Transcript Slide 1

Septic Tank Drainfield Site Suitability
and Evaluation Workshop
Montana State University Extension Service,
Department of Land Resources
an Environmental Sciences,
in cooperation with
Montana Department of Environmental Quality
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1-Introduction DEQ Septic.ppt
Analysis of Septic System Failure
Karen Deal
MSU Extension Service
MSU Graduate Studies
May 1998
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Goal: identify correlations between septic system failures and improper
site evaluation, improper installation, or insufficient system maintenance
Conducted interviews with local engineers, installers, pumpers, and
homeowners to determine what factors contribute to septic system
failure
In general, septic system failure occurs when the soil within the
absorption field (leach bed, drain field) no longer accepts effluent from
the septic tank at the same rate at which it is generated. This is called
hydraulic failure. As long as sewage is not backing up into the house, the
system is working. However, failure of treatment could be occurring while
the system is mechanically functioning. This would be the case if
insufficiently-treated wastewater were discharged to a shallow aquifer, and
reached nearby wells or surface waters.
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The main factors associated with septic system failure
are: or construction, tight soils, inappropriate drain field
location, system age, improper maintenance, drainfields
unlevel or too deep.
Collectively, these circumstances leading to system failure,
hydraulic failure, and those leading to treatment failure,
are often a consequence of inadequate site
characterizations being performed by site evaluators
who are unaware of such factors as soil structure,
structural discontinuities, and correct soil characteristic
assessments.
When evaluating the drainfield location, landscape
positions, slope positions and evidence of seasonal
saturation by groundwater are sometimes overlooked,
not recognized, or disregarded.
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Improper Site Evaluation
Since site evaluations create the foundation for the completion of a working septic
system, these evaluations should be as correct and thorough. For permitters,
the abililty to interpret data, recognize limitations, and properly assess is critical
Examples:
Incorrectly performing site evaluations such as altering and/or incorrectly
completing and reporting percolation test results, where required.
Very low hydraulic conductivity; inaccurately distinguishing soil textural
differences
Inexperienced people evaluating soils and the hydraulic capacity of the soil is
not being calculated correctly or taken into account properly in the design phase
Surface drainage was not diverted from the drainfield
Slope and landscape positions were disregarded, leading to surface drainage
and direct infiltration of the trenches
Conventional systems installed in saturated and/or poorly drained clay soils
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where traditional systems should not be constructed
Specific Locations of Concern
Site characteristics that must be looked for when determining a site’s suitability
for a particular septic system – the most frequent causes of failure related to site
evaluations:
Reported by soil scientists and inspectors: include 1) gravelly soils, 2) clay soils,
and 3) high (shallow, close to the land surface or trench bottom) groundwater,
contributing to saturated soil conditions in the lower portion of the soil
profile of drain fields.
Additionally, steep slopes require appropriate designs; and percolation rates of
the soil must be considered (either from texture or measurement).
Failure of treatment in areas that have gravelly soils which constitute a poor
filter media or a conduit to shallow groundwater.
Soils with low percolation rates – particularly soils which are dominated by clay,
have obvious textural discontinuities, or are consolidated
Collectively…. 1) seasonal high groundwater infiltrating the trenches, 2) the soils
are not adequately permeable, or 3) the trenches and/or pipes are not sloped to
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produce appropriate distribution of effluent or proper infrastructure is not provided
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Will this
site work
properly?
Landscape
X
X
X
X
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Changing Technology
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What we plan to cover
Basic soil science
The soil formation process
Variability of soil across the landscape
Assessing and evaluating soil properties
Landscape effects on drain field conditions
Water movement and behavior in soil
Chemical movement and behavior in soil
Using ‘soil survey’ for site evaluation
Information management
Field site visit
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Montana DEQ Septic Tank Drain Field - Wastewater Treatment Site Evaluation Form
Evaluator: _______________________________________________ Phone: _______________________
Fax:_______________________
Email: _________________________________________________________________
Mailing address:_________________________________________________________
_________________________________________________________
Property owner or entity for whom evaluation being completed:
_____________________________________________________________
Site location (physical location or
address):______________________________________________________________________________
Site location (GPS lat/long coordinates or legal
description):________________________________________________________________
Excavation method (circle appropriate option):
backhoe pit
hand dug pit
auger hole
exposed
construction pit other
none
Date on-site evaluation completed: __________________________________________________
Weather and lighting at time of evaluation: ____________________________________________
Average annual rainfall at site vicinity (inches):
_____________________________________________________
General vegetation/land use in site vicinity (circle appropriate option):
grassland
sage/grass prairie
timbered forest
suburban/pasture
irrigated agriculture
dryland farming sedges/reeds
Site Overview and Preliminary Assessment Checklist
within floodplain YN evidence of past flooding (distance from proposed site:
feet)YN evidence
of recent alluvial/ colluvial deposition (distance from proposed site:
feet)YNslope (% or feet/100
feet) at proposed siteavailable suitable area for proposed drain field (acres, L x W (in feet))coarse gravel,
cobble, boulders on surface (if present, percent cover:
)YNproposed drain field location within
landscape*
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Questions?
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