One important feature of karst areas is the absence of surface water which often leads to groundwater being the main source of supply. Drilled wells can be difficult to construct in these areas and karst springs remain critical as a method of harnessing the supply. Most of Ireland's largest springs are in karst regions, including the Kedrah springs near Cahir, Co. Tipperary; Dower Spring near Castlemartyr, Co. Cork; Two Gneeves Spring near Kanturk, Co. Cork; Cong Springs in Co. Galway; and Killeglan Spring, Co. Roscommon.
The large karst spring at Aughclogeen north of Galway city.
The spring is the source of water for a considerable
surrounding area (David Drew)
Karst springs have been used for water supply from the earliest times of human existence. They can range from small seasonal seepages to very large, reliable springs which may be the source of a substantial river. Many springs, both large and small, have been named, often in honour of saints, and in some cases have given their names to towns and villages, such as Patrickswell or Ballintubber. Many springs probably had religious significance in pre-Christian times.
The area around the mouth of a spring is often very boggy, especially in winter, and this can lead to pollution of the spring. Engineers use a variety of methods to combat this: for eaxample where the spring mouth is essentially a cave, a wall is built around the spring mouth and the intake pipes are installed deep into the cave; where the water wells up through broken rock or gravels the spring flow is often confined within a square or circular wall containing the intake and the spring is allowed to overflow in a controlled manner.
Bored wells in karst limestones can present several problems to the hydrogeologist and engineer. Firstly, it is difficult to identify the best place to drill as a successful borehole depends on meeting sufficient water-bearing fissures. If these fissures are a few metres apart and nearly vertical it can be easy to miss them. In well developed karst areas, a dry well can be drilled a few metres away from a successful one. Traditionally, this difficulty was resolved by using the services of a water diviner. Today, there are scientific methods using geophysical instruments which are often successful in identifying the presence of fissure zones. The second difficulty is that if large fissures are encountered, the limestone rock can be unstable and may collapse into the borehole. Competent drillers can overcome this, but they need to be ready for such conditions. An associated problem is that the fissures can contain large amounts of loose sediment which can enter the well, damage the pump and render the water undrinkable. Again, this can usually be solved by proper construction of the well, including the installation of well screen, casing and cement, and by careful development of the well, involving controlled pumping over a lengthy period.
As with a spring, it is important to emphasise the need for extreme watchfulness with regard to protecting the well from pollution, and this begins with drilling the well in a location where pollution is least likely.
In spite of all the problems (actual and potential) there are many thousands of successful boreholes in karst limestones throughout Ireland, delivering reliable quantities of good quality water to tens of thousands of people.