A groundwater vulnerability screening methodology for Northern Ireland

A requirement of the EU Water Framework Directive is the assessment of the risk of groundwater contamination within those groundwater bodies identified in each Member State. In order to carry out the risk assessments, knowledge of the vulnerability of groundwater is necessary. The report is in two parts: first, a description of the groundwater screening methodology is made. This methodology was originally developed for use in Scotland, but has now been adapted for use in Northern Ireland, taking into account local data availability. Second, the creation of the GIS-based 1:250 000 scale groundwater vulnerability map using suitable data is described. Groundwater vulnerability is defined as the tendency and likelihood for general contaminants to reach the water table after introduction at the ground surface. All groundwater is to some degree vulnerable and the screening tool produced for the current project is designed to reflect the ability of contaminants to reach the water table surface across Northern Ireland. It is not intended as a complete solution to risk assessment and should be used as a regional guide to the possible degree of specific site investigation required at any locality. The screening methodology applies to the situation where contamination from the land surface leaches vertically downwards to the water table within the uppermost aquifer at a particular locality. The groundwater vulnerability assessment is, therefore, influenced by several factors that relate to the pathway element of a typical hazard – pathway – receptor risk assessment. In this case, the pathway is characterised by the geological and hydrogeological characteristics of the soil layer, the underlying superficial deposits and bedrock. The pathway between the ground surface and the water table can affect the degree of attenuation of contaminants. Factors that can influence attenuation include: • The permeability and clay content of the superficial deposits. • The thickness of the superficial deposits. • The mode of groundwater flow in bedrock aquifers (fracture or intergranular flow). • The permeability and clay content of intergranular bedrock aquifers. • The depth to the water table in both superficial and intergranular bedrock aquifers. It is the above factors that determine the vulnerability classification. Vulnerability has been divided into five categories, with Class 1 areas having the lowest risk of groundwater pollution and Class 5 the highest. One of the main principles adopted for the current methodology was how attenuation could be affected by the nature of groundwater flow. It is assumed that only in geological deposits where there is significant or total unsaturated intergranular groundwater flow that attenuation can occur. Where contaminants move to the water table through unsaturated fractured bedrock, the methodology assumes that no attenuation of pollutants can take place. It is the recognition of the hydrogeological characteristics within the pathway instead of the ‘importance’ of a particular aquifer that results in the final vulnerability map of Northern Ireland showing significant areas of Classes 4 and 5 within upland and certain other regions. This reflects the common occurrence of igneous and metamorphic rocks within these areas where the potential for attenuation of contaminants in the pathway is very limited

A hydrogeological assessment of the Sherwood sandstone aquifer within the Moneymore region of Mid-Ulster, Northern Ireland

The geology and hydrogeology of the Lagan Valley is well documented and the hydrogeological properties of the Sherwood Sandstone well known. A recent groundwater supply investigation in the Moneymore area has advanced our understanding of the Sherwood Sandstone aquifer in the Mid-Ulster region. Unlike the same aquifer unit in County Antrim, the Mid-Ulster unit is poorly understood owing to it not having been used for water supply when compared to its Lagan Valley counterpart.Tetra Tech were involved in the specification, design and drilling of two deep boreholes for the purpose of abstracting groundwater from the Sherwood Sandstone aquifer near Moneymore. The first of two boreholes (BH01) was drilled to 100 metres below ground level (mbgl) and the second (BH02) to a depth of 260 mbgl. The two boreholes are approximately 10 metres apart from each other, but their respective open hole response zones are 20 metres apart vertically. Despite this, the two boreholes had little to no connectivity as demonstrated by the pumping test data. This is consistent with observations made during drilling, with intermittent mudstone and sandstone bands of rock confining water at depth. This demonstrates local compartmentalisation within the Sherwood Sandstone aquifer in the Mid-Ulster region.Combined pumping tests and monitoring of observation wells nearby allowed the aquifer properties to be calculated. Currently, long term monitoring within the area is ongoing to confirm the results of the hydraulic assessment. The results to date suggest that the Triassic sediments of the Mid-Ulster sandstone aquifer have lower transmissivity and storativity relative to the Lagan Valley system. Similar to the Lagan Valley, the Mid-Ulster aquifer reaches a thickness in excess of 260m and compartmentalisation permits the abstraction from upper (unconfined) and lower (confined) portions of the same aquifer. The data gathered as part of this investigation will provide valuable information to future generations who wish to avail of this natural resource within the Mid-Ulster region.<br/

Phosphorus in groundwater-an overlooked contributor to eutrophication?

This paper presents the first international assessment of phosphorus concentrations in groundwater, using data from the Republic of Ireland, Northern Ireland, Scotland, England and Wales. Phosphorus is considered to be the main limiting nutrient in most freshwater ecosystems. Controlling phosphorus inputs is thus considered the key to reducing eutrophication and managing ecological quality. Very little attention has been paid to evaluating transfers via groundwater due to the long-held belief that adsorption and metal complex formation retain the majority of potentially mobile phosphorus. In each country, ecologically-important phosphorus thresholds are exceeded in a significant number of groundwater samples. The relative contributions of potential sources for these elevated concentrations are currently unclear but there is evidence to suggest that they are at least partly anthropogenic. The results suggest that groundwater P concentrations are such that they may be a more important contributor to surface water phosphorus than previously thought. Copyright &cedil; 2008 John Wiley &amp; Sons, Ltd

Coronal Heating as Determined by the Solar Flare Frequency Distribution Obtained by Aggregating Case Studies

Flare frequency distributions represent a key approach to addressing one of the largest problems in solar and stellar physics: determining the mechanism that counter-intuitively heats coronae to temperatures that are orders of magnitude hotter than the corresponding photospheres. It is widely accepted that the magnetic field is responsible for the heating, but there are two competing mechanisms that could explain it: nanoflares or Alfv\'en waves. To date, neither can be directly observed. Nanoflares are, by definition, extremely small, but their aggregate energy release could represent a substantial heating mechanism, presuming they are sufficiently abundant. One way to test this presumption is via the flare frequency distribution, which describes how often flares of various energies occur. If the slope of the power law fitting the flare frequency distribution is above a critical threshold, $\alpha=2$ as established in prior literature, then there should be a sufficient abundance of nanoflares to explain coronal heating. We performed $>$600 case studies of solar flares, made possible by an unprecedented number of data analysts via three semesters of an undergraduate physics laboratory course. This allowed us to include two crucial, but nontrivial, analysis methods: pre-flare baseline subtraction and computation of the flare energy, which requires determining flare start and stop times. We aggregated the results of these analyses into a statistical study to determine that $\alpha = 1.63 \pm 0.03$. This is below the critical threshold, suggesting that Alfv\'en waves are an important driver of coronal heating.Comment: 1,002 authors, 14 pages, 4 figures, 3 tables, published by The Astrophysical Journal on 2023-05-09, volume 948, page 7