By Rob May, Prairie Farm Rehabilitation Administration, Watrous
Wet wells are installed next to surface water supplies to provide a quick and convenient method for housing a water distribution pump.
Monitoring of wet wells with direct intakes into dugouts, or with filters such as gravel or sand between the wet well and the dugout, has shown that oxygen levels in the wet wells are considerably lower than in the dugout water.
The turnover of fresh water with sufficient oxygen levels -- greater then 3 mg/l -- is very slow in wet wells. For example, a family of four uses about 1000 litres/day. A 762 mm wet well with 4 metres of water in it would have a volume of 1800 litres. This means replacement of the water in the wet well would require about two days.
Dissolved and suspended organic material is concentrated from the runoff water and reagitation in dugouts. Organics entering a wet well are further concentrated by the well's settling effect.
Decomposition of the organics will take place in the dugout and continue to occur in the wet well. Insufficient oxygen levels will cause the organics to decompose in an anaerobic or anoxic state. This results in hydrogen sulphide gas formation, lower pH, water high in organics, and resuspension of nutrients located at the bottom of the wet well.
Complete elimination of the wet well, such as a direct intake with jet pump, may also have its merits. In some cases, though, it is difficult to omit, and for this reason, alternate wet well designs are being considered.
To avoid the problems associated with a large diameter wet well, groundwater well technology has been applied to a surface water problem. A 50 mm or 75 mm floating direct intake is still used, but a 152 or 200 mm PVC (polyvinyl chloride) well casing replaces the 762 mm or greater CSP (corrugated steel pipe), used to house the submersible pump. (refer to Vol. 1 No. 1,
A test wet well has been installed at a site where these anaerobic/anoxic problems have previously been observed. The existing 127 mm MDPE (medium density polyethylene) intake pipe was extended and attached to a 152 mm PVC casing. Rigid insulation was placed around the PVC casing, which was installed in the existing 762 mm CSP.
Pit run gravel insulates and stabilizes the new wet well. The pump is hooked up to the existing distribution system with a pitless adaptor. When the 3/4 hp, 40 l/min submersible pump is turned on, the water level in the 152 mm PVC casing drops 5 cm, where it stays until the pump is turned off. This has the effect of drawing water from above and below the pump, ensuring a fresh supply of water to the wet well.This project was supported by PFRA and Myron Teneycke, a producer in Young, SK.
