W2B-037 Groundwater - Ecology-0.txt
Enfo - promoting care of the environment GROUNDWATER
Did you know that 99.5% of the world's water cannot be used because it is either saline or is locked up in glaciers and ice sheets? Most of the remaining water is present in rocks as groundwater and less than 0.01% is present in rivers and lakes. In many parts of the world where rivers and lakes are either absent or are polluted, groundwater is often the only source of fresh water. In Ireland about 25% of the water we use comes from groundwater and this figure will undoubtedly rise as we become more developed. Groundwater is a resource that we need to protect because once it is polluted it cannot be used for many years.
WHAT IS GROUND WATER?

Groundwater is part of the hydrological or watercycle (Diagram 1). The cycle begins with water evaporating from the oceans and forming clouds. When the clouds cool - such as when they rise over mountains - the water is released as rainfall. Some of the rain is evaporated or is used by plants and goes straight back into the atmosphere. Some flows directly into rivers and streams and from there flows back into the oceans and some percolates through the soil and into the underlying rocks - the water in the rocks is groundwater.

Once in the rock the water moves slowly through the pores or cracks and emerges at ground level by coming out at springs or by flowing into rivers.

A body of rock which contains appreciable quantities of water is called an 'aquifier'. The 'water table' in a aquifier is the level below which the pores or cracks are completely filled with water - the saturated zone. Above the water table - the unsaturated zone - the pores are filled with both air and water. In many parts of Ireland 'perched' water tables exist where impermeable material (material which will not allow water through), such as boulder clay, overlies an aquifier. In these situations water cannot get through and you get two watertables, one in the soil and one in the aquifier beneath.

You can bring groundwater to the surface by drilling into saturated rock and pumping the water out. The amount of water that can be extracted depends on the number and size of the pores in the rock. If the rock is 'tight', such as granite, little water can be pumped out but if the pores or cracks are numerous and large, such as in fissured limestone, you can extract useful amounts of water.
A MAJOR POTENTIAL FOR DEVELOPMENT
Approximately 1,140 mm of rain falls in Ireland each year of which about 360 mm percolates through the soil and into groundwater (this is called 'groundwater recharge'). The amounts of groundwater recharge vary in Ireland, with most in the north-west (513 mm per year) and least in the south (350 mm per year).

50% of Ireland's groundwater is considered 'developable' meaning that we could extract it relatively easily. At present we extract less than 2% of the developable groundwater and of this, public bodies use 36%, industry uses 37% and the remainder is used for rural domestic supplies. Most groundwater is extracted in the east of the country and least in the west and north-west. Some midland and western counties have a very high dependence on groundwater - County Roscommon, for example, uses groundwater for about 80% of its water supply.

WHY GROUNDWATER NEEDS PROTECTION

In general the quality of Irish groundwater is good but because we have no national survey of groundwater, like An Foras Forbartha's national survey of rivers and lakes, it is difficult to accurately assess groundwater quality.

Although it is usually more difficult to pollute groundwater than rivers and lakes (because the soil can either stop the pollutant reaching groundwater or help to reduce its concentration) many of our activities can have an impact on groundwater quality. All too frequently we have adopted an attitude of 'out of sight, out of mind' and this has resulted in groundwater being polluted and wells having to be abandoned. Because groundwater moves slowly, once it is polluted it can remain so for many years.

In rural areas septic tanks and farmyard activities can pollute groundwater while in and around urban areas refuse dumps would be just one source of contamination.

A septic tank consists of a watertight container which separates liquids from solids and provides a limited amount of organic digestion, and some sort of soakage system for disposal of the liquid overflow. Because the effluent from septic tanks is not fully treated it can cause pollution.

If you have a septic tank and it is polluting the water in your well it is probably because the tank is located too close to the well. Your septic tank can also pollute groundwater if it is situated in an area where the soil is too thin or too waterlogged to allow proper soakage of the effluent. Problems of pollution may also arise if you have used a soakage pit filled with stones rather than a proper pipe distribution system or if you do not empty your tank regularly. Diagram 2 shows a layout for a septic tank that minimises groundwater pollution.

The use of silage for winter feeding has increased dramatically in Ireland over the last 20 years. When you make silage, an effluent with a great potential for pollution is generated. Unless you put a concrete slab beneath the silage and drains at the sides to collect the effluent, pollution of groundwater and rivers and lakes can occur.

Dungheaps in farmyards can cause similar problems to those described for the silage and you need to take similar precautions to prevent pollution.

In urban areas one of the greatest threats to groundwater quality is that posed by refuse dumps. Each one of us generates about a quarter of a tonne of refuse each year which has to be disposed of - usually by landfill.

Domestic refuse generates a noxious leachate when water is passed through the landfill. This leachate can cause severe pollution of groundwater unless the landfill is located in a suitable area and designed and managed properly.

To avoid pollution of groundwater, dumps need to be located where there is a thick unsaturated zone so that the leachate can be reduced in concentration before it reaches groundwater. The thickness of the unsaturated zone depends on the type of material beneath the dump - thicker in sands and gravels than in clays. Also, after each day's dumping the refuse should be covered with material which won't allow water into the refuse, because the more water you allow into the refuse the more leachate you generate.

Even where there is a thick unsaturated zone and where you cover the refuse daily some contamination of the groundwater will occur. It is imperative that we locate refuse disposal sites only in areas where the groundwater resources have been proven to be of no great importance.

In addition to refuse dumps, leaky sewers, leaking underground storage containers and accidental spillages have caused pollution of groundwater beneath urban areas (Diagram 3).

GETTING A SUPPLY OF GROUNDWATER

If you are not part of a water supply scheme and are considering a source of water then you might consider digging or boring a well and extracting groundwater. The accompanying chart shows the steps involved in getting a supply of groundwater.

To determine how much water you will need you should allow 230 litres (50 gallons) of water a day for each person in your household. The requirement for your livestock varies from about 140 litres (30 gallons) a day for a dairy cow to about 20 litres (4 gallons) a day for 100 chickens. Having determined the total daily requirement you should then see what your peak usage would be because it is this that your well will have to yield without going dry. It is recommended that your hourly well yield be at least one sixth of your daily needs.

Traditionally people have gone to water diviners to find out if there is groundwater beneath their land. Although a water diviner may be able to indicate where you should drill he will not be able to tell you how deep you will have to drill or how much water you are likely to get. This information can be obtained from the Geological Survey and has the added advantage of being free of charge!

The type of well you use will depend to a large extent on the depth to the permanent watertable and you can check this out easily using a mechanical excavator. If you have to use a bored well check out the reputation of the well driller with those of your neighbours who have had wells drilled by him and make sure you get a written contract that specifies the terms and conditions of the drilling agreement.

Once the well is drilled and the yield tested, the well should be sealed against pollution (Diagram 4) and disinfected. A few days after disinfection you should send a sample of water from the well to the local Health Board for analysis. The driller should send copies of the borehole log and the results of the well test to the Geological Survey. The results of the water analysis should also be sent to the Geological Survey. This information helps them to build up a regional picture of groundwater quality and quantity and so helps the next person coming along looking for information.

The water quality problems you are likely to encounter include hardness (lime) and iron and iron bacteria. These problems are fairly easy to solve and the Geological Survey will be able to help you if you encounter such problems.
