GeoSierra Environmental's expertise in design and construction of successful installation of PRBs extends well beyond on-site performance. Our effectiveness in laboratory analysis, state of the art office facilities, custom PRB construction equipment, and specialized workshop, all complement our full line of services to our clients.
Each PRB design activity requires additional data beyond conventional site characterization; namely, column reactivity data and iron permeability design data. These data are generated from laboratory tests conducted from site groundwater and soils. Laboratory column tests quantify the degradation reaction rates and pathways (daughter products) of the particular contaminant specie in the presence of iron filings. These tests also address additional issues such as potential precipitation and clogging of the reactive barrier or potential passivation based on the geochemistry from the site groundwater. The design methodology for each PRB incorporates a probabilistic fate and transport model for VOC natural attenuation downgradient of the PRB. Probabilistic distributions for the design input parameters (formation hydraulic conductivity, groundwater flow gradient, VOC concentrations, VOC degradation half lives, iron PRB porosity and iron PRB effective thickness) are developed, resulting in computed probabilistic distributions for PRB effluent VOC concentrations, while taking into consideration the lifetime expectancy of the PRB.
GeoSierra Environmental also conducts specialized hydrogeological testing utilizing hydraulic pulse interference to confirm groundwater flow velocities for PRB design. Additional testing that can also be conducted includes subsurface imaging and tracer studies for contaminant investigation or for QA/QC of PRB construction.
Column Treatability Tests & Design
Bench scale treatability tests determine the applicability of a particular reactive treatment system for a site. Laboratory column iron reactivity tests are performed on site groundwater to quantify the degradation rates of volatile organic compounds or metals in the presence of iron. The column test also identifies any potential for precipitation or clogging of the iron as a treatment medium or passivation due to existing groundwater geochemistry and provides design data for the final design of an iron PRB. A typical column treatability test requires approximately five gallons or twenty liters of site groundwater and five kilograms or ten pounds to be shipped to the laboratory.
Important Considerations and Limitations of PRBs
When designing a PRB, it is critical to evaluate ALL site data including the hydraulic properties of the site surrounding the PRB and any additional geochemistry (i.e., nitrates, sulfates, etc.) Other considerations include the location of the PRB and the locations of monitoring wells used to evaluate performance. Installation of a PRB at the downgradient edge of a contaminant plume versus mid-plume will provide significantly different results and expectations on treatment of the plume. Battelle has evaluated several PRB sites for longevity and hydraulic performance and have concluded that mid-plume PRBs often require five years before they experience a stable, clean front that will adequately assess the PRB's performance (Battelle, October 2001). Therefore, it is up to the responsibility of the engineering environmental consultant to ensure that the environmental contractor has used sound industry standards in their design and installation methods in accordance with the methods and practices outlined by the USEPA and additional guidance documents.
Although PRBs have been proven over the last 20 years to be a valuable in-situ method of groundwater treatment, their overall success requires an accurate conceptual site model (CSM) to be developed. Treatment of groundwater through a ZVI PRB is largely the result of direct contact with flowing groundwater, although the addition of highly reactive ZVI into the groundwater also creates strong reductive zones capable of supplemental reduction without direct contact. Groundwater concentration reductions downgradient are the result of a clean water front flowing downgradient. PRBs that are installed mid-plume do not treat downgradient sorbed mass that may exist due to previously flowing contaminant concentrations through various lithological units. Silts and clays may desorb mass at a greater rate than a PRB's clean water front may be yielding within a mixing zone local to where desorption may be occurring. This is a primary challenge in evaluating PRB performance short term (less than two years) and is dependent on many factors. Other major considerations when selected a PRB approach, particularly one that uses ZVI as a reactive media, is the quality of data used in development of the CSM. Many changes to site specific parameters will affect the performance of the PRB including groundwater velocities, gradients, groundwater elevational increases whereby untreated water flows above the PRB, concentration changes, angles of entry through the PRB, changes in groundwater geochemistry, presence of possible ZVI passivators such as nitrates and biofouling, groundwater temperatures, and a host of other factors. It is critical that the consultant proposing the use of a PRB understand these variables, as the design and installation relies on data that has been collected by others and essentially installing media where requested. The ultimate success of these projects depends on the quality of the CSM data collection, and whether careful consideration and knowledge has been shared to minimize the possible negative impacts with changes to any of the above parameters. Projects often include factors of safety to offset undesirable results but large variations in select parameters can result in significant PRB design changes.
As the saying goes in the environmental industry, the performance of your remedial action is only as good as the data provided!