7A Karst Hydrogeology Assessment
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River Deel (Crossmolina) Drainage Scheme in association with 7 A KARST HYDROGEOLOGY ASSESSMENT 7A.1 INTRODUCTION A h drogeological assessment of the River Deel was commissioned b OPW in December 2015 during the options assessment stage for the River Deel Flood Relief Scheme. ,he initial purpose of the assessment was to assess the potential for a diversion channel between the River Deel and -ough Conn to impact on low flows in the River Deel. In 2015. there was a deficit of documented evidence of /arst activit in 0orth 1a o. including the catchment of the River Deel. Anecdotal evidence suggested a connection between the River Deel and the 2pond3 labelled on Ordnance Surve 1aps at 1ullenmore. A /e consideration in December 2015 was the potential for a diversion channel to affect low flow conditions in the river. which forms part of the River 1o SAC and is home to a range of protected species such as White Clawed Cra fish. Atlantic Salmon and -ampre Species. all of which are among the 4ualif ing interests of the River 1o SAC and Freshwater Pearl 1ussel. a species listed on Annex II of the E5 6abitats Directive that is not among the 4ualif ing interests of the site. 7arst flows are unpredictable and site-specific in character. 7arst activit can var in time and space so it is essential that an understanding be developed of where water goes underground. its underground routes and the possibilities of a new channel adversel affecting these /arst flows. or intersecting underground conduits. In order to understand the nature of the local /arst h drogeolog . its relationship with low flows in the River Deel. and to assess impacts from a flow diversion channel for Crossmolina in sufficient detail. it was deemed necessar to carr out further h drogeological investigation. ,he main ob8ective of the investigation was to establish where water sin/s underground. in what 4uantities. and to determine if all of it emerged at the 1ullenmore Springs. and if not. then where. ,he /arst h drogeological assessment which culminated in the preparation of this report was led b Dr David Drew with assistance from 9onathan Reid. R an 6anle consulting engineers. Environmental Impact Assessment Report Appendix 7A-1 River Deel (Crossmolina) Drainage Scheme in association with Cr ssm lina T wn Ballycarr n Gauge L ugh C nn Nephin Figure 7A.1: Catchment of River Deel (Outlined in black, ource EPA) 7A.2 ASSESSMENT METHODOLOGY 7A.2.1 Requirement f r Further Investigati ns Further Investigation re4uired as part of the assessment was initiall identified in the following areas: • Preliminar site investigation to help determine the extent of /arstification in the catchment. • Catchment wal/over and collecting anecdotal evidence of the presence of /arst. • Flow measurement along the River Deel to determine the location and 4uantities of sin/ing water. and at the 1ullenmore Springs and other potential outlets for groundwater. • Anal sis of historic mapping and relevant topographical features. and • Identification of sin/holes along the River Deel. d e tracing and tracer anal sis. 7A.2.2 Assessment Staging ,he h drogeological assessment of the River Deel was carried out in two stages as follows: • Stage 1: An initial assessment of the extent of /arstification (open conduits). using the location and measurement of springs in the wider River Deel catchment and the location of other /arst features. o ,his wor/ could be underta/en under winter conditions (to greatest benefit in combination with some preliminar site investigation). o ,as/s involved in Stage 1 were to include catchment wal/over. information gathering. flow measurement. anal sis of historic mapping and low flow data and preliminar geotechnical investigation. Environmental Impact Assessment Report Appendix 7A-2 River Deel (Crossmolina) Drainage Scheme in association with o ,he output from Stage 1 is an assessment of the catchment of the River Deel under static h drogeological conditions. Static conditions in the case of the River Deel include periods of medium to high flows. which preclude a more detailed anal sis of interaction between river flows and the /arst groundwater s stem re4uires lower flows. ,he more detailed anal sis ta/es place at Stage 2. • Stage 2: ,he second stage was to be carried out during low flow conditions using the information obtained at Stage 1 to investigate the d namics of the groundwater h drolog of the area. for example where water sin/s underground and where it reappears. flow rates. variations with stage. etc. o ,his assessment was to be underta/en under low water conditions when the groundwater s stem was not overwhelmed b medium to high river flows. o ,as/s involved in Stage 2 were to include catchment wal/over during low flow conditions. d e tracing. tracer anal sis. flow measurement. bath metric surve s. and preliminar geotechnical investigations along the ban/s of the River Deel. the 1ullenmore Springs and in the vicinit of the diversion channel. o ,he output from Stage 2 is an assessment of the catchment of the River Deel under d namic h drogeological conditions. Stage 1 will provide a basic understanding of the /arst h drogeolog under static conditions. but not sufficient detail to enable a h drogeologist to understand the nature of the local /arst h drogeolog and its relationship with low flows in the River Deel. ,his can onl be understood following Stage 2. an assessment of the catchment of the River Deel under d namic h drogeological conditions. 