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APPENDIX A MODEL PARAMETER IMPACT SENSITIVITY ANALYSIS Three parameters have the greatest impact on groundwater contamination They are organic carbon partition coefficient (kms), infiltration velocity or recharge rate (q), and the fraction organic carbon content (fes) The k,,c for the chemicals of concern in this project have been obtained from published sources Recharge rate and f,,, are site-specific Recharge rate is mainly controlled by annual precipitation, surface coverage, and existence of impermeable bamer layers in the soil column, whereas fa varies with sedimentary facies (Figure A-1) A.1 Sensitivity to Organic Carbon Partition Coefficient The partition coefficients (ka) for xylenes, ethylbenzene, and naphthalene are well known and widely accepted in technical circles They are provided in the VLEACH Manual Because 1 diesel is a mixture and, therefore, represented by the physio-chemical parameters for each discrete compound in its mixture, one specific organic carbon compound was selected for modeling purposes We used modified napthelene properties as described by personnel at the CRWQCB A.2 Sensitivity to Recharge Rate The mean annual precipitation in Stockton is 14 inches. Location I is completely beneath a building and Location H is beneath concrete and asphalt The ground surface is covered at both sites We examined contaminant mass loading to groundwater at various recharge rates The result at 14 inches per year represents the worst case scenario which only occur if locations I and II were completely exposed to rainfall and there were no impermeable layers within the soil column. The modeling result for the recharge rate of 2 inches per year corresponds to a situation when most precipitation is diverted as run-off through drainage systems In this case, the subsurface is least affected from infiltration The model results with recharge rates of 4 and 8 inches per year correspond to intermediate situations when partial infiltration occurs The results of the sensitivity analysis for recharge rates are shown in Figures A-2 through A- 5 Results indicate that the higher the recharge rate, the higher the contaminant mass loading spike will be, and the earlier the spike will occur Our conclusions are based on the results in Table 5 which were obtained under the assumption of full recharge rate (i.e , infiltration) MZ/04-06-95/UST/95-0089 APP 1 i