County Laois Groundwater Protection Scheme

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					Decision-making on Location of On-site Wastewater
                Treatment Systems


Difficult Sites : Difficult Decisions


                    Donal Daly
              Groundwater Section
           Geological Survey of Ireland
                                              1
         Some Context!!


                                       Sewage pipe!!




          holiday house in west of
                   Ireland

Sinéad                               The Stray Cat
                                                       2
The 2-D View of rural Ireland




My problem: I can “see” the underground
        beneath on-site systems.




                                          3
Good Planning Requires Putting the 3rd Dimension
              into Decision-making




                                              4
My objective today: To utilise my ‘problem’
of ‘seeing’ the underground as a basis for
              providing advice.
                                              5
  FÁS Course: “Site Suitabilty Assessment for On-site
 Wastewater Treatment Systems” organised by GSI & EPA
1. Course is aimed at providing
   knowledge/expertise/competency to enable the 3
   questions to be answered in a professional way,
   using EPA site Characterisation Form as the
   framework
2. Course recommended in DEHLG Circular Letter
   5/03
3. Chartered geologists & engineers need Course, in
   my view
4. Recommendation: Co Co set up a panel of
   consultants
5. “Large” course (200) planned for May 2006
                                                   6
        Site Suitability Assessment
      The 3 Critical Issues/Questions
    Will “sure we get fly to them
1. The “sure can’t we to give the moon
   The the effluent hadaway underground (the
                     at the surface?
   without ponding response”!
 applicant) a fighting chance” syndrome
   (i.e. hydraulic issue)
      My recommendation: always approach sites
But: with these 3 issues in mind.
Recommendation to Co. Co.:
 2. Will the effluent be treated adequately before
  Septic answer is “yes” to greatest cause of
      If tank systems are reached?
    a -not accept ‘site all of them, site
  Do relevant
watershould be receptor is is suitable subject
        borne diseases in US.
                suitable.
    (i.e. attenuation issue) ’ conclusion.
to site improvements
      My must be realistic.
  Solutionassumption: discharge to surface water
  Theis not “minimum PisafterO distances” in
  Require evidence and H2 don’t disappear;
 3. Will allfeasible orseparationimprovement
       constituents N,     prohibited. (Keep be
works pathogens(i.e. poorly draining areas
      mind - undertaken
microbialare streamsan issue before considering
    complied with? in site restrictions issue)
  Advanced systems summer.)
site often dry up in are not a panacea; they should
not be seen as an excuse to say a site is OK.    7
    Topography & Landscape Position
In Poorly-draining areas, small streams are
  intermittent or summer flow is a small % of
  winter flow




                                                8
       Risk-based Decision-making
       “Source – pathway – receptor”
   Source       pathway        Receptor
                               ‘at risk’


Effluent
              “ground”     Groundwater,
                           aquifers, wells,
                           springs, streams,
                           lakes, beaches, shell
                           fish areas, wetlands

                                                   9
                                                Issue 3
      Will all “minimum separation distances” be complied with?
                        (i.e. site restrictions issue)

Type of system          Watercours    Wells/     Lake     Any        Site    Road    Slope
                         e/ stream   springs*           Dwelling   boundar          breaks/
                                                         house        y               cuts



                               sites can fail this 4test. 4
      A significant number of 50
Septic tank;      10    10           7     3
prefabricated
      Recommendations:
intermittent filters;
mechanical aeration
systems.
           Measure out the distances on site map, and if
       necessary on site 30
In situ intermittent 10         50    10     3    4      4
filters; percolation
       Be particularly careful if wells are present
area; polishing
filters
      nearby                                                                         10
               (i.e. site restrictions issue)

A significant number of sites can fail this test.
Recommendations:
 Measure out the distances on site map, and if
necessary on site
 Be particularly careful if wells are present
nearby



                                                    11
        Site Suitability Assesment
           The 3 Critical Issues
1. Will the effluent get away underground
   without ponding at the surface?
   (i.e. hydraulic issue)

2. Will the effluent be treated adequately before
   a relevant receptor is reached?
   (i.e. attenuation issue)
   Issue 2 can usually be ‘engineered’ if
    Will all “minimum separation distances” on
3. necessary. Yet, most emphasis is placed be
   complied with?
   this!!!!!
   (i.e. site restrictions issue)
   Issue 1 frequently cannot be ‘engineered’. 12
Issue 2 can usually be ‘engineered’ if
necessary. Yet, most emphasis is placed on
this!!!!!

Issue 1 frequently   cannot be ‘engineered’.


