Following on from our last issue, consultant hydrogeologist James Dodds discusses technical issues relating to underground water

PROTECTING WATER QUALITY

IN THE LAST ISSUE of Drilltalk, we discussed where water comes from to replenish the underground aquifers. We also mentioned that the source of the recharge water, and the materials it passes through, modify its chemistry and quality.

Water is always pumped for a purpose. Use varies widely, from drinking water—where quality is paramount—to an industrial requirement where purity is not a concern. Water quality is controlled by its overall chemistry, and no water is pure. Indeed, drinking absolutely pure water would be unhealthy and unpalatable.

Natural groundwater contains a wide range of chemical constituents in greater and lesser proportions. These include sodium and magnesium, carbonate, sulfate and chloride, and calcium—all of which are necessary for good health.

In addition to these, there is a wide range of minor traces, the commonest being iron and manganese. How these elements interact with each other is dependent on complex chemical processes, but it is the interactions that result in the taste and feel of the water that we drink.

In industrial processes, the water chemistry may be important in its usage, storage or heating. In food industries, constituents may be added or removed to provide a water with a standardised chemistry that will behave and taste predictably. This means that the chemistry of the water is very important.

Water quality can only be judged against a defined standard. Most common in the UK is the drinking water standard, based on several EC water quality directives. These provide limits for various constituents of the water and its physical properties to ensure that it is safe to drink. But the standards do not define the water chemistry, as some unharmful key constituents are not included.

The most common cause of poor quality water is contamination from surface or near-surface activities. Materials on or in the ground may leach pollutants that dissolve in the recharge water and are carried down to the aquifer. Chemical and physical processes may act on the pollutants and remove them from the water. But if the contaminant load is too high, or the geological conditions allow rapid transport, then contamination or pollution of the aquifer will occur.

To minimise the risk of contamination it is important that surface activties are controlled, and this is the reason for the myriad of regulations controlled by the Environment Agency and others. These deal with the disposal and storage of wastes and potentially polluting substances, and objectives of the controls are discussed in the EA document The Policy and Practice for the Protection of Groundwater.

Some key points should be observed to minimise the risk of borehole contamination: