Chapter 7 - Soils, Geology and Hydrogeology€¦ · Mayo County Council N60 Balla to Claremorris Realignment at Heathlawn Environmental Assessment Report 84 7 Soils, Geology and Hydrogeology

Chapter 7 - Soils, Geology and Hydrogeology

• Appendix 7.1 – Soils, Geology and Hydrogeology

• 32103901/EAR/Figure 7.1- Hydrogeological Features Identified

• 32103901/EAR/Figure 7.2 - Geology, Soils and Hydrogeology - Near Surface Soils

• 32103901/EAR/Figure 7.3 - Geology, Soils And Hydrogeology - Overburden Geology(Subsoil)

• 32103901/EAR/Figure 7.4 - Bedrock Aquifer

Mayo County Council N60 Balla to Claremorris Realignment at Heathlawn

Environmental Assessment Report

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7 Soils, Geology and Hydrogeology

7.1 Introduction

This chapter considers and assesses the soils, geology (superficial and bedrock) and hydrogeology aspects including local groundwater resources and groundwater dependent karstic features that exist in the vicinity of the proposed development.

7.2 Methodology

The following assessment includes the review of previous assessment and screening reports, the design of the proposed development, the site-specific factual ground investigation report and ground permeability testing results, available online data including the information held by the Geological Survey of Ireland (GSI) and from the Environmental Protection Agency (EPA) website. Reference for all sources of information used is provided in Section 7.9.

The desk study information was completed by information gathered during a site walkover in May 2012. Information collected during this survey is described in Section 7.3

The assessment covers a study area of 500m radius, measured from the centreline of the proposed development in accordance with the NRA Guidelines on Procedures for Assessment and Treatment of Geology, Hydrology and Hydrogeology for National Road Schemes, 2008.

7.3 Existing Geological Environment

The area of interest of this assessment is located in the gently undulating rural lands of south County Mayo, between the towns of Balla and Claremorris. The most notable features within the study area are the Balla Turlough and an Un-named turlough. These turloughs are further described, geologically in Section 7.3.1, and hydrogeologically in Section 7.3.4.

7.3.1 Solid Geology

The 2011 ground investigations did not intercept bedrock along the scheme and the assessment of the bedrock geology is thus based solely on desk studies and available online GSI information.

The geological records made available from the GSI Online Mapping database indicated that the solid geology underneath the site is composed of various calcareous sedimentary rock formations. The Kilbryan Limestone Formation is located beneath the North West of the scheme and composed of dark nodular calcarenites and shales, followed by the Lough Carra Member which is characterized by thick-bedded pales and pure limestones. The Burrishoole Member which is a cross-bedded peloidal limestone underlies the middle sections of the proposed works. The South West section of the scheme is underlined by the Creagh Member, composed of fine-grained dark limestones and thin shales.

The GSI Online Mapping application shows an Un-named fault (Structure Code 11F1-2) crossing the N60 road in a north east-south west direction approximately at Ch.1,500. No other known fractures or structural discontinuities of bedrock were identified within the study area.

Mayo County Council N60 Balla to Claremorris Realignment at Heathlawn Environmental Assessment Report

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One karst feature had been identified on the GSI karst database within the study area, namely the Pollavady Turlough (hereafter the Unamed Turlough), indicated approximately 120m northwest of the scheme. According to the information provided on the GSI Online Mapping Karst Database, the feature is characterised by limestone and muddy lithologies. Based on consultations undertaken with the local residents and visual evidences gathered from site walkovers, several small karst related features were identified within the study area. Depressions in ground, temporary ponding areas, sinkholes and land collapses characterise the ground condition adjacent to the road between Ch. 2,200-3,130 as shown on 32103901/EAR/Figure 7.1: Feature Identified within the Study Area. 7.3.2 Near Surface Soils

The online Environmental Protection Agency (EPA) Envision Beta Map soil layer indicates that there are two main soil groups represented beneath the proposed works. The Irish Forest Soils (IFS) classifications for these soils are:

� Deep poorly drained soils (BminPD), these soils are predominantly derived

from calcareous parent material; and � Deep well drained soils (BminDW), again mainly derived from calcareous

parent material.