                                               13
Scenario 1: Thick low permeability subsoil
1.    Vulnerability = “Low” = >10m low K
     subsoil
2.   Soil = gley
3.    Rushes + many field drains
                                    Percolation
4.    Subsoil classed as CLAY       test hole!!!
5.    Trial hole examined in summer; no
     water but mottling evident to 0.6m
     bgl. In winter, water in hole.
6.    “P” = 42; “T” >100
                                               14
15
Scenario 1: Thick low permeability (K) subsoil
           Recommended Approach
1. Conceptualise site in terms of S-P-R
2. Main issues
    Targets at risk ?          SW (& ponding)
    Why?            Low K subsoil & high water table
     Pathways?                Sideways (horiz.)
     Hydraulic issue?         Yes
     Attenuation issue?       Yes, but can be
                              resolved
    Potential threat to the environment (from P.) &
    human health (from pathogens)
                                               16
Scenario 1: Thick low permeability (K) subsoil
              Possible Solutions
1. Main perceived problem        high water
   table
2. Usual proposed solution      “P” value
   passes, therefore ‘site improvements’ the
   soln.
   Lower the water table by drainage
   Build a mound
   Replace the subsoil

                                           17
Scenario 1: Thick low permeability (K) subsoil
       Will proposed solutions work??
1. Actual Problem: Subsoil permeability (K)
   is low
2. Will lowering the water table solve this?
      No!!! It merely creates a temporary storage
     area for effluent; it does not change the K.
3. Will a mound work?
     No. It treats the effluent but the hydraulic
     issue is unresolved.

                                               18
Scenario 1: Thick low permeability (K) subsoil
       Will proposed solutions work??
1. Will removal of subsoil & replacement
   with ‘suitable’ subsoil work?
     No, in most instances. Just acts like a bath
     tub.
     Might work where subsoil is shallow (say<2m).
     But, depends on underlying bedrock K, to
     resolve hydraulic issue.
     This needs to be evaluated in poorly
     productive aquifer areas using drilling &
     testing & calculations
                                               19
Scenario 1: Thick low permeability (K) subsoil
       Will proposed solutions work??

    Conclusion: Discharge to groundwater is
      not feasible in vast majority of sites
       in this setting; it cannot physically
                      occur!!!!
        Therefore, site is unsuitable.
          Are such areas common??
              Unfortunately, Yes.
                                           20
21
22
23
              Trial hole




Percolation
 test hole




                           24
25
26
Subsoil
classed as
‘CLAY’ using
BS5930




       27
 Start of     My conclusion:   Next day
percolation    site is not
   test
                suitable




                                   28
     Scenario 2: Thick “lowish” permeability
                     subsoil
1.     Some Rushes + some field drains

2.     Subsoil classed as SILT/CLAY

3.     No water in hole; slight evidence of
      mottling, but uncertain

4. “P” = 42; “T” = 73

5. Main Issue: permeability is marginal;
   possible high water table; hydraulic
   problems                                   29
Scenario 2: Thick “lowish” permeability subsoil
           Possible Solutions & Decision
1. Lower the water table by drainage
      This will only work if there is an outlet. OK on sloping
       Compare these on flat sites.
      sites; will not workfirst 2 scenarios.
2. Build a mound Where subsoil consists of CLAY,
     Conclusion:
     with “T that >~90, percolation to
     May help ”invalue it keeps no solutionpipes close to
     ground level, thereby maximising use of topsoil to
     hydraulic issue; site is unsuitable.
     enable effluent to flow away underground. But does
      not resolve hydraulic issue on flat sites for winter
       Where “
      situations.T”<~90 & “P” <50, hydraulic issue
      might be resolved where site is sloping.
3. So, these sites are suitable in certain
   circumstances; unsuitable in others; depends
   largely on topography.
4. Decision based on visual assessment + judgement!
                                                             30
  Scenario 3: Thin (~1m) free draining soil &
    subsoil over a poorly productive aquifer
1. Main issues
    Targets at risk?             SW & GW
   Why?        Thin subsoil & low K bedrock
    Pathways?       Downwards and/or Sideways
    Hydraulic issue?             Perhaps
    Attenuation issue?           Yes, but can be
                                 resolved
   Potential threat to the environment (from
   P.) & human health (from pathogens)
                                           31
  Scenario 3: Thin (~1m) free draining soil &
                   a Co. Co.:
   subsoil over to poorly productive aquifer
   Recommendation
1. Key Issue
    Require evidence that bedrock can enable
        Permeability of
     effluent to get away.top of bedrock; this is the
       layer that enables the effluent to move a way
      This requires specific site investigation, including
       underground
     permeability testing, water table measurements,
     permeability testing, water table measurements,
2.   etc, can this be
     How and calculations assessed?
       Examining rock outcrops and rock in trial pit
       Visual assessment >> Looking at indicators such
       as vegetation & presence of drainage ditches
3. Some sites suitable; others unsuitable

