Following on from our last issue, consultant hydrogeologist James Dodds discusses technical issues relating to underground water.
WHERE DOES THE WATER COME FROM and how does it get there?

In previous issues of Drilltalk, we’ve discussed what groundwater is, how it moves, and the importance of an aquifer’s ability to store water. But where does the water come from in the first place—and how is it replenished?

When many types of sedimentary rocks such as sandstone, limestone and chalk are originally deposited they are formed under water from saturated sediments. As these sediments are subjected to heat and pressure and the rocks form, this water can be retained in the rocks. This is called formation water and is almost always very saline due to its extreme age. In order for water in an aquifer to be useful, formation water must have been flushed out by fresh, relatively recent water.

The origin of all fresh groundwater is rainfall in one way or another. Rain falling from the sky is affected by several processes before it becomes groundwater, and only a proportion of rainfall makes it through to the water table.

The most important effects are the use of the water by plants, and evaporation from vegetation and the ground surface (evapotranspiration); runoff across the surface to drains, streams and rivers, shallow flow in the soil and thin perched aquifers toward drains and ditches.

What is left becomes recharge. In the UK this generally varies between 20% and 40% of annual rainfall, although higher and lower values can be encountered in certain catchments. The recharge replenishes the aquifer year after year, but is dependent on rainfall, so in dry years recharge is less.

The amount of recharge is partly dependent on the ‘right type of rain’: Short, heavy rainstorms, particularly after a dry period, result in a high level of runoff as the rainwater cannot seep into the ground fast enough. Steady, prolonged rain through the winter when plant usage is low and temperatures are low—reducing evapotranspiration to a minimum—maximises the recharge. So during the grey cold, rainy weeks of autumn and spring, think of the benefits to your groundwater supply. If the ground is not frozen, some of the highest recharge rates come from melting snow.

Recharge of rainfall isn’t the only source of water. Leakage of water from rivers and canals can contribute significantly to the groundwater resource, as can leakage from water mains and sewers. Studies in the UK have shown that leakage from water mains and sewers under urban areas can compensate for the ground being covered by the tarmac, concrete and buildings which prevent direct recharge from rainfall.

Recharge water from the different sources will have different qualities. As water moves through the soil it picks up chemicals applied to the soil and will leach or flush contaminants as it passes through. Sewers have obvious contaminant loads. The impact of surface activities on the quality of recharge waters, the quality of groundwater and therefore the quality of your borehole water will be discussed in the next issue.

James Dodds, JDIH (Water & Environmental) Limited, 01332 865866 , www.jdih.co.uk.