The 2011 ground investigations encountered the topsoil strata up to the depth of 0.4m. 7.3.3 Overburden Geology (Subsoil)

Subsoil thickness can vary considerably over the area, from very thin cover where the rock is close to the surface, to more significant depths. According to the GSI sub-soil map (www.gsi.ie) the study area is underlain by glacial till, derived mainly from the underlying limestone formations. See 32103901/EAR/Figure 7.3: Overburden Geology (Subsoils). The till deposits are diverse materials that were largely deposited glacially or post-glacially and exhibit a wide range of characteristics due to the variety of parent materials and different processes of deposition. Glacial deposits were reported to lie beneath the proposed development in the 2011 ground investigation borelogs, comprising granular and cohesive sediments including clay, silt, sand and gravel. The cohesive glacial deposits, described as firm to stiff brown sandy gravelly clay and silt, generally occur in the upper parts of the shallow soil profile. These deposits were encountered usually to depths of between 1 and 2m and occasionally exceeded 3m. Granular glacial deposits are commonly described as grey silty to very sandy gravel with cobbles and boulders. Brown, silty to very gravelly sands have been confirmed beneath the scheme. These granular deposits were encountered to the maximum depth of 3m. The granular and cohesive glacial sediments are often interbedded, and the logged subsoil geology showed vertical and horizontal heterogeneity beneath the scheme. The GSI subsoil map also indicates two bedrock outcrops within the study area. These exposures are located 360m and 180m north of the scheme, at Ch.1,660 and


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Ch. 4,200 respectively. See 32103901/EAR/Figure 7.3: Overburden Geology (Subsoils). The 2011 ground investigations did not encounter solid geology. The thickness of the subsoil lithologies is therefore unknown along the proposed development but proved to be over 4m at one location (maximum depth investigated during the ground investigation). However, the online GSI geological map indicates bedrock exposures in the vicinity of the scheme, suggesting that drift thickness is highly variable.

7.3.4 Groundwater

This section describes the baseline conditions identified through the gathering and review of available online information from the GSI database, ground investigation report and previous screening reports. (a) Bedrock Aquifer Classification

The majority of the solid geology underlying the proposed development is classified by the National Draft Aquifer Map as a regionally important karstified (conduit) aquifer (Rkc) of the Dinantian Pure Bedded Limestone unit. See 32103901/EAR/Figure 7.4: Bedrock Aquifer. The bedrock between Ch.1,600-1,800 has been identified as a locally important aquifer (LI) with having moderate productivity only in local zones. This aquifer is of the Dinantian Lower Impure Limestone unit. The bedrock within the study area has a high karstic potential. The aquifer is characterised by underground channels, caves and other voids that were formed by dissolution of the limestone by percolating groundwater flows. No bedrock groundwater level information is available as the 2011 investigations did not reach bedrock.


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(b) Vulnerability

The depth to bedrock (i.e. soil and subsoil thickness) is a critical factor in determining groundwater vulnerability of a site. The vulnerability of groundwater to pollution is assessed using a matrix developed by the GSI (refer to Table 7.1).

Subsoil Thickness

Hydrogeological Characteristics

Subsoil Permeability (Type) and Thickness Point Recharge.

Unsaturated Zone

High Permeability (Sand/Gravel)

Moderate Permeability (Sandy Till)

Low Permeability (Clayey Till, Clay, Peat)

(>30m Radius)

(Sand/ Gravel Aquifers Only)

0.0 – 3.0 m Extreme Extreme Extreme Extreme Extreme 3.0 – 5.0 m High High High N/A High 5.0 – 10.0 m High High Moderate N/A High >10.0 m High Moderate Low N/A High Notes: i) N/A = not applicable ii) Permeability Classifications relate to the engineering behaviour as described by BS5930 iii) Release point of contaminants is assumed to be 1-2 m below ground surface iv) Outcrop and shallow subsoil (i.e. generally <1.0m) areas are shown as a sub-category of extreme vulnerability.

