Texas Soils Research Could We Do the Same

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Texas Soils Research Could We Do the Same?

• Dr. Bruce J. Lesikar
• Texas AgriLife Extension Service
• Southwest Onsite Wastewater Conference
• January 30 31, 2008

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Overview

• Background water movement
• What is a soil loading rate? Comparing Apples
  and Oranges
• What to measure?
• Getting the data

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Water Movement - Designing at the Boundaries

• Water movement from a trench
• Water movement through the soil treatment area or
  drain field
• Water leaving the soil treatment area

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Definitions

• Long-term acceptance rate (LTAR) (gpd/ft2).
  design parameter expressing the rate that
  effluent enters the infiltrative surface of the
  soil treatment area at equilibrium, measured in
  volume per area per time, e.g. gallons per day
  per square foot (gpd/ft2).
• Linear Loading Rate (LLR) (gpd/ft) quantity of
  effluent applied along the length of a lateral,
  trench or bed, typically expressed as volume per
  unit length per unit time (e.g. gallons per foot
  per day).

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Hydraulic Loading LTAR

• Loading rate 0.2 g/ft2-d
• Annual loading 0.2 g/ft2-d x 365 days 73
  g/ft2- yr
• Depth of water 73 g/ft2- yr / 7.48 g/ft3 9.8
  ft/yr or 118 inches/ yr

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Linear Loading Rate

• Do you account for side wall loading?
• Three foot wide trench has 3 ft2 of absorptive
  area per foot of trench
• Loading rate to bottom 3 ft x 0.2 gallons per
  day/ft2 0.6 gpd/ft

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Definitions

• Areal Loading Rate (ALR) (gpf/ft2) quantity of
  effluent applied to the footprint of the soil
  treatment area (or the absorption area of an
  above-grade soil treatment area) expressed as
  volume per area per unit time, e.g., gallons per
  day per square foot (gpd/sq. ft.).
• Contour Loading rate (CLR) (gpd/ft) cumulative
  total of effluent applied to the soil profile at
  the down gradient end of a dispersal system
  installed on a slope, expressed as volume per
  unit length per unit time along the contour
  (e.g., gpd/ft.).

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Contour Loading Rate

• Assume slope is downward on this drawing
• Sum the linear loading rate for the 4 trenches 3
  foot wide.
• CLR 4 x (3ft2 x 0.2g/ft2-d) 2.4 g /d linear
  foot

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Areal Loading Rate
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Areal Loading Rate

• Add distance between trenches to the total area
  where effluent is being distributed
• Accounts for rainfall, evapotranspiration and
  percolation components of soil treatment area.
• Compare systems on a more direct loading basis.
• ALR (3ft2x0.2g/ft2-d)/ 9ft2
• ALR 0.067 g/ft2-d
• What about drip?

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Composition of Soil

• Inorganic materials
• Organic materials
• Air
• Water
• Microbes

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Hydraulic Loading Rate

• Textural classification
• Sand
• Silt
• Clay
• Structure
• Loading Volume per surface area time (gallons
  per square foot)
• Trench Infiltrative surface
• Drip field Surface area

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Organic Loading Rate

• Biomat
• Oxygen transfer capability
• Loading
• Mass per surface area (mg per square foot)
• Mass per emitter (mg per emitter)
• Trench Infiltrative surface
• Drip field Surface area

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Theoretical Hydraulic Acceptance Rates for Soils
Receiving Wastewater
Acceptance Rate
Long-term Acceptance Rate
Time
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Water Movement from a Trench

• Restricting water movement
• Biological
• Chemical
• Physical
• Unsaturated conditions outside the biomat
• Water moves outward and downward in all
  directions from a ponded trench

Biozone
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Biomat sidewalls

• Biomat develops along the bottom and then around
  the trench
• Ponding levels use sidewalls
• Excessive ponding depths create saturated flow
• Narrower allows more surface area
• Narrower allows better O2 transfer

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Water Movement From A Drip Emitter

• Saturated Flow
• Water moving in all directions from the emitter
• Water moving along the lateral and then out into
  the soil
• Unsaturated Flow
• Dispersal in all directions

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Modeled Flow Pattern Around a Drip Emitter
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Water Movement From An Operating Drip Emitter
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Water Movement Through the Drip Field
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Determining Soil Loading Rates

• Conduct a field study to characterize changes in
  soil hydraulic properties caused by application
  of septic tank effluent through subsurface drip
  systems.
• Site data Wastewater loading
• Soil data -
• Water movement -
• Soil chemical condition -

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Site Description
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Operation Data
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Applied Effluent Quality
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Soil Sample

• Seven replicates of undisturbed soil cores 7.6 cm
  diameter by 7.6 cm long were obtained from three
  different locations and from three different
  depths around the drip emitter

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Collecting Soil Samples
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Location of the Soil Samples
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Calculating Water Movement Rate

• In lab process
• Double ring infiltrometer
• Amoozemeter reading
• Perk tests?
• Water loss from a trench?

Double ring infiltrometer
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Location of Soil Samples in Soil Profile for
Chemical analysis
30 cm
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Soil Core Sample Tests

• Hydraulic conductivity was tested using constant
  head method, and
• Soil retention curve was determined using
  pressure cell method.

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Pore Size Distribution

• Pore size distribution was determined from
  retention data using the following equation
• r is the mean pore radius, h is the capillary
  potential, ? is the water surface tension, ? is
  the contact angle between liquid and solid, g is
  the acceleration due to gravity, and ? is the
  density of water.

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Results of Soil Hydraulic Properties Analysis
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Pore Size Distribution at 3 cm Above the Emitter
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Pore Size Distribution at 7 cm Below the Emitter
Pore Radius Interval, ?m
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Pore Size Distribution at 30 cm Below the Emitter
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Change in the Concentration of Selected Chemicals
in the Irrigated Area Compared to That in the
Control Area ().
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Saturated Hydraulic Conductivity
Soil Surface
Drip Line
Emitter
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Summary

• What are you trying to measure?
• Make sure you are comparing apples to apples not
  apples to oranges.
• Data is important
• Wastewater loading quantity, quality, area
• Soils
• Water acceptance rate
• Field tests versus lab tests
• Dollars available