The Houston Ship Channel is critical to the City of Houston and the surrounding region's economy as a navigation route and industrial water supply. In order to maintain and improve its water quality, discharges to the Channel are regulated by the State of Texas' Commission on Environmental Quality (TCEQ).
Phosphogypsum, a by-product of phosphoric fertiliser production, is landfilled as "gypsum stacks". Stormwater which falls within a stack complex is considered contaminated and cannot be released. Ponds, built atop the stacks, are used to contain the water and maintain a surge capacity volume equal to or greater than a 2S-year, 24-hour rainfall event. The only other significant mechanism of water balance reduction is the approximately 0.7 million gallons of stack water consumed daily during normal plant operations.
Stack closure is initiated when the supporting foundation's safe bearing capacity under the stack has been reached. Typically, at closure, a new stack's surge pond is available to contain the old stack's remaining water balance. However, this facility is scheduled to shut down and a new stack will not be constructed. To avoid a potential discharge of gypsum-impacted water into the Channel, the option of initiating pre-emptive closure of the active gypstack complex has been selected.
RPS is working with Tampa, Florida based consulting firm Terra Environmental to design and implement the closure system. To date, approximately 90 acres have already been closed, while an additional 60 acres has been designed and put out to bid. The system consists of a combination of grass lined side slopes, a concrete base road system, and HOPE lined drainage channels, piping and ponds. In order to maintain segregation between clean stormwater runoff and gypsum impacted water, elevated drainage channels are being constructed into the side slopes. In order to address required surge capacity, 75 million gallons of HOPE lined pond capacity has been designed and constructed into a closed-portion of the gypstack complex. The pre-emptive closure will remove approximately 500 million gallons from the final water balance by the end of the active life of the stack in 2011, and provide the future surge capacity required for the final closure sequences.
For the project, RPS consultants are designing the final grading, drainage and roadway systems, preparing the construction plans and technical specifications, performing the earthwork modelling and watershed drainage impact analyses, arranging and negotiating for local permits, providing land surveying, wetland delineation, stormwater pollution prevention plan development and HELP modelling, and performing construction inspection and QA/QC.