Table 7.1: GSI matrix relating to vulnerability of Groundwater to Pollution

The thickness of soil and sub-soil deposits can mitigate against the risk of potential surface pollutants by acting as a protecting filtering layer, attenuating any infiltrating contamination before it interacts with the bedrock aquifer. As mentioned previously, the ground investigations did not prove the full thickness of the superficial deposits, and based on the encountered sediments types and the definitions shown in Table 7.1, the vulnerability of the karstic aquifer is inferred to range from High to Moderate. The GSI groundwater vulnerability mapping shows that the proposed development is underlain by ground water of High Vulnerability, with a sections classified as Moderate Vulnerability between Ch.1,275 and Ch.1,600. (c) Superficial Aquifer Classification

According to the information indicated on the GSI online database, the glacial subsoil strata within the study area is not classified as an aquifer. Limited groundwater information is available for the subsoil deposits, as only one groundwater observation borehole was installed in drift deposits during the ground investigations, and two rounds of monitoring were undertaken in August and September 2011. The borehole is located in the middle section of the scheme at Ch. 2,860. The groundwater level was recorded at 1.2 m below ground level (mbgl) and 0.9 mbgl respectively. Groundwater strikes and seepages within trial pits were also observed at two locations: 1.6mbgl at Ch. 2,220 and 2.9mbgl at Ch. 3,680. Yet other locations where infiltration tests were undertaken demonstrated dry conditions exist at least up to 1.5 to 2.6m deep in seven areas, despite the tests being undertaken subsequent to a heavy rainfall period.


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Table 7.2 summarises the infiltration test results.

Pit location Average infiltration rate (m/min)

Ch 1,050 No result due to rapid percolation

Ch 1,050 0.676241

Ch 1,800 0.109231

Ch 2,800 0.055736

Ch 2,925 0.109231




Ch 4,200 0.430542

Ch 4,350 1.513414

Table 7.2: Infiltration Test Results

As reported in Table 7.2, rapid infiltration was noted at pits located at Ch.1,050, Ch. 3,200 and Ch. 4,200. (d) Hydrogeological Characteristics

The hydrogeological characteristics of the geological units of the study area are summarised in Table 7.3.

Geological Unit Geological Characteristic

Hydrogeological Characteristic

Hydrogeological Sensitivity

Dri

ft

Topsoil Porous cohesive and granular sediments with organic material.

Not considered as an aquifer, infiltrating water from rainfall is inferred to percolate through and reach the underlying subsoil deposits.

N/A

Glacial till

Composed of variable sediments including clay, silt, sand and gravel.

Poor groundwater potential due to high heterogeneity and variable permeable nature.

Low sensitivity except where sand and gravels zones are known to be present and well connected (Medium)

Bed

rock

Karstic limestones of the Dinantian Pure Bedded Limestone Unit and Dinantian Lower Impure Limestone unit

Comprising sequences of karstified limestones and calcareous shales.

High to moderate groundwater potential due to high level karstification.

High

Table 7.3: Hydrogeological characteristics drift and bedrock units

No further conclusion can be drawn on short term aquifer behaviour based on the available data. (e) Karstic Groundwater Dependent Systems

Two groundwater dependent karstic features were identified within the area of interest, namely the Balla Turlough and a small Un-named turlough. These are shown on 32103901/EAR/Figure 6.1: Surface Water Features.


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Turloughs are topographic depressions in karst which are seasonally inundated, primarily from groundwater source and support important terrestrial ecological communities characteristic of wetlands. Turloughs sustain water only during wet conditions, mainly during the autumn to spring period. The base of the turloughs is commonly covered by subsoil deposits, the thickness of which is among the key controlling factors of the hydraulic conditions. The Balla Turlough is located at the North West end of the scheme and lies in a north-south hollow. It is of high ecological value for and classified as a candidate Special Area of Conservation (cSAC). The proposed development runs along the south east boundary and slightly within the cSAC area associated with Balla Turlough (chainage 1,000 to 1,280). Based on information collected from the National Parks & Wildlife Service (NPWS) website, the Balla basin comprises marl deposits. Considerable accumulation of peat is also noted to be present. Furthermore, it was indicated that during wet periods, the water rises mostly on the western side and from the southern end of the basin, where more permanent water bodies are also known. Based on satellite images of the study area, the Balla Turlough is modified by several drains, installed to remove water from the basin more rapidly than would naturally occur. The Un-named turlough is located approximately 120m north of the proposed development at Ch1,700. There was no further information available on this feature at the time of writing this report. (f) Private Water Supply (PWS)

