In situ anaerobic bioremediation (ISAB) and in situ chemical oxidation (ISCO) can be useful for treating a variety of groundwater contaminants. However, to be effective, the treatment reagent must be brought into close contact with the target contaminant. This project developed a set of tools to assist remediation system designers in developing effective, reasonably efficient systems for distributing aqueous amendments for in situ treatment of contaminants. The project focused on the development and application of tools for the design of ISAB systems using soluble substrates (SS) and emulsified vegetable oil (EVO) and ISCO systems using permanganate (MnO4-).
In the first phase of work, currently available numerical models were used to understand the effects of site conditions (e.g., permeability (K), site heterogeneity) and design variables (e.g., location of wells, injection rates, volumes, amount of reagent, etc.) on reagent contact efficiency (CE). Results from 3-dimensional numerical simulations in heterogeneous aquifers were used to develop relationships between reagent distribution and amount of fluid/reagent injected. This information was then used to develop simple, spreadsheet-based design tools to assist in planning injection systems for in situ aquifer treatment with EVO, SS, and MnO4-. Using these tools, designers can evaluate the effect of different design variables (e.g., well spacing, amount of reagent, injection volume, etc.) on remediation system cost and expected performance.
Anaerobic Bioremediation with Emulsified Vegetable Oil
Sensitivity analysis results showed that aquifer volume CE is primarily controlled by the volume of EVO injected and the volume of water injected to distribute the EVO. Simple curves were developed to estimate CE and then incorporated into spreadsheet-based tools to compare the cost and performance of different designs. Capital and life-cycle costs appear to be relatively insensitive to site conditions, for both barrier and area treatment. Total costs are often higher for large, wide, deep sites. Unit costs are often higher for smaller sites due to the proportionately higher fixed costs associated with planning, design, and monitoring. Errors in estimation of the maximum oil retention (ORM) by the aquifer material can have a major impact on cost and performance for both barrier and area treatment. ORM should be directly measured on field or laboratory samples whenever possible.
Anaerobic Bioremediation with Soluble Substrates
Anaerobic bioremediation with SS is most effective when the substrate concentration is greater than some minimum concentration (Cmin). However, CE will vary over time as substrate is depleted or washed out of the target treatment zone and as additional substrate is injected. To account for these variations in time, the steady-state, volume average contact efficiency (CESS) was used as the primary measure of SS injection performance. Sensitivity analysis results showed that CESS greater than 50% can be achieved by periodic injection of 0.1 to 0.25 pore volumes of SS solution containing 20 to 100 times the Cmin required for effective treatment. Operating costs can be reduced by injecting substrate less frequently, but can result in a dramatic drop in CESS. Results from these sensitivity analyses were incorporated into a spreadsheet-based tool to allow designers to compare different designs, and used to evaluate costs and performance for a range of site conditions. In most cases, increasing CESS also results in increasing costs. The highest ratio of CESS to cost often occurs when CESS is between 70% and 80%.
Chemical Oxidation with Permanganate
MnO4- transport and distribution in the subsurface is controlled by reactions with the target contaminant and non-productive reactions with Instantaneous Natural Oxidant Demand (NODI) and Slow Natural Oxidant Demand (NODS). Model simulations indicate that the two parameters with the greatest impact on aquifer CE are: (1) the mass of MnO4- injected; and (2) the volume of water injected. When small amounts of MnO4- are injected, the reagent is rapidly consumed and pollutant removal efficiency does not increase with time after the first 30 days. However, when larger amounts of MnO4- are injected, the reagent can persist for several months resulting in a gradual increase in CE with time.
For constant MnO4- mass, increasing fluid volume injected initially results in improved treatment efficiency. However, further increases in the fluid volume injected result in little additional benefit. Conversely, when fluid volume is held constant and MnO4- mass is increased, treatment efficiency steadily increases, due to downgradient drift/dispersion of MnO4-. However, increasing the mass of MnO4- injected may also increase the amount of MnO4- that migrates out of the target treatment zone. The Conceptual Design of In Situ Chemical Oxidation (CDISCO) design tool was developed within a Microsoft Excel spreadsheet to allow users to design MnO4- injection systems based on a user-specified minimum oxidant concentration and contact time, and evaluate the effect of different alternatives on costs.
The tools developed in this project are intended to help remediation system designers improve reagent distribution at a reasonable cost. However, designers should be aware of the numerous other factors that can lead to poor treatment including improper reagent selection, unfavorable environmental conditions, absence of required chemical catalysts or bacteria to facilitate the reaction, and chemical or biological inhibitors. Contaminant treatment efficiency will also be lower if the contaminants are primarily located in lower K zones that are not effectively contacted by the treatment reagent. Nonetheless, improving the fraction of aquifer contacted by the reagent should improve contaminant treatment efficiency at many sites.
Users should also be aware that these design tools are strictly focused on improving reagent distribution. In virtually all cases, the tools predict that CE can be increased by injecting more reagent with more water to distribute the reagent. While this should improve reagent distribution, it may also increase the potential for adverse secondary impacts, possibly including displacement of the contaminant and changes in aquifer geochemistry. These potential impacts need to be carefully considered before implementing any in situ remediation approach.