Bedrock aquifer classification is based firstly on the rock type (lithology), and the 1:100,000 simplified bedrock map is used as a basis. This map is known as the Rock Unit Group map, which can be viewed on the webmapping viewer
Yield is one of the main concerns in aquifer development projects, yields from existing wells are conceptually linked with the main aquifer categories:
- Regionally important (R) aquifers should have (or be capable of having) a large number of ‘excellent’ yields: in excess of approximately 400 m3/d (4,000 gph).
- Locally important (L) aquifers are capable of ‘good’ well yields 100-400 m3/d (1,000-4,000 gph).
- Poor (P) aquifers would generally have ‘moderate’ or ‘low’ well yields - less than 100 m3/d.
However, existing well yield information is often difficult to use because reliable, long term yield test data are quite rare (particularly for the less productive aquifers). In practice, then, the following criteria are used in aquifer classification:
- Permeability and transmissivity data from formal pumping tests, where discharge and water levels readings have been taken over a period of many hours or days.
- Productivity data from wells where either formal pumping tests have been undertaken or where at least one combined reading of discharge and drawdown data are available. The GSI has developed the concept of ‘productivity’ as a semi-quantitative method of utilising limited well test data (Wright, 2000). A ‘productivity index’ is assigned to a well from one of five classes: I (highest), II, III, IV, and V, using a graphical comparison of well discharge with specific capacity.
- Occurrence of springs with ‘high’ flows (greater than 2160 m3/day total flow).
- Occurrence of wells with ‘excellent’ yields (greater than 400 m3/day discharge).
- Hydrological information such as drainage density where overlying strata are thin, and baseflows or flows in rivers (better aquifers will support higher baseflows and summer flows).
- Lithological and/or structural characteristics of geological formations which indicate an ability to store and transmit water. Clean washed and sorted sands and gravels for example, are more permeable than poorly sorted glacial tills. Clean limestones are also more permeable than muddy limestones. Areas where folding and faulting has produced extensive joint systems tend to have higher permeabilities than areas where this has not occurred.
- Aquifer assessments from Groundwater Protection Schemes in neighbouring counties and from existing reports.
All seven factors are considered together; productivity and permeability data are only given ‘precedence’ over lithological and structural inferences where sufficient data are available. Data from neighbouring counties in similar geological environments are included.
Some bedrock units have been grouped if they are of similar geological age and have similar lithological/structural characteristics. In considering the classifications provided, it is important to note that:
- The bedrock aquifer classifications are based on the bedrock units mapped by the Bedrock Section of the Geological Survey of Ireland at 1:100,000 scale.
Irish hydrogeology is unusually complex and variable. As a consequence, there will often be exceptionally low or high yields that do not conform with the aquifer category given.
- The top few metres of all bedrock types are likely to be relatively permeable, even in the poor aquifers.
- There may be localised areas where recharge is restricted. This could occur, for example, where the vulnerability is low, or where a small portion of the rock unit has been faulted away from the main body of the unit. In these situations, the development potential even of regionally important aquifers may be limited. In considering major groundwater development schemes at particular sites, it will be important to consider the long term balance between recharge and abstraction, as well as the aquifer potential.