4 minute read
Umetco Former Mine Site
Hot Springs, Arkansas, United States
Creating a thriving aquatic habitat from an open-pit mine. From 1967 to 1985, Umetco conducted mining activities across 150 hectares of this 200-hectare property in the Ouachita Mountain ecoregion, resulting in low pH and mobilized zinc in the surface water. Acid rock drainage further degraded the water quality, and mine spoil areas and unimproved roads increased sedimentation in headwater streams. From 2015 to 2019, Umetco installed permanent, natural treatment systems to restore water quality and improved existing stormwater drainage to promote overland flow into lined, limestone riprap channels. This change limited water infiltrating into buried mine spoils and promoted conveyance of clean water to nearby streams, one such restored by adding cobble substrate and strategically placing downed trees to stabilize the banks and to create riffles and pools. The project team also engineered a unique, three-layer sediment cover in a 52-meter deep former mine pit lake, encapsulating sludge containing metal hydroxides and neutralizing pH in sediments and overlying water. Upland habitats restored with native wildflowers and grasses now provide food for a variety of pollinators and support ground-nesting songbirds. This project emphasized passive engineering solutions to improve water and habitat quality and to minimize maintenance. In response, terrestrial and aquatic biological communities are thriving.
Article cover: Pedestrian access bridge overlooking the capped former mine pit lake. (Photo by Dow)
Producing Efficiencies
Using a georeferenced barge attached to cables anchored at the sides of the 6.5-hectare former mine pit lake, the team applied three layers of sediment cover along the bottom of the lake. The placement of an engineered cover this deep in a mine pit lake had not been successfully attempted previously, but quality assurance monitoring has demonstrated that the cover is working. The resulting water quality has been outstanding, and the population of native fish species is thriving, also in part due to the habitat enhancements using on-site downed trees to promote pool-and-riffle features.
Using Natural Processes
The project team created approximately 0.8 hectare of wetlands at the outflow from a former mine pit lake to create habitat and to reduce metals concentrations in the water prior to its entering an adjacent stream. Additionally, the team planted native wetland and upland plants with an emphasis on plants used by pollinator species. In the year since the wetland creation, the water quality entering the stream has dramatically improved. Fish species have returned; and pollinator species, such as the tall poppy mallow (Callirhoe leiocarpa) and plains coreopsis (Coreopsis tinctoria), have emerged.
Broadening Benefits
The Umetco team created over 80 hectares of improved habitat and prepared the site for long-term biological productivity. With the restoration complete and water quality improving throughout the 200-hectare site, the project team is now developing a plan to return the site to beneficial community use. The system of trails, used in the past to monitor fence lines, and installed wooden and steel pedestrian bridges could allow the public to enjoy the site; and the team hopes to demonstrate to the public that a properly restored former mine site can become an amenity for the community at large.
Promoting Collaboration
The project team developed three-dimensional conceptual site models to help clearly explain to regulatory partners the project challenges and their plans to address them effectively. Although public access to the site was denied for safety reasons during construction activities, the team posted informational signs at the site entrance to inform the public of the transformation taking place. Currently, they are working with the local community and conservation groups to transfer ownership and management of the property to provide this natural space to the public for decades to come.
