Some of the rain that falls on Ireland filters through the soil, subsoil and rocks and becomes groundwater.  Much of that water moves slowly underground and remains there for long periods before emerging as base flow in rivers or as springs. Some is extracted for drinking, particularly for single houses in the country, and often receives little or no treatment prior to use.

Groundwater is protected against pollution from the surface if there is sufficient depth of overlying soil and subsoil but, if that depth of cover is small, then groundwater is vulnerable from septic tank effluent and from slurry spreading.  Much of the underlying rock in Ireland is limestone that is easily dissolved by water and this has created cracks and fissures in the rock that allows pollutants from the surface to move quickly down to the underlying water body.  Hence much of Ireland’s groundwater resources are vulnerable to pollution.

The pollutants of most concern to groundwater in Ireland are Nitrates from fertiliser use on land and microbial contamination from septic tanks and slurry spreading.  Septic tank systems are used extensively for sewage treatment in the rural areas of the country and, while the tank itself separates out gross solid material, the real treatment occurs in the soil and subsoil of the percolation area downstream of the tank.  Almost all animal manures are recovered as fertiliser on farm land and this material has the potential to be highly polluting with a very high microbial count.

Nitrate Contamination

Nitrates concentrations in groundwater
Courtesy EPA

The graph shows the nitrate concentrations in groundwater from 1995-2015.  The limit set by regulation for nitrate in drinking water is 50 milligrams per litre but the EU Directive also set a guide value of 25 milligrams per litre as an indicator of contamination.

The graph shows that a small number of samples exceeded the 50 limit but a larger number breached the guide value.  Nitrate contamination is more common in the South and Southeast of the country where farming is more intensive.


Phosphate Contamination

The average phosphate concentration in groundwater was below the Threshold Value, 0.035 mg/l P, at 92% of the monitoring locations during 2010–2015 (Map 5.3). Of the 16 monitoring locations with higher average concentrations, six were greater than 0.05 mg/l P.

Although most groundwater monitoring locations had average phosphate concentrations below the river environmental quality standard of 0.035 mg/l P, there is potential for groundwater in certain areas to contribute a significant proportion of the phosphate load to surface waters (EPA, 2017).

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