Preferential Ground-Water Flow: Evidence from Decades of Fluorescent Dye-Tracing

Transcription

1 Preferential Ground-Water Flow: Evidence from Decades of Fluorescent Dye-Tracing Martin H. Otz & Nicholas A. Azzolina Nano Trace Technologies GSA Fall Meeting 2007

2 What is FDT? In fluorescent dye tracing (FDT) surface/ ground water is colored by organic fluorescent dyes In most aquifers dye detection limits of less than 10 ppt (or ng dye / L H 2 O) can be reached

3 What s wrong with using Darcy s Law? Fluorescent dye-tracing (FDT) tests show that ground-water seepage velocities can be orders of magnitude faster than calculated by Darcy s Law FDT data show that classical assumptions of groundwater flow are often wrong! Real-world conditions mandate the use of tracing techniques to determine accurate flow paths & seepage velocities

4 Hydraulic conductivity and advection Q = K I A v a = K I / n e Clay Till Silt Sand Gravel <10-8 cm/s (< ~0.03 ft/yr) ~10-6 cm/s (~0.03 ft/d) ~ cm/s (~0.03 to 0.3 ft/d) ~ cm/s (~3 to 30 ft/d) >10-2 cm/s (> ~ 30 ft/d)

5 Hydrodynamic dispersion homogeneous -isotropic Real world: heterogeneous Direction of ground water motion

6 Theoretical breakthrough maximal effective flow velocity dominant effective flow velocity (= main dye concentration) effective average linear velocity mean flow velocity (= 50% of the recovered tracer amount has passed the sampling site Kaess, 1998

7 Real world breakthrough W21: 1st dye test, second appearance 82 days after uranine injection RFI first appearance 12 days after uranine injection detection limit 0 21-Sep 4-May 14-Apr 25-Mar 24-May 13-Jun 1-Sep 12-Aug 23-Jul 3-Jul Uranine dye

8 Aquifers are dynamic open systems! URA EOS SRG SRB NAP PYR Date ppt ppt ppt ppt ppt ppt 21. Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Sep Sep Sep Sep Sep Sep Sep Sep Sep Fluctuations of tracer c often match with precipitation

9 Dye tracing for the longest railway tunnel on Earth

10 Results FDT AlpTransit dye traveled 16 km (10 miles) in 42 days

11 Cross-section west to east How much faster? 20x

12 Oil-contaminated fluvial setting How much faster than Darcy velocity? 350x

13 FDT in fractured bedrock How much faster than Darcy velocity? 650x

14 FDT in a clayey unconfined setting How much faster than Darcy velocity? 110,000x

15 Some numbers Triassic anhydrite/dolomite aquifer Darcy = 21.6 m/day FDT = m/day Heavily oil-contaminated fluvial aquifer Darcy = m/day FDT = 5.55 m/day Fractured bedrock aquifer Darcy = m/day FDT = 11.1 m/day Clayey unconfined aquifer Darcy = 4.7 x 10-6 m/day FDT = 0.52 m/day Average difference between Darcy calculations and FDT results is 450%!

16 GW usually follows old river channels!

17 Conclusions More than 75% of all tracer test show that ground water moves faster than the Darcy velocity and does not move in the direction of the hydraulic gradient Hydrogeologic conceptual models need to consider fast flowing preferential flow paths Fluorescent dye-tracing is one of several techniques to estimate these GW flow paths

18 Questions Contact:

19 Acknowledgements DR. GEORG WYSSLING AG Lohzelgstrasse 5 CH-8118 Pfaffhausen Switzerland

20 Preferential Ground-Water Flow: Evidence from Decades of Fluorescent Dye-Tracing [GSA Fall Meeting (Spoken text)] by Dr. Martin H. Otz, Nano Trace Technologies, Gartenstrasse 6, CH-3252 Worben, Switzerland , Nicholas A. Azzolina, Nano Trace Technologies, P.O. Box 3898, Ithaca, NY, USA, , 1. Numerous fluorescent dye-tracing tests show that the ground-water seepage velocities in many different types of unconsolidated sediments and fractured rocks can be orders of magnitude faster than calculated standard measurements of hydraulic properties. These tracer tests show that the classical advection-dispersion hypothesis is in most cases inadequate for characterizing solute transport in heterogeneous media. Since 1986 I have been involved in hundreds of fluorescent dyetracing tests where the ground water flow didn t follow the predicted direction and even worse was much faster than claimed by the involved engineers and scientists. 2. This slide summarizes what dye-tracing is about. In fluorescent dye tracing surface and/or ground water is colored in by organic fluorescent dyes. Extremely low detection limits certainly make this technology a method of choice to trace water flow paths. 3. For more than 20 years we and other fellow scientists have tested the hypothesis that fast-flowing preferential ground-water flow paths through secondary porosity features are the dominant process for conducting water through aquifers, and that Darcy s law based on a granular porous media only poorly predicts travel times with precision in the real world. 4. As we all know the first formula comes out of Darcy s Law. Discharge equals hydraulic conductivity times hydraulic gradient times cross-sectional area. Hydraulic conductivity (which is a measure of permeability) depends on both the primary porosity of the aquifer matrix (particle size) and the secondary porosity caused by structural features (like joints, faults, solution cavities and so on). If we divide the Darcy velocity with the effective porosity (the porosity of an aquifer that actually can conduit water) then we receive our seepage velocity also called advection or average linear velocity and it is a measure of how fast ground water flows in porous media. This is actually a formula I regularly use and I always get disappointed as soon as I see the tracer results. Of course we have other parameters that play a significant role such as hydrodynamic dispersion. 5. Hydrodynamic dispersion is the mechanical mixing along the flow path. So if we take a look at these water particles in different colors then we realize that solutes in an aquifer take different pathways and cover different distances over time. Some are faster, some are slower and often their paths meet. But due to the fact of the ubiquitous secondary porosity in every aquifer setting there is an incredible increase of the seepage velocity. Either it is as karst conduits in weathered limestone, fracture flow or

