What Can Models Tell Us About On-Site Systems? David Radcliffe & Larry West University of Georgia Presented at the On-Site Wastewater Treatment Conference October 25-27, 2005 North Carolina State University

How On-Site Systems Function Many of questions about on-site systems concern water flow in trenches Many of questions about on-site systems concern water flow in trenches Flow is predominately in 2 dimensions (2D) Flow is predominately in 2 dimensions (2D) Includes unsaturated zones Includes unsaturated zones Multiple layers including a biomat Multiple layers including a biomat Spatial scale is on the order of m's Spatial scale is on the order of m's Especially suitable to analysis using 2D numerical models for saturated and unsaturated flow Especially suitable to analysis using 2D numerical models for saturated and unsaturated flow

2D Numerical Models EPA has called for use of these types of models to understand on-site systems EPA has called for use of these types of models to understand on-site systems May be especially suitable for comparing systems with different geometries May be especially suitable for comparing systems with different geometries Models may not predict accurate absolute values unless they are calibrated Models may not predict accurate absolute values unless they are calibrated But they are likely to show accurate relative differences between systems But they are likely to show accurate relative differences between systems

2D Numerical Models Models may be a powerful training and teaching aid Models may be a powerful training and teaching aid User interface provides 2-D graphics that are relatively easy to understand User interface provides 2-D graphics that are relatively easy to understand Animated plots show changes over time Animated plots show changes over time

HYDRUS-2D State-of-the-art 2D numerical model State-of-the-art 2D numerical model Developed by scientists at US Salinity Lab (Simunek et al. (1998) Developed by scientists at US Salinity Lab (Simunek et al. (1998) Recent improvements in code and user-interface Recent improvements in code and user-interface Fast and relatively easy to use Fast and relatively easy to use Still takes a lot of time to run problems right Still takes a lot of time to run problems right Users need good knowledge of soil physics Users need good knowledge of soil physics Input of soil hydraulic properties Input of soil hydraulic properties Can use texture as input and get soil properties from database Can use texture as input and get soil properties from database Or input Ksat and water retention properties of soil horizons Or input Ksat and water retention properties of soil horizons

HYDRUS-2D Beach and McCray (2003) first to apply HYDRUS-2D to on-site trench flow Beach and McCray (2003) first to apply HYDRUS-2D to on-site trench flow Radcliffe et al. (2005) describes our recent work on effects of gravel Radcliffe et al. (2005) describes our recent work on effects of gravel

Models Models should be able to tell us something we don’t know already Models should be able to tell us something we don’t know already Quantify concepts that we already have in mind Quantify concepts that we already have in mind Surprise us with new concepts Surprise us with new concepts Identify research gaps Identify research gaps This is especially likely to happen in 2D This is especially likely to happen in 2D Intuition (at least mine) is not so good Intuition (at least mine) is not so good

Objective Demonstrate what we have been able to learn about how on-site systems function using HYDRUS-2D Demonstrate what we have been able to learn about how on-site systems function using HYDRUS-2D

Topics 1D Infiltration 1D Infiltration Borehole infiltration Borehole infiltration Biomat effects Biomat effects Trench interactions Trench interactions Gravel effects Gravel effects Sidewall effects Sidewall effects Consumptive use of water Consumptive use of water Potential future uses Potential future uses

1D Infiltration Soil 100 cm deep and 100 cm wide Soil 100 cm deep and 100 cm wide Two comparisons using Cecil soil Two comparisons using Cecil soil Uniform soil Uniform soil Cecil Ap (Ksat = 230 cm/day) Cecil Ap (Ksat = 230 cm/day) Layered soil Layered soil Cecil Ap to depth of 40 cm Cecil Ap to depth of 40 cm Cecil Bt2 below (Ksat = 2.47 cm/day) Cecil Bt2 below (Ksat = 2.47 cm/day) Free water at soil surface Free water at soil surface Show simulations for time = 0 to 0.07 days Show simulations for time = 0 to 0.07 days Numerical models and grid system Numerical models and grid system

100 cm Soil surface

T = 0.00 day

T = 0.01 day

T = 0.02 day

T = 0.03 day

T = 0.04 day

T = 0.05 day

T = 0.06 day

T = 0.07 day

40 cm

T = 0.00 day

T = 0.01 day

T = 0.02 day

T = 0.03 day

T = 0.04 day

T = 0.05 day

T = 0.06 day

T = 0.07 day

Topics 1D Infiltration 1D Infiltration Borehole infiltration Borehole infiltration Biomat effects Biomat effects Trench interactions Trench interactions Gravel effects Gravel effects Sidewall effects Sidewall effects Consumptive use of water Consumptive use of water Potential future uses Potential future uses

