REV101480

• Permit No: M1974015
• IBM Index Class Name: Revision
• Doc Name: EXHIBIT G WATER INFORMATION
• Type & Sequence: AM6
• Media Type: D
• Archive: No

• • ~~i1C
Mr. Scott Gudahl
January 28, 1997
Page 3
In general, wells founded in shallow alluvial aquifers are susceptible to water quality changes due
to the lack of an overlying protective low permeability layer. Such alluvial aquifers are particularly
susceptible to bacterial contamination due to livestock and leach fields in the area. Total coliform
analyses have resulted in positive detections for several of Western Mobile's monitoring wells, the
Zapf domestic well and the Shupe cistern. Water from wells tapping the alluvial aquifer should be
treated.
WATER TABLE IMPACTS RELATED TO MINE DEWATERIN
Dewatering of mine pits will lower the water table around the pit, an effect termed "the cone of
depression". The amount of water level decline (drawdown) decreases with distance from the
dewatered gravel pit. The distance that the cone of depression will extend beyond the pit is a
function of several factors, including the transmissivity' of the aquifer and boundary conditions
(such as a surface water feature or bedrock boundary of the alluvial aquifer). Experience has shown
that cones of depression during dewatering may extend a distance of 500 to 1,000 feet from the
perimeter of the pit or until reaching a major surface water feature such as St. Vrain Creek, an
unlined pond, the South Branch, or a large unlined ditch. Consequently, 500 to 1,000 feet from a
dewatering pit, the water table remains at its normal: level. For example, the water level 500 feet
east of the active mine pit (Plate 1) has declined about four to six feet as indicated by the water
level in monitoring well PZ-33. The water table is back to near normal conditions 750 feet
southeast of the active pit as indicated in monitoring well PZ-31.
Where dewatering impacts wells, wetlands or surface water features, the impacts can be mitigated
by strategically applying dewatering discharge water or other available water back to the aquifer
(recharge) via unlined ditches or infiltration galleries.
Dewatering may also increase seepage losses from ditches. Seepage losses could be mitigated by
recharge as indicated above or by lining the ditch.
WATER TABLE IMPACTS RELATED TO MINE RECLAMATION
The site reclamation plan includes a slurry wall parallel to St. Vrain Creek and the South Branch, as
well as lined and unlined ponds. The slurry wall parallel to the creek and South Branch will tend to
maintain preexisting groundwater levels on the upgradient (north) side of the wall, and will have
the desired effect of minimizing depletions from the St. Vrain Creek and South Branch. Impacts to
wetlands, riparian areas, and wells can be mitigated by a variety of means. For example, an
infiltration system along with strategically constructed slurry walls.
In areas of lined ponds, water levels will tend to rise on the upgradient side of the pond, and will
decline on the downgradient side of the pond. This declining water level on the downgradient side
of a lined pond is known as a "shadow effect". This shadow effect could be measurable as much as
Transmissivity is a technical term to describe the water yielding properties of an aquifer. The higher
the transmissivity the more water an aquifer will supply to a well or pit.