In L&DA’s water production courses we compare and contrast the benefits and drawbacks of Surface and ground waters. The UN World Water Development Report 2022 “Groundwater – making the invisible visible” will look at many aspects of groundwater from its use on small islands to its resilience and sustainability. Perhaps we need to know some more basic aspects of Groundwater?
Small quantities of groundwater can be found in many areas. Exploitable quantities are usually associated with limestone, chalk, sandstone, greensand or gravel deposits; although there are other suitable geological formations. Gravel deposits are usually fairly shallow (0 -10 m) and occur in existing or ancient river beds. The others can be found at various depths, up to several hundred metres and may themselves be several hundred metres thick.
The basic quality of the water abstracted (pollution aside) is a function of the geochemical nature of the aquifer, the formations the water has flowed through to get there, the time the water spent in the aquifer and the influence of water from adjacent aquifers and sources.
Groundwaters, especially from deep, porous strata such as chalk and sandstone, are often of consistent chemical quality and good microbiological quality. Rain water can take several years to penetrate to the abstraction point. During this time filtration, solution, precipitation and ion exchange processes can all take place thereby affecting the chemical quality. Bacteria and other micro-organisms die off or are filtered out. The major exception is gravel where the shallow nature and probable influence of a river mean the quality can be much more variable.
Waters abstracted from chalk or limestone (both based on calcium carbonate) or deposits cemented together by calcium carbonate (such as sandstone) yield waters which are hard but otherwise of excellent quality, frequently requiring marginal chlorination as the only form of treatment. However, sometimes the water in an aquifer contains very high hardness, chloride, or sulphate concentrations due to the influence of adjacent formations and is not suitable for use.
Some waters are rendered unsuitable for other reasons or may require treatment before being used for supply. Mine waters commonly contain iron and are of low pH. Gravels often have anaerobic conditions leading to solubilisation of iron and manganese or the water may be bacteriologically unreliable.