Dames & Moore, 1999 - USDA Forest Service

preliminary report 1997). Sample locations are shown on Figure 5.4-2. Analytical data are summarized
in Table 5.4-4. These studies identified the general character of the seeps and delineated two distinct seep
chemistries: (1) acidic to highly acidic, iron-dominant seeps from the base of the tailings piles, and (2)
chemically discriminate seeps of low iron concentrations and high zinc and copper concentrations
(Kilburn and Sutley, 1996). In general, water chemistry results for seeps sampled by the USGS are
comparable to data collected during the RI.
5.4.2.2 Background Groundwater Quality
As stated previously in Section 5.1.3, evaluation of representative background groundwater quality at the
Site is a difficult component to assess due to the relatively complex hydrogeologic conditions.
Groundwater occurs in both bedrock and the overlying glacial soil and alluvium. Groundwater movement
within the bedrock in the watershed is anticipated to be limited to fmctures in the bedrock. The intent of
the RI was to collect background data from wells and seeps located outside of the observed influence of
the Holden Mine. Samples were collected from monitoring wells HV-3 located in Holden Village,
HBKG-1, and HBKG-2 during May and September 1997 to assess background groundwater quality.
Two seeps were sampled to assess background, SP-26 in July and September 1997 and SP-27 in May
1998.
Monitoring well HV-3 (H-3) is located on the north side of Railroad Creek and topographically above
Holden Village (Figure 5.4-1). The well monitors either glacial till or colluvium. Metal concentrations
detected in samples collected from well HV-3 in May and September 1997 were below MTCA Method A
and B levels. Beryllium was not detected in the samples, but the detection limits (0.2 to 1 Cc/L) reported
were above the MTCA Method B level (0.0203 pg/L). Metal concentrations did not appear to fluctuate
seasonally with the exception of potassium and sodium, which decreased from the spring to the fall. The
pH was neutral (6.6 and 6.5 SU).
Well HBKG-1 was originally installed as a background' well, however, its location is directly
downgradient of the maintenance yard and water chemistry indicates that the groundwater at this location
is affected. This well was not considered further for background groundwater quality assessment. The
data collected from this well is discussed in the maintenance yard section.
Monitoring well HBKG-2 is located to the east of the ball field and campground, adjacent to a road that
historically crossed Railroad Creek during the operation of the Holden Mine. However, it has been
reported that waste rock was used as fill material to construct the road. Therefore, groundwater samples
collected from this well may not be representative of background water chemistry. Cadmium, copper,
and zinc concentrations were below MTCA levels; however, in May, the concentrations were elevated 3
to 15 times above those detected in HV-3 indicating that the groundwater collected at HBKG-2 may be
affected. Concentrations were similar in September 1997. The data from HBKG-2 was not considered
further for assessment of background groundwater quality.
A reconnaissance was perfom'ed on the south side of Railroad Creek upvalley between the Site and Big
Creek to locate seeps that may be indicative of groundwater quality in the area unaffected by mining
activities at the Holden Mine. Two seeps, SP-26 and SP-27, were identified to assess background
groundwater quality. SP-26 is located adjacent to Railroad Creek near sampling station RC-6, west of the
Site (Figure 5.4-1). Samples were collected from seep SP-26 in July 1997 and September 1997. A
DAMES & MOORE