Transcript Document

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
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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
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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
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Risk Based Approach: Hazard-PathwayReceptor.
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;
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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
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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
Typical ICW Influent - Farmyards
Typical ICW influent - Wastewater
Domestic ICW Influent
Ammonium-N concentration
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Sustained influent concentrations
exceeding 100mg/l and flux
concentrations in excess of 280mg/l may
cause vegetation die-off.
Typical Discharges - Farmyards
Typical Discharges - Wastewater
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
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ICWs should not be considered for sites:
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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).
Restrictions (contd.)
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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 wetwoodland bunded area??)
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Surface Water Protection
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ICW should be of sufficient size and/or
receiving water should have sufficient
assimilative capacity (see below)
Groundwater Protection
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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.
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
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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).
Site Assessment - Practical
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Desk Study & collation of information
 Visual Assessment:
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Site Tests:
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Characterisation of wastewater (farmyard
inventory/PE and volumetric range).
Evaluate receptor sensitivity and location.
Trial holes.
Soil characteristics and particle size analysis.
Decision Process/Recommendations.
Desk Study
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Prelim. consultation with client:
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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
Data Collation
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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
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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
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Discharge (m3/yr) = (A+B+C) – (D+E) where,
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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).
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:
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Min 3 sets of samples over 3 months (incl.
July – Sept)
Mixing of discharge with river water
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Cds = (Qu x Cu) + (Qd x Cd)
(Qu + Qd)
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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.
Groundwater
Limiting parameter – Ammonium-N
 EPA and GSI – Aquifers & Vulnerability maps –
groundwater protection zones.
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If Vulnerability maps not available check:
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National aquifer maps (GSI).
Relevant River Basin District project.
Soil & subsoil maps (Teagasc).
Outcropping bedrock & karst (GSI).
Groundwater Response Matrix – Appedix A
(Guidance Document).
If Surface Discharge not available:
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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
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Designated (and candidate) NHAs,
SPAs, SACs, protected structures,
archaeological sites – consult with:
Client
 LA
 NPWS
 DECLG
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Visual Site Assessment
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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
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Site visits – client present (on wet days):
Component sources of waste.
 Estimate total volume of waste.
 Discuss all water management activities
with client.
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Take Photographs.
Visual Assessment - Topography
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Scope Topographical Survey to:
Aid design.
 Examine discharge options.
 Assess landscape fit.
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Wet sites have advantages but:
Biodiversity impact/benefits?
 Additional inflow?
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N.B. Check for field drains.
Visual Assessment - Surface Waters
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Identify receiving waters:
Channel width, depth, debris marks.
 Est. assimilative capacity.
 Water quality info – samples.
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Other water features:
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Lakes, wetlands, streams, ditches and N.B.
land drains.
Photographs.
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
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Verify (or amend) desk study:
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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:
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Area of ICW 0.5ha
0.5 – 1.0ha
1.0 – 1.5ha
1.5 – 2.0ha
More than 2.0 ha
- min 3
- min 4
- min 5
- min 6
- min 7
Record soil data to BS 5930 (Ref. Appendix D,
Guidance Document)
Soil Subsoil Characteristics
Particle Size Distribution Test
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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
Discharges Glaslough ICW
‘The Pure Drop’ (Autumn 2010)
No Discharge (1 May 2011)
Contact Information
[email protected]
 Tel. 047 88811
 Mob. 087 6470832
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