Dames & Moore, 1999 - USDA Forest Service

In order to provide comparative flow conditions between stations in Railroad Creek, the following
assumptions and estimates were made for the loading analysis:
Measured flow was used if flow conditions were not changing during the sampling round.
Flow was estimated during dynamic flow conditions such that flow was consistent with
downstream flow relationships between stations.
. Creek drainage and seep concentrations were assumed to be representative for the flow
conditions encountered or estimated during the loading period.
The results of the loading analysis for May and September 1997 are presented in Tables 6.6-1 and 6.6-2,
respectively. The analysis was performed by dividing Railroad Creek into two reaches. Reach 1 included
the creek from RC-I to RC-4. Reach 2 included the creek from -RC-4 to RC-2. The tables show loading
calculations for magnesium, zinc, cadmium, copper and iron. For each parameter, the flow was multiplied
by the concentration for each source to yield the load. The loads were then added to give the cumulative
load at each station. At RC-4 and RC-2, the cumulative load from each source was compared to the total
load measured in Railroad Creek. Total load in Railroad Creek is the load calculated using the water quality
data for samples collected at RC-4 or RC-2. The commutative load for each reach was subtracted from the
total load of each reach. The results are referred to in the tables as "Reach 1 Balance" and "Reach 2
Balance." The tables also show the total balance. In general, these balances account for non-point-source
discharges (groundwater) or (if negative) can indicate load loss due to flow loss or chemical effects. The
balances also incorporate the uncertainties in the measurements of flows. The results for the ApriYMay
1998 loading analysis are shown in Table 6.6-3. Percent loading for 1997 is provided by location on Figure
6.5-20.
6.6.13 Loading Analysis and Mass Balance Results - 1997
Conservative Parameter - Magnesium
As a first step to confirming the validity of the mass balance approach, a chemically conservative parameter,
rather than a heavy metal, can be used to calibrate or verify the site-specific water balance discussed in
Section 4.4. The term "conservative parameter" is described in Section 6.3.3.1. The purpose is to confirm
the water mass balance using a parameter for which the total load can be accounted for. The parameter
selected in this case was magnesium. The main reason for selecting this parameter is that magnesium is
released by weathering processes at the Site and is always detectable. Other conservative parameters such as
chloride are not released by weathering and are frequently non-detectable, and therefore are not appropriate
for this purpose.
In Table 6.6-1 (May 1997 calculation), the incoming magnesium load at RC-I was 5098 mg/s. In Reach 1
(defined as receiving mine and support area drainage - RC- I to RC-4), the source load additions totaled 986
mg/s, for a cumulative load of 6084 mg/s. The greatest proportion of the load originates from P-5, followed
by SP-23. The measured load in Railroad Creek at RC-4 was 6656 mg/s. The deficit between the two
(cumulative at RC-4 and measured at RC-4) was +572 mg/s. If this load is applied to the inputs from
groundwater calculated using the water balance (0.9 cfs, 25.49 Us), the required concentration was 22.5
ma, as shown in Table 6.6-1. Magnesium concentrations were generally less than 10 mg/L in the portal
drainage, lagoon and seeps. The calculated concentration was, therefore, greater than the e-ed
17693405-019Uuly 27.1999;4:11 -RAFT FINAL RI REWRT