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1 <br /> Working To Restore Nature <br /> 66.44 Dissolved Hydrocarbons in Groundwater <br /> Subjective analyses of groundwater in site monitoring wells indicated that no wells exhibited any <br /> ' free floating hydrocarbons during the last monitoring in April, 1994. However, groundwater <br /> from wells MW3 and MW5 exhibited gasoline odors at that time. Free floating petroleum <br /> product was observed in groundwater in well MW5 in December, 1990 and a product sheen was <br /> noted as recently as December 1993. <br /> Current analytical results indicate that TPHg and benzene was detected in four of the eight <br /> monitoring wells No TPHg or benzene was detected in wells MW4, MW6, MW7, or MW8. <br /> Benzene concentrations detected in wells MW 1 and MW2 were marginally above the detection <br /> limit of 0.5 parts per billion (ppb) Isoconcentration maps showing the extent of dissolved <br /> benzene and TPHg in April 1994 are shown on Figures 7 and 8, respectively. Histoncal <br /> ' groundwater analytical results are presented in Table 2 <br /> Based upon the extent of dissolved petroleum hydrocarbons in groundwater beneath the site and <br /> assuming a soil porosity of 20% within the saturated zone, the volumes and masses of <br /> ' contaminants beneath the site were calculated to be approximately 2.5 gallons (16 2 pounds) of <br /> ' TPHg and 0 15 gallons (1.1 pounds) of benzene. Standard calculation sheets are included in <br /> Appendix III. <br /> 1 <br /> �5 <br /> Toxicity, Persistence and Potential for Contaminant Mi ration � <br /> ' The toxicity of residual contaminants at the site can be discussed in terms of occupational <br /> exposure limits established by National Institute for Occupational Safety and Health (NIOSH). <br /> ' A short term exposure limit (STEL) is designated by a 15-minute time-weighted average (TWA) <br /> exposure in air that should not be exceeded at any time during a workday. Immediately <br /> ' dangerous to life or health concentrations (IDLHs) represent the maximum concentration in air <br /> from which, in the event of respirator failure, one could escape within 30 minutes without a <br /> ' respirator and without experiencing any escape-impainng or irreversible health effects. Benzene <br /> has been classified by the American Conference of Governmental Industrial Hygienists (ACGIH) <br /> 150098UMPORTSWAP 0744 DRF 14 <br /> 1 <br />