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Site Assessment – Desk Study, Field Survey, Soil Structure. Dan Doody Senior Engineer (Retired) Monaghan County Council. Site Assessment – Key Issues. Environmental Function of ICW – Natural treatment processes. Minimum ‘hard engineering’. Low/zero energy requirements.

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Site Assessment – Desk Study, Field Survey, Soil Structure

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Site Assessment – Desk Study, Field Survey, Soil Structure

  • Dan Doody

  • Senior Engineer (Retired)

  • Monaghan County Council


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Site Assessment – Key Issues

  • Environmental Function of ICW – Natural treatment processes. Minimum ‘hard engineering’. Low/zero energy requirements.

  • Social Considerations – Existing land use. Stakeholder interests in site and linked land and water resources.

  • Landscape fit – Integration into the environment - Detailed land survey.

  • Biodiversity – Complement local ecology. (Wetland plants, diverse habitats).


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Site Assessment

  • Baseline information:

    • Can ICW be safely constructed?

    • Any negative impacts on environment/receiving waters?

    • Site data for appropriate design.

    • Info to regulators – planning/discharge licenses.

    • Info to contractors – Include ground conditions to allow cost estimate.


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Site Assessment - General

  • Location of ICW

    • If in (or discharging to) a SAC, SPA, NHA – An Appropriate Assessment (Art 6 of Habitats Directive) will be required.

    • Otherwise - field visit to describe habitats in accordance with the Heritage Council’s guide to habitats in Ireland.

    • Generally favour sites with low current biodiversity value.


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Site Assessment - General

  • Risk Based Approach: Hazard-Pathway-Receptor.

    • Hazard: water vectored pollutants, construction/operation of ICW.

    • Pathway: significant linkage (e.g. field drains, gravel seams, karst) between hazard and receptor.

    • Receptor: Surface Water, Groundwater, Natural/built heritage.


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Site Assessment - General

  • ‘Absolute Protection is not possible’.

  • Mitigate risk by;

    • Low velocity/high residence time – wetland configuration of adequate functional area.

    • Subsoil of sufficient depth and of the required impermeability.


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Site Assessment - General

  • ICWs afford additional protection:

    • High impedances to infiltration by biofilms, humus and organic matter.

    • Biological feedback mechanisms secure water retention.

    • Wetland soils provide an effective processing medium (e.g. for denitrification of nitrate-N and ammonium-N).


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Site Assessment - General

Terrestrial Soils v. Wetlands Soils


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Typical ICW Influent - Farmyards


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Typical ICW influent - Wastewater

Domestic ICW Influent


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Ammonium-N concentration

  • Sustained influent concentrations exceeding 100mg/l and flux concentrations in excess of 280mg/l may cause vegetation die-off.


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Typical Discharges - Farmyards


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Typical Discharges - Wastewater


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Precipitation/Evapotranspiration

  • Rain related inflow may be many orders of magnitude greater than that originating from direct sources.

  • Conversely there are likely to be drought periods when there will be no surface water discharges.


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Restrictions

  • ICWs should not be considered for sites:

    • Within 60m up-gradient of a potable well.

    • Within the inner protection zone of a public groundwater supply (300m up-gradient where protection zone not identified).

    • Within 25m of dwelling.

    • Under mature trees.

    • Where there is a risk of collapse (swallow holes/karst features).

    • Of Natural Heritage value (without an appropriate assessment).


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Restrictions (contd.)

  • ICWs should not be considered for sites:

    • Where ICW may negatively impact Cultural Heritage value.

    • Where adequate land area not available.

    • Close to watercourses (10m from ponds 1&2, 5m from subsequent ponds).

    • Liable to flooding.

    • Where neither surface discharge nor exfiltration is possible (additional wet-woodland bunded area??)


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Surface Water Protection

  • ICW should be of sufficient size and/or receiving water should have sufficient assimilative capacity (see below)


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Groundwater Protection

  • 500mm thickness of subsoil under ponds with a max permeability of 1x10-8m/s underlain by a further 500mm of subsoil.

  • On a regionally important aquifer where the groundwater vulnerability is high/extreme, 750mm of soil enhanced to 1x10-8m/s underlain by a further 250mm.

  • On highly permeable sand/gravel/fractured rock in hydraulic contact with the water table – 750mm of low permeability subsoil with upper 500mm enhanced to provide a permeability of 1x10-8m/s.


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Groundwater Protection (contd.)

  • Where a risk exists of catastrophic leakage (karst geology/mined areas) increase depth of subsoil to 1500mm – or reject the site.

  • Geomembrane-lined ponds to be underlain by 100mm of subsoil & 50mm of protective fine sand and overlain by 200mm of low to moderate permeability subsoil.


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Competency of Assessor

  • To collect and interpret recorded and field info.

  • To make a visual assessment (Is a specialist needed?).

  • To assess the impact on aquatic receptors and site values.

  • To design the wetland (including understanding terrestrial and wetland soil ecology and their bio-geochemical processes).


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Site Assessment - Practical

  • Desk Study & collation of information

  • Visual Assessment:

    • Characterisation of wastewater (farmyard inventory/PE and volumetric range).

    • Evaluate receptor sensitivity and location.

  • Site Tests:

    • Trial holes.

    • Soil characteristics and particle size analysis.

  • Decision Process/Recommendations.


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Desk Study

  • Prelim. consultation with client:

    • Current wastewater practices.

    • Approx. volume and composition of waste.