21 man-made conduits as part of subsurface construction. This tremendous increase in ground water velocity is evident in more than 75% of all tracer tests. 6. Here are classical breakthrough curves (all have a different dispersion coefficients) the dye concentration are plotted versus a timeline. The effective average linear velocity can be calculated and is in between the dominant effective flow velocity and the mean flow velocity. In an ideal world, which is homogeneous isotropic and no ground water level fluctuations we would end up with a nice Gaussian curve. 7. Unfortunately in the field we very rarely end up with such nice breakthrough curves often the main breakthrough curve is followed by a bunch of less intense smaller breakthrough curves. As our world is an open system this is no surprise. In this case precipitation allowed the ground water to use the main preferential flow paths that were inactive under the days without rain. 8. What is noteworthy in this case is not only that all of the dyes made it to the well, but the dynamics how they reached the well. The well was pumping at a constant rate. The fluctuations of the different dye concentrations mimic the local precipitation pattern. Up and down, up and down. In case of no rain the preferential flow paths (or conduits within the aquifer) towards the well cease to function and the well gets its water mainly from a deeper aquifer. But not during days of rain. It is important to note that even in adjacent wells preferential flow paths can lead to large differences in ground water travel times. 9. The next few slides will show you 4 different aquifer settings and hopefully convince you that ground water flow is mainly preferential in nature. The first location is in Switzerland. Since 1999 the longest railway tunnel, which will cross the entire alpine range at its base is being under construction. It will be an integral part of the European high speed train system and an expedite route for north-south cargo freight. Let s focus on this yellow highly weathered Triassic zone here, which is wedged in between two granitic massifs. 10. What is the problem? 2 rivers flow on the surface to the west and they lose around 150 gallons a second to the highly weathered Triassic sedimentary rocks beneath. Despite decades of research and the believe that the lost water can be found in the springs to the west water chose to follow a different way. 10 miles in 42 days to the east and not the west crossing under a deep mountain valley. 11. This cross-section along the ground water flow paths should clarify my previous explanations. Water moved 20 times faster compared to the calculations of the hydrogeologists. 12. Let s take a look at the results of a dye tracing test done in a heavily oil-contaminated fluvial aquifer. Fluorescent dye was injected here and within 12 days the dye showed up in W21. Most amazing was the discovery that the dye didn t simply steer towards the active pumping wells but was deviated considerably indicating preferential flow paths and inhomogeneities in the subsurface probably due to the construction infillings. The main concentration of dye traveled 350 times faster compared to the calculated seepage velocity.

22 13. The result of this fluorescent dye-test amazed the involved parties as they have never thought that dye would reach the other side of the road within 2 weeks. A fracture analysis showed and indicated preferential flow paths prior to the tracer testing. Nevertheless years of research failed to conclusively indicate how fast and in which direction the ground water moved. The red dye up here reached the recovery wells down here within 2 days and not 2 months as previously postulated. 14. In some wells at this site fluorescent dyes were found within as little as 6 days even traveling through a relatively impermeable silty clay formation. Within 81 days, the main concentration of the fluorescent dye reached a monitoring well only 42 m away. The previously determined average linear velocity using a measured saturated hydraulic conductivity was more than 100,000x slower than the observed travel times. Another indication for the presence of macropores or fracture networks in the study area, demonstrating again the importance of preferential GW flow paths. 15. In this slide I have listed all seepage velocities of the 4 case studies I have presented before. We realized that the average difference between the Darcy calculations and the fluorescent dye-tracing results is off by more than 450%. This is simply incredible and cannot be neglected! 16. Before I conclude my talk here. Don t forget a very important feature. It is the presence of ancient now inactive river channels. The main ground water volume might still use these pathways as indicated by the underground crossing of this orange dye here. 17. Our findings suggest that most conceptual hydrogeologic models need to include the possibility of fastflowing, preferential groundwater flow. These can only be clearly defined using tracing techniques to determine the ground-water flow paths, effective seepage velocities, and accurate permeability estimates.