Estimation of Injected Carbon Longevity and Re-oxidation Times at Enhanced Reductive Bioremediation Sites

Abstract:

Addition of an organic substrate to provide an electron donor and carbon source can be very effective at stimulating enhanced reductive bioremediation (ERB) of chlorinated solvents, energetics, and other groundwater contaminants. However, the quantity of electron donor added is usually based on an individual’s or company’s “rule of thumb” rather than considering site-specific conditions such as groundwater velocity, carbon source, and upgradient electron acceptor concentrations, potentially leading to unnecessarily large amounts of carbon injected. Mass balance estimates indicate that over 99% of electrons donated go to electron acceptors other than the primary contaminants. Thus, injecting excessive amounts of organic carbon can lead to a persistent reducing zone, releasing elevated levels of dissolved manganese, iron, methane, and sometimes arsenic. Monitoring data on carbon injections and electron acceptors were collected from 33 ERB sites. Two approaches were then used to evaluate carbon longevity and the time required to return to near-oxic conditions at an ERB site. The first method employed a simple mass balance approach, using such input parameters as groundwater velocity, upgradient electron acceptors, and amount of carbon injected. In the second approach, a combined flow, transport and geochemical model was developed using PHT3D to estimate the impact of ERB on secondary water quality impacts (SWQIs; e.g., methane production, iron mobilization and transport, etc.) The model was originally developed for use in estimating SWQIs released from petroleum sites, but has since been modified for use at ERB sites. The ERB site to be studied is a perchlorate release site in Elkton, Maryland where 840 lbs of an emulsified vegetable oil was injected. The results from the simple mass balance approach and PHT3D model will be compared and used to identify conditions where the simplified approach may be appropriate.