An observation published in 1986 shows how badly the underground formation was leaking while Quail Creek Reservoir filled near St. George.
The report was written by two members of the Brigham Young University geology department, Paul Dean Proctor and Willis H. Brimhall. The article, "Silver Reef Mining District, Revisited: Washington County, Utah," was published by the Utah Geological Association in 1986.This past New Year's Day, the dike burst at Quail Creek Reservoir, about nine miles from St. George, causing an estimated $12 million in damage to homes, bridges, farm land, livestock, roads and a natural gas pipeline.
Officials of the Utah Division of Water Resources have said they were aware of geological problems at the site.
In an attempt to stop water from flowing underground, the Washington County Water Conservancy District, owners of the reservoir, drilled hundreds of grouting holes in the dike and filled them with concrete. The district also added a new cutoff trench to halt the flows.
In their report, Proctor and Brimhall show a geologic map indicating that the area where the dike was built is part of the Shnabkaib Member of the Moenkopi Formation. This member, or type of geological formation, is described as "interlayered gray gypsum beds and pink to red siltstones and shales (having) mound-like weathering with powdery gray expanded soil above gypsum beds."
The fractured underground formation, plus gypsum that might have dissolved upon contact with water, are two of the main possibilities being examined as possible causes of the dike failure. An independent panel has been reviewing the data in St. George, during sessions that at first were open to the public but are now conducted in secret.
The Shnabkaib Member was described as 480 feet thick and being "more resistant to erosion than the underlying Middle Red Member" of the Moenkopi Formation.
The most telling comment in the report is: "Of special interest is a relatively recent heavy flow of water which now issues from the cream-colored sandstones just north of Highway 17 on the east limb of the fold (the Virgin Anticline). The likely source of the water is the newly constructed and filling Quail Creek Reservoir 11/2 miles north."
That was published after then-Gov. Scott M. Matheson dedicated the completed reservoir project on Sept. 20, 1985. Even after that time, work was carried out in an effort to seal it better.
The area where water was seeping rapidly from the underlying rock was directly downstream from the Quail Creek Dike. If water continued to work its way through fractured rocks beneath the dike, that might have eventually triggered the collapse.