Dames & Moore, 1999 - USDA Forest Service

and the northwest comer of tailings pile 2 (Lambeth, 1998). Figures 4.3-3c and 4.3-3d display the results of
the mapping.
4.3.9.2 RI Findings
The physical and chemical characteristics of the interstitial iron-oxide precipitates found in a cemented to
partially cemented condition (ferricrete) adjacent to Railroad Creek were evaluated as part of the RI.
Test Pit Excavations
Four backhoe excavated test pits were completed as part of the ferricrete assessment (Figure 4.2-6b). The
test pits were excavated within the highwater channel or wetland floodplain of Railroad Creek. The test pits
were generally 3 to 4 ft wide and from 3.5 to 7.0 ft deep. Test pit logs are found in Appendix C. Water was
encountered in all of the test pits.
Based on the test pits and the visual inspection of ferricrete exposures along the south bank of Railroad
Creek, ferricrete varied considerably in thickness, hardness, degree of cementation, texture, grain size
distribution, sorting and color. In places (i.e., along the south bank of Railroad Creek adjacent to tailings
pile 1) ferricrete was observed as a non-sorted conglomerate (i.e., grain supported) or diamictite (i.e., matrix
supported) of angular and rounded cobbles and wood pieces (i.e., timber) in a matrix of well indurated rusty
red-brown cement. In other places (in the wetland area just east of tailings pile 3), interstitial iron-oxide
precipitate was observed and consisted of well-sorted, laminated to cross-bedded sands and pebbles (with
very little fines) cemented weakly by a film of iron oxide.
Although predominantly cemented by fenic oxide as indicated by the red, brown and rust colors, other
colors such as green, blue-green, yellow and white (especially in the vicinity of DMTPIE-1) indicated that
ferric, copper and aluminum precipitates could also be present (refer to Section 5 for a discussion of the
chemistry of the ferricrete). The ferricrete exposed in DMTPIE-1 was observed to contain lenses of
uncemented light yellow to orange-brown sands and silts, and pockets of dark gray to blue gray clay.
During the field reconnaissances, the thickness and distribution of ferricrete was observed to be highly
variable. It is reported that the USFS attempted to remove andfor break up the ferricrete during Site
rehabilitation efforts between 1989 and 199 1 (PNL, 1992). Referring to Figure 4.3-3d, field observations at
RC-9 indicated that a cemented vertical ledge greater than 4 feet thick formed the south bank of the channel.
This ledge was observed to be almost continuous (although the thickness varied) along the south bank
adjacent to tailings pile I from RC-9 to just upstream of the confluence with Copper Creek (see stream
reach 1-E on Figure 4.2-21 and Figure F-4 in Appendix F which displays a schematic cross-section for reach
1-E). The presence of relatively mature trees adjacent to the ledge suggests that the ferricrete has been
present in the area of tailings pile 1 for a relatively long period of time.
The relatively hard, thick and indurated ferricrete was not observed downstream of the Copper Creek
.confluence. However, iron-staining was observed along the entire Railroad Creek channel from midway of
tailings pile 1 to downstream of tailings pile 3. In addition, the channel deposits observed in the test pits
east of tailings pile 3 had zones of staining and relatively weak to moderate cementation.
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17693-005-0 19Uuly 19. 1999;4:5 1 PM;DRAR FINAL RI REPORT