Four PWS wells (PWS 1, PWS 2, PWS 3 and PWS 4) and one spring (PWS 5) have been identified along the scheme and are shown on 32103901/EAR/Figure 7.1: Features Identified within the Study Area, based on data collected during site surveys and information gathered from the local residents. PWS 1 is located approximately 50m south of the existing N60 road at Ch.1,800. The well is approximately 43m deep and used for domestic and livestock purposes. PWS 2 is abandoned and is now backfilled with stones. PWS 2 is therefore not considered as a receptor. PWS 3 is located outside the study area, approximately 550m south of the scheme at Ch.2,400. The well is not in use and the property is connected into the municipal supply. For these reasons the well is not considered as a receptor. The exact location is unknown for PWS 4; however, land owner consultation indicated that the well is possibly located 230m south west of the proposed Heathlawn Junction. The supply is not used by the residents, therefore PWS 4 is not considered as a receptor. A spring (PWS 5) was also identified at the site surveys. The spring is located approximately 400m north east of the scheme at Ch. 1,750. It is used for livestock purposes and understood to be discharging into the Un-named turlough.


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7.4 Appraisal Method used for Assessment of Impacts

The methods used for assessment of impacts is based on “Guidelines on Procedures for Assessment and Treatment of Geology, Hydrology and Hydrogeology for National Road Schemes” published by the National Road Authority in 2008. 7.4.1 Construction Phase Impacts

(a) Matrix of Impacts

(i) Importance

The importance or sensitivity of groundwater in the study area was determined using the criteria set out in Table 7.4.

Importance Criteria

Extremely High Attribute has a high quality or value on an international

scale

Very High Attribute has a high quality or value on a regional or

national scale

High Attribute has a high quality or value on a local scale

Medium Attribute has a medium quality or value on a local scale

Low Attribute has a low quality or value on a local scale

Table 7.4: Criteria for Rating Importance of Hydrogeology Attributes

Although no Source Protection Zones are present within the study area, groundwater below the site (groundwater within the karstic bedrock aquifer) is considered of Very High importance. This is due to the fact that the majority of the site is underlain by a regionally important bedrock aquifer of high karstic potential. The Balla Turlough SAC has an importance of Extremely High, due to its classification of being a Natura 2000 site of International Importance. (ii) Magnitude

The magnitude of impacts was determined using the criteria set out in Table 7.5 below.

Magnitude of

Impacts Criteria

Large Adverse Results in loss of attribute and/or quality and integrity of attribute

Moderate Adverse Results in impact on integrity of attribute or loss of part of attribute

Small Adverse Results in minor impact on integrity of attribute or loss of small part of attribute

Negligible Results in an impact on attribute but of insufficient magnitude to affect either use or integrity

Table 7.5: Criteria for Rating the Magnitude of Impacts

No beneficial impacts on geology, soils and hydrogeology are usually produced by road schemes. This is also the case of the proposed development.


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(iii) Significance

The significance of impacts was determined using the criteria set out in Table 7.6 below.

Importance of Attribute

Magnitude of Impact

Negligible Small Adverse Moderate Adverse

Large Adverse

Extremely High Imperceptible Significant Profound Profound

Very High Imperceptible Significant /

Moderate

Profound /

Significant Profound

High Imperceptible Moderate / Slight

Significant /

Moderate

Severe /

Significant

Medium Imperceptible Slight Moderate Significant

Low Imperceptible Imperceptible Slight Slight / Moderate

Table 7.6 Criteria for Rating the Significance of Impacts

7.4.2 Construction Phase Impacts

(i) Soils and Geology

The construction works will involve the excavation of topsoil and subsoil deposits in areas of proposed road cuttings. It is not anticipated that the underlying limestones will be intercepted. Table 7.7 indicates that the soil and subsoil deposits will be affected by the future construction works. The maximum excavation depth in the road cutting areas is expected to be 4 m, as reported in Table 7.7. The proposed works will also require the construction of embankments above the current ground level. Based on the baseline information collected, the potential impact on soils, subsoil and solid geology is as follows:

� Soils derived from calcareous parent material occur beneath the proposed works. The soils have a low importance due to the widespread and common abundance of these lithologies within the study area. The removal of soils in areas of road cuttings and the compaction of the soil strata in embankment areas is of negligible magnitude; therefore the overall Significance of Impact on the soil is Imperceptible.

� Heterogeneous glacial till deposits underlie the soil strata. The importance of the glacial till is low if they are intercepted, they are of low sensitivity and the magnitude of impact is negligible. As a result, the overall Significance of Impact on the subsoil deposits is Imperceptible.