7A.2.. Study Area ,he stud area for the investigation encompassed the /arst region in the catchment of the River Deel upstream of Crossmolina. ,he precise extent of the stud area was refined as /e pieces of information were obtained during Stage 1 and the initial phase of the Stage 2 assessment. Environmental Impact Assessment Report Appendix 7A-3 River Deel (Crossmolina) Drainage Scheme in association with Cr ssm lina T wn L ugh C nn Nephin Car0 nifer us 1re2car0 nifer us (not shaded) Pure Bedded limestone 5pper impure bedded limestone Quartzite. Gneiss and Schist Sandstones Igneous (Ox mountain Granodiorite) Sandstones shales and limestones Catchment Boundar Figure 7A.2: Bedrock Geology of River Deel Catchment (Outlined in black, ource GSI) 7A.. STAGE 1 ASSESSMENT 7A...1 Stage 1 Investigati ns ,he Stage 1 Assessment involved wal/over surve s. inspection of /arst features. review of mapping. plotting of /arst features. on site conductivit mapping and flow estimating. Flow measurement along the River Deel consisted of monitoring flows in the river. which remained at normal to high levels following the December 2015 flood event and a relativel wet start to 201A. It was not anticipated that low flow measurement would be possible until Stage 2. Flows were also estimated but not measured at the 1ullenmore Springs. Due to the timeframes involved and procurement restrictions. it was not possible to mobilise a geotechnical contractor to site in time to provide input to the Stage 1 6 drogeological Assessment. ,he outcome of the Stage 1 Assessment is set out below. followed b further investigation and information gaps which were identified at the conclusion of the Stage 1 Assessment. Environmental Impact Assessment Report Appendix 7A-4 River Deel (Crossmolina) Drainage Scheme in association with Dinantian Sandstones, Shales and Limestones Dinantian Pure Dinantian Dinantian Upper Bedded Limestone Sandstones (impure) Limestone (Ballina Formation) CROSSMOLINA OPW 6 drometric Gauge Ball carroon ,obermore 1ullenmore Springs Pollnacross ,he Richmond 2Pollgorm3 Bridge Pool -ough Agawna ,obernagawna Springs Car0 nifer us (not shaded) Pure Bedded limestone Sandstones 5pper impure bedded limestone Sandstones shales and limestones Catchment Boundar -iDAR Extents Figure 7A.3: Refined Study Area, howing extent of available -iDAR map.ing 7A...2 Mullenm re Springs As noted above. prior to the stud . there was a deficit of documented evidence of /arst activit in 0orth 1a o. including the catchment of the River Deel. Anecdotal evidence suggested a connection between the River Deel and the 2ponds3 labelled on modern Ordnance Surve 1aps at 1ullenmore. In fact. the townland name 1ullenmore is recorded in the 1A04 Calendar of Patent Rolls of 9ames I and it is therefore li/el that a mill was operating in the localit from at least that time. ,he ponds labelled on the modern Ordnance Surve maps. and which were not identified as a /arst features on historical Ordnance Surve mapping are in fact springs. referred to in this report as the 1ullenmore Springs. An iron mill is believed to have been located at this site as earl as the mid-18th centur . ,his ceased production when the timber in the vicinit was exhausted and was in a state of ruin b 1800. Griffith3s Caluation (1848-18A4) records that an iron mill had formerl been located within the townland. ,he soils in the townland are described as heav cla s and the tenants were recorded as being of middling circumstances and residing in stone houses. ,he A-inch OS map of 1840 (Figure 7A.4) shows a corn mill. with two corn /ilns to the north. ,he 25-inch OS map of 1D00 (Figure 7A.5) shows a number of unnamed buildings Environmental Impact Assessment Report Appendix 7A-5 River Deel (Crossmolina) Drainage Scheme in association with at this location and. while a lime /iln is indicated to the southeast. there are no traces of the two corn /ilns previousl indicated in the area to the north. ,he histor of the area is closel connected to the 1ullenmore Springs and the stead water suppl that the provided to the various milling activities (as an iron mill and a corn mill) that have been underta/en at this location over the centuries.