Erosion and Sediment Control
East Gypsum Stack Closure at Mississippi Phosphates Corporation
The Mississippi Phosphates Corporation (MPC) manufactured Diammonium Phosphate (DAP) fertilizer at its Pascagoula facility from the late 1950’s thru December 2014 when it declared bankruptcy. The EPA Region 4 Superfund Division assumed financial responsibility of daily operations in February 2017 after resources from the bankruptcy trust were depleted. The East Gypsum Stack (EGS) contained > 750 Million Gallons (MGs) of water with acidic pH (≈2-3) and high levels of nutrients (phosphorus/nitrogen) that was contained in a series of ponds and ditches with very poor structural integrity. Because of the large footprint (350 acres) of acid generating material exposed in the EGS, one inch of rainfall produces about 9 MGs of impacted water that must be stored and subsequently treated before discharge to the bayou. An average of 2 to 4 million gallons per day (MGD) of water is treated via lime precipitation to prevent an uncontrolled release of untreated water to the adjacent Grand Bay Estuary Reserve on the east and Bayou Casotte on the west.
In April 2018, the EPA Administrator signed an Action Memo that approved a 3 phase, $72 Million closure plan for the EGS that is designed to improve the quality and reduce the quantity of contact water and leachate that requires treatment. Construction funding will be provided by EPA’s Remedial Action Priority Panel. A Value Engineering (VE) study was conducted to compare traditional closure methods using liner and two feet of cover soil versus an innovative strategy that employs an engineered geosynthetic turf cover system. EPA selected geosynthetic turf in its final design because it is expected to save an estimated $6 million, including $4.6 million on construction costs and $1.4 million in operations and maintenance costs (since it does not need to be mowed, fertilized, etc.). An estimated 42,700 truck trips for hauling cover soil will be eliminated, thereby reducing the environmental carbon footprint of the closure work. Lastly, the geosynthetic turf can be installed more quickly, resulting in fewer days of ongoing wastewater treatment at the site (which currently averages $50,000 per day, but will be reduced as each phase of stack closure is completed).