� The bedrock strata are likely to remain unaffected by the proposed earthworks.

� There is also a risk of enhancing ground collapses, in particular in the vicinity of Cutting 2.


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(ii) Hydrogeology

Screening of Road Cuttings

A key aspect of the proposed construction works in terms of potential dewatering effect is whether cuttings will intercept the water table. Cutting areas also have the potential to cause a decline in groundwater quality as the protective overburden thickness is reduced in these areas. Table 7.7 below details the excavations required for the proposed works. The table only indicates cuttings that are exceeding the depth of 1m.

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Cutting 1 1,260-1,430

3.6 >4 >1.5 No/Possibly

Porous topsoil and heterogeneous cohesive and granular glacial sediments

Cutting 2 2,860-3,050

4.1 >4 >1.5 No/Possibly


Cutting 3 3,240-3,540

2.4 >4 unknown

No/Possibly


Proposed L-65426-0 (Side Road 1) Cutting

10-100 1.7 >4 >1.5 No/No


Proposed L-65426-0 (Side Road 1) Cutting on Mainline *

1050-1150

3.4 >4 >1.5 No/No


Proposed L-5539-0 (Side Road 5) cutting

10-110 1.65 >4 unknown

No/No

Porous topsoil and heterogeneous cohesive and granular glacial sediments and boulders.

* Side Road 1 extends laterally, parallel to the main alignment, into a raised area. The total excavation depth increases in this extension as a result of locally raised topographical conditions; however excavation base level remains the same, at the level of the existing road.

Table 7.7: Excavation Areas Deeper than 1m within the Study Area


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Groundwater Quality

In the event of accidental spillages during construction, contamination may migrate through the upper unsaturated zone reaching the groundwater table in the subsoil deposits and impair groundwater quality, unless appropriate measures for control of discharge and drainage are taken. The importance of the superficial aquifer is considered as low while the importance of the bedrock aquifer is very high. The significance of impact is anticipated to be imperceptible on both the superficial and bedrock aquifer, as contaminants concentrations would be expected to attenuate during vertical migration. The overall potential impact of significance on groundwater quality is therefore assessed as Imperceptible. However there are a number of specific groundwater receptors that need to be assessed in the context of potential accidental spillages:

� Potential magnitude of impact on PWS 1 (medium importance) during construction is assessed as negligible (because of its depth). This results in a potential Slight significance Imperceptible of impact.

� Potential magnitude of impact on PWS 5 (medium importance) during construction is assessed as negligible (because of its distance from the scheme). This results in a potential Imperceptible significance of impact.

� The potential impact on groundwater quality supporting the turlough groundwater dependant systems will be a combination of potential on superficial groundwater, bedrock groundwater and run-off water quality. This results in a potential Moderate / Slight significance of impact.

Groundwater Flow and Yield

Local groundwater flow direction and gradient can be affected in areas of excavations that reach and extend below groundwater levels. The lowering of groundwater levels and consequent dewatering of shallow aquifers can affect nearby surface water features (e.g. streams or turloughs) and ecosystems that interact with groundwater by entirely or partially reducing their baseflow component. Based on available groundwater data, Table 7.7 indicates that it is possible that groundwater may be intercepted by some of the excavations/cuttings along the proposed development. However, should groundwater be intercepted, the information gathered at this stage suggests that the effect would be minimal and localised and would only affect the superficial aquifer (low importance). This would therefore result in a significance of impact of Imperceptible within the superficial deposits. The bedrock aquifer is not affected by the proposed earthworks, therefore no impact is predicted on bedrock groundwater flow. No groundwater is expected to be directly intercepted within the turloughs. However groundwater potentially intercepted at Cutting 1 may generate a slight and localised effect within the most eastern part of the Balla Turlough. In the absence of mitigation, this would potentially result in a temporary Small Adverse magnitude of impact on the hydrogeological regime of this groundwater dependant system during construction. As a consequence, significant potential significance of impact is predicted on the most eastern end of the Balla Turlough SAC/pNHA during construction. No impact on the groundwater dependant system associated with the Un-named Turlough is expected.


