Frequently Asked Questions about Water Resources and Groundwater
Do you ever wonder where your drinking water comes from?
But where does all this water come from?
How much fresh water is really available to us?
When rainfall becomes groundwater, how does it actually move through the ground?
How does this affect your drinking water?
So, how clean is Ireland’s groundwater?
What are Groundwater Protection Schemes?
How to Disinfect your Well
Frequently Asked Questions about Groundwater Web Mapping Site
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Do you ever wonder where your drinking water comes from?
It’s a good question to ask every now and then. If you live in a rural area, chances are your water comes from a well that pumps water from under ground - groundwater. In other areas, your water may come from a river or lake - surface water. In fact, a quarter of Ireland’s water supplies come from groundwater sources - in some areas, this figure increases to 80%! Groundwater doesn’t just provide drinking water, in Ireland, it contributes significantly to rivers by providing base flow, especially in the summer time. Base flow can be defined as the water that seeps into streams from the ground.
Here in Ireland, many days you only have to look out your window to find the answer to that - from precipitation, or rainfall. Some of this water evaporates into the atmosphere or is lost by transpiration from plants, some runs into streams eventually flowing to the sea, and some soaks into the ground and percolates downwards until it meets the water table. Water below the water table is called groundwater. This water flows slowly through the ground until it seeps into rivers, or comes out of the ground as springs, or is pumped to the surface from wells.
The amount of fresh water that is available to humans is actually quite small; therefore we need to conserve and protect whatever we have, especially if we want our children’s children to have clean, safe drinking water. The Earth is unique in the fact that it is covered by so much water. Most of it, however, makes up the oceans - 97% in fact. Does that mean that humans depend on the remaining 3% to get our drinking water from? Well, no actually, because 2.15% of the Earth’s water is held in ice sheets and glaciers, like in the Arctic. This means that there’s only 0.85% of the Earth’s water that is available to us as a fresh water drinking source. The majority of this water is held under ground, as groundwater. Only a very small percentage forms rivers and lakes. Considering how precious this resource is, it is extremely important that it is well protected.
The water is stored in pores and fractures within the rock. Pores are the spaces between the grains. Above the water table, in the unsaturated zone, these pores are filled mainly with air. Below the water table, in the saturated zone, they are filled with water. It’s from the saturated zone that wells abstract water. Rocks vary in their ability to store and transmit water. Those that are capable of yielding water to wells in usable quantities are called aquifers. Some aquifers, usually those made of sand and gravel, transmit water through the pores, or spaces between the grains. Other aquifers, usually those composed of bedrock, transmit water through fractures in the rock.
In Ireland, the majority of our aquifers are limestone, which is made up of calcite, or calcium carbonate (CaCO3), from shells, shell fragments and calcite crystals. Fractures in these rocks store and transmit water for wells for domestic, group and public supply wells. A unique feature of limestone is that it dissolves in weak acid solutions. As rainwater falls to the ground and comes into contact with topsoils, it becomes a weak acid solution called carbonic acid (HCO3). When this comes into contact with the limestone, the limestone dissolves to form calcium bicarbonate (Ca(HCO3)2), which is washed away. This allows small fractures to widen, which then allows more water through, which allows more limestone to be dissolved, which then makes the fracture widen more... This process is called karstification
One of Ireland’s best examples of an area that is karstified is the Burren in County Clare. Karstified areas are characterised by caves, sinkholes, springs, and a lack of continuous rivers. In karst areas, there is a lot of interaction between the surface water and the groundwater. Streams are fed by groundwater springs, which then may “disappear” underground, only to reappear at the surface at another spring kilometres away. Also, groundwater flows in these areas very, very quickly. In a well developed karst system, the water can flow up to 200 meters in an hour! Large areas of karst limestone are present in counties Galway, Clare, Mayo, Roscommon and Sligo. Smaller areas are present in counties Cork, Tipperary, Kilkenny, Leitrim and Kerry.
