The following is from the gsi website:
GSI Web - One off housing wastewater
Where an on-site system is in the zone of contribution of a well, the likelihood of contamination and the threat to human health depend largely on five factors:
• the thickness and permeability of subsoil beneath the invert of the percolation trench;
• the permeability of the bedrock, where the well is tapping the bedrock;
• the distance between the well or spring and the on-site system;
• the groundwater flow direction; and
• the level of treatment of effluent.
Table 3: Recommended Minimum Distance between a Receptor and a Percolation Area or Polishing Filter
CLICK ON THE IMAGE TO VIEW A LARGER VERSION IN ANOTHER WINDOW
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Notes :
1. Depths are measured from the invert level of the percolation trench.
2. Depths and distances can be related by interpolation: e.g. where the thickness of silty, sandy CLAY is 1.2 m, the minimum recommended distance from the well to percolation area is 40 m; where the thickness is 3.0 m, the distance is 30 m; distances for intermediate depths can be approximated by interpolation.
3. Where bedrock is shallow (<2 m below invert of the trench), greater distances may be necessary where there is evidence of the presence of preferential flow paths (e.g. cracks, roots) in the subsoil.
4. Where the minimum subsoil thicknesses are less than those given above, site improvements and systems other than conventional systems, as described in EPA (2000), may be used to reduce the likelihood of contamination.
5. If effluent and bacteria enter bedrock rapidly (within 1-2 days), the distances given may not be adequate where the percolation area is in the zone of contribution of a well. Further site specific evaluation is necessary.
6. Where bedrock is known to be karstified or highly fractured, greater depths of subsoil may be advisable to minimise the likelihood of contamination.
Figure 1: Relative Location of Wells
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References
DELG/EPA/GSI, 1999. Groundwater Protection Schemes. Department of Environment and Local Government, Environmental Protection Agency and Geological Survey of Ireland.
EPA, 2000. Wastewater Treatment Manuals: Treatment Systems for Single Houses, Environmental Protection Agency.