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No cutting is proposed in vicinity of PWS 1 nor PWS 5, and therefore no yield impact is expected on these supplies. 7.4.3 Operational Phase Impact

(a) Soils and Geology

Soils and geological formations are considered to remain unaffected by the operation of the scheme with the exception of ground stability which remains a long term problem. (b) Hydrogeology

(i) Groundwater Quality

Based on the success of infiltration tests reported in Table 7.2, the drainage design is a combination of several soakaway trenches, filter drains and a discharge via a swale system to the Un-named turlough. Pollutant removal will be a combination of drainage circulating through grassed surface water channel and diesel interceptors. Traffic flows with the proposed development in place will remain as experienced on the N60 currently and therefore the proposed drainage design constitutes an improvement of the current situation with regards water quality. During the operational phase, the magnitude of impact is anticipated to be small on the superficial aquifer and negligible on the bedrock aquifer as contaminants concentrations would be expected to attenuate during vertical migration. The potential impact of significance on groundwater quality including supporting the turlough groundwater dependant system is therefore assessed as Slight. A small potential impact on the groundwater quality of PWS 1 and PWS 5 is expected. This results in a potential Imperceptible significance of impact. (ii) Groundwater Flow and Yield

There are no additional impacts anticipated on the superficial aquifer flow and yield, nor the identified PWS. The potential level of impact determined during the construction phase also applies to the operational phase. 7.4.4 Do-Nothing Scenario Impact

In the event that the proposed development is not constructed there will be no impacts on geology and hydrogeology, except due to future engineering works which would be controlled under the relevant planning process and the existing risks of ground collapse.

7.4.5 Worst Case Scenario Impact

If the proposed development is operated under worst case traffic conditions there would be no additional negative impacts on geology and hydrogeology, provided that the mitigation measures described are fully implemented. 7.4.6 Indirect Impacts

No indirect impacts are anticipated with regard to soils and geology.


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In the absence of mitigation, adverse indirect impact on hydrogeology and groundwater dependent sensitive ecological habitats may occur as contaminants migrate into the aquifer as a result of accidental spillages. Measures to mitigate this occurrence are discussed in Section 7.5 below. 7.4.7 Cumulative Impacts

Other proposed infrastructure projects in the locality include proposals for the upgrade of the Balla to Manulla section of the N60, proposals to extend the Lough Mask Regional Water Supply Scheme from Balla to Kiltimagh, and the planned extension of the burial ground at Balla (Objective O/HC-BG 1). However as none these projects have planning approval or are in construction, cumulative impacts are not anticipated.

7.5 Proposed Mitigation and Avoidance Measures

7.5.1 Construction Phase Mitigation

Mitigation measures to prevent or reduce the potential for contamination of local groundwater during construction or operation proposed for surface water protection and detailed in Chapter 6: Hydrology and Drainage, Section 6.5, are also applicable to protect groundwater. The grassed surface water channel, interceptor ditch and filter drain with 25m centre outfalls and petrol interceptor (as detailed in 32103901/PDD/Figure 01) between Ch1000 and Ch 1500 on the south western side of the carriageway, will be constructed and operating prior to excavation of Cutting 1 (Ch 1260 to 1340). These drainage features are adjacent to Balla Turlough SAC, to facilitate the controlled discharge of any intercepted groundwater back into the turlough habitats in the case of the water being intercepted during excavations. The outfalls to the turlough will be open during construction only, and closed during operation. As part of the EOP the contractor will be required to institute and maintain a construction and demolition waste management plan complying with the DoEHLG “Best Practice Guidelines on the Preparation of Waste Management Plans for Construction and Demolition Projects”. Soils re-use and management will be addressed as an element of the Waste Management Plan. The excavated materials, depending on their geotechnical and environmental suitability, will be re-used in the construction of embankments and in general landscaping measures. This will allow the importation of material to site is minimised where possible. Hazardous materials required during the construction stage will be stored with all fuel, oils and chemicals stored in secure areas on an impermeable base, at least 10m and 100m away from the scheme drainage system and turloughs, respectively. Fuel storage areas will be bunded to provide adequate retention capacity in the event of a leak or spillage occurring; With regards to the Construction Industry Research and Information Association (CIRIA) 2001 publication “Control of Water Pollution from Construction Sites, Guidance for Consultants and Contractors” all construction works will be completed in line with the recommendations of this publication. This will serve to mitigate against any soil pollution occurring.


