ARCHIVED REPORTS XR0000211 - Laserfiche WebLink

Home Browse Search

SECTICMSEVEN aneenalnues and KIMIteti®ns use Tlus type of sampling approach is likely to overestimate the chemical concentrations to which a receptor would be exposed and the potential health impact to the receptors evaluated Anal ' Samples were analyzed using California State Certified Laboratory procedures and were subjected to limited review, to obtain data surtable for decision-making However, it should be understood that sample analysis is subject to uncertainties associated with precision, accuracy, and detection of chemicals at low concentrations Analytical precision and accuracy are evaluated through laboratory QA programs Uncertainties associated with precision and accuracy of analysis are generally random errors which may lead to over- or underestimation of risks These errors are typically of low magnitude (well below an order of magnitude) compared to other sources of uncertainty in the risk assessment Due to the hrruts of analytical methodologies and the complexity of matrixes for environmental samples, some chemicals present in low concentrations in samples may not be detected, leading to a possible underestimation of risk This potential source of uncertainty is addressed during data evaluation, as described below 7.11 Data Evaluation The 95%UCL concentrations of chemicals detected in each medium were complied for use in the risk assessment It was assumed that a chemical not detected in a given sample was actually present at one- half of its detection limit if that chemical was present in any sample in a given media This assumption is probably conservative and may lead to overestimation of risk, particularly for those chemicals reported with low frequency and low concentrations 7.2 CHEMICAL FATE AM®TRANSPORT MODELING UNCERTAINTIES ' The models used to estimate the fate and transport of chemicals for this risk assessment are consistent with those recommended by EPA in its Superfund Exposure Assessment Manual (EPA 1988) and ASTM RBCA (1995) Because of uncertainties in modeling, EPA-recommended models and input parameters are conservative and produce exposure concentrations that should not underestimate the health risks Assumptions of the modeling are discussed in Section 3 0 Modeling assumptions were conservative and were designed to be protective of groundwater and human health Conservative assumptions include degradation half lives of 100,000 days (typical half lives for BTEX are 8,500 days for benzene, 230 days for ethylbenzene, 28 days for toluene, and 365 i T 11987%973000NA%STOCKTOMREPORTISHOREP#1 D0G18-FEB4XWr-4008NA%SNA 7-2