Remedial Action Plan and Closure Program for Coronet Industries

Location
Plant City, FL

Award Period
2006 and 2018 – 2022

Background

In May 2022, APTIM completed the closure of a multi-phase remedial action plan and closure program for Coronet Industries’ process water impoundment facility in Plant City, FL. APTIM was chosen as the successful contractor via a competitive bid process to perform closure of a former process water impoundment, Pond No. 6, in 2006 under a lump sum contract. Following the success of the closure of Pond No. 6, the remainder of the sitewide remedial action was approached by developing a multi-phase remedial action plan (RAP) throughout the 2,500-acre property. APTIM was selected for implementation RAP-1 remedial action and returned to the site in 2018, operating under a cost-plus-negotiated-fee contracting mechanism. Coronet retained APTIM to continue with the subsequent phases (RAP-2, RAP-3, and RAP-4) as a sole source contractor. The final remedial action, implementing RAP-4, was completed in May 2022.

Project Challenges 

The key to the success of closure of the impoundments during all the phase of work was contingent on stabilized haul roads into the sediment and water management. Failure to properly manage both or either of these could lead to catastrophic events resulting in safety issues and the overall success of the project. 

Concrete for the haul road had to be placed in a manner where it keyed into a stable material under the sediment. Once the stable bottom is defined, the placement of concrete rubble with the proper angle of repose, fanning out from top to bottom on both sides, is critical for the stability of the road. Once constructed, the haul roads require continual detailed inspections and maintenance as required. 

Both surface water and groundwater are encountered while working in the impoundments. Prior to commencing work, the area need to be surveyed to determine the best location to install sumps—for both groundwater and surface water management—that will not interfere with construction activities. The area around the impoundments needs to be graded with berms and channels to divert surface water from entering the impoundments. All water needs to be directed far away from the work area, as well as the access points, via channeling and/or mechanical pumping. 

Project Requirements

Services Provided

Pond No. 6 closure addressed a 60-acre former process water impoundment. The closure consisted of constructing a multi-layer capping system over structurally unstable pond sediments, upwards of 24 feet deep. This was done to eliminate direct contact and to reduce infiltration of surface water to the underlying sediments and groundwater. Impacted material from other areas throughout the site were deposited into Pond No. 6 and included in the pond closure. A series of stabilized haul roads, constructed with concrete rubble, were constructed to provide access into the pond in support of the closure and capping activities.

The multi-layer cap construction consisted of:

Additionally, a groundwater interceptor drain system was installed around the perimeter of the of the cap to control lateral migration of potential constituents of concern in the shallow aquifer from the cap area and, potentially, off site. The interceptor system consisted of copolymer perforated sock pipe, installed a deep trencher and interconnected with a series of manholes. Each manhole was equipped with a submersible pump, which was operated of an automated float system and remains in operation today. A network of surface water control ditches were constructed to manage stormwater runoff from the cap.

RAP-1 consisted of the closure of the east portion of Pond 1S, a 6-acre former process water impoundment. A concrete rubble stabilized haul road was constructed to divide the pond into two sections, east and west. Closure of Pond 1S consisted of constructing a multi-layer cap as previously proven successful during the closure of Pond 6 with the addition of a panel drain system on the HDPE liner. We constructed final grade to a 2 percent slope, providing positive stormwater drainage off the cap. Concrete structures and drainage piping were installed to capture stormwater and convey it to stormwater management ditches.

RAP-2 consisted of the closure of three areas with varying levels of ground stability: Ponds 2/2A, 48 acres of former process water impoundments; west side of Pond 1S, a 14-acre former process water impoundment; and a 20-acre process ditch area. All three areas were cleared of vegetation, where accessible, with the use of specialized low ground pressure clearing equipment. A series of concrete rubble stabilized haul roads were constructed to provide access into the ponds. A multi-layer cap system was constructed in the former ponds over the areas with soft sediments, while just a 40-mil HDPE cap was installed over the stable areas.

Large mud waving became a safety concern while installing the sand layer on the geotextile in a portion of Pond 2 due to the depth and volume of soft sediments. The cap was re-engineered where mud waving caused issues to consist of two layers of geotextile covered with a layer of sand without a 40-mill HDPE liner system. The impoundments were restored as surface water retention basins. The areas surrounding impoundments were graded to provide positive sheet flow of stormwater. Concrete structures along with drainage piping were installed to capture stormwater and convey it to stormwater management ditches and ponds.

RAP-3 consisted of the closure two formed process water impoundments with varying levels of ground stability: West side of Pond 4 at 3 acres and Pit S at 4 acres. Closure of the portion of Pond 4 consisted of:

Pit S presented the potential for multiple safety concerns due to the volume/depth of unstable sediments and standing water. Therefore, it was not feasible to construct a multi-layer cap system. An innovative approach was developed to dewater the pit and use a sand slinger broadcast sand cover over the sediment from the banks of the pit at a uniform depth of 24 inches. Depth of the sand was confirmed via lidar survey pre- and post-sand placement with drone technology. The impoundment was restored as a surface water retention basin.

RAP-4 consisted of the closure of the east portion of Pond 4, a 14-acre former process water impoundment. Vegetation was cleared from the pond with the use of specialized low ground pressure clearing equipment. Two stabilized haul roads, constructed with concrete rubble, were installed to provide access throughout the pond to support the closure and capping activities. Pond closure consisted of constructing the proven effective multi-layer cap over the sediments in the pond. During construction of the cap, an overflow channel was constructed in the berm of the pond to accommodate for the potential of high stormwater levels in the pond upon completion of capping the sediments. The impoundment was restored as surface water retention basin.

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