Dames & Moore, 1999 - USDA Forest Service

'
iron added is precipitated and is, therefore, not accounted for under the dissolved load. The decrease in load
between RC-7 and RC-2 may reflect oxidation of ferrous to ferric iron. As oxidation occurs, ferric iron is
precipitated. The September calculation indicated a similar conclusion.
Dissolved iron is contributed primarily h m groundwater baseflow along the tailings piles. Iron loads are
highest during the spring when flows are the highest. Iron loads decline through summer and fall after seeps
have begun to dry up and groundwater recharge is no longer occurring. Iron discharge is delayed relative to
zinc, cadmium and copper because iron loads are 'from groundwater baseflow, not seeps or drainages. The
delay in iron loading reflects an extended period of infiltration through the tailings piles with subsequent
discharge through the base of the piles into the streambed of Railroad Creek. This also suggests that seep
flow is sustained by bank storage and groundwater flow associated with the alluvial materials beneath the
tailings piles, and is not necessarily directly related to recharge through the piles.
.
6.6.1.4 Loading Analysis and Mass Balance Results - Spring 1998
The loading of copper, zinc, cadmium and iron in Railroad Creek was estimated from the flow
measurements and water quality results obtained during the 1998 May sampling round. The loading
estimates were developed for all of the sampling locations with concurrent flows that were either
measured or could be reasonably estimated from flow relationships between stations (see Section 4.3).
The load values were developed for comparison with the May 1997 values and are assumed to be
approximate given that flow timing was not considered in the calculation. Table 6.6-3 shows the May
1998 load estimates and is compared herein to Table 6.6- 1.
In general, the May 1998 total load in Railroad Creek was larger for copper, zinc, and cadmium relative
to 1997, and the iron load was lower. However, the pattern of loading was similar to 1997 with the
majority of the load for copper, zinc and cadmium accounted for by the portal drainage, and the majority
' of the iron loads occumng between RC-4 and RC-2 from groundwater. Additionally, copper, zinc and
iron loads decreased downstream of their sources, similar to 1997. Very little of the metals load observed
at RC-2 appeared to be generated upstream of the Site, as indicated by low relative loads observed at
upstream stations in Holden Creek, Big Creek and at RC- 1 I.
The larger load values for copper, zinc and cadmium may be a result of the earlier sampling time relative
to the first flush from the 1500-level main portal drainage as compared to the 1997 May sampling event.
The 1997 May samples were collected during the initial part of snowmelt; however, based on the Railroad
Creek hydrograph at RC-4, a melt event had already occurred by the time the samples were collected; '
sampling had been completed in April 1997 before, but not during, the event. In 1998, however, the
samples were collected earlier in the melt period, during the first snowmelt event '(see Section 4.3). The
lower iron loading may reflect lower groundwater yields because of a delay in groundwater discharge to
Railroad Creek, given that the sampling in 1998 occurred earlier in the melt period.
6.6.1.5 Loading Analysis and Mass Balance Results - Additional Source Areas
Waste Rock Piles and Mill Area
Additional source areas that contribute metals load to the site include the East and West Waste Rock piles
and the mill area (Figure 6.5-20). The loading of dissolved cadmium, copper, iron, and zinc from these
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17693-0OS-019Vuly 27. 1999;4:11 WRAFT FINAL RI REPORT