Southwest Treatment AreaThe Coliseum Boulevard Plume (CBP) is a groundwater plume that contains trichloroethylene (TCE). TCE is a common solvent that is used for many purposes, including metal cleaning and laundry stain removal. TCE was used by the Alabama Department of Transportation (ALDOT) for asphalt testing until the mid-1980’s.

Groundwater in the southwest part of the CBP is managed in several ponds. These ponds provide for nature to degrade and remove TCE in the water through uptake by plants and treatment by exposure to sunlight and air.

The Southwest Treatment Area (SWTA) is a 100-acre former commercial sand and gravel mining operation located west of the Chisholm neighborhood and east of Jackson Ferry Road. When the mining excavations reached the groundwater, the groundwater was pumped out of the mine excavations. The pumping caused the surrounding groundwater, including the southwest portion of the CBP, to flow into a pond that is constructed to maintain low groundwater levels.

Surface mining activities were discontinued in 2009 and the ALDOT purchased the SWTA to continue pumping to capture the groundwater in the southwest part of the CBP.

The following maps show the CBP and the SWTA ponds.

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Southwest Treatment Area FeaturesA groundwater collection ditch is connected to a pond, called a Dewatering Pond, which controls the groundwater levels. The Dewatering Pond was used in the mining operations to control groundwater levels and was retained as one of the corrective measures for the CBP. As groundwater flows into the collection ditch, the TCE in the groundwater is treated by vegetation and exposure to sunlight and air.

Water is pumped from the Dewatering Pond to a Transfer Pond, which is a larger pond where water is stored and exposed to sunlight and air for treatment. From the Transfer Pond, water can be pumped to either the Auxiliary Treatment Basin or the Discharge Pond. Water from the Discharge Pond flows through a Discharge Ditch into a Natural Wetland before it leaves the site through a City drainage culvert.

TCE is monitored at various locations in the treatment ponds. TCE concentrations are reduced between the Dewatering Pond and the Transfer Pond. No TCE has been found in the Discharge Pond or the Discharge Ditch. ADEM specifies operating requirements and discharge limits required for compliance. The system is designed to meet ADEM requirements. Monitoring and routine observations show that the system treats the TCE in the surface water prior to it leaving the site.

ReclamationAfter ALDOT bought the property in 2009, the property was reclaimed following Alabama Department of Industrial Relations requirements. Reclamation eliminated high and unstable earthen walls. Photographs below show the change of the SWTA from a mining operation to a natural treatment system. Groundwater that is collected in the ponds becomes surface water and TCE is treated in the ponds.

Dewatering Pond used for Sand and Gravel Mining Dewatering Pond after Reclamation

Dewatering Pond

A surface water pump in the Dewatering Pond maintains low groundwater levels so that the groundwater from the southwest portion of the CBP moves to the treatment ponds. Groundwater is collected in a ditch that flows into the Dewatering Pond. The surface water pump and groundwater collection ditch are shown on the photographs below. TCE in the groundwater is treated as it becomes surface water.

Dewatering Pond and Pump Groundwater Collection Ditch

Dewatering Pond

The Dewatering Pond pump is operated by an automated control system. The volume of water pumped and time of operation is recorded continuously (see graph below). This information is maintained on a computer and is used to monitor the system.

Dewatering Pond Control Panel for Automated Pumping System

Example graph from the Control system

Transfer Pond

Surface water in the Dewatering Pond is pumped to the Transfer Pond. Surface water is treated in the Transfer Pond by exposure to sunlight and air. Pumps in the Transfer Pond move water to the Discharge Pond using an automated control system.

Discharge from Dewatering Pond into the Transfer Pond

Transfer Pond Control Panel for Automated Pumping System and Pumps

The Auxiliary Treatment Basin (ATB) was formerly used to settle silt and clay out of water during the mining operations. A valve box constructed between the Transfer Pond and Discharge Pond allows some or all of the water to be diverted to the ATB for additional treatment if needed. The ATB also provides additional water storage for system maintenance activities.

Auxiliary Treatment Basin

ATB Under Construction ATB Vegetated (Winter 2011)

Valve Box to Divert Flow

Discharge Pond

Water from the Transfer Pond is pumped to the Discharge Pond for final treatment. Water enters an Inlet Basin and naturally moves through the wetland plants to the Outlet Basin. Two pipes in the Outlet Basin allow the water to flow into the Discharge Ditch. The Inlet Basin and wetland plants are shown below.

Inlet Basin in the Discharge Pond Wetland Treatment Cell

Natural Wetlands

Water flows from the Discharge Pond into the stone-lined Discharge Ditch. The water flows through the Discharge Ditch into the Natural Wetlands.

Outlet from the Discharge Pond to Discharge Ditch End of Discharge Ditch to the Natural Wetlands

Natural Wetlands and Flood GateThe 15-acre Natural Wetland provides habitat for wetland trees, birds and aquatic plants and animals. Water movement through the wetlands varies due to natural diversions created by beaver dams and aquatic plants. Water is released from the natural wetland through a City floodgate and drainage culvert. ADEM requires ALDOT to monitor TCE and water quality at the floodgate. There have been no violations of ADEM requirements.

Natural Wetlands Flood Gate

Southwest Treatment AreaThank you for visiting our site and taking the time to review this element of ALDOT’s work on the Coliseum Boulevard Plume. If you have any questions, please do not hesitate to contact us at:

CONTACT INFORMATION Email: cbpinfo@dot.state.al.us

Call: Project Information Line 334-353-6635

View of the Transfer Pond Natural Wetlands