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Groundwater Pollution

Week 03 – 0324 Continuity and Flow Nets

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1