Lecture 2bSoil Texture, Water & Septic Tanks Drain

Fields• Soil Texture

influences the water content of soils.

What happens to water when it is added to the soil?

Soil Permeability – how we describe water movement through the

soil

• Hydraulic Conductivity – movement of water through the soil = permeability

• Determines the suitability of the soil for a septic tank drain field.

Permeability – Water Transmission

• Sandy soils will have fast water transmission and low water retention

• Clay soil will have slow water transmission and high water retention.

Porosity and Permeability

• Porosity is a measure of the open space within soil or rock

• Pore spaces represent the reservoir for holding water.

• Permeability is a function of the sizes of particles, pores, and the way they are arranged.

• Permeability is how quickly water will flow through the soil

• The straighter and larger the pores, the faster the permeability.

• Clays tend to reduce the porosity and permeability of soil material due to the small pores.

• Generally, surface horizons have a larger porosity and subsoils have smaller porosity and reduced permeability

Depth to Water Table

• Many landscapes have soils that are going to be in contact with the water table.

• Soils will show “Wet Soil” characteristics when they develop in this type of landscape position.

• The depth to the water table is independent of soil texture.

The soil here will beSaturated when theWater table is high

In areas with high water tables - water in the basement or the pit for the basement may occur. Determine the oxidation-reduction

status of the soil for clues about this condition. High chroma (bright) – well drained soil

Low chroma – gray, bluish, or gray green – anaerobic conditions & high water table is likely

• A• Bg• Cg1• Cg2

Basements & Water Tables• Water in basement : need to provide drainage

tile around the basement next to outer wall, water collected by sump pump in basement

Basementgravel Tile

New House Construction

This conditionMay eventually lead to a wet basement

Sewage Disposal• For cities and towns

sewage is collected and treated in a waste water treatment plant.

• The US has spent may billions of dollars upgrading and building these facilities to clean up our surface waters.

• Some ocean communities still use ocean outfalls for their waste.

Septic Tank Systems

• In rural areas not serviced by sewer systems, a method of disposal of common household and human wastes is necessary.

• For these areas an anaerobic treatment process called a septic system is employed.

Septic Systems

The septic system consists of a series of four steps.

1. Septic Tank (collects the wastes from the house)

2. Biological process occurring in the settling tank. (converts solids to liquids)

3. Dosing to the absorption field. (effluent is leaked into the soil from pipes with holes)

4. The absorption field. (soil converts the waste water to clean water)

• Have you ever been to a place that used a septic tank drain field for sewage waste?

• How did you know this?

Test to determine soil suitability for a septic tank drain field

• Percolation test : calculate perc rate by measuring time it takes water to drop 1 inch in a pre-wetted hole = time/drop in water level = min/inch

• see Minnesota Guide to onsite sewage treatment systems for details.

• http://septic.umn.edu/publications/index.html

Soils and the Absorption Field• Soil Texture - determines

permeability : too slow and field fails, too fast and ground water polluted

• Depth to water table - less than 3 feet and effluent will pollute groundwater.

• Depth to bedrock or impermeable layer - water moves laterally on rock and is not cleaned

• Slope - steep slopes allow effluent to seep out hill side.

• Soil organisms decontaminate the effluent by killing pathogens.

• Nutrients absorbed by soil and used by organisms and plants growing on the soil.

Septic Systems• If the complete septic system is

working correctly the homeowner may never have to think where the waste is going.

• However, major failure of the system could cause major problems and they all fail with age. (20 to 30 years)

• Too slow percolation will cause the sewage to back up into the house or move over the surface of the soil.

• Too fast will allow insufficient time for microbes in soil to clean water

Septic Systems

• They system can also fail if the biological organisms are no longer working to metabolize the waste.

• Detergents and bleaches in sufficient quantity can kill off the organisms.

Septic Tank & Wells

Leaching Fields• Leaching fields contribute Nitrogen to the surrounding

soil and eventually water table. As more fields are added to the area, the water table’s Nitrate levels increase.

• This process is faster with soils that have rapid permeability. The Anoka Sand plain is an example of this kind of environmental problem.

Alternative Systems

• Traditional septic systems have proved to be an effective way to treat sewage when space and good soil conditions exist.

• However -- When space is limited or soil conditions are poor, homeowners may need a modified treatment system.

• Poor soil conditions include clay textures, soils that are poorly drained, soils shallow to bedrock or soils that are sand and too permeable.

Mound System – used when soils are not suitable

Mound System for Septic tank Drain field

Other Alternative Systems a Sand Filter

• http://www.bae.umn.edu/~septic/LCMR/todd.html• http://www.extension.umn.edu/distribution/

naturalresources/DD7672.html

• The homeowners wanted to try out a new system and maximize their smaller lot by avoiding the use of a mound.

• A sand filter system was chosen because of its reliability, flexibility, and level profile.

• A single-pass sand filter system pre-treats septic tank effluent by filtering it through sand before sending it to a soil treatment system.

Alternativeto septic tank drain fields - flush toilets that don’t use water.

Compost ToiletsSun Mar & Envirolet

• A composting toilet must perform 3 completely separate processes

• 1) compost waste and toilet paper quickly and odorlessly

• 2) ensure finished compost is safe

• 3) Evaporate Liquid

Composting Units• Separating solid wastes from the

toilet and delivering them to a composting unit reduces household water use up to 40 percent.

• Composting removes many pathogens while nutrients are stored in the safe compost which can be used on the garden.

• Some systems use worms, while others use bacteria and aeration. The composted material must be removed periodically.

• Cost – $1200 -3000