Dames & Moore, 1999 - USDA Forest Service

an "A" series well, it appears to be screened within tailings pile materials rather than the alluviallreworked
till based on review of the boring log for this well (Appendix B).
When two or more monitoring wells are completed at adjacent locations in different hydrostratigraphic
units, those monitoring wells can be used to compare water levels in the units and assess the direction of
potential groundwater flow between the units. Referring to Figure 4.4-1, clusters of adjacent wells occur
at locations PZ-I, PZ-2, and PZ-3 on tailings pile 2, and PZ-4, PZ-5, and PZ-6 on tailings pile 3.
Figures 4.4-12, 4.4-13, and 4.4-14 show groundwater levels measured in groundwater monitoring wells at
locations PZ- 1 (south side tailings pile 2), PZ-3 (north side tailings pile 2), and PZ-4 (south side tailings pile
3), respectively (Figure 4.4-1). Pressure headstwater levels in the "A" series wells exhibit a decrease in
water level from May through September 1997. Water levels in the "B" and "C" series wells react in a
similar manner to each other, remaining relatively constant, with slight rises seen in June or July. Pressure
headslwater levels in the "A" series monitoring wells do not exhibit the same trend as water levels in the "B"
and "C" series monitoring wells, as would be expected if all three series of monitoring wells were completed
in a ,fully hydraulically connected unit.
Throughout the Site, monitoring wells completed in native material indicate the same water 'level trend as
described for the "A" series wells above. Groundwater level declines were large enough in two "A" series
wells (PZ-1 A and PZ-4A) to result in their being dry in September 1997.
"B" series monitoring wells, completed near the base of the tailings, showed the trend described above. One
exception was monitoring well TP3-7B, which indicated decreasing water levels throughout the monitoring
period. This monitoring well was identified during the initial well assessment as having a loose surface seal.
It is possible that the downhole seal at this well has failed, and that surface water is allowed to drain into the
well.
Water level data for the RI were collected over a period of approximately five months (mid-May to late-
September). This interval is believed to include the'period of highest groundwater levels (mid-May to mid-
June) and of relatively low groundwater levels (September).
4.4.3.3 Groundwater Flow
Groundwater flow directions can be established from water-level elevation maps by drawing flow lines that
run perpendicular to the groundwater elevation contour lines. Figures 4.4-6 through 4.4-9 are water level
elevation maps based on measurements from wells completed in the alluvium, colluvium, and the
alluvium/reworked till unit, respectively. Figures 4.4- 10 and 4.4- 1 1 are water level elevation maps based on
measurements from monitoring wells screened in the tailings.
Groundwater Flow in Native Materials
The May water-level elevation map (Figure 4.4-6) shows a strong component of groundwater flow from the
Site north toward Railroad Creek. Groundwater to the south of Railroad Creek is recharged by snowmelt
from the southern side walls of the valley and flows primarily north toward Railroad Creek. Much of the
groundwater flow is discharged to Railroad Creek, as described below; however, some groundwater likely
flows longitudinally down valley beneath the creek. Groundwater gradients are relatively high, ranging as