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Johnston hears technical briefing on CIWW capacity, conservation actions and possible large purchases of treatment capacity
Summary
City staff and Foth engineers presented long-term water-demand projections, CIWW expansion options and immediate conservation actions after CIWW staged shortages. The council was asked whether to pursue capacity purchases (options include 630,000 gallons already bought and a recommended 700,000-gallon request at the Grimes expansion) knowing costs can run into millions per million gallons.
City staff and a consulting engineer presented an extended briefing on water use trends, regional capacity options and near-term conservation steps tied to Central Iowa Water Works (CIWW) operations.
Matt (city staff) summarized 20 years of population and water-use data, noting an approximate historical growth rate of 3.65% annually and that average-day use has risen with population while maximum-day purchases have trended down after irrigation-rate changes. He told council the city must plan capacity for peak-day demand, not just average-day consumption.
Nathan Whipple of Foth explained the demand graph and confidence bands used to plan purchases: the city currently has a contractual CIWW supply of about 6,690,000 gallons and has already purchased 630,000 gallons from a Sailorville expansion. Foth presented scenarios tied to two upcoming regional expansions (Grimes, coming online ~2028, and a West plant ~2032). The consultant outlined confidence intervals (80%/95%/99%) and how annexation assumptions and historical peaking factors drive higher projected max-day demand. He said the city could request 700,000 gallons from the Grimes expansion to reach higher confidence, but warned that if all other communities also buy in, Johnston might receive only its minimum allotment.
Councilors questioned why the confidence bands start above recent max-day purchases; staff explained the model uses historical peaking factors (which include multi-year outliers) and adds expected annexation demand. Staff and the consultant estimated the capital cost of purchased capacity is large—on the order of $15,000,000 per million gallons in recent estimates—and advised council to weigh rate impacts. Staff proposed working with financial adviser PFM to model rate effects of different purchase options.
Separately, staff reported recent CIWW conservation actions tied to high nitrates in the Raccoon and Des Moines rivers and capacity stress: CIWW briefly moved through stages 1–3; stage 3 enacted a mandatory ban on lawn irrigation for member communities. The city suspended nonessential uses, shut off sprinklers in some municipal facilities, and flagged properties for continued irrigation. Staff described an enforcement path: outreach and warnings, then possible shutoffs if automatic-irrigation systems do not comply after notification (staff said a 48-hour compliance window would be used before disconnection steps are considered). The council did not take a final purchase decision at the meeting but asked staff to return with rate-impact modeling and recommended target confidence levels for future purchases.
The staff presentation and follow-up work will return to council for policy guidance on whether to request specific allotments in upcoming CIWW expansions and how to fund them through rates or other means.
