Over the last few months, Catchment Unit staff have given…
WFD Characterisation: Assessing Groundwater
Anthony Mannix from the EPA Catchments Unit gives an overview of why groundwater is so important, and how its quality and quantity was assessed as part of the WFD Characterisation process.
Why is groundwater important?
Groundwater is important both as a drinking water source and as a pathway to surface water, and for ecosystems. Over 90% of small private supplies have a groundwater well1 and more than a quarter of large public supplies are from groundwater2.
Groundwater provides a proportion of flow to rivers depending on the properties of the aquifer and the overlying soil and subsoil in the catchment. The proportion of flow to rivers from groundwater is greater during dry periods when groundwater base flow dominates.
Water Framework Directive Monitoring of Groundwater
Groundwater quality and quantity is monitored in Ireland for the Water Framework Directive (WFD). There are 513 groundwater
bodies across Ireland. The design of the monitoring network is based on a conceptual understanding of the hydrogeological system and pressures. The groundwater quality network focusses on monitoring points with large zones of contribution (ZOC) because the water quality is less affected by localised pressures. In this regard, monitoring points with daily abstraction rates greater than 100 m3 per day and large springs were prioritised for the groundwater quality network. There are 255 groundwater quality monitoring points for WFD assessment. There are also 24 spring flow monitoring sites, and 135 groundwater level monitoring boreholes.
What are the key issues for groundwater in Ireland?
Elevated nutrient concentrations (phosphorus and nitrogen) continue to be the most widespread water quality problem in Ireland3 across the water categories. The drinking water standard for nitrate is 50 mg/l NO3 and this relates to potential for harm to human health, and there is an associated groundwater WFD threshold value of 37.5 mg/l NO3.
In 2016 the mean nitrate concentration exceeded the Irish groundwater WFD threshold value concentration of 37.5 mg/l NO3 at six of the EPA WFD monitoring sites, with two sites having mean concentrations greater than the Drinking Water Standard4. Where concentrations are greater than the drinking water standard then treatment or blending to reduce the concentration is necessary before using as a drinking water supply. Exceedance of the threshold value indicates action may be required to avoid exceeding the drinking water standard.
Groundwater can be an important pathway for the movement of phosphorus from diffuse and small point sources to water ecosystems, particularly in catchments with thin soils or where bedrock outcrops at the surface. The Irish groundwater WFD threshold value of 0.035 mg/l P is considered when assessing the contribution of phosphorus in groundwater to surface waters. (7.7%) of EPA WFD groundwater sites were in breach of their respective threshold values for nitrate phosphate in 20164.
Escherichia coli (E. coli) is a bacterium used as an indicator of faecal
contamination of water. It is detected widely in groundwater in Ireland (found at 42% of EPA WFD groundwater monitoring sites in 20164). Its detection is an indication of contamination of groundwater by faecal matter and that pathogens (disease causing organisms) may be present. This highlights the necessity for testing of drinking water supplies from groundwater for microbial contamination, and for the provision of adequate treatment. This is particularly relevant for households and other private supplies which may not have treatment in place for microbial contaminants. The EPA recommends that all drinking water supplies are monitored at least once a year for E. coli, with greater frequencies required depending on the supply type.
There may be other localised water quality issues for groundwater, for example pesticides or hydrocarbons like oil, diesel or petrol. Sites that are polluted from point sources may impact on groundwater immediately downhill from the pollution source. Smaller sites generally do not have an impact at a groundwater body scale. Large point sources potentially impacting on groundwater are assessed based on site specific data collected as part of site characterisation, or regulatory data from licenced sites submitted to the Office of Environmental Enforcement of the EPA as discussed below.
During dry periods, water levels can fall below the shallow wells in particular, resulting in dry wells. Large wells abstracting significant quantities of groundwater could impact on surface water ecosystems by reducing the flow of water to them. However, at a WFD groundwater body scale there have been few issues with quantity of groundwater in Ireland as discussed below.
Groundwater WFD Status
WFD status classification for groundwater is based on five chemical and four quantitative tests developed to assess whether the WFD objectives are met.
The chemical tests investigate water quality impacts by looking for:
- evidence of saline or other intrusions
- exceedances of quality standards and thresholds that would result in failure to achieve the environmental objectives of associated
- surface waters
- groundwater- dependent terrestrial ecosystems
- or drinking water protected areas
- evidence of deteriorating trends in quality (general chemical test)
- The quantitative tests investigate over-abstraction of groundwater by looking for:
- evidence of saline or other intrusions due to change in groundwater levels,
- impacts on the environmental objectives of associated surface waters and groundwater-dependent terrestrial ecosystems due to alterations in groundwater levels,
- exceedence of the available groundwater resource by the longterm annual abstraction through assessing water balances.
