Gravel pit reservoirs Colorado's water storage solution

Gravel pit reservoirs emerged in the mid-1980s as a solution to water storage in the Front Range of Colorado. The stratigraphy within gravel pits in the Front Range generally consists of sand and gravel deposits covered by thin clayey sand overburden. Beneath the sand and gravel exists claystone bedrock. When the aggregates are mined to the claystone floor, a storage "bucket" is formed by simply sealing the gravel highwalls. The earliest lined pits consisted of below grade earthen slope liners founded on bedrock and extending upward to the original ground surface. Soil-bentonite slurry walls constructed around the perimeter of a gravel mine property were introduced in 1990 as a lining system. Slurry wall liner systems allowed an owner to claim storage in both surface water and groundwater present in the void space of gravel deposits inside the slurry wall. Additional developments occurred when above-grade dams were constructed around gravel pits. This resulted in the increase in the size of a specified project from 1,000 to 2,000 acre-feet (1.2 to 2.4 million m(3)) to 6,000+ acre-feet (7.4 million m(3)). As the population of the Colorado Front Range grows, economic water storage is becoming a greater challenge. Increased competition for reservoirs has caused miners to coordinate with water storage purchasers at the conception of mine investigation work. This paper presents the development of gravel pit reservoirs through the presentation of multiple case histories. Many successful projects and a handful of failures caused by geologic conditions and improper construction techniques are discussed.