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About Groundwater

About Groundwater. SCGEO 2106 Week 4. Peter Dahlhaus. Evapotranspiration. Precipitation. Interception. Pond Storage. Throughfall. Interception Storage. Overland Flow. Infiltration. Soil moisture storage. Interflow. Runoff. Channel storage. Throughflow. Groundwater recharge.

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About Groundwater

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  1. About Groundwater SCGEO 2106 Week 4 Peter Dahlhaus

  2. Evapotranspiration Precipitation Interception Pond Storage Throughfall Interception Storage Overland Flow Infiltration Soil moisture storage Interflow Runoff Channel storage Throughflow Groundwater recharge Groundwater discharge Return flow Groundwater storage Baseflow

  3. When does water become groundwater? Soil moisture Unsaturated zone (Vadose zone) Saturated zone (Phreatic zone)

  4. pressure +’ve -’ve (pressure) (suction) Hydrostatic increase

  5. Water rises in a column of soil due to capillarity. The ‘suction’ is due to the surface tension between the water molecules and the soil particle surfaces. Small diameter Greatest height Surface tension

  6. Capillary Fringe: Silty Clay ~ 1 metre Fine Sand ~ 0.1 metre Gravel ~ 0.001 metre

  7. About groundwater.... Groundwater is stored in the spaces and voids in the rock mass, such as the pore spaces between the grains and particles, or in fractures or in cavities. For groundwater to move, the spaces or voids need to be interconnected. The pathways can be very torturous and complex, like a three-dimensional maze. Groundwater moves at varying speed but is usually very slow. Velocity ranges from a few microns per year (in a clay) to hundreds of metres per day (in a very open-fractured rock). “Underground rivers” don’t really exist. (Rivers might disappear underground into a cave, but that’s not the same as groundwater. Deep Leads are buried rivers, but the surrounding rocks are saturated with groundwater as well).

  8. Groundwater storage Volume of voids (Vv) Total volume (Vt) Porosity (n) = Effective porosity (permeability) enables an aquifer/rock unit to store, transmit and release water

  9. Calcarenite (dune limestone) Barwon Heads • Primary porosity is made at the same time as the rock – sands, gravels, sandstone, limestone Scoria Mt Buninyong

  10. Secondary porosity is made when rocks are fractured or “dissolved” by later processes Limestone cave Port Campbell Fractured rhyolite Wannon Fractured basalt Dunnstown

  11. Groundwater storage Specific Yield is the ratio of the volume of water drained under gravity to the volume of saturated rock. Volume drained (Vd) Total volume (Vt) Specific yield (Sy) = Specific Retention is the ratio of the volume of water retained after gravity drainage to the volume of saturated rock. Volume retained (Vr) Total volume (Vt) Specific retention (Sr) = Specific yield + specific retention = porosity Sy + Sr = n

  12. Groundwater storage A core one metre long and 10cm diameter is extracted from an aquifer. Saturated weight is 19.65kg It is left to drain (by gravity) and then weighed as 17.29kg It is then oven dried (105oC) to a constant weight of 16.90kg. Calculate specific yield and specific retention and porosity. 1 m 0.1 m

  13. Groundwater storage A core one metre long and 10cm diameter is extracted from an aquifer. Saturated weight is 19.65kg It is left to drain (by gravity) and then weighed as 17.29kg It is then oven dried (105oC) to a constant weight of 16.90kg Total Volume (Vt) = 0.00786m3 Weight of water drained = 2.36kg Volume of water drained (Vd) = 2.36L = 0.00236m3 Specific yield (Sy) = 0.00236/0.00786 = 0.3 = 30% Volume of water retained (Vr) = 0.39L Specific retention (Sr) = 0.00039/0.00786 = 5% Porosity (n) = Sy + Sr = 35% 1 m 0.1 m

  14. The Saturated Zone The watertable is usually a subdued replica of the land surface Springs, seeps, swamps, rivers & lakes occur where the groundwater intersects with the land surface Unsaturated zone (Vadose zone) Saturated zone (Phreatic zone)

  15. The amount of groundwater in storage changes with the seasons Water tables fluctuate with seasonal input (recharge)

  16. Movement of water Groundwater flows from higher elevations to lower elevations. It travels from where it enters the system (recharge) to where it leaves the system (discharge)

  17. Unconfined aquifer • Open to the surface • Broad recharge area • Includes most aquifers

  18. Aquifer conditions Unconfined – open to the surface. Confined – sandwiched between less permeable beds. Fractured rock – water stored in fractures.

  19. Confined aquifer • “Sandwiched” between less permeable beds • Recharge area is limited to aquifer outcrop • Source of artesian water

  20. Aquifer – carries water in useable quantity No covering layer = Unconfined

  21. Confining beds make up the non-aquifers and may be referred to as: aquifuge - an absolutely impermeable unit that will not transmit any water, aquitard - a low permeability unit that can store groundwater and transmit is very slowly, and aquiclude - a unit of low permeability located adjacent to a high permeability layer.

  22. An aquifer system is the complete 3-d package of aquifers and confining beds http://campuswaterquality.ifas.ufl.edu/images/floridianaquifer.jpg

  23. Groundwater Head groundwater bore ground level Pressure Head Total head  Static head Elevation Head Australian Height Datum (AHD)

  24. Hydraulic Gradient (i) = (h1 – h2)/L Hydraulic gradient shows flow direction

  25. Three point Problems Three points are needed to fix a plane in space Hydraulic gradient = 0.05 N ΔL = 100m ΔH = 5m Bore B RLgw = 49m Bore A Elevation = 52m SWL = 10m RLgw = 42m Flow direction RLgw 50m 100m 0 RLgw 45m Scale Bore C RLgw = 39m RLgw 40m

  26. Vertical Gradients Groundwater flow is three-dimensional

  27. DARCY’S EXPERIMENT Q a Cross sectional area (A) Q a to the head loss over a distance (i)

  28. Darcy’s Law Q = kiA k = Hydraulic Conductivity

  29. Hydraulic Conductivity (k) Transmissivity (T)

  30. km m mm mm

  31. Constant Head Permeameter Falling Head Permeameter

  32. Hydraulic conductivity is often varied in a single aquifer

  33. Homogeneity / Heterogeneity

  34. http://www.regione.emilia-romagna.it/wcm/geologia_en/Sections/Water_resources/rel_scentifiche/094_err_case_study/fig_01.jpghttp://www.regione.emilia-romagna.it/wcm/geologia_en/Sections/Water_resources/rel_scentifiche/094_err_case_study/fig_01.jpg Deltas, alluvial plains, lacustrine deposits, paludal deposits and glacial sediments are examples of heterogeneous aquifers. http://ess.nrcan.gc.ca/gm-ces/bulletin/bulletin_v3_2_e.php

  35. Isotropy / Anisotropy

  36. Isotropic Homogeneous Isotropic Heterogeneous Anisotropic Homogeneous Anisotropic Heterogeneous

  37. Reality: Most aquifers are heterogeneous and anisotropic in three dimensions. The degree of variation depends on the scale of the investigation. As you zoom out the variations become less important. http://www.kgs.ku.edu/Hydro/Publications/2005/OFR05_29/gifs/fig12.jpg

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