Protection schemes in karst areas
The Geological Survey of Ireland, the Department of Environment and Local Government and the Environmental Protection Agency have jointly developed a methodology for the preparation of groundwater protection schemes. Schemes based on this methodology are gradually being drawn up and implemented by local authorities. Each scheme involves a county-wide map of groundwater protection zones, based on an aquifer map and a map of the natural vulnerability of the groundwater to pollution. This is accompanied by guidelines for controlling various sources of pollution (e.g. septic tank systems).
Karst is included in the protection scheme in two ways: firstly as part of the aquifer categorisation; and secondly, as a factor in vulnerability mapping.
Karst limestones are shown as a separate, distinct aquifer category on aquifer maps, as a means of drawing attention to the variability in the hydrogeological characteristics, the difficulties in locating successful wells, the focused nature and high velocities of groundwater flow, and the low level of pollutant purification in karst areas.
Summary of Components of Groundwater Protection Scheme
- The vulnerability map mainly depends on the nature and thickness of subsoils (unconsolidated material overlying bedrock, dating from the Ice Age and the last 15,000 years since the end of the Ice Age). Two factors relating to the subsoil cover are worth noting in karst areas:
- The irregular bedrock surface arising from limestone solution (with buried solution hollows) means that the subsoil thickness is often very variable over short distances, so the degree of protection provided is hard to predict.
- Where subsoil deposits are absent, limestone weathering gives rise to a thin or patchy soil, which provides little or no protection to the underlying groundwater.
The greatest vulnerability arises where water can move underground directly at particular points, bypassing the protective soil or subsoil cover, e.g. where stream water sinks underground (swallow holes) or along streams where water is lost over a longer reach (often where an artificial channel has been created by arterial drainage).
One element in a groundwater protection scheme is the drawing up of source protection zones around major public water supply sources. Each source's catchment area (or Zone of Contribution) must be identified. In karst areas this is particularly difficult, since the underground pathways are highly unpredictable. Where a sinking stream is thought likely to re-emerge at a spring, it is sometimes possible to trace the water from sink to spring by using special dyes or other tracers. The speeds at which water travels underground in karst systems, as revealed by dye tracing, can be several km per day, as compared with perhaps one or two metres per day in non-karstified aquifers. This emphasises the importance of avoiding pollution as rapid flow means a pollutant can quickly reach a water source without any chance for the harmful matter to be filtered out, absorbed or degraded.
Extremely vulnerable areas defined in the national protection scheme methodology include:
- Areas where bedrock is exposed at the surface or where the subsoil cover is less than 3 m thick.
- Areas within 30 m of karst features (including along the area of loss of losing or sinking streams) and within 10 m on either side of losing or sinking streams upflow of the area of loss.