|
Mr. Frank Barone • -2 - • 22 February 2006
<br /> Human Health
<br /> In its 18 August letter, Regional Water Board staff requested that MSRM identify suitable
<br /> concentration thresholds for constituents that may pose a health threat to residents in an adjacent
<br /> downgradient development through the vapor inhalation pathway. In Geomatrix' 2001 Human
<br /> Health Risk Assessment, 1,2-dibromo-3-chloropropane (DBCP) and 1,2,3-trichloropropane
<br /> (1,2,3-TCP) contribute about 90 percent of the total risk to residents. Geomatrix proposes to
<br /> compare the concentration of DBCP and 1,2,3-TCP in indicator wells to a threshold value that
<br /> will be protective of the indoor air inhalation exposure pathway of residents overlying the plume.
<br /> Since these two constituents are the primary drivers for human health risk, Geomatrix proposes
<br /> to evaluate the concentrations of these two constituents at the monitoring wells on OxyChem's
<br /> downgradient property boundary (MW-2, MW-5B, and MW-7) and in the proximate off-site
<br /> downgradient monitoring wells (MW-26 and MW-27). Geomatrix proposes threshold values
<br /> from the San Francisco Bay Regional Water Quality Control Board's February 2005 Screening
<br /> for Environmental Concerns at Sites with Contaminated Soil and Groundwater, Interim Final,
<br /> Volume 2 (Environmental Screening Levels) and from USEPA's 2002 Office of Solid Waste and
<br /> Emergency Response Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway
<br /> from Groundwater and Soils (USEPA Vapor Intrusion) and selecting the more stringent of the
<br /> two standards. The proposed threshold for DBCP is 170 ug/L and for 1,2,3-TCP is 1,500 ug/L.
<br /> Regional Water Board staff concur with the strategy of using DBCP and 1,2,3-TCP as indicator
<br /> constituents, but also believe that Geomatrix should include 1,2-dichloropropane (1,2-DCP) as
<br /> an indicator constituent. 1,2-DCP is also mobile,volatile, could pose a health risk through vapor
<br /> inhalation, and is present in groundwater beneath the downgradient development. We concur
<br /> with the DBCP threshold of 170 ug/L, do not concur with the 1,2,3-TCP threshold of 1,500 ug/L,
<br /> and propose a threshold of 280 ug/L for 1,2-DCP. The compound 1,2,3-TCP is not listed in the
<br /> Environmental Screening Levels, but there is another threshold available from the Department of
<br /> Toxic Substance Control (DTSC). The Johnson and Ettinger model as modified by DTSC
<br /> (Vapor Intrusion Guidance Interim Final, December 2004), suggests that a groundwater
<br /> concentration of 15 ug/L at about five feet below ground surface provides an estimated cancer
<br /> risk of 2-in-one million, whereas it estimates that 1,500 ug/L 1,2,3-TCP provides a 2-in-10,000
<br /> risk protection. Therefore,MSRM should use 15 ug/L 1,2,3-TCP as the threshold value. The
<br /> compound 1,2-DCP is listed in the Environmental Screening Levels, which suggests a threshold
<br /> of 280 ug/L as protective of the inhalation pathway for indoor air. The Johnson and Ettinger
<br /> model as modified by DTSC suggests that 280 ug/L of 1,2-DCP in groundwater provides a 4-in-
<br /> one million risk protection. The thresholds for implementation of a contingency plan that
<br /> MSRM needs to use are summarized in Table 1 below.
<br /> Since monitoring wells MW-23, MW-25, MW-28, and MW-29 are also downgradient
<br /> monitoring wells within the residential subdivision that is proximate to OxyChem's
<br /> downgradient boundary, MSRM should add these monitoring wells to the group of monitoring
<br /> wells evaluated for threshold concentrations of DBCP, 1,2,3-TCP, and 1,2-DCP.
<br />
|