SITE INFORMATION AND CORRESPONDENCE

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r _ Received: 11 / 9/99 11 :24AM; 616 227 4349 -y COAA/SJOA EPU; Page 3 - 2iov-09-1999 10: 19 SWRCB/CLEAN WATER PRGMS 916 227 4349 P.03i35 from the 85-90 foot depth. It.is our opinion that the low levels of MTBE that have been sporadically detected in groundwater from some of the Site shallow wells are consistent with atmospheric washout. • The lateral extent of soil impacts has been identified.Hydrocarbon impacts in the soil appear to be limited to the area around the former product islands. 0 The lateral extent of groundwater impacts has been identified and is constrained by data from the monitoring well array. The downgradient extent of impacts to groundwater by TPHg is less than 100 ft from the Site property line. The downgxadient extent of Site benzene impacts to groundwater at concentrations greater than I ug/L are less than 80 ft. from the Site property line. • Chemical analytic data from Site soil sampling indicate that the vertical extent of soil impacts at the Site have been identified and do not extend below 70 ft bgs. Concentrations of TPHg and benzene are significantly attenuated below 50 to 60 ft bgs. THE PLUME IS STABLE AND iSELF REMEDIATING • An examination of 9 years continuous monitoring data for the site indicates that the groundwater plume is stable and has not substantially moved. Exhibits A and B illustrate the distribution of benzene in Site,shallow groundwater in May, 1990 and February, 1999. As shown, the maxunum extent of benzene in shallow groundwater has remained essentially unchanged over 9 years. Analysis of well elevation data indicates that there is an upward gradient at the Site. 'Pais calculation is based on quarterly monitoring data (3 quarters) collected after allowing the new well cluster to stabilize and utilization of current (February, 1999) well survey data which accounts for land subsidence which has occurred in the area. Concentrations of petroleum hydrocarbons in groundwater have declined significantly over the 11-year period of the monitoring record. • Multiple lines of biogeochemical data (including dissolved oxygen, nitrates, holo (II), sulfates, and dissolved methane data)presents strong evidence to indicate that the groundwater plume is undergoing microbially mediated aerobic and anaerobic biodegradation- As would be expected in a microbially active system, Exlu'bit C illustrates that nitrate is being depleted near the core of the plume as it is used as an ahernate electron acceptor to facffitate microbial degradation,of the Site petroleum hydrocarbons. Exhibit D illustrates that the concentrations of soluble ferrous iron are elevated near the core of the plume as ferric iron is reduced by microbially mediated biodegradation of Site petroleum hydrocarbons. I It is our opinion that the petrol=a hydrocarbon plume at the Site is adequately characterized to understand the lateral and vertical extent of impact. The groundwater impacts have been monitored for over 11 years. The data indicate that the plume is impactti ag a very limited volume of soil and groundwater. Site data also indicate that the plume is stable (not moving or expanding) and that the plume is self-remediating through the mechanisms of natural attenuation and biodegradation. Itis our opinion that water quality objectives Pecs: 2/3