When we think of water pollution our minds tend to conjure dystopian images of large scale industrial development, and the associated discharge of treated or untreated effluent into our watercourses. In reality much pollution to the water resources of the World occurs in a much less dramatic, but more widespread manner. Because infrastructural investment for treating waste water is primarily limited to urban population centres, much of the Worlds rural housing is not connected to a public sewer system. Consequently rural housing developments, in both the developed and developing World, cannot ensure the safe treatment, and monitoring of waste water, in the same way that urban municipal sewer systems can.
A good case study to illuminate this increasing threat to clean water supply is the Republic of Ireland, which unlike more urban European countries has up to a third of the population living in the open countryside, in individual dwellings not connected to a public sewer.
The wastewater from such rural settlement patterns is disposed of by a number of different methods. These include aeration systems, filter systems, and the purification of waste water through reed bed installations. The latter involves the use of "Phragmites Australis" reeds whose extensive root systems are able to break down organic compounds. This provides a good removal of organic components, and suspended solids, and some de-nitrification also takes place. (EPA 2000).
However, by far the commonest method is the use of septic tanks, and in 2005 there were some 350 000 of these systems installed in Ireland (Sherkin Comment 2005). The Geology and Hydrology of Ireland is almost uniquely unsuitable for the widespread use of septic tanks. Around half of the Republic of Ireland is underlain by Limestone rock. Because of its particular characteristics of extensive jointing, the presence of large voids, and rapid underground drainage, Limestone can allow waste water to travel large distances without it being cleaned by filtration. If the surface cover of soil is not adequate to filter chemical and biological contaminants from waste water, then septic tanks can lead to the contamination of ground and surface water supplies.
The potential sources of pollution hazards, in rural Limestone areas, include dumping in sink holes and depressions, the spreading of farm slurries on shallow soils, unlined silage storage, farmyard runoff, as well as contamination from septic tanks. Pollutants then move rapidly from the land surface, through fractures and conduits in the limestone, and down to the water table. The pollution of groundwater then makes its way to surface streams and lakes. (Karst Working Group 2007).
It is the responsibility of the homeowner to ensure that the wastewater treatment system is installed in accordance with the planning conditions and that it is properly maintained on a regular basis, to ensure that it does not cause pollution to the environment or to drinking waters. However there is no monitoring, by the EPA or other bodies, as to the condition of rural septic tanks. Problems include poor design, poor siting, and faulty installation of on site treatment systems. A particular problem in Ireland is the common practice by builders of diverting the storm drain into the septic tank they are installing, and thus overloading the system when it rains. (Sherkin Comment 2005).
Environmental Protection Agency. (2000) Landfill Manuals, Landfill Site Design. Dublin: EPA. [Accessed 31st October 2008] Available from World Wide Web: <http://www.epa.ie/downloads/advice/licensee/epa%20landfill%20site%20design.pdf>
Karst Work Group. (2007) Karst Booklet. Dublin: Geological Survey of Ireland. [Accessed 5th January 2009] Available from World Wide Web: http://www.gsi.ie/Programmes/Groundwater/Karst+Booklet/
O'Leary, G. (2005) On-Site Treatment of Waste Water. Cork: Sherkin Comment, p11.