Borehole Infiltration Soil 100 cm deep and 100 cm wide Soil 100 cm deep and 100 cm wide Borehole 5-cm radius; 30-cm deep Borehole 5-cm radius; 30-cm deep 10 cm of water ponded in borehole 10 cm of water ponded in borehole Same soil comparisons Same soil comparisons Uniform soil with properties of Cecil Ap Uniform soil with properties of Cecil Ap Layered soil Layered soil Cecil Ap to depth of 40 cm Cecil Ap to depth of 40 cm Cecil Bt2 below 40 cm Cecil Bt2 below 40 cm Run simulations from time = 0 to 0.08 days Run simulations from time = 0 to 0.08 days Research gap Research gap Symmetry in borehole flow Symmetry in borehole flow

100 cm 30 cm Soil surface

T = 0.00 days

T = 0.02 days

T = 0.04 days

T = 0.06 days

T = 0.08 days

40 cm

T = 0.00 days

T = 0.02 days

T = 0.04 days

T = 0.06 days

T = 0.08 days

Research Gap How best to measure Ksat using borehole measurements in layered soils? How best to measure Ksat using borehole measurements in layered soils? May be difficult to obtain steady flow in a layered soil using long times to reach “equilibrium” May be difficult to obtain steady flow in a layered soil using long times to reach “equilibrium” As wetting front expands it encounters new horizons As wetting front expands it encounters new horizons No abrupt drop when this happens No abrupt drop when this happens May be best to use shorter times May be best to use shorter times

Topics 1D Infiltration 1D Infiltration Borehole infiltration Borehole infiltration Biomat effects Biomat effects Trench interactions Trench interactions Gravel effects Gravel effects Sidewall effects Sidewall effects Consumptive use of water Consumptive use of water Potential future uses Potential future uses

Biomat Effects Uniform loam soil (Ksat = 25 cm/day) Uniform loam soil (Ksat = 25 cm/day) Trench without biomat Trench without biomat Trench with biomat (Ksat = 0.05 cm/day) Trench with biomat (Ksat = 0.05 cm/day) Simulations run to Simulations run to 0.35 days (without biomat) 0.35 days (without biomat) 4.0 days (with biomat) 4.0 days (with biomat) Research gap Research gap Symmetry in trench flow Symmetry in trench flow

400 cm 300 cm 90 cm 150 cm No biomat Soil surface

T = 0.00 days

T = 0.05 days

T = 0.10 days

T = 0.15 days

T = 0.20 days

T = 0.25 days

T = 0.30 days

T = 0.35 days

With biomat

T = 0.00 days

T = 0.50 days

T = 1.00 days

T = 1.50 days

T = 2.00 days

T = 2.50 days

T = 3.00 days

T = 3.50 days

T = 4.00 days

4a T = 0.85 day 4b T= 2.5 day

Research Gap Biomat properties? Biomat properties? Ksat Ksat Thickness Thickness Water retention properties Water retention properties Do biomats reduce macropore flow below trench? Do biomats reduce macropore flow below trench?

Axis of Symmetry

Topics 1D Infiltration 1D Infiltration Borehole infiltration Borehole infiltration Biomat effects Biomat effects Trench interactions Trench interactions Gravel effects Gravel effects Sidewall effects Sidewall effects Consumptive use of water Consumptive use of water Potential future uses Potential future uses

Trench Interactions Loading rate of 2 cm/day to trench bottom Loading rate of 2 cm/day to trench bottom 0.5 gal/ft 2 /day 0.5 gal/ft 2 /day Two uniform soils Two uniform soils Clay loam with Ksat = 6 cm/day Clay loam with Ksat = 6 cm/day Sand with Ksat = 712 cm/day Sand with Ksat = 712 cm/day No biomats No biomats Run simulations to 12 days Run simulations to 12 days Research gap Research gap

T = 0 day Clay loam

T = 1 day

T = 2 day

T = 3 day

T = 4 day

T = 5 day

T = 6 day

T = 7 day

T = 8 day

T = 9 day

T = 10 day

T = 11 day

T = 12 day

T = 0 day Sand

T = 1 day

T = 2 day

T = 3 day

T = 4 day

T = 5 day

T = 6 day

T = 7 day

T = 8 day

T = 9 day

T = 10 day

T = 11 day

T = 12 day

Comparison Clay loam after 12 daysSand after 12 days

Research Gap What is the optimum spacing between trenches? What is the optimum spacing between trenches? How is affected by texture and slope? How is affected by texture and slope?