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7.5.2 Operational Phase Mitigation

The proposed development drainage design, including petrol interceptors, soakaways and filter drains will be regularly maintained by the local authority or appointed maintenance contractor.

7.6 Residual Impacts

Imperceptible residual impacts to the geology and soils affected by the proposed development are expected as a result of the scheme construction and operation. Negligible impact on groundwater levels and flow is anticipated and slight positive impact on groundwater quality is expected due to road runoff treatment measures.

7.7 Assessment Conclusions

The assessment indicates that the impact on soils and drift geology will be Imperceptible. No impacts are expected on the solid geology. No sites or features designated or identified as being of geological interest will be affected by the proposed development. Potential impacts are identified for groundwater flow and quality sustaining the karstic groundwater dependent turloughs present within and adjacent to the study area. During construction and operation, mitigation measures will be put in place and the residual impact will therefore be reduced to Imperceptible.

7.8 Limitation to Assessment

The information used for the assessment is based on information from the GSI website, available publications and available ground investigation data.

7.9 References

1. Environmental Protection Agency; http://gis.epa.ie/betazone/envision/ 2. Geological Survey of Ireland (GSI) online geology data; www.gsi.ie; 3. N60 Heathlawn Road Improvement Scheme (C) Ground investigation

Factual Report, September 2011; 4. N60 Heathlawn - Road Improvement - Ground Permeability Testing 5. N60 Balla to Claremorris Road Improvement Scheme - Heathlawn Section-

Ecological Assessment & Appropriate Assessment Screening Report, 2011;

6. N60 Castlebar to Claremorris Road Improvement Scheme - Heathlawn Section-Ecological Assessment & Appropriate Assessment Screening Report, 2011;

7. N60 Balla to Claremorris Road Project at Heathlawn-preliminary Design Plans. Jacobs, 2011

8. National Parks & Wildlife Services; http://www.npws.ie/media/npwsie/content/images/protectedsites/sitesynopsis/SY000463.pdf, 1997

9. National Road Authority (NRA), Guidelines on Procedures for Assessment and Treatment of Geology, Hydrology and Hydrogeology for National Road Schemes, 2008

10. Working Group on Groundwater Sub-comittee on Turloughs: Guidance on the Assessment of Pressures and Impacts on Groundwater Dependent Terrestrial Ecosystems, Risk Assessment Sheet GWDTERA2a – Turloughs, March 2005.

Mayo County Council N60 Balla to Claremorris Realignment at Heathlawn

Environmental Assignment Report

Appendix 7.1 – Soils, Geology and Hydrogeology

Association 33 (Shallow Brown Earths and Rendzinas 60% Grey Brown Podzolics 25%, Gleys 10%, Peats 5%)

This soil parent material consists of shallow glacial till of mainly Carboniferous limestone composition and occurs on flat to undulating relief. The predominant soil consists of a shallow gravelly loam to silt loam texture and is classified as a Brown Earth of high base status. The topography is undulating to flat with gentle slopes.

The predominant soil a shallow Brown Earth, which is well to excessively drained. This soil has a gravelly loam to silt loam texture and is classified as a Brown Earth of high base status. The soil profile displays a dark surface horizon containing about 20% clay and 35% silt overlying a pale-brown subsoil. This rests on a limestone bedrock.

These soils have a moderately wide use range. The soils are suitable for both tillage and grassland farming. Where soil depth permits cereals and root crops are grown. Due to excessive drainage a moisture deficit can occur in dry seasons, limiting production. Pastures can be grazed over a long season without poaching, suitable for intensive sheep enterprises. Grassland and tillage are suited with the exception of outcropping limestone bedrock and boulder strewn and scrub areas.

� These tables are the draft issue (May 2012).� The landowner name columns are temporary columns for reference

purposes only. They will be removed from the final draft.� LP ref. no. column separately identifies the affected land parcels owned by

the same landowner.

A provisional assessment of the agricultural impact has been carried out on three farms based on a desktop study and roadside survey, and one farmer did not wish to disclose full details of the area of the affected area, the affected area has been estimated for these land holdings. They are marked with an * in the table.

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Documents

Chapter 7 - Soils, Geology and Hydrogeology€¦ · Mayo County Council N60 Balla to Claremorris Realignment at Heathlawn Environmental Assessment Report 84 7 Soils, Geology and Hydrogeology