Dames & Moore, 1999 - USDA Forest Service

not originating h m the underground mine. As noted later in this section, no other unaccounted sources of
metals loading are noted in Railroad Creek between RC-6 (upstream of Seep SP-26) and RC-4 (downstream
sampling station). .
In July and August, influx of water into the mine is assumed to be, on average, significantly less than
during the snowmelt period. By fall, the influx of water into the mine from upslope is assumed to be
entirely from rainfall. As these months experience lower precipitation, overland flow through the 800-
and 1100-level waste rock piles is significantly reduced. Seeps downslope from Honeymoon Heights
were noted to flow only in response to rainfall events during the fall 1997 field program.
Underground Mine Water Chemistry
Comparison of seepage chemistry from the Honeymoon Heights drainage area suggests that the upslope
waters, represented by seep SP- 14 (which includes SP-14, SP- 14 Lower, and SP- 14 Upper), have been only
moderately influenced by the mine or natural occurrences of mineralization. Metals and sulfate
concentrations for seep SP-14 appear to be chemically comparable to other waters at the Site, but with
significant dilution (pH: 4.5 to 6.1, Cu: a to 1410 pgL, Zn:5 to 1610 p a, S04:a.5 to 22 m@). Seep
A-I, collected from the 1100-level portal, appears to have little or no influence from the mineralized
bedrock (Cu: 120 p a, Zn: 257 pgL, Cd: 2.3 pa, S04:40 mg/L) and does not reflect water quality of the
mine pool as noted at the 1500-level main portal drainage (P-1). Water from this seep appears to be derived
from rainwater and snowmelt that infiltrates through the overlying surficial materials to the adit.
The only direct indicator of underground water chemistry and processes is provided by the 1500-level
main portal drainage (P-1). However, comparisons of portal drainage water quality with water quality
data collected from other sources indicated that the portal drainage water chemistry most closely
- resembles the water draining from the mill building. The USGS (personal communication with Jim
Kilburn, 1999) indicated that numerous typical secondary sulfate minerals were identified in the mill
, building in 1995 and 1996 (Table 6.1-1). These minerals contain iron, copper, zinc, manganese,
aluminum and sulfate and vary from highly soluble to sparingly soluble (Alpers et al. 1994). The minerals
are formed when concentrated groundwaters derived from contact with oxidizing mineral concentrates
and residual ore become chemically over-saturated with minerals. This can occur by mixing of different
solutions and evapo-concentration.
By comparison, the same minerals are probably present in the underground mine. This is a reasonable
assumption since the mine probably contains exposures of ore-type material which could not be extracted
due to requirements for ground support, or because the volume was not justified by mining economics.
Sulfates are formed on the walls of the underground mine by oxidation and are then rinsed during
flushing events. Evaporation of water during drier periods (primarily winter) allows the salts to
accumulate.
Experience at other underground mines within similar geologic conditions indicates that these salts may
be present throughout the mine wherever water emerges and can evaporate such as where fractures and
drill holes intersect mine walls and where water is locally ponded and then drains. Since water flow likely
occurs unpredictably on some fractures and not others, these locations-of stored oxidation products are
randomly distributed. Significant deposits of salts are generally identified opportunistically rather than by
1769M05-019Uuly 27,1999.4:ll PWRAFT FINAL RI REPORT