Water Table Impacts From Mining

Open pit gravel mining operations greatly disrupt the water table in the mined areas. Groundwater will have to be pumped down in order to provide for a dry pit from which heavy machinery can be used to extract the gravel.  Individual pit areas of up to 20 acres are allowed. If two pits are mined simultaneously the total area of both pits can be 35 acres. Dewatering pits of this size will likely draw down the water table in the proposed gravel mining area of the St. Vrain Valley.

Gravel mining operations by previous permit owner, Lafarge, were conducted in the St. Vrain Valley. During that time the water table was drawn down considerably and artesian springs and shallow wells went permanently dry. Options to mitigate the future loss of potable well water include trucking water to the homes affected, constructing a pipeline from Longmont to provide Longmont City water to the valley, or drilling deeper wells to provide a lower quality water which required extensive treatment to make the water quality acceptable.

Trucked in potable water will require each home to have a suitable cistern or water tank to store several days’ supply of water. The water trucks will increase traffic Hygiene Road and other roads in the valley. Constructing a water main to connect with the City of Longmont would force homeowners to have to purchase Longmont city water. The cost of this when used for agricultural purposes would be unsustainable for small family farmers–and would undermine their water rights, infrastructure that residents have invested in to be sustained by the St. Vrain River. Deeper wells miss the sweet water in the St. Vrain aquifer and produce water with high dissolved mineral content which requires treatment to make it potable. The salinity of one deep well is so high that it cannot be used for irrigation and is at the upper limit for stock water.

The proposed gravel mining operation may produce up to 100 acres of open water in the valley after the gravel pits are mined. The proposed mining area is nearly 500 acres, so approximately 20 percent of the valley’s foraging areas may be converted to ponds. The overall impact of mining such an extensive portion of the valley could cause massive disruption to the groundwater since the water table will have to be drawn down below the bottom of the mining pits which may be as deep as 30 feet. Pumping dry a 20-acre pit will require massive effort to keep water from reaching the pit through the St. Vrain natural aquifer and will have an area of influence far greater than the 20-acre pit. Even if mining operations are kept at a minimum of 1,000 feet from existing wells, the dewatering procedures may dry up or decrease the yield of existing wells.

Below is a photograph of a small pond in the valley which is fed by groundwater. The surface of the pond is the top of the current water table. The historic 2013 St. Vrain flood dropped the water table by several feet and it has not recovered, even during periods where snow runoff is at its prime in spring. Prior to the floods, the groundwater would typically cover the rocks.

Damage done to the watershed during the 2013 flood has not yet been repaired resulting significant lower overall groundwater table in the St. Vrain Valley, even without a gravel extraction operation.