundwater must also be carefully checked, as the microbes require a pH level in a range from five to seven. As they break down CHCs, the microbes generate volatile fatty acids that reduce the pH level of the groundwater and could undermine their effectiveness. Where pH levels are found to be
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too low, a ‘buffer’ can be added to raise them to within the optimal range.
Inorganic indicators It is also necessar y to check other geochemical parameters such as the levels of nitrates and sulfates. This helps to identify and understand the complex geochemical dynamics in the aquifer and possible presence of other microbes in the aquifer that could influence remediation reactions; these other microbes can compete for the injected food source. Information on these factors enables the practitioner to adjust the food supply to ensure that the dehalorespirators are still well supplied.
Total organic carbon Regularly sampling and measuring the total organic carbon (TOC) in the aquifer is crucial, as this is an indicator of the substrate (food) on which microbes sur vive and thrive. The TOC levels should be measured both spatially and temporally to determine whether more substrate needs to be injected and where. Aquifers – and even zones within a single aquifer – will differ in terms of their hydraulic properties and flow velocities. During the characterisation phase, prior
to full-scale implementation, specific measurements should be undertaken to provide insights into the aquifer dynamics that will later influence substrate migration. The focus needs to be on the zone being treated and on ensuring that the substrate is finding its way to the targeted area and is being retained in the treatment zone for long enough that the microbes can utilise it efficiently. This highlights the impor tance of making allowance in the project conceptualisation and characterisation phase for identifying preferential groundwater flow paths in the aquifer, which leads to flow heterogeneity within the system. These flow paths could take the injected substrate in unintended directions and deprive the microbes of an adequate food source.
Changes in concentration Groundwater samples also need to be analysed to measure the concentration changes of CHCs within and downgradient of the biobarrier over time. Samples are collected ever y few months from carefully located monitoring holes. Indeed, the frequency and timing of the sampling regime can be viewed as a science in itself. Similarly, the sampling methodology is