Well, imagine....if a lot of water can move through a karst system quickly, what happens if pollutants get into the water? In many cases, it only takes a small amount of pollution to make water unsafe for drinking. In Ireland, the main sources of groundwater pollution are septic tanks, farmyards, and urban areas. Other sources, such as waste disposal sites, industrial spills and leakages have detrimental effects on a local scale. It is probable that organic and inorganic fertilizers have increased nitrate levels in wells, particularly in the south and south-east of Ireland. While pollutants from septic tanks and farmyards may seem mild compared to pollutants from industrial areas, there are many septic tanks and farmyards in Ireland. Therefore, you should be aware of the potential threat they can pose to clean drinking water. Septic tanks and farmyards contain faecal bacteria (and viruses), nitrate and phosphate, amongst other chemicals. Faecal bacteria and nitrate present the biggest threats to human health. The most common health problem associated with faecal bacteria is diarrhoea, but typhoid fever, infectious hepatitis and gastrointestinal infections can also occur. The consumption of nitrate rich water by young children may give rise to a condition known as methaemoglobinaemia, or ‘blue baby syndrome’.
In Ireland, human activities have not yet caused the same degree of pollution problems to groundwater as in most other EU countries. However, an increasing number of localised problems are coming to the attention of the GSI and groundwater researchers, mainly where wells are polluted by septic tanks and farmyards. Beneath some septic tanks and farmyards are pockets of contaminated groundwater. As these sources are associated with houses, water wells are often located nearby, and may be in or close to the polluted pockets of groundwater. So, although the actual quantity of polluted groundwater is small, it can result in significant health risks. In many areas, at least 30% of private domestic and farm wells are contaminated, in some vulnerable areas more than 50% are contaminated at some time during their use. Most of these wells are not just contaminated chemically but are polluted by faecal bacteria, and probably viruses.
Pollution of groundwater in Ireland tends to be mainly microbiological rather than chemical - in fact, wells and springs with levels of chemicals greater than the EU MAC (maximum admissible concentration for drinking water) and arising from human activities are not common. However, there is still some contamination caused by human activities, even though the concentrations are lower that the EC MAC. The main chemical contaminants in Ireland are nitrate, ammonia, potassium, chloride, iron and manganese. There is some evidence to show that there are occasional pollution problems from refined oils, pesticides and solvents; however, more data is needed in this area.
To protect our groundwater resources the Groundwater Section of the GSI currently produces Groundwater Protection Schemes on a county-wide basis. These consist of the following suite of maps and an accompanying report:
The GSI currently produces Groundwater Protection Schemes on a county-wide basis to map and protect the groundwater resources within that county, and to protect specific groundwater sources of drinking water supplies. These schemes are based on the concepts outlined in the National Groundwater Protection Schemes, which will be available for download soon. The groundwater protection schemes are based on information provided by a suite of maps, including:
- Groundwater protection zones,hydrogeological data
- Outcrop and depth to bedrock
- Subsoils geology
- Bedrock geology
The groundwater vulnerability map indicates the likelihood of groundwater being polluted, and is produced by combining permeability and depth to rock information. The groundwater protection zone maps shows the type of groundwater resources and the vulnerability of these resources to pollution by combining the vulnerability map with the aquifer map. This ‘final’ map helps county planners locate future potentially polluting activities (such as landfills, septic tanks) to minimise the risk to groundwater contamination.
To date, groundwater protection schemes are available for Counties Tipperary (South), Waterford, Offaly, Limerick, Meath, Wicklow, Clare, Laois, Cork (South), Kilkenny, Roscommon, Monaghan, and Tipperary (North). By early 2004 the Donegal scheme will be completed. From County Wicklow onwards, the maps are produced at the 1:50,000 scale; earlier maps are produced at the 1:63,360 (1in = 1 mi) scale. Maps and reports for the completed protection schemes are available for review at the GSI.
Want a more detailed explanation of Groundwater Protection Schemes?
Want to know what counties have Groundwater Protection Schemes?
How to Disinfect your Well
To disinfect the well we recommend you use the following method;
You will need
(1) 2 lb (900 gm) of chloride of lime (containing about 35% available chlorine)
(2)1 lb (450 gm) of “High test Hypochlorite” (50 - 70% available chlorine).
(3) Laundry bleach (Sodium Hypochlorite, 3 - 5% available chlorine) 2 gallons (9 litres) of 3% or 1 gallon (4.5 litres) of 5% strength.
For (1) or (2) dissolve in 5 gallons (23 litres) of water. For (3) make up the solution to 5 gallons (23 litres). Mix thoroughly.
Pour half the solution into the well, start the pump and open all taps until water from each tap has a distinct smell of chlorine. Then stop the pump and add the rest of the solution. Allow to stand for 12 - 24 hours, then pump to waste until the smell of chlorine disappears.