Human and Ecological Risk Assessment - Earthjustice

Appendix LTime to Peak Concentration at Receptor WellWhat follows is a description of how the peak concentrations and their arrival times were treatedto create the plots presented in this appendix, including the treatment of zero-value observations.L.2 MethodologyGiven a constituent managed in a particular type of WMU (e.g., arsenic in landfills), allinfiltration rates and their corresponding peak concentrations and arrival times predicted by themodel were extracted from the input/output data for simulations across all waste types in whichthe selected constituent was found. The triplets of data needed to prepare the graphs—infiltrationrate, peak concentration, and arrival time—were then filtered from the data and segregated byliner type. Zero-value observed concentrations were treated in the following manner:• Zero-value observations corresponding to zero-value infiltration rates were assigned anarbitrary value of -1, effectively excluding those data from the graphs. This wasappropriate, because when infiltration is zero, there is no plume and no contaminantsenter or are transported in groundwater. Only the composite liner scenarios producedzero-value infiltration rates.• Zero-value observations corresponding to non-zero infiltration rates were assigned anarbitrary arrival time greater than (>) 10,000 years, the maximum simulation timeframe.These data points are also not shown in the plots, as only times up to 10,000 years werevisible.Table L-1 shows the distribution of zero-value concentration observations by WMU andcomposite liner scenario. The total observations in this table include data points with a modeledarrival time of >10,000 years and those with zero infiltration rates. Note that for surfaceimpoundments, there are fewer model runs (observations) for thallium because thallium resultsare not available for ash and coal waste streams because of very limited data in the CCWconstituent database.Table L-1. Distribution of Zero-Value ConcentrationsTotalObservations a(Number)ZeroInfiltrationRates(Number)ZeroInfiltrationRates b(%)CompositeLinerObservations(Number)Composite LinerZero InfiltrationRates(%)WMU TypeLandfill 29,717 3,538 11.9% 4,847 73.0%Surface Impoundment 19,825 500 2.5% 1,406 35.6%(As, B, Se) cSurface Impoundment 9,905 389 3.9% 1,130 34.4%(Tl) da Per constituent across all waste types and liners.b Out of all observations.c Observations for arsenic, boron, cobalt, and selenium; all were modeled in both ash and ash and coal wastestreams managed in surface impoundments.d Observations for thallium only, which was detected only in ash waste streams managed in surfaceimpoundments.After zero infiltration rate observations were filtered from each data set, percentiles ofarrival time of the peak observed concentration were plotted on the y-axis by liner type andWMU (Figures L-1 through L-21). The x-axis range for landfills is 0 to 10,000 years. ForApril 2010–Draft EPA document. L-2