Dames & Moore, 1999 - USDA Forest Service

main source of zinc load (82 percent .of load added downstream of RC-1, not including Copper Creek).
Other sources account for less than 6 percent individually.
A similar approach was used for the September 1997 calculation. A contribution fiom groundwater was
added in Reach 1 to account for the difference between load observed at RC-I and RC-2 (63 mgls) that
could not be accounted for by load contributed by P-5 (1 6.9 mg/s). In September, non-point sources appear
to represent the majority (77 percent) of the zinc load added to Railroad Creek between RC-I and RC-2.
The total balance (non-point source discharge, i.e., groundwater) for zinc between RC-I and RC-2 in
September 1997 (63 mg/s) is comparable to the total balance in May 1997 (56 mg/s).
The cadmium balance for May 1997 indicates that P-5 is the main source (5.1 mg/s, 70 percent of load
added by the Site) and tha! the total balance is the next largest source (1 7 percent). In September, 1997, the
contribution from non-point sources was 80 percent compared to 17 percent fiom P-5, though the load h m
non-point sources remained comparable (1.23 mg/s in May compared to 0.24 mg/s in September). The
findings are comparable to zinc and indicate that p od drainage accounts for the majority of load entering
Railroad Creek in May. In September, most load enters Railroad Creek through groundwater. The
groundwater contribution appears to be relatively stable.
Non-Conservative Heavy Metals - Copper and Iron
Mass balances for copper and iron provide limited information because they are readily removed h m
solution by precipitation changes (as seeps mix with Railroad Creek) and adsorption.
, The copper load observed at RC-4 in May 1997 (374 mg/s) can primarily be accounted for by P-5 (225
I.*
,% mg/s) with lessor contributions by SP-23 (97 mgfs) and RC-1 (16 mg/s). The balancing (groundwater) load
,,-.
(20 mg/s) was negligible compared to the other loads. Copper load appeared to decrease between R-4 and
'
RC-2. The load required to balance was -48 mg/s. Overall, a small negative load balance of -28 mg/s was
.-". indicated for RC-I to RC-2 after accounting for known sources. This is consistent with removal of copper
solution by pH adjustment and adsorption, which occurs continually along Railroad Creek as low pH waters
mix with surface water but is particularly likely in Reach 2 when iron is added by the tailings pile seeps.
The negative balance does not preclude the addition of copper through groundwater sources. The balance
indicates that the net effect of addition through groundwater and removal by attenuation mechanisms is net
removal.
The September 1997 calculation indicated a positive load balance requknent in Reach 1 and negative load
balance for Reach 2, although overall, a positive balhcing load of 2.6 mgfs was obtained and is the largest
source of load to Railroad Creek. Qualitatively, the calculations for May and September 1997 are similar.
Differences between May and September reflect changes in P-5 water quality and quantity and removal of
copper prior to mixing with Railroad Creek.
The calculation for iron indicated a load loss between RC-1 and RC-4 in May. Iron concentrations were
very low and the decrease was probably due to near detection limit (0.02 mg/L) concentrations in Railroad
Creek. Significant load increases were observed between RC-4 and RC-7, followed by a decrease between
RC-7 and RC-2. The load added by surface seeps was not sufficient to account for the load increase
observed, hence a balancing load of 1412 mg/s was added. The calculated iron concentration for the load is
24 mg/L. This concentration of iron is lower than observed in seeps which indicates that a portion of the
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1769340541 9Uuly 27.1999:4: 11 -RAFT FINAL RI REPORT