Human and Ecological Risk Assessment - Earthjustice

Appendix LTime to Peak Concentration at Receptor WellAppendix L. Time to Peak Concentration at Receptor Wellfor Selected CCW ConstituentsL.1 IntroductionThis appendix presents plots of arrival times for the peak well concentrations used tocalculate groundwater-to-drinking-water risks for selected CCW constituents (arsenic III and V,boron, cobalt, selenium IV and VI, and thallium 1 ). The arrival times are plotted as cumulativedistributions for surface impoundments and landfills. These constituents were selected torepresent the chemicals with the highest estimated risks and to span the range of mobility in thesubsurface.Groundwater pathway modeling conducted in support of the CCW risk assessmentconsisted of probabilistic fate and transport simulations of mostly metal constituents present inthree different waste types (ash, ash and coal, and fluidized bed combustion wastes) managed inlandfills and surface impoundments. Three liner designs were also considered: no liner; a 3-footclay liner; and a composite liner (a composite of geomembrane, geosynthetic clays, and/orcompacted clays), assigned to each CCW waste management unit (WMU) based on liner typedata in the EPRI database (see Appendix B). The predicted groundwater concentrations wereused to estimate potential risks to humans and the environment exposed to the modeled CCWconstituents.Among the inputs to the model were distributions of infiltration rates of water through thelandfills and surface impoundments corresponding to each of the three liner types. Among theoutputs generated by the groundwater pathway fate and transport model were the peakconcentration observed at the receptor well and the time at which the peak was observed. Foreach probabilistic simulation scenario (a constituent in a particular waste type managed in aparticular type of WMU), approximately 10,000 sets of model inputs generated an equivalentnumber of groundwater observations. Some were non-zero concentrations, others were zero. Forthese zero-value observations, the model also assigned a value of zero to arrival time. Zero-valueobservations can be attributed to zero-value infiltration rates (which occur only for WMUs withcomposite liners); in that case, no mass leaves the WMU and there is no time of travel. Zerovalueobservations can also be attributed to fate and transport conditions that retard themovement of a constituent from the WMU through the subsurface to the extent that the dissolvedcomponent was not observed within the established maximum allowable timeframe (10,000years). In this case, the time of travel is greater than 10,000 years.To better understand the time frames in which risks associated with exposures tocontaminated groundwater may occur, an analysis was performed to graphically representdistributions of arrival time of the peak groundwater concentrations at the nearby drinking waterwell. The analysis was performed across all waste types with respect to liner and WMU type.1 Thallium was not modeled in the surface impoundment scenario, and thus no arrival times were calculated here.April 2010–Draft EPA document. L-1