Acidity Testing – Avoid pH complications before they happen
Every organism has a pH range within which growth is possible and typically a well-defined optimum pH for maximum growth rates and activity. Although organisms that thrive in low pH (acidophiles) and high pH (alkaliphiles) environments have been isolated, most microorganisms are classified as neutrophiles with pH optimums between pH 6 and 8. Many important bacterial groups responsible for biodegradation of chlorinated solvents belong to this neutrophile category. For example, the pH for optimal growth of Dehalococcoides ethenogenes strain 195, the only known bacterium capable of complete reductive dechlorination of PCE to ethene, is between pH 6.8 and 7.5. Likewise, optimal pH ranges for several Desulfitobacterium species capable of reductive dechlorination of chlorinated ethenes and ethanes are near neutral. Thus maintenance of a circum neutral pH in subsurface environments can be an important factor in promoting reductive dechlorination.
In pristine aquifer systems, low groundwater pH is relatively uncommon although oxidation of sulfides can lead to pH values as low as 4 to 5 under natural conditions. More often, pH excursions are a direct result of site activities including:
co-contamination with strong acids used in a wide variety of industrial processes,
source area treatment using Fenton’s-based in situ chemical oxidation, and
organic acid production following subsurface injection of an electron donor (e.g. HRC, EOS, molasses, etc.) designed to stimulate biological reductive dechlorination.
Aquifer pH can be increased by the subsurface circulation or injection of a dissolved base or alkaline material. The added alkalinity, however, will be consumed by groundwater acidity and acidic mineral surfaces in the aquifer matrix. Consequently, determination of groundwater and soil acidity is a key component of remediation system design. The acidity and buffering analysis provides the equivalents of base needed to overcome aquifer acidity and maintain a near neutral pH required for optimum biological activity.
Ideally, pH Buffering analysis is performed on both soil (aquifer materials) and groundwater samples at each location to determine the acidity of both acidic minerals and native groundwater. Two 250 mL amber glass bottles containing the sample (aquifer material or groundwater) are submitted to Microbial Insights. Acidity testing results are displayed on a plot of pH as a function of milliequivalents of base per kg of soil (meqv/kg soil). At the study site shown, existing subsurface pH is 5.1 which is well below the optimum range for Dehalococcoides ethenogenes suggesting possible inhibition of reductive dechlorination. Based on acidity testing, 75 meqv base/kg soil would be necessary to consume acidity and achieve a pH of 7. Armed with this acidity value, the base equivalents of the chosen alkaline material (provided by manufacturer or vendor), and the size of the treatment zone, site managers can then calculate the mass of alkaline material required and injection parameters.