Laserfiche WebLink
,�Rensro� UNITED STATES ENVIRONMENTAL PROTECTION AGENCY <br /> sg <br /> Region 9 75 Hawthorne Street +* <br /> San Francisco CA 94105-3901 <br /> December 14, 1993 <br /> s -r_' <br /> Marshall Cloud r <br /> Environmental Protection Office tv <br /> n <br /> DDRW-Tracy =+ <br /> P.O. Box 960001 w <br /> crpcvt9nA, C 95296.0250 <br /> RE: Comments on Montgomery Watson's (MW) Groundwater Modeling Interim Technical <br /> Memorandum for DDRW-Tracy dated October 1993 <br /> Dear Marshall, <br /> This letter transmits EPA's comments on the above referenced Technical Memorandum. In <br /> general, EPA agrees with the approach presented in this memorandum. However, we have the <br /> following comments. <br /> 1. This document describes the uppermost geologic horizon for modeling as the hydraulically <br /> conductive Upper Horizon of the Upper Tulare Formation. However, the siltier saturated layer <br /> designated as "above Upper Horizon" by both Woodward-Clyde Consultants (WCC) and by <br /> MW should be considered the shallowest model layer. At first thought, the Upper Horizon <br /> would appear to be adequate as the shallowest model layer. However, the fact is that the <br /> wells most contaminated with tetrachloroethene (PCE) or trichloroethene (TCE) or both <br /> (monitoring wells LM25A [TCE=197 µg/L], LM30A [PCE=251 µg/L, TCE=34 pg/L], and <br /> LM32A [PCE=153 pg/L, TCE=55 µg/L] from MW's April 1993 monitoring round) are <br /> screened partially or entirely within the finer-grained sediments above the Upper Horizon. <br /> These Ilner-gra [ic Sedia7i�mt tend 4-4, Co^Stiwents such ac PCE and other volatile and <br /> non-volatile contaminants to a greater extent than the less adsorptive sediments in the Upper <br /> Horizon. The geometric mean of the hydraulic conductivities for those "above Upper <br /> Horizon" sediments was about 2.6 feet per day, as determined in Section 3 of WCC's <br /> November 1992 Final Operable Unit No. 1 Remedial Investigation/Baseline Risk Assessment <br /> Report DDRW-Tracy, California. The potentiometric surface for most of the wells depicted in <br /> the cross-sectional representations in Figures 7 and 8 lies within those finer-grained sediments. <br /> [Note: The text of this MW report also states that the finer-grained sediments are not barriers <br /> to groundwater flow. But TCE, PCE and other contaminants such as organo-chlorine <br /> pesticides and trace metals tend to be concentrated in these above Upper Horizon sediments, <br /> such as in wells LM15A and LM103A. Can a hydrologic model, as currently proposed, <br /> adequately account for transport of the greater masses of PCE and TCE and, probably, other <br /> 1 <br />