Dames & Moore, 1999 - USDA Forest Service

the surface waters. Some of the dissolved metals precipitate out of the water column and settle on the
' bottom of Railroad Creek.
Seasonal metals concentrations indicate that copper, zinc, and cadmium are at their highest levels during
spring snowmelt (h4ayIJune) when seeps and discharge horn ke portal drainage are at their highest, as
discussed previously in Section 5. Iron concentrations are at their highest during periods of lower flow. In
addition, copper, zinc and cadmium increase the most dramatically between stations RC-I and ~d-4, and
iron (and to a lesser extent zinc) concentrations increase most dramatically between stations RC-4 and RC-
7. These observations indicate that copper, cadmium and zinc enter Railroad Creek fiom the portal drainage
and associated seeps and drainages north of the tailings piles, and that iron is introduced primarily fiom
seepage and groundwater flow from the tailings.
6.6.1.2 Mass Balance Calculation Method
The metal loading to Railroad Creek was computed using flow measmment data and analytical results for
water samples collected over a tweday period in 1997. Specifically, loading was calculated as the
measured concentration for each metal for each source times the flow rate of each source at the time the
sample was collected. The individual source loading was compared to the computed loading for the same
constituent at the same time within Railroad Creek. Loading is reporled as mass per unit time. For the
purposes of this study, the loading is reported as milligrams per second (mgs), which are common units for
reporting loading results.
Flow data used to compute the loading fiom the seeps and seeprelated drainages were estimated andor
measured directly in the field at the time that the water quality sample was collected. Field flow
measurement methods used were previously described in Sections 3 and 4. Flow measurement accuracy is
directly related to the flow volume measured at each station and the measurement tools utilized. The ./
accuracy of the flow measurements for the surface water in Railroad Creek ranges between 5 and 7 percent
using a flow meter (Swoffer or Price AA), and 10 to 12 percent utilizing a bridgeboard, as used at RC-2 and
RC-4 during high flow conditions.
The accuracy of seep flow measurements at SP-6 through SP-9, SP- 14 through SP-19, SP-21, SP-23A and
SP-23B was estimated to be e5 percent.
Seep flow estimates for SP-1 through SP-5; SP-1OEN through SP-13; and SP-24 through SP-26 are
assumed to be accurate within f 50 percent of the actual value because of limited amount of flow and the
difficulty in capturing and measuring flow emerging as diffuse seepage. Although the accuracy of these
measurements is marginal, the loading analysis will demonstrate that the load of select metals at these seep
locations were an insignificant proportion to the overall load of select reaches of Railroad Creek.
It should be noted that the flow in Railroad Creek was dynamic and was changing during the period that
seeps were sampled, and also during the time that Railroad Creek was sampled (particularly during the May
and July sampling rounds). Consequently, the flow measurements recorded at the time of seep sampling did
not always reflect comparative flow conditions relative to RC-2.
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17693405-0 19UuIy 27. 1999;4: 11 PMDM FINAL RI RE#IRT