City leaders hear technical case for near‑ and far‑field modeling of proposed Inner Harbor desalination plant

City staff and the design‑build team told the City Council on March 25 that they are continuing detailed hydrodynamic work — including both near‑field and far‑field modeling — as part of the Kiewit design contract for the proposed Inner Harbor desalination plant.

The city’s chief operating officer for Corpus Christi Water, Drew Mollie, summarized work already performed to support permitting and said the draft discharge permit issued by the Texas Commission on Environmental Quality (TCEQ) reflects independent modeling the agency performed in parallel with the city’s consultants. “Because there was congruency and because there was an agreement between the two different models, we were able to get our draft discharge permit in December of 2023,” Mollie told council.

GHD and Kiewit described the modeling they are performing as part of the plant design. GHD’s Chris Benjamin said three‑dimensional hydrodynamic modeling is needed to design diffusers and predict plume fate under a range of intake salinities, temperatures and operating recoveries. “We’ve taken all the different factors and all the monitoring data that’s in the permits … and we’re using that holistically to make sure that we can accurately model the discharge under different scenarios,” Benjamin said. Technical director Jason Cochlin and aquatic ecologist David Buzan of Freese and Nichols reviewed the city’s earlier field data collection — acoustic Doppler current profiling, water‑quality sampling and a 2019 white paper setting salinity thresholds for species — and said the data underpinned the near‑field analyses used in the TCEQ review.

Why it matters: public commenters have repeatedly asked for an independent far‑field study to estimate whether concentrate (brine) discharged into the Inner Harbor could measurably change salinity across the Corpus Christi Bay system. The Kiewit/GHD team emphasized that “near field” refers to the regulated mixing zones set by TCEQ (initial dilution, aquatic and human‑health zones out to roughly 400 feet from a diffuser), while “far field” denotes modeling beyond those regulatory zones to predict where and how concentrate might disperse across the broader estuary. The city manager and the design‑builder said both analyses are necessary to demonstrate regulatory compliance and to inform diffuser design.

What the city presented: Mollie and the city manager explained the history of environmental work and funding for the project, including SWIFT (Texas Water Development Board) loans and the bonds already sold to fund the program. Kiewit and GHD described their modeling software and approach: CoreMix for near‑field jet/dilution, and three‑dimensional hydrodynamic models (e.g., EFDC/MIKE or comparable grids) for far‑field transport. GHD said diffuser design and pre‑treatment (dissolved air flotation, oxidation) reduce biological oxygen demand and remove particulates and metals prior to discharge, a factor in the team’s ecological assessment.

Public comment in the hearing was large and focused on the modeling request and on who should perform far‑field work; speakers asked for an independent third‑party modeler without ties to the project. Several environmental, recreation and fishing community organizations presented letters and petitions calling for a robust, independently contracted far‑field modeling study that would include all proposed intake/discharge sites. The design‑builder responded that a far‑field model is already part of Kiewit’s contracted work and that Freese & Nichols, EPA, TCEQ and other reviews have produced consistent, low parts‑per‑thousand changes in basin‑scale scenarios run to equilibrium.

Outcome and next steps: Staff recommended the council allow the design‑builder (Kiewit with subconsultant GHD) to continue near‑ and far‑field modeling that is already in the design scope. The council did not record a final policy vote on whether to separately procure an additional third‑party far‑field contract during the meeting. Several council members asked for clearer timelines and cost estimates for the comprehensive far‑field modeling; staff said the Kiewit design team was about halfway through its modeling work and expected to finish in roughly two months. The council and staff also discussed public requests for an independent model; one outside contractor submitted a proposal to the council during the meeting, and councilmembers asked staff to return with cost and procurement options if the council wished to pursue a separate third‑party engagement.

Ending: The design‑build team said they will finish the hydrodynamic modeling needed to complete diffuser design and to meet TCEQ permit conditions; councilmembers asked staff to bring clearer cost and schedule figures back to the council for any additional third‑party modeling procurement.