In this project we are developing push-pull tests to evaluate the potential for aerobic cometabolism of chlorinated aliphatic hydrocarbons (CAHs), such as trichloroethene (TCE) using gaseous cometabolic substrates such as propane. Specific tests are designed
- To determine the transport characteristics of nutrients, substrates, and CAHs and their transformation products,
- To determine whether indigenous microorganisms have the capability to utilize selected substrates and transform targeted contaminants,
- To determine rates of substrate utilization and contaminant transformation, and
- To optimize combinations of injected nutrients and substrates to maximize rates of contaminant transformation.
Transport characteristics (e.g., retardation factors) of substrates, contaminants, and, in some cases their transformation products are needed to compute substrate utilization and contaminant transformation rates and are also needed as input to site-scale groundwater flow and contaminant transport modeling and these are obtained using Transport Tests. Transport Tests are conducted in a way that minimizes the potential for substrate utilization or contaminant transformation. Biostimulation Tests are designed to stimulate microbial activity. Rate of substrate utilization and contaminant transformation are determined using Activity Tests, which are conducted under conditions that promote the expression of indigenous microbial activity. In our approach, Transport Tests are conducted first (Figures 2). Then a series of Biostimulation Tests is conducted to stimulate microbial activity (Figure 3). Finally one or more Activity Tests are conducted to demonstrate aerobic cometabolic activity of the indigenous microorganisms by monitoring the rate of consumption of injected nutrients (e.g., nitrate) and gaseous substrates (e.g., propane and oxygen), the production of defined products from injected surrogate compounds (e.g. the production of ethylene oxide from injected ethylene and the production of propylene oxide from injected propylene), and the production of defined CAH oxidation products (e.g. the production of cis-DCE epoxide) (Figure 4).
Figure 1. Push-pull test sequence.
Figure 2. Example extraction phase breakthrough curves for a push-pull Transport Test.
Figure 3. Measured propane, oxygen (DO), nitrate, and bromide concentrations during three field Biostimulation Tests.
Figure 4. Push-pull-Activity Test showing in situ production of ethylene oxide from injected ethylene.