Dames & Moore, 1999 - USDA Forest Service

Accuracy
The accuracy of the bedrock aquifer baseflow contribution is based on the accuracy of the portal drainage
flow measurements and assumptions regarding the average portal drainage flow (see above), and on the
assumption that there are no other unobserved significant sources of flow from the bedrock aquifer
entering the creek. Although it is possible that a fracture system capable of transporting groundwater
underlies the creek bed and contributes groundwater baseflow to the creek, several considerations justify
the assumption that a significant bedrock source other than the portal drainage is not present. These
considerations include the following:
i3<
The presence of low permeability glacial till underlying the creek and overlying a large
part of the bedrock near the valley floor would likely restrict groundwater flow from a
bedrock fracture system.
A fracture system large enough to contribute significant amounts of groundwater (0.1 cfs)
to the creek would likely be expressed on the valley sidewalls as well, with resultant
emergent springs which have not been observed.
The loading analysis (see Section 6.0) can account for the observed changes in water
'
chemistry using the baseflow sources.
4.4.4.10 Results of Water Balance Analysis
The results of the water balance for each reach are tabulated and summarized in Tables 4.4-9 and 4.4-10
.for the average MayfJune 1997 period (31 days), and the average September 1997 period (30 days),
respectively. These periods coincide with the flow net analyses presented on Figures 4.4- 15 and 4.4-17.
Figures 4.4-20a through 4.4-20d provide detailed schematics of site flow paths, which are based on
groundwater and surface water observations. The water balance equations provide a simplified picture of
these pathways as illustrated in Figures 4.4-2 1.
The theoretical outcome of the water balance analysis is that both sides of the equation are equal. '~hus,
the sum of the water balance components listed in Tables 4.4-9 and 4.4- 10 should equal zero. A non-zero
sum indicates error in the estimates of the water balance components andlor that not all flows are
accounted for. If the summation is negative, too much water is accounted for; in other words, the error is
positive in a downstream direction. If the sum is positive, too little water is accounted for; in other words,
the error is negative in a downstream direction.
In general, the flow in Railroad Creek is much greater than potential flow gains or losses from the site.
Summation of the error in the water balance tabulations for summer and fall in each reach are well below
the measurement error of flow in Railroad Creek (i.e., less than 5 to 7 percent of the flow at RC-4). The
water balance accuracy is not sufficient to rule out small water sources not specifically identified as a
component of the inflow, or to evaluate the accuracy of the individual component inflow estimates.
Since error in the water balance is within the measurement error of Railroad Creek, and the magnitude of
the inflow components is generally (with the exception of Copper Creek) well below the error in
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17693-005-019Uuly 19. 1999;4:51 PM:DRAFT FINAL RI REPORT