Topics 1D Infiltration 1D Infiltration Borehole infiltration Borehole infiltration Biomat effects Biomat effects Trench interactions Trench interactions Gravel effects Gravel effects Sidewall effects Sidewall effects Consumptive use of water Consumptive use of water Potential future uses Potential future uses

Gravel Effects Covered in more detail in afternoon talk Covered in more detail in afternoon talk Gravel vs. chamber systems Gravel vs. chamber systems Gravel masking Gravel masking Embedded gravel Embedded gravel Cecil soil Cecil soil Research gap Research gap

Gravel system Chamber system

Gravel Masking ChamberGravel Pressure Head (cm) 30 cm 60 cm

Gravel Masking After 2 days: 0.31 vs cm/day Ratio of 1.15

Gravel Masking

Embedded Gravel Chamber Gravel Pressure head (cm)

Embedded Gravel Effect After 2 days: 0.31 vs cm/day Ratio of 1.50

Embedded Gravel

Research Gap To what extent are gravel particles embedded? To what extent are gravel particles embedded? In trench bottom In trench bottom In trench sidewall In trench sidewall What is the effect of gravel “fines”? What is the effect of gravel “fines”?

Topics 1D Infiltration 1D Infiltration Borehole infiltration Borehole infiltration Biomat effects Biomat effects Trench interactions Trench interactions Gravel effects Gravel effects Sidewall effects Sidewall effects Consumptive use of water Consumptive use of water Potential future uses Potential future uses

Sidewall Effects Also covered in more detail in afternoon talk Also covered in more detail in afternoon talk How much water exits trench via sidewalls compared to bottom? How much water exits trench via sidewalls compared to bottom? Earlier work done by Keyes, Tyler and Converse Earlier work done by Keyes, Tyler and Converse Our work by Shelby Finch Our work by Shelby Finch graduate student working with Larry West graduate student working with Larry West Pulsed system Pulsed system 3 pulses per day 3 pulses per day simulations shown for day 7 during first pulse (13 time steps) simulations shown for day 7 during first pulse (13 time steps) Research gap Research gap

190 cm 180 cm 60 cm 45 cm 30 cm trench

T = days

T = days

T = days

T = days

T = days

T = days

T = days

T = days

T = days

T = days

T = days

T = days

T = days

T = days

92% of flow out of trench is through sidewall

Research Gaps How high on sidewall do biomats form? How high on sidewall do biomats form? Are sidewall biomat properties different from bottom biomats? Are sidewall biomat properties different from bottom biomats? Ksat Ksat Thickness Thickness Water retention properties Water retention properties Would tall narrow trenches work better than conventional design? Would tall narrow trenches work better than conventional design?

Topics 1D Infiltration 1D Infiltration Borehole infiltration Borehole infiltration Biomat effects Biomat effects Trench interactions Trench interactions Gravel effects Gravel effects Sidewall effects Sidewall effects Consumptive use of water Consumptive use of water Potential future uses Potential future uses

Consumptive Use Recent Atlanta TV program: Recent Atlanta TV program: “The real problem for the man down stream is the man-made enemy, a million septic tanks under Atlanta's recent growth which do not put water right back into the river. You take it and put it into the aquifer and it goes into the ground water system and into south Georgia.” “The real problem for the man down stream is the man-made enemy, a million septic tanks under Atlanta's recent growth which do not put water right back into the river. You take it and put it into the aquifer and it goes into the ground water system and into south Georgia.” Is water use consumptive (lost)? Is water use consumptive (lost)? Only evaporated water and water transpired by plants (evapotranspiration) is lost Only evaporated water and water transpired by plants (evapotranspiration) is lost

Consumptive Use How much septic discharge goes to “evapotranspiration” in a typical on-site system How much septic discharge goes to “evapotranspiration” in a typical on-site system Assumed Assumed Grass roots to 3 ft Grass roots to 3 ft Trench bottom at 4 ft Trench bottom at 4 ft Weather data for Athens in 1995 Weather data for Athens in gal/day; 300 ft trench 450 gal/day; 300 ft trench Made runs with and without on-site system Made runs with and without on-site system

Consumptive Use Water Balance Component (cm) Without Septic With Septic Difference Septic drain inflow Rainfall Grass transpiration Soil evaporation Storage change -437 Deep drainage % of septic drain inflow becomes deep drainage

Topics 1D Infiltration 1D Infiltration Borehole infiltration Borehole infiltration Biomat effects Biomat effects Trench interactions Trench interactions Gravel effects Gravel effects Consumptive use of water Consumptive use of water Potential future uses Potential future uses

Potential Future Uses Tall narrow trenches vs. conventional design Tall narrow trenches vs. conventional design Effect of soil layers and installation depth Effect of soil layers and installation depth Hillslope effects on trench interactions Hillslope effects on trench interactions Groundwater mounding beneath trenches Groundwater mounding beneath trenches Nitrate and bacteria leaching Nitrate and bacteria leaching Dissolved oxygen concentrations Dissolved oxygen concentrations

Conclusions Models can tell us things we don’t know Models can tell us things we don’t know Quantify processes Quantify processes Identify research gaps Identify research gaps Surprise us occasionally with new concepts Surprise us occasionally with new concepts Models have potential as training/teaching tool Models have potential as training/teaching tool Fast Fast 2D images and animation 2D images and animation Models promote application of soil physics! Models promote application of soil physics!