Dames & Moore, 1999 - USDA Forest Service

groundwater levels.are highest in the deep wells beneath the tailings, and high flow occurs at the 1500-
level portal drainage. During the May round when flows are the highest, the .portal drainage is the
primary source of loading of cadmium copper and zinc to Railroad Creek. Seeps SP-23 and SP-23~are
the two next highest point sources that are estimated to contribute cadmium copper and zinc during May;
however, this load drops to zero later in the year as seep SP-23 dries up.
Iron enters Railroad Creek primarily by groundwater associated with the tailings piles. Iron loads are
greater in September than May. Iron loads enter Railroad Creek downstream of the load sources (i.e..
portal drainage) for cadmium, zinc and copper, which enter the creek as surface flows or seeps.
Additional source areas located at the West and East waste rock piles and the mill area are not significant
loading sources to Railroad Creek. Metals loading at SP-21 is insignificant but may account for the
unaccounted loads noted in September.
8.4.5.2 Sediment
Railroad Creek was generally found to be relatively sediment poor due to the relatively steep gradient and
the occurrence of storm events which transport the natural sediment downstream to Lake Chelan. The
natural sediment is being generated by the stream erosion processes of ~ai'lroad Creek.
The tailings piles generate iron-oxyhydroxide precipitate which forms due to diffuse groundwater flow from
the tailings into the stream substrate. In addition, wind and precipitation-related erosion of the tailings pile
slopes results in a relatively minor amount of tailings materials being delivered into Railroad Creek. Both
the precipitate and tailings, in addition to the native sediments. are transported downstream both by normal
streamflow and higher energy streamflows during storm events.
8.5 RISK CHARACTERIZATION
The following discussion summarizes the findings of the human health and ecological risk assessments. A
detailed discussion is presented in Section 7.0 of this report.
8,S.l Human Health
The combined results of the screening level human health assessment and the site-specific human health risk
assessment indicate that the environmental conditions at the Site and Holden Village do not pose an
unacceptable risk to potentially exposed populations, i.e., residents, recreational users of the Site, and USFS
personnel. An evaluation of the cumulative risks for each potentially exposed population also indicates that
cumulative cancer and noncancer risks are below MTCA allowable cumulative risk levels. These
conclusions are based upon very conservative screening criteria and site-specific assumptions which, for all
practical purposes, overestimate the risk posed by the Site.
8.5.2 Ecological
The results of the Ecological Risk Assessment are summarized below. Hazard quotients are presented for
each of the receptors. HQs less than 10 are considered low or no risk, HQs greater than 10 and less the 100
are considered intermediate risk, and HQs greater than 100 are considered high risk.
17693-005-019Uuly 28, 1999;10:24 AM;DRAFT FMAL RI REPORT