Clifden-Castlebar GWB: Summary of Initial Characterisation
Clifden-Castlebar GWB: Summary of Initial Characterisation
Clifden-Castlebar GWB: Summary of Initial Characterisation
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1 st Draft Inishturk, Inishb<strong>of</strong>in, Inishshark <strong>GWB</strong> Description August 2005Inishb<strong>of</strong>in, Inishturk, Inishshark <strong>GWB</strong>: <strong>Summary</strong> <strong>of</strong> <strong>Initial</strong> <strong>Characterisation</strong>.Hydrometric AreaLocal Authority32Galway, MayoCo Co’sAssociated surface water features Associated terrestrial ecosystem(s) Area(km 2 )Lakes: Loughnabraud, Church Lough, B<strong>of</strong>in, Coastal lagoons on Inishshark and Inishb<strong>of</strong>in (O’Riain,Bawna, Gowlansgower, Loughnagrooaun, 2004). ~ 17Namucka, Coolnaknick.Several unnamed streams.TopographyThe land surface <strong>of</strong> the islands is characterised by several hills mixed with low-lying flat areas. The islands are bordered by a rockycoastline interspersed with rare sandy beaches. Elevations range from 10-170 mAOD. There are a number <strong>of</strong> streams and lakes on each<strong>of</strong> the islands. The islands are grouped together under one <strong>GWB</strong>, as they have similar geology, geomorphology, hydrogeology and lieclose to each other.AquifercategoriesThe main aquifer category is:Pl: Poor aquifer which is generally unproductive except for local zones.DischargeRecharge Overlying StrataGeology and AquifersMain aquiferlithologiesKey structuresKey propertiesThicknessLithologiesThickness% area aquifernear surfaceVulnerabilityMain rechargemechanismsEst. rechargeratesLarge springsand largeknownabstractions(m 3 /d)Main dischargemechanismsHydrochemicalSignatureInishb<strong>of</strong>in and Inishshark are composed <strong>of</strong> Precambrian Quartzites, Gneisses & Schists. Inishturk is composed <strong>of</strong>Ordovician Metasediments.The rocks in the <strong>GWB</strong> have undergone several episodes <strong>of</strong> deformation, comprising intense folding and faulting.The main structural trend is E-W. The Renvyle-B<strong>of</strong>in Slide is a major E-W trending structural feature. Bedrockstrata dip northwards 30-70 0 .There are no data available. The aquifer lithologies and structure are similar to the nearby <strong>Clifden</strong>-<strong>Castlebar</strong> <strong>GWB</strong>.Data for that <strong>GWB</strong> indicate low transmissivities – in the range <strong>of</strong> 0.7-20 m 2 /d. In the vicinity <strong>of</strong> faults, transmissivitymay be higher. Storativity is expected to be low (
1 st Draft Inishturk, Inishb<strong>of</strong>in, Inishshark <strong>GWB</strong> Description August 2005• The land surface is characterised by a relatively hilly terrain.• The <strong>GWB</strong> is composed primarily <strong>of</strong> low transmissivity rocks.• Groundwater flow is expected to be concentrated in fractured and weathered zones and in the vicinity <strong>of</strong> fault zones.• Diffuse recharge is expected to occur via rainfall percolating through the subsoil and rock outcrops.• Flow paths are likely to be short (30-300 m) with groundwater discharging rapidly to nearby streams and small springs.• Flow directions are expected to follow topography.• The rock units are generally <strong>of</strong> low permeability, thus baseflow to rivers and streams is likely to be relatively low.Attachments Figure 1.ConceptualmodelInstrumentationInformationSourcesDisclaimerStream gauges: NoneEPA Water Level Monitoring boreholes: NoneEPA Representative Monitoring points: NoneMorris, J.H., Long, B., McConnell, J.B. Archer (1995). Geology <strong>of</strong> Connemara. An introduction to the physicalstructure, ancient environments and modern landscapes <strong>of</strong> parts <strong>of</strong> northwest Galway and southwest Mayo, toaccompany the bedrock geology 1:100,000 scale map series, sheet 10, Connemara. Geological Survey <strong>of</strong> Ireland MapSeries Report.O’ Riain, G., (2004). Water Dependent Ecosystems and Subtypes Draft Report. WFD Support Projects. CompassInformatics in association with National Wildlife and Parks Service (DEHLG).Note that all calculation and interpretations presented in this report represent estimations based on the information sourcesdescribed above and established hydrogeological formulae.Figure 1. Location and boundaries <strong>of</strong> <strong>GWB</strong>