    • Clients rationale for wastewater management.

    • Provide the client with an understanding of ICWs (incl. environmental benefits).

    • Budget costs and approx. land area.

    • See ‘Site Assessment Form’, Appendix C. of Guidance Document


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Data Collation

  • Site Information.

    • Targets at risk?

    • Site Restrictions? – NPWS.

    • Location options?

    • Topography – OSI maps.

    • Weather/Climate data (www.met.ie)


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Surface water receptors

  • Flow and quality data from EPA, OPW LA, otherwise calculate flow by:

    • Empirical formula or

    • CAD model.

    • Direct flow measurements.


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Discharge from ICW

  • Discharge (m3/yr) = (A+B+C) – (D+E) where,

    • A = initial volume for treatment (m3/yr).

    • B = Intercepting (paved/roof) area (m2) x annual rainfall (m).

    • C = ICW area (m2) x annual rainfall (m).

    • D =ICW area (m2) x annual evapo-transpiration & interception (m).

    • E = ICW area (m2) x annual infiltration rate (to ground) (m).


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Receiving Water Quality data

  • BOD,MRP,SS, Ammonium-N, Nitrate-N, Nitrite-N, (+ if available, Q-rating & WFD river body status).

  • May be available from EPA or LA – if not:

    • Min 3 sets of samples over 3 months (incl. July – Sept)


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Mixing of discharge with river water

  • Cds = (Qu x Cu) + (Qd x Cd)

    (Qu + Qd)

    • Qu = river flow u/s.

    • Cu = pollutant concentration in river u/s.

    • Qd = discharge flow.

    • Cd = pollutant concentration in discharge.

    • Cds = pollutant concentration in river d/s.

  • Average flows (ICW discharge prop. to rainfall)

  • S.I. No.272 of 2009 surface water and S.I. No.9 of 2010 groundwater.


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Groundwater

  • Limiting parameter – Ammonium-N

  • EPA and GSI – Aquifers & Vulnerability maps – groundwater protection zones.

    • National aquifer maps (GSI).

  • If Vulnerability maps not available check:

    • Relevant River Basin District project.

    • Soil & subsoil maps (Teagasc).

    • Outcropping bedrock & karst (GSI).

  • Groundwater Response Matrix – Appedix A (Guidance Document).


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If Surface Discharge not available:

  • Consider discharge to ground.

    • If discharge less than 5m3/day detailed assessment not required.

    • If greater than 5m3/day – guidance on detailed assessment to be published by EPA in 2011.


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Natural & Cultural Heritage

  • Designated (and candidate) NHAs, SPAs, SACs, protected structures, archaeological sites – consult with:

    • Client

    • LA

    • NPWS

    • DECLG


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Visual Site Assessment

  • If no insurmountable problems from desk study carry out visual assessment to:

    • Verify (or amend) desk study.

    • Assess on-site hazards.

    • Evaluate surface/ground water receptors.

    • Location options for ICW.


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Visual Assessment - Wastewater

  • Site visits – client present (on wet days):

    • Component sources of waste.

    • Estimate total volume of waste.

    • Discuss all water management activities with client.

  • Take Photographs.


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Visual Assessment - Topography

  • Scope Topographical Survey to:

    • Aid design.

    • Examine discharge options.

    • Assess landscape fit.

  • Wet sites have advantages but:

    • Biodiversity impact/benefits?

    • Additional inflow?

  • N.B. Check for field drains.


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Visual Assessment - Surface Waters

  • Identify receiving waters:

    • Channel width, depth, debris marks.

    • Est. assimilative capacity.

    • Water quality info – samples.

  • Other water features:

    • Lakes, wetlands, streams, ditches and N.B. land drains.

  • Photographs.


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Visual Assessment - Groundwater

  • Check existing wells/boreholes within 300m (incl. direction of flow).

  • Groundwater levels (trial holes).

  • Any groundwater quality data?

  • Karst features – swallow holes, ‘dolines’

  • Road cuttings, open excavations, river banks (ground conditions).

  • Photos.


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Utilities, Natural & Cultural Heritage

  • Verify (or amend) desk study:

    • Archaeological input?

  • Trees, o/h lines, houses, schools, churches.

  • Set-back distances.

  • Downwind receptors.

  • Evidence of flooding.

  • Can ICW be constructed?


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Trial Holes/Trenches

  • Trial Holes – min 2-3m below bases of ponds (plus depth to rock & water table).

  • No. of trial holes:

    • Area of ICW 0.5ha- min 3

    • 0.5 – 1.0ha- min 4

    • 1.0 – 1.5ha- min 5

    • 1.5 – 2.0ha- min 6

    • More than 2.0 ha- min 7

  • Record soil data to BS 5930 (Ref. Appendix D, Guidance Document)


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Soil Subsoil Characteristics


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Particle Size Distribution Test

  • A PSDT indicates permeability:

    • Clay content – lab test to BS 1377 (particles greater than 20mm diameter removed)

    • Clay content 13% or greater for 1x10-8m/s.

    • If 10% - 13% clay content, enhance subsoil to achieve permeability of 1x10-8m/s.

    • No of PSDTs related to size of ICW (similar to trial hole scenario).


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Summary – ICW Decision Issues


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Discharges Glaslough ICW

‘The Pure Drop’ (Autumn 2010)

No Discharge (1 May 2011)


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Contact Information

  • [email protected]

  • Tel. 047 88811

  • Mob. 087 6470832


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