Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | AUGUST 2015 | 35 In the know Technically speaking: boreholes B oreholes play a vital role in public and private water supplies; in some cases, the same source has provided a reliable and cost effective supply of water for over a hundred years. However, like everything else, wells and boreholes don't last forever and they benefit from a little love, care and understanding to ensure continued service and to maximise their efficiency. What follows is a practical guide to installing and rejuvenating a water supply borehole. Installing a supply borehole The key to installing a reliable and efficient borehole is understanding the hydrogeology of the site, and assessing the aquifer potential and aquifer properties. The geology of the UK is highly variable, but we are fortunate to benefit from some excellent aquifers on both a local and regional scale. The major Chalk, Limestone and Sandstone aquifers found in the South and Midlands in particular provide substantial yields, potentially in the region of 1,000m 3 / day or more. Good yields can be obtained from other formations too, and at a local scale there might be several options for developing an independent water supply. The depth of a borehole is determined by the local geology. In the UK, a borehole can range from tens of metres to hundreds of metres, all depending on the depth and Love your borehole For water supplies large or small, boreholes can provide years of service if given the right care and attention Dr PhIL ham PrinciPAl HydroGeoloGiST, envireAU WATer Principal bedrock aquifers in the UK (map courtesy of the UK Groundwater Forum) properties of the target aquifer. There is no maximum, but as a general rule of thumb, most water supply boreholes in the UK are less than 300m in depth. The diameter of a borehole is primarily a function of the required borehole yield, which is controlled by the size of pump which will physically fit down the hole. For example, to deliver a yield of 100m 3 /hour, a submersible pump with a 150mm (6") diameter motor is required and therefore the recommended minimum diameter of the finished borehole would be 200mm (8"). The diameter of the borehole will also have an impact on borehole efficiency, and there are benefits in drilling larger diameter boreholes when targeting specific aquifer types. Borehole design Having understood the hydrogeology of a site and the depth of the target aquifer, the borehole needs to be designed. Commercial boreholes typically comprise a permanent, grouted steel casing at the top of the borehole to provide a sanitary seal and prevent ingress of poor quality surface waters. The casing may also be used to case out 'unwanted formations' that could be unstable or yield water of a poor quality. Below this, a borehole may be 'open-hole' (unlined) or lined with a slotted well screen and casing, dependent on the stability and nature of the target aquifer formation. A number of drilling methods exist and a range of materials can be used in the construction of a borehole, depending on depth and the exact geology likely

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