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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

1-Introduction DEQ Septic.ppt

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Analysis of Septic System Failure

Karen DealMSU Extension Service

MSU Graduate StudiesMay 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 where traditional systems should not be constructed

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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 produce appropriate distribution of effluent or proper infrastructure is not provided

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Will this site work properly?

Landscape

X

XX

X

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Changing Technology

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What we plan to coverBasic soil science

The soil formation processVariability of soil across the landscape

Assessing and evaluating soil propertiesLandscape effects on drain field conditions

Water movement and behavior in soilChemical movement and behavior in soil

Using ‘soil survey’ for site evaluationInformation management

Field site visit

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Montana DEQ Septic Tank Drain Field - Wastewater Treatment Site Evaluation FormEvaluator: _______________________________________________ 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 noneDate 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/reedsSite Overview and Preliminary Assessment Checklistwithin 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?