Dames & Moore, 1999 - USDA Forest Service

prediction. Therefore, inspection of the underground workings to identi@ specific types of salts and
accumulations of salts is unlikely to be successful in characterizing the salts because not all parts of the
workings are accessible due to physical barriers (backfill and collapsed workings) and safety concerns
(ground stability, air quality, hidden hazards).
The pathway from the sources to the soluble salts involves contact with the host rocks and mixing with
other waters. The host rocks are primarily composed of alumino-silicates and isolated occurrences of
marble. This results in addition of aluminum, magnesium, calcium, potassium and sodium to the water as
has been observed throughout the Site. The addition of these elements largely occurs in proportion to
sulfate, which is a surrogate for the release of acid. These are expected to be extremely complex
pathways.
Significant seasonal effects are apparent in the 1500-level main portal drainage at station P-1. This is
reflected in pH changes. Lowest pHs are observed in the spring, and pH increases through the summer.
This is accompanied by decreasing aluminum, copper and zinc concentrations, but sulfate does not
change significantly. The lower pH in the spring is probably caused by flushing of acidic salts from rock
surfaces and Fractures in the spring, and by local recharge of pools within the underground mine that have
evaporated through the previous summer and winter. This acid load is less effectively neutralized by
contact with neutralizing minerals and reflects the decreases in pH at the portal. During the summer, pH
increases due to much lower acid load, longer contact times with neutralizing minerals in the underground
mine and mixing with alkaline groundwater, resulting in more complete neutralization.
The minor variations in sulfate concentrations suggest that the source of the water does not change
significantly (that is, the pool within the underground mine continues to supply the water observed at P-
I). The variation in metal concentrations in P-1 drainage is related to the differences in pH, and
precipitation of oxyhydroxides rather than changes in source release. Source release may increase in the
spring due to flushing, as described above, but the pH variations produce the same effect.
A metals-loading analysis of surface water discharging from the portal drainage was performed using the
data collected during the RI. The flow data and chemical data collected from the 1500 level main portal
drainage (station P-1) during the May, July and September 1997 rounds were used in the analysis. A
summary of the chemical data is provided in Section 5 of this report. Historical flow and chemical data
from 1982, 1983, and 1991 were also evaluated for use in the loading analysis; however, the data were
not used in the loading analysis due to the uncertainties associated with the accuracy .of the flow
measurements recorded for these periods.
The results of the loading analysis are shown in Figure 6.5-5. Zinc and copper exhibited similar loading
trends, with the highest loads occurring in May and a decline in concentration through September. Copper
loads are generally lower than zinc and decrease more significantly in the late summer and fall. Data
indicate that the dissolved iron loads discharged at the portal are approximately one order of magnitude less
than corresponding zinc loads and continue to decrease to September.
sulfate loads were approximately one order of magnitude greater than corresponding metal (zinc, copper,
iron) loads. The patterns of sulfate discharge are very similar to those observed for the metals. The mass of
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