                                                       32
Weathered zone as fluid pathway




                     Weathered zone at
                     top of bedrock
                     aquifer




                                    33
     Scenario 4: Cutover Bog Site

 Typical Scenario
 Flat
Drained; drainage channels around site
Grassland
High water table (usually<1m bgl in winter)
“P” tests pass; “T” tests cannot be
undertaken
Peat usually underlain by ‘impermeable’ lake
clays
                                        34
35
        Scenario 4: Cutover Bog Site

1. Main issues
    Potential threat to the environment (P.) &
   human health
   Targets at risk          SW (& ponding)
   Why?        Peat has a low permeability
    Pathways?       Sideways (horizontal)
    Hydraulic issue?             Invariably
    Attenuation issue       Yes, but can be
                            resolved
                                           36
       Scenario 4: Cutover Bog Site
             Site Suitability??
1. Site will usually be unsuitable

2. An exception may be on a relatively high
   area on the edge of the bog, where the
   drains are maintaining a low water table.
   But would need a ‘T’ value to check
   permeability.



                                         37
Scenario 5:
Floodplains




              38
39
Scenario 6: Thin soil on karstic bedrock




                                       40
Scenario 6: Thin soil on karstic bedrock




                                           41
Karst – sinking stream




         Co. Roscommon
                         42
43
Karst




 Co. Roscommon
                 44
Karst




        45
Karst – collapse features




       Co. Roscommon        46
47
48
    Scenario 6: Thin soil on karstic bedrock
1. Main issues
    Targets at risk?        GW & SW (via GW)
   Why?        Karstic limestone
    Pathways?       Downwards (vertical)
    Hydraulic issue?             No
    Attenuation issue?           Yes, but can be
                                 resolved
   Potential threat to the surface water (P.)
   via groundwater & human health (pathogens
   in drinking water)
                                           49
Scenario 6: Thin soil on karstic bedrock


Pass effluent through as thick a subsoil or
polishing filter as possible -- at least
1.8m thick, and greater where nearby
wells are present. This is the key!
Therefore, mound probably required.
Make sure hydraulic loading is not too high
Make sure water from wells is treated!!

                                        50
      Scenario 7: Large developments with
            discharge to groundwater
1. Main issues
    Targets at risk?          GW & SW (via GW)
   Why?         high liquid & contaminant loading
    Pathways?        Downwards (vertical)
    Hydraulic issue?           Yes, usually
    Attenuation issue?         Yes
   Potential threat to the surface water (P.)
   via groundwater & human health (pathogens
   in drinking water)
                                             51
     Scenario 7: Large developments with
          discharge to groundwater
 Recommendation
1. Scaling-up of EPA Manual requirements
   not (remotely!) sufficient/adequate.
2. A comprehensive site investigation needed:
    involving drilling;
    permeability testing;
    installation of piezometers;
    hydraulic gradient determination;
    calculations showing effluent can migrate
    away.
                                                52
       Site Suitability Assessment The 3 Critical Questions

 1. Will the effluent get away underground
    without ponding at the surface?
    (i.e. hydraulic issue)
    1. Will the effluent be treated adequately before a
       relevant receptor is reached?
    (i.e. attenuation issue)
 2. Will all “minimum separation distances” be
    complied with?
    (i.e. site restrictions issue)
Recommendation:
  Training via FÁS course vital
Recommendation:
  Use these questions as the framework for your
recommendation/decision                                       53
            Conclusions
The primary factors determining the
 answers to the hydraulic and
 attenuation issues are:
   The permeability of the subsoil;

   The thickness of the subsoil;

   The permeability of the bedrock,
   where the soil/subsoil is shallow
                                       54
                 Conclusions
 Difficult Sites in Galway
1. Lack of soakage main issue (hydraulic
   issue) i.e surface water at most risk
   from new houses
       Difficult Decisions in Galway
 2. Karst hydraulic issue is not may be
Solving the and peatland areas always feasible
   problematical
Some sites are NOT SUITABLE; you may
need to say “No”!
Therefore, planning policy for these areas needs
to take this into account                      55