On January 29, the city of Casper's staff were alerted to a 17-foot-deep sinkhole on Midway Drive they described as "large enough to fit a Volkswagen Beetle."

The sinkhole was immediately covered with metal plates and fenced off for three months until a contractor performed the repair.

A memo to council notes that the sinkhole could have easily occurred under Wyoming Boulevard, Poplar Street, or adjacent to the river.

"Staff are grateful the recent sinkhole occurred in an innocuous location and have developed an action plan should another sinkhole arise" as noted in a memo to council.

Depending on the location, structural failures in the NPSSI could result in raw wastewater entering a property or flowing into the river.

At their pre-meeting on May 20, the council is being asked to authorize submitting an application to the Wyoming State Loan and Investment Board for a $2.8 million loan for the North Platte Sanitary Sewer Interceptor (NPSSI) rehabilitation Phase II Project.

A Brief History

During the late 1970's oil boom, engineers projected significant population growth for the Casper-area and designed the sewer interceptor to serve a regional population of 300,000.

Casper's population growth was nowhere near that big. So it was left with an oversized, underused NPSSI. That resulted in low flow conditions, causing a buildup of hydrogen sulfide gas inside the pipe. Certain bacteria turn the H2S into sulfuric acid, which eats away at the concrete, making the pipe thinner, weaker, and rougher.

When enough concrete has corroded away ( over 1 "), the rebar in RCP becomes exposed; at this stage the pipe has lost approximately half its strength. As corrosion of the RCP continues, the pipe can develop cracks, holes, or even collapse if not addressed.

A handful of years ago, the City built a Corrosion Control Facility off Indian Paintbrush Drive to inject Ferrous chloride into the sewer interceptor pipe, which slows the corrosion. This cost around $3 million.

In 2022 the City secured an $8 million loan to fund the first phase of a rehabilitation project, but with inflation, the funding didn't cover all the high priority areas.

The Midway Drive sinkhole was the second NPSSI snafu, the first occurring in 2015, underscoring the urgent need for repairs.

Financial Considerations

Council's FY25 budget already includes $5.05 million for Phase II of the project. They are hoping to get a $2.8 million loan at a 1.25% interest rate to be paid off over the coming 30 years using Regional Wastewater fees. That means the average customer's bill will increase by about 31 cents per month, starting in January 2026 if all goes according to plan.