Human and Ecological Risk Assessment - Earthjustice

Section 3.0Analysisflow is greater than the available groundwater flow, then all of the mass available in thegroundwater is assumed to be transferred to the surface waterbody. It is important to note thatwhile a mass transfer is assumed to take place between the two systems, mass is not actuallyremoved from the groundwater—it is still available to be observed at a receptor well placedbeyond the groundwater-surface water interface.To ensure that an unrealistic transfer of mass from the contaminated groundwater into thesurface waterbody does not occur, the available groundwater flow is compared to the streamflow. If the groundwater flow exceeds the stream flow, all of the stream flow is assumed to befrom groundwater discharge and the total concentration in the stream is equal to the groundwaterconcentration.The waterbody considered in the CCW risk assessment is a river, stream, or lake locateddowngradient of the WMU. As described in Appendix C, the flow characteristics anddimensions for this waterbody were determined by site-specific stream flow data, the width ofthe groundwater contaminant plume as it intersects the waterbody, and established relationshipsbetween flow and stream depth. The stream segment modeled in this assessment was assumed tobe homogeneously mixed.Simple equilibrium partitioning models were used to estimate contaminant concentrationsin the water column, suspended and bed sediments (see Section 3.7.1), and aquatic organisms(see Section 3.7.2). Special modeling provisions for aluminum are described in Section 3.7.3.3.7.1 Equilibrium Partitioning ModelThe primary surface water model used to estimate groundwater impacts on waterbodies isa simple steady-state equilibrium-partitioning model adapted from models in EPA’s IndirectExposure Methodology (IEM; U.S. EPA, 1998c) and Human Health Risk Assessment Protocol(HHRAP; U.S. EPA, 1998d). This model is based on the concept that dissolved and sorbedconcentrations can be related through equilibrium partitioning coefficients. This model was usedfor all constituents except aluminum, which was modeled based on a solubility approach (seeSection 3.7.3). Although these models have not been specifically peer reviewed in thisapplication, they have been subject to the Agency’s peer review process as part of thedevelopment of the IEM and HHRAP.The model partitions the total mass of chemical contaminant in the waterbody into fourcompartments:• Constituents dissolved in the water column• Constituents sorbed onto suspended solids• Constituents sorbed onto sediment particles at the bottom of the waterbody• Constituents dissolved in porewater in the sediment layer.Table 3-9 provides the partitioning coefficients used by the surface water model to estimatecontaminant partitioning between water and suspended solids in the water column and betweenApril 2010–Draft EPA document. 3-37