The current WFD groundwater status (2010 to 2015) is based on monitoring data for the EPA WFD groundwater monitoring network. It also considered information from the surface water WFD status assessments, and assessment of licenced sites from the Office of Environmental Enforcement of the EPA.
The picture of the groundwater quality status based on area is good. 99% of the area of the country is at Good status and less than 1% is at Poor status. 45 of the 513 groundwater bodies are at Poor status.
Thirty-six of these failed the general chemical test. These poor status bodies make up a small area nationally as most are small waterbodies delineated for waste and industrial licenced sites and historic mines. There was an increase in the number of poor status GWB for this test from the previous assessment. This was due to improved information and technical approach rather than due to deterioration. Nine water bodies failed due to the impact of groundwater quality on surface water ecology with groundwater contributing more than 50% of the load to cause a breach of the river phosphate Environmental Quality Standard.
One waterbody was at poor quantitative status due to the impact of drainage on a Groundwater Dependent Terrestrial Ecosystem (GWDTE).
The characterisation process for groundwater looked for evidence of impact across the areas outlined above for the groundwater status classification. While the structure of the status tests was used as a guide for characterisation, the approach was expanded to use evidence and forms of assessment additional to those used for status assessment. The aim was to determine the risk of failing to meet the objectives for each waterbody. The contribution of nutrients from groundwater to surface waterbodies was particularly focused on.
Groundwater contribution of phosphate to rivers
The connection between each river and the groundwater body or bodies contributing water to them was assessed. Where the surface water ecological status was at risk or review and phosphorus was the significant issue then the associated groundwater bodies were assessed.
Several elements were included in the assessment to determine if the groundwater contribution to surface water was likely to be significant including:
- Groundwater phosphate concentrations
- Groundwater phosphate trends
- Surface waterbody risk
- Surface waterbody significant pressure
- Aquifer characteristics
- Pollution Impact Potential maps generated using the Catchment Characterisation Tool and the modelled loadings per hectare via the groundwater pathway from the SLAM model 5,6 , (Mockler et al. 2016, 2017)
These details were assessed the combination of each groundwater body with each overlying surface water body. The assessment was repeated several times for most groundwater bodies as they intersected with multiple surface waterbody catchments. The groundwater quality information, aquifer characteristics, and the LAM model results in particular were used to determine if a significant contribution from each groundwater body to surface water was likely.
The following table shows the groundwater risk assigned when related to the surface water risk and the likely significance of phosphate (P) via the groundwater pathway to surface water.
|Surface Water Risk
groundwater P contribution
groundwater P contribution
contribution not likely
|Not at Risk
|Review/Not at Risk
|Not at Risk
|Not at risk
|Not at Risk
|Not at Risk
|Not at Risk
Surface water is the receptor for phosphate so the groundwater bodies are not At Risk for this assessment if the associated river waterbodies are not at Risk. An exception was when there was a significant upward trend in Groundwater P concentrations that could contribute to river waterbodies becoming less than good status.
Groundwater contribution to transitional and coastal waters
Where transitional and coastal waters had impacts potentially caused by nitrogen the contribution from groundwater was assessed. This looked at:
- The proportion of water coming from groundwater using the recharge7 to groundwater
- Nitrate concentration for the groups of groundwater bodies in the catchment
- The nitrate concentrations in rivers associated with the groundwater bodies in the catchment
This information was used to compare estimated loads of nitrogen via the groundwater pathway to surface water in the catchment. These were compared with the other surface water inputs by the EPA Catchments Unit to determine the significance of the groundwater load.
|Number of Groundwater Bodies At Risk
|Phosphate contribution to associated Surface Water Bodies
|Chemicals from Industrial Licenced Sites
|Ammonia from Waste Licenced Sites
|Contamination from Historic Mines
|Average nitrate concentrations in a monitoring point >
37.5 mg/l NO3
|Phosphorus contribution to Groundwater
Dependent Terrestrial Ecosystems (GWDTE)
|Quantitative impact on GWDTE
Almost half of the At Risk classification for groundwater bodies are due to the contribution of phosphorus to At Risk surface water bodies. Measures to address the surface water significant pressures will address pressures on the groundwater bodies.
Most the remaining groundwater bodies At Risk of not achieving their environmental objectives are related to point sources (waste and industrial licenced sites and historic
mines). Measures to remediate the problems are managed through the EPA’s licence enforcement process.