Salt Lake City Restores Mill Creek Well Re-drilling an old well restores a water supply source. Scott Cardwell, P.E. April 13, 2001
The original 12-inch well was 237.5 feet deep and had a flow of 1500 gallons per minute (gpm). The new 20-inch diameter well is 545 feet deep and has a capacity of over 2500 gpm. The ground elevation at the well site is about 4560 feet above mean sea level. The normal water level in the new production well, when the well is not being pumped, is 186 feet below the ground surface. The level drops to 236 feet when the pump is being operated at its capacity. The water was tested and it meets all drinking water standards. Prior to drilling the new production well, the City drilled a 6-inch monitoring well about 14 feet west of the production well. It was drilled to a depth of 725 feet. This hole was used to evaluate the subsurface layers and locate the water bearing layers. This information was used to identify the right locations for screens to be placed in the production well. The total screen length installed was 110 feet. The production well was drilled to a diameter of 28-inches. A 20-inch casing and screen was installed in the middle of the drill hole. Gravel was placed between the outside of the drill hole and the casing. The gravel filters out any fine sand particles that might be moved with the water that flows to the well during pumping. During the pump testing of the final production well both the monitoring well and the old well, located 67 feet south, were used to measure the drawdown of the groundwater. These measurements gave information about groundwater flow conditions at the well and enabled the City to choose a safe sustainable pumping rate. This well will be a valuable addition to the City's water sources and will play a vital role in providing dependable water supply for Salt Lake City's customers, particularly during droughts. It will provide over 3.5 million gallons per day (MGD) when it is in use. The cost of drilling the new replacement well was about $254,000. The building and pumping equipment cost another $393,000. The new production well was placed into operation in April 2001.
The new well was drilled using a more modern reverse circulation rotary method. Reverse circulation drilling has advantages over the cable tool method of drilling. This method uses a circulating drilling fluid to return cutting to the surface. The process for reverse circulation drilling involves a rotating drill string with a bit on the end that chips (roller) or scrapes (blade) the cuttings from the bottom of the hole. The cuttings are then returned to the surface via the interior of the rods.
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