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Groundwater e xploration & exploitation

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  1. Groundwater exploration & exploitation

  2. Bores are drilled for many purposes: urban water supplies, geothermal, salinity monitoring, contamination studies, rural water supply, mine dewatering, geotechnical investigations, etc., etc.

  3. Steps Determine the purpose of the project Desktop study/research Field reconnaissance Permits and legal issues Site selection Drill/driller selection Drill bore Bore construction Bore development Pump selection Bore maintenance

  4. Purpose Determine the purpose of the bore Monitoring bore? Production bore? Salinity? Domestic? Contamination? Stock? Extraction? Irrigation? Town water? Industrial? • Bore purpose sets the parameters for: • Water quality (chemistry, salinity, etc.) • Water quantity (volume required) Only certain aquifers will be able to supply the quality and quantity required

  5. Field reconnaissance • Access for drill rigs • Infrastructure • Regulations

  6. Drill bore • Hit water • Quality suited to purpose? • Quantity suited to purpose? Decision Do you construct the bore?

  7. Bore construction Where to set the screens: Lithology (bore log) Geophysics (log)

  8. Bore construction Selection of materials: PVC = cheap, inert, long lasting, lower strength Stainless steel = expensive, inert, long lasting, high strength Mild steel = cheap, rusts, high strength Bronze = expensive, inert, long lasting, high strength Aggressiveness of groundwater (pH, chemistry) Depth of bore (deep bores = casing collapse) Wall materials (soil? rock? Strength)

  9. Wound Wire Screens

  10. Sintered HDPE Screens PVC Screens

  11. Gravel Packing

  12. Bore Development

  13. Electric submersible pumps

  14. Some useful terms to know: • Cone of depression • Drawdown • Radius of influence • Specific capacity

  15. Cone of depression can be used to measure aquifer parameters in a pumping test. One bore is used for the pumped bore Nearby bores are used to measure the cone of depression which develops around the pumped bore over the duration of the pumping test

  16. Pumping Test layout Pumping bore Observation bores Watertable

  17. Pumping Test layout Pumping bore Observation bores Watertable

  18. Pumping Test layout Observation bores must be in the right aquifer! Pumping bore Observation bores      upper aquifer lower aquifer

  19. Pumping Bore Considerations What interval is open to the aquifer? Is it fully penetrating? Or partially penetrating? selection is based on purpose of test and degree of complication added by other factors Does it have the discharge capacity? Can the discharge be disposed of? Is there opportunity for drawdown data collection?

  20. Observation Bore Considerations Distance from pumping well Open interval Accessibility for measurement tape pressure transducer Interference from pumping

  21. Test Operation - Considerations Collection of antecedent water levels Monitoring of barometric pressure if expected drawdowns are small (say less than a metre) Discharge rate may need to be adjusted during the test

  22. Data Collection Frequency 0 Drawdown 10 min 1.0 0.1 min 2.0 0.1 1.0 10 100 Time At increasingly greater intervals (logarithmic)

  23. Recovery Data Useful(after pumping ceases) Drawdown Recovery 0 Drawdown 5 Pump on Pump off 10 Time

  24. Theis Solution C.V. Theis (1900 – 1987) In 1935 C.V. Theis published a solution to calculate Transmissivity and Storativity of an aquifer from the shape of the cone of depression measured during a pumping test -u r2S Q ¥ e ò u s = = du p u T t T 4 4 u • Drawdown = s • Pumping rate = Q • Radial distance between pumping and observation wells = r • Transmissivity = T • Storativity = S Aquifer Parameters of interest

  25. The common form of the Theis equations are: Q = W(u) Transmissivity T 4 p s Storativity S 4Ttu = r2 W(u) is known as the “well function” and is found using a look-up table (usually in the appendices of a groundwater textbook)

  26. Theis assumptions Ideal confined aquifer Fully penetrating bore Horizontal layers of infinite extent Uniform K and Ss Horizontal, radial flow only to the well

  27. Graphical Solution Log scale Log scale

  28. Graphical Solution Pumping test data

  29. Theis ideal curve (This is a plot of the well function values in the look-up table)

  30. Overlay curves

  31. Match point Match curves

  32. W ( u ) Solve equations for T and S Q = T p s 4 W(u) and u are the matchpoint values on the Theis ideal curve; and t and s are the matchpoint values on the pumping test data curve 4 Ttu = S 2 r

  33. Summary of Pumping Tests “ideal” confined aquifer boundary impermeable unconfined aquifer Drawdown (s) leaky aquifer constant head boundary Time (t) The shape of the pumping test graph tells us about the aquifer type and boundaries

  34. Jacob straight line method

  35. Ds Find (s) over (1 log cycle)

  36. Measurement of t0 t0

  37. 2 . 25 T t o S = 2 r Calculation of T and S. 2 . 3 Q T = 4 p D s

  38. Advantages of Pumping Tests • Measure parameters in situ • Average parameters over a large volume • Measure T and S simultaneously Disadvantages of Pumping Tests • High cost • Non-uniqueness of T and S results • Disposal of pumped groundwater (especially if saline or contaminated)

  39. Slug or Bail Tests: Alternatives to Pumping Tests Single bore test No pump required Solid steel slug used to raise the water level in the bore, or bore is bailed to lower the water level in the bore Measure time required for the water level to recover to its original state Rate of recovery is proportional to K (hydraulic conductivity)

  40. Slug test Initial conditions Piezometer Ground Watertable

  41. Slug test Start of test Ground Raises watertable in the piezometer Watertable Add a steel slug

  42. Slug test Measure the recovery Ground Watertable in the piezometer recovers to its original level Watertable

  43. Single bore (recovery) test K = hydraulic conductivity. R = radius of well filter pack (screen). r = radius of the well casing. L = screen length. T37 = time for water level to recover to 37% of the initial change. M.J. Hvorslev (1895 – 1989) r2ln(L/R) K = In 1951 M.J. Hvorslev published a solution to calculate the Hydraulic Conductivity of an aquifer from the time taken for a raised or lowered water table in a piezometer to recover to its original level 2LT37 (For L/R > 8)