09-13-1999 Council Packet - Laserfiche WebLink

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• Replacement of the rubber lineshaft bearings. The bearing retainers may also have been replaced. • A quick cleaning of the well by airlifting. • The 1989 pump repairs had a construction cost of $17,200, and did not include any significant well inspection, maintenance, or motor repairs. Well Construction A copy of the well construction diagram is attached for your reference. Well No. 1 was drilled in 1971 and is finished in the Jordan Sandstone Formation, which is the most widely used aquifer in the Twin Cities area. There are several unusual features about Well No. 1. Most Jordan wells terminate with an open hole or a developed cavity in the Jordan Sandstone, which is overlain by a thick limestone formation in most areas. The limestone acts as a ceiling and prevents the collapse of surface materials into the well cavity. At Well No. 1, the overlying limestone formation is absent. Therefore, Well No. I was finished with a screen and gravel pack out of necessity. Screened wells require occasional maintenance to clean build-up and deposits fi-om the screen and gravel pack. Such deposits can contribute to water quality problems. Based on the available records, only minimal cleaning has been required since the well was drilled. A cleaning of the screen and gravel pack should be performed if inspection of the screen indicates excessive deposits. Well No. 1 has a 24-inch outer casing to a depth of 273 feet and a 12-inch casing to a depth of 315 feet The screen extends down to 385 feet. The annular space outside of the screen and between the casings is filled with a gravel pack to the surface. By today’s standards, gravel packs must extend no more than 10 feet above the screen, and annular spaces between casings must be completely filled with grout to provide a good seal into the aquifer. In addition, grouted wells usually have longer service lives. Due to the construction of the well, Well No. 1 could become susceptible to contamination firom shallow groundwater if the outer well casing deteriorates. When the pump is running, the water level is drawn down approximately 200 feet. If the outer casing were deteriorated, the higher external pressure of shallow groundwater would cause the shallow groundwater to leak into the gravel pack. If the outer casing is not vented, a vacuum would be created in the gravel pack, which would increase the leakage rate. Due to '.he age and construction of the well, the condition of the inner casing and screen should be determi.ied by a downhole video camera inspection. Unfortunately, there is no practical way to determine the condition of the outer well casing. At some fumre time, it may be possible to remove the gravel pack and fully grout the space between casings. This would be a difficult and expensive project. Such a project would be warranted if surficial contaminants show up in significant quantities in the well. We are not aware of any such contamination at this time. Well No. 2 There are several factors about Well No. 2 that relate to Well No. 1: • Well No. 2 is 270 feet deep, and pumps from sand and gravel deposits just above the Jordan Sandstone. Therefore, a difference in water quality between Well No. 1 and Well No. 2 is not surprising. • Well No. 2 has a single casing and is not grouted. The televising performed in 1997 showed that the casing is in generally good condition. Therefore, the Well No. I outer casing should be in generally good condition since the casings penetrate the same soils, subsurface materials, and are of the same age.