Some key issues in the design of water wells in unconsolidated and fractured rock aquifers

The key well design issues considered in this paper are the length of the intake section; the hydraulic efficiency of the well; the length of grouted upper casing for wells in fractured rock aquifers and the potential trade-off between well yield and security against pollution; and the economics of well design. For wells in thick, relatively uniform unconsolidated aquifers, the well depth and screen length can be estimated using a simple discharge-drawdown relationship. This approach can help avoid constructing unnecessarily deep wells if alternative guidance to screen the bottom third of the aquifer was followed in such situations. Hydraulic efficiency is an important consideration in well design: the paper highlights that whereas screen entrance velocity has been a topic of much discussion in the literature, well upflow velocity has received less attention, but can be an important contributor to well losses in small diameter screens. In fractured hard rock aquifers, there may be a compromise required in well design between maximising well yield by exploiting shallow fracture zones whilst also providing adequate sanitary protection to the well by installing an upper grouted casing. Recent data from Ireland on the distribution of hydraulic conductivity with depth in poorly productive fractured rock aquifers are used to calculate the reduction in well yield that would result from increasing the length of the grouted upper well casing. Economic aspects of well design are especially important where there are a large number of wells to be drilled and/or where wells are required in poor rural communities in developing countries. The principles of cost-effective boreholes for developing countries are summarised, noting the opportunities for small-diameter shallow wells constructed with inexpensive manual or lightweight mechanical drilling rigs.