Successful PRBs involve more than just on-site performance. Effective PRB remedies rely on an extensive design and build process that includes collecting subsurface data and utilizing laboratory analysis.
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.
Once the remedy is designed, bench scale treatability tests confirm if it will be effective at the 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.