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Johnston hears water-capacity options and enacts enforcement for conservation under CIWW stages
Summary
Staff presented CIWW capacity projections and recommended buying capacity to maintain a high confidence level; council also received an update that CIWW enacted stage 3 mandatory irrigation restrictions due to high nitrate levels and capacity strain, and Johnston staff began enforcement.
City staff and an engineering consultant updated the council on Johnston’s water-use history, near-term supply options and the conservation response after Central Iowa Water Works (CIWW) moved through staged restrictions this month.
Staff reported Johnston’s current contractual supply of about 6.69 million gallons per day and noted the city previously purchased 0.63 million gallons of capacity in the Sailorville (Grimes) expansion. Consultant Nathan Whipple (Foth) presented demand projections and confidence bands based on historical max-day versus average-day peaking factors. That analysis shows a range of future needs and two key CIWW expansion opportunities: the Grimes (Sailorville) expansion (capacity coming online in the late 2020s) and a planned West treatment plant (estimated online around 2032). Staff said the estimated market cost to purchase additional capacity is substantial — roughly $15 million per 1,000,000 gallons of capacity — and asked the council for feedback on how aggressive to be in purchasing capacity to achieve 80–99% confidence bands for future peak demand.
Separately, staff described recent conservation stages enacted by CIWW. The city reported that CIWW initially placed member communities under voluntary cuts, then moved to stage 2 and then a stage 3 mandatory ban on lawn irrigation after demand approached or exceeded CIWW’s production capability and nitrate levels in the rivers limited treatment capacity. Staff said city parks, street sweeping and hydrant flushing were curtailed, the splash pad closed, and the city notified large water users. Parks staff and police were assigned additional patrols; staff reported 11 reported violations over the weekend, five properties were tagged for probable continued irrigation, and roughly 100 accounts were under review for possible enforcement follow-up.
Council members asked detailed questions about the peaking factors that drive the projections, the role of annexed areas in demand estimates and whether commercial customers are the largest irrigation users; staff said commercial and institutional users (e.g., school district, large sites) are often high-volume accounts and that residential irrigation nonetheless drives peak-day demand. Staff recommended moving forward with analysis to model rate impacts and financing for specific capacity-purchase scenarios and asked the council whether it wanted to pursue a five- or 10-year confidence-target approach. Staff also noted the CIWW shortage-response and the need to track enforcement and possible future changes to codes or agreements.
Council did not take a final purchase decision at the meeting; staff will return with cost and rate-impact scenarios if council wants to pursue purchasing additional CIWW capacity.
