By Colin McCandless
A collaborative undertaking to protect the Mill Creek Sink, a sinkhole connected to an underwater cave network located in the City of Alachua, is now providing critical stormwater control and treatment to help improve water quality for thousands of North and Central Florida residents.
The Mill Creek Sink Water Quality Improvement Project involved a voluntary public agency effort among Alachua City officials, county officials, environmental engineers and the Suwannee River Water Management District to plan the construction of a treatment wetland to treat surface and stormwater runoff prior to it overflowing into the natural wetland habitat connected to the Mill Creek Sink. A ribbon cutting ceremony held on May 31 celebrated its completion, which took several years to finish.
The project entailed building a natural stormwater management system that treats and filters runoff from the interstate and other nearby commercial properties that were polluting the sink. Its primary purpose is reducing nitrogen loading to the Santa Fe River Basin (currently listed as impaired for nitrate-nitrogen) and, ultimately, the Floridan aquifer. It is also designed to remove other pollutants including heavy metals and suspended solids.
Why protecting Mill Creek Sink is important
Mill Creek Sink is a vital natural resource because it helps recharge the Floridan aquifer, an 82,000-square-mile reservoir holding billions of gallons of the state’s fresh drinking water. Groundwater in the Floridan aquifer is the source of more than 1,000 springs in North and Central Florida and provides water for more than 90% of its residents.
Treating urban stormwater runoff before it enters the sink is important because it can collect harmful pollutants and debris as it travels across impervious surfaces such as roads and parking lots. Some of these pollutants may include nitrogen, phosphorus, sediment, heavy metals and byproducts of car exhaust. Mill Creek Sink absorbs pollutants from stormwater runoff, which can then seep into the aquifer and threaten the water supply. So, protecting surface water at Mill Creek in turn protects the aquifer.
Adding to the project’s urgency, pollutants have already contaminated much of the water in the Floridan aquifer, led by the toxic nitrate-nitrogen. Nitrates are nutrients necessary for plant growth, but water containing elevated nitrate-nitrogen levels flowing from the aquifer into Florida’s springs can harm the ecosystem.
Reducing contaminants from stormwater runoff
The Mill Creek Sink Project was constructed in the City of Alachua, near the intersection of Interstate 75 and State Road 441, where stormwater runoff from adjacent parking lots and roads previously received minimal treatment prior to flowing through the project site toward the Mill Creek Sink system.
It mitigates the runoff contaminant issue by providing a stormwater management system to create additional treatment for runoff flowing into the Mill Creek Sink. A collection barrier between these contaminants and Mill Creek Sink provides nature time to slowly filter groundwater by percolating through loose sandy soils and porous limestone bedrock.
The project consisted of three lined conveyance swales, two pre-treatment basins and a treatment wetland basin designed to collect and treat runoff from the interstate and existing commercial business drainage structures. Additionally, it involved planting 1.2 acres of native plant species to help process nutrients as well as provide suitable wildlife habitat. Along the project’s northern limits, the three basins deliver additional stabilization, surface water containment and access for management activities and public recreational viewing.
The state-of-the-art avoidance, minimization and mitigation plan provides a natural approach to preserving precious water resources entering the sink and the water supply. Interpretive signage will be placed at intervals along the project property area for public education and engagement.