What Can Models Tell Us About On-Site Systems?

1. What Can Models Tell UsAbout 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

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

3. 2D Numerical Models • 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 • Models may not predict accurate absolute values unless they are calibrated • But they are likely to show accurate relative differences between systems

4. 2D Numerical Models • Models may be a powerful training and teaching aid • User interface provides 2-D graphics that are relatively easy to understand • Animated plots show changes over time

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

6. HYDRUS-2D • 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

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

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

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

10. 1D Infiltration • Soil 100 cm deep and 100 cm wide • Two comparisons using Cecil soil • Uniform soil • Cecil Ap (Ksat = 230 cm/day) • Layered soil • Cecil Ap to depth of 40 cm • Cecil Bt2 below (Ksat = 2.47 cm/day) • Free water at soil surface • Show simulations for time = 0 to 0.07 days • Numerical models and grid system

11. Soil surface 100 cm 100 cm

12. T = 0.00 day

13. T = 0.01 day

14. T = 0.02 day

15. T = 0.03 day

16. T = 0.04 day

17. T = 0.05 day

18. T = 0.06 day

19. T = 0.07 day

22. 40 cm

23. T = 0.00 day

24. T = 0.01 day

25. T = 0.02 day

26. T = 0.03 day

27. T = 0.04 day

28. T = 0.05 day

29. T = 0.06 day

30. T = 0.07 day

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

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

34. Soil surface 30 cm 100 cm 100 cm

35. T = 0.00 days

36. T = 0.02 days

37. T = 0.04 days

38. T = 0.06 days

39. T = 0.08 days

42. 40 cm

43. T = 0.00 days

44. T = 0.02 days

45. T = 0.04 days

46. T = 0.06 days

47. T = 0.08 days

50. Research Gap • 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” • As wetting front expands it encounters new horizons • No abrupt drop when this happens • May be best to use shorter times