Factors Controlling Riffle-Scale Hyporheic Exchange Flows in a Gaining Stream

1. Factors Controlling Riffle-Scale Hyporheic Exchange Flows and Their Seasonal Changes in a Gaining Stream: A Three-Dimensional Groundwater Flow ModelR.G. Story, K.W.F. Howard, and D.D. Williams2003 Geology 230 Kent E. Parrish, P.G., C.Hg February 14, 2013

2. Lecture Outline • Introduction • Site • Field Methods • Model Description • Results • Discussion • Conclusions • Limitations

3. Introduction lifeinfreshwater.org.uk Hyporheic Flow Riffle-pool: Basic unit of exchange area

4. Introduction Harvey and Bencala, 1993 • Used a model to predict that, in mountain streams, exchange can occur even during aquifer discharge • Discontinuities in stream gradient (>20% slope) • But they did not describe other properties of streambed or aquifer that are required to allow exchange • Did not attempt to find relationships between hydrological or geological parameters of the system and vertical or lateral extent of exchange flows

5. Introduction Tracer Studies versus Modeling • Tracers have been main tool to compare storage sizes and exchange rates • But too many factors to simply describe system • Tracers cannot distinguish between surface and subsurface storage • Tracers operate from surface water perspective but hyporheic flow is through porous medium Color represents a take-home point

6. Introduction Past Modeling Efforts • Hadn’t attempted to evaluate range of controlling factors for exchange • Had simulated streams as only one cell wide in models • Had been two-dimensional models

7. Introduction This Article’s Objectives • Identify hydrological and geological conditions that are required for hyporheic exchange to occur during aquifer discharge • Identify key factors that are sufficient to explain seasonal changes in exchange flows • Describe differences in vertical versus lateral exchange flows and paths in different parts of streambed

8. Site Speed River, southern Ontario • Gravel bed • Flows across undulating glacial terrain • Low topo relief (2 - 5 m/km) • Dolomite aquifer bedrock 20 m below ground surface • Domomite overlain by low-K till, kame, and outwash deposits (K = 10-7 m/s (0.028 ft/d) to 10-8 m/s [0.0028 ft/d)

9. Site Speed River, southern Ontario • Stream lies in recent alluvium 1-1.5 m deep and 5-10 m wide on each side of the stream (K = 2 x 10-4 m/s [57 ft/d]) • At Site, stream is 6 m wide; 0.15 – 0.35 cm deep in summer • Summer baseflow = 0.1 m3/s • Winter baseflow = 2 – 3 times summer baseflow

10. Field Methods Field Studies • Used nested minipiezometers (dia. 1.3 cm) • Each piezo had single 5 mm opening • Nest consisted of piezos 0, 20, 40,60, 80, or 100 cm below stream bottom • Nests installed about 1 m apart in two transects • Across stream at upstream end of riffle • Along axis of stream between upstream and downstream end of riffle

11. Field Methods Field Studies • Measured hydraulic head distributions in 3-D in one 13-m-long riffle site

12. Field Methods Field Studies • Data collected over four seasons (Aug 1996 to July 1997 • Additional measurements over high and low base flow periods until November 1998

13. Field Methods Field Studies • NaCl tracers used to confirm flow directions interpreted from piezo heads • Injected in up-welling and down-welling zones, at center and near sides of stream channel • Measured EC • Time to peak EC was used to calculate K • K = - (vne)/i; where, v = flow path length/tracer travel time • ne = effective porosity of sediments • i = hydraulic gradient between release and detection points

14. Field Methods Field Studies • Measured temperature variations over 24-hour period in stream channel and across upper transect • Measured every 3 hours • Criterion for when surface water reached measurement depth: when piezo temp cycle had amplitude > 10% of stream channel temp cycle • Criterion based on Silliman et al. (1995)

15. Field Methods Field Studies • Time delay between temp peaks in stream channel and in each piezo was used to calculate a first-order travel time estimate for surface water down-welling • NaCl not conservative tracer so only first-order estimate possible

16. Model Description • Model Domain 1,000m x 500 m • E and W boundaries were Speed River catchment edges • N and S boundaries parallel to groundwater flow • Grid cell sizes • 8 m x 8 m across domain • Refined to 1 m x 1 m at the riffle • 12 model layers • Dolomite aquifer bottom, Kx,y = 10-6 m/s (0.28 ft/d); Kz = 10-7 m/s (0.028 ft.d)

17. Model Description • Kx,y of Layer 12 dolomite estimated from specific capacity tests • Layers 1-3 till and outwash with K = 3 x 10-8 m/s (0.008 ft/d) estimated from slug tests • Layers 4-11 till and outwash with K = 10-6 m/s (0.28 ft/d) and 10-5 m/s (2.8 ft/d) “calibrated” layers 1-3 to match vertical gradients across piezos and historical stream discharge data (Water Survey of Canada, 1992) • 3-9 horizontal 0.25 m thick near stream • High K zone (est. via salt tracers) along and 1.5 m beneath stream K = 2 x 10-4 m/s (57 ft/d)

18. Model Description

19. Model Description Note thin, constant thickness cells. Allowed finer-scale modeling

20. Model Description Stream as constant heads

21. Model Description

22. Model Description Aquifer Recharge • Estimated from stream discharge records • Recharge = summer and winter base flow / catchment area • Applied as constant flux to top model layer Model Application • Steady state runs (36) • Varied parameters to simulate winter/summer conditions • Stream heads winter and summer (factor of 2) • Groundwater discharge doubled (field-based) [raised heads by 2 m and doubled aerial recharge. Then doubled groundwater discharge again • K varied over 2 orders of magnitude (field-based)

23. Field Results

24. Field Results Temp expressed as % of variations in stream temperature

25. Model Results Key Model Factors (model most sensitive to these) • Hydraulic conductivity • Boundaries of hyporheic zone • Head difference between upstream and downstream ends of riffle • Flux of groundwater entering the alluvial zone from the sides and beneath • Steeper Summer stream gradient causes increased exchange flux • Hyporheic flow travel times related to both flow velocity and distance

26. Model Results Vertical versus lateral exchange flows • Vertical exchange in the channel occurred more consistently than later flows into the stream banks • Downwelling extended to the bottom layer of the alluvial deposits in majority of simulations

27. Model Results Note flow direction change when K exceeded threshold

28. Model Results

29. Model Results Summer Heads (2 x Steeper stream gradient than Winter) Hyporheic Flux vs K

30. Model Results Winter Heads Hyporheic Flux vs K

31. Model Results Summer Heads (Steeper stream gradient) Hyporheic Zone Depthvs K

32. Model Results Winter Heads Hyporheic Zone Depthvs K

33. Model Results Summer Heads (Steeper stream gradient) Hyporheic TravelTime vs K

34. Model Results Winter Heads Hyporheic TravelTime vs K

35. Model Results Little exchange

36. Model Results High exchange

37. Model Results Summer Heads (Steeper stream gradient) Hyporheic Flux vs K

38. Model Results Winter Heads Hyporheic Flux vs K

39. Model Results Upstream Transect All Seasons

40. Conclusions • Low-gradient streams, riffle-scale exchange flows are possible only when high-permeability materials (Kx,y =10-5 m/s [2.8 ft/d)]) • Moderate- to low-permeability catchment Kx,y = 10-6 m/s (0.28 ft/d) to K = 3 x 10-8 m/s (0.008 ft/d)with alluvial sediments surrounding the stream • Amount of exchange flux, lateral and vertical extent of surface water penetration, and travel times through hyporheic zone determined by three parameters: • K of the alluvium • GW flux to the alluvium • Hydraulic gradient between riffle ends

41. Conclusions • Exchange flows tend to be stronger but more variable at the sides than at the center of the stream channel • Hydraulic conductivity of the streambed can vary by up to 40% with season due to changes in water temperature

42. Model Limitations • Model not calibrated (fatal flaw) • Homogenous K in streambed • No bank storage (not transient model) • Isotropic conditions in high K zone around stream (unrealistic) • Use of constant head cells allows unrestricted flow into/out of model. Can lead to unrealistic water balance. • Drastic changes in model descretization can lead to numerical dispersion (unrealistic and instable results)

43. Suggested Improvements • Calibrate the model using field data • Perform more rigorous sensitivity analysis • Produce table(s) and figure(s) of the calibration and sensitivity analysis • Check and present internal water balance of calibrated model • Improve model descretization • Simplify figures

44. References Harvey, J.W. and K.E. Bencala, 1993, The effect of streambed topography on surface-subsurface water exchange in mountain catchments, Water Resources Research, v. 29, p. 89-98. Silliman, S.E., J. Ramirez, and M.G. Scafe, 1997, The hydrogeology of southern Ontario, Ontario Ministry of Environment and Energy, Toronto, Canada. Water Survey of Canada, 1992, Historical streamflow summary-Ontario, Inland Waters Directorate, Water Resources Branch, Department of the Environment, Ottawa, Ontario, Canada.