Dames & Moore, 1999 - USDA Forest Service

.a I.
Other Related Findings
Comparison of Railroad Creek Locations with Respective Reference Locations
Data collected for the benthic invertebrate community metrics were evaluated using the W test to
determine if the data were lognormally or normally distributed. The results from these distribution fits
were then utilized for statistical comparison in accordance with the pairings of Railroad Creek locations
and their respective reference locations previously described. The results of the distribution fit for each
pair of locations are presented in Table 4.6-2c while the statistical comparisons of the Railroad Creek
location pairs with the respective reference pairs are presented in Table 4.6-2d. Because lognormal and
normal fits and "no fit" conditions were encountered during the distribution fit evaluations, both
parametric and nonparametric tests were used on the Railroad Creek and reference locations for the
comparisons. The parametric test (independent t-test with 95 percent confidence) was performed if the
reference and Railroad Creek metrics were found to be both either lognormally or normally distributed.
A nonparametric test (Wilcoxin Rank Sum test with 95 percent confidence) was performed if the
distribution fits from both data sets were not identical or the evaluation indicated no fit. Results of the
statistical comparisons indicated that the percent dominant taxa were similar between both groups (RCl
and RC-6 reference compared with RC9 and RC-7 affected and SFAC-1 and BC-1 reference compared
with RC-5a and RC-10). The only other comparison indicating similarity was the ratio of shredders in the
comparison of RC 1 and RC-6 reference with RC-9 and RC-7.
Comparison of Species and Sampling Stations
Benthic invertebrate communities in Railroad Creek at the upstream reference sites are similar to
communities in other streams outside the drainage (Bridge Creek, South Fork Agnes Creek, and Company
. Creek). Other sampling locations at and downstream of the Site are variously affected. The number of taxa
and organisms are reduced at all stations at or downstream of the former operations. A more careful
analysi$ of trophic and habitat requirements for the organisms present at the various sites indicate that the
primary effect immediately downstream of the tailings piles is a reduction in habitat capability for benthic
invertebrates. This conclusion is based on several factors including the following:
, .
• Benthic communities at the reference sites contain the hll spectrum of trophic types
(scrapers, shredders, collectors, filter feeders, and predators) as well as habitat usage (free
ranging, burrowers, net spinning, upper stone surface).
This hll range and similar density is also found in upstream unaffected (by iron
oxyhydroxide precipitate) portions of RC-9. This sampling station is downstream of the
portal drainage where the portal water will be hlly mixed with Railroad Creek water and
yet the upstream portion of this sampling station shows no apparent toxicological effect
from that discharge. The concentration of metals of concern does not change appreciably at
sampling stations downstream of RC-9.
The lower portion of RC-9 (affected by iron oxyhydroxide precipitate) and all other
downstream sites except RC-3 demonstrate an affected benthic community. These
sampling stations (except RC-3) have communities without large free ranging predators,
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17693-005-019Uuly 19, 1999;4:Jl PM;DRAFT FINAL RI REPORT