2022-09-26_PERMIT FILE - M2022042 (6)

Home Browse Search

Windsor East Mine, Exhibit G – Water Information September 2022 Page 3 | 11 1.2 HYDROGEOLOGIC SETTING 1.2.1 Geology The geology mapped at land surface beneath the site consists of quaternary age valley-fill deposits comprised of sand, silt and gravel primarily (Figure G-2). The areas of higher elevation around the site are typically comprised of quaternary terrace deposits. Boring logs from installation of monitoring wells immediately east of the property were reviewed for details on the site geology. The Parsons sand and gravel mine has 12 monitoring wells on the property, including MW-12 which is the nearest to the Windsor East property, located between the Parsons property and the Windsor East property (Figure G-3). The boring logs generally indicate that the geology consists of a 10-30 ft thick layer of unconsolidated alluvial sediments overlying siltstone bedrock. The upper 4-14 feet of the alluvial sediments are typically finer-grained silts and clays and may be only partially saturated in many locations. The lower 4-20 feet of the alluvium consists of sands and gravels which are expected to be highly transmissive of shallow groundwater. The shallow bedrock of the Fox Hills Sandstone consists of weathered, consolidated sedimentary rock varying from claystone to siltstone as observed in the boreholes advanced on the Parsons mine property to the east. In comparison to the alluvium, the weathered bedrock is expected to be several orders of magnitude less transmissive due to the consolidated, finer-grained properties. Bedrock was encountered in the boreholes advanced at the Windsor East mine property between 15 and 22 ft below ground surface (bgs), and 13ft bgs in the nearest Parsons Mine borehole (Parson MW-12). 1.2.2 Groundwater Groundwater at the site represents a combination of water that flows through the high-permeability valley-fill alluvial deposits parallel to the Cache la Poudre River and water which infiltrates in the surrounding higher-elevation recharge areas to the north and south of the river, typically associated with the agricultural fields that dominate the land use of the area. Infiltrating water in these areas of higher elevation drain at the lower-lying erosional valleys formed by streams and rivers of the area, resulting in flow patterns that resemble a muted form of the land surface topography. The erosional valleys are typically underlain by the higher-permeability sands and gravels deposited by historic flood events and form channeled zones through which groundwater can flow more rapidly. South of the Windsor East property, the Cache la Poudre River is the surface-water feature for local groundwater discharge. Within several hundred feet of the river, groundwater flows in a direction that is near parallel to the river due to the constant interaction with the river stage within the porous sands and gravels. As a result, while groundwater beneath the site is typically slightly higher than that of the river and flows toward the river, the flow direction of groundwater flow is generally parallel to the Cache la Poudre River from west to east at close proximity (Figure G-5). Some minor component of upward groundwater flow from the deeper bedrock may occur, but this is likely to be negligible compared to the influence of the river and the underflow within the valley-fill alluvium. The direction of regional shallow groundwater flow is therefore toward the southeast but changing to a near easterly direction near the river. Based on water-level measurements in the monitoring wells installed at the Windsor East mine property, the water table is approximately 8-11 feet below land surface. 1.3 EXISTING AND PLANNED WELLS 1.3.1 Existing Monitoring Wells A network of monitoring wells was installed in 2010 to characterize the groundwater conditions at the Parsons mine east of the Windsor East property (Figure G-3). The boreholes for the wells were drilled to the bedrock contact and the wells were constructed using 8-10 foot screened intervals between the water table and the bedrock contact. These wells generally show the direction of groundwater flow in an easterly direction, in equilibrium with the river water elevations, although subject to the influence of the Parsons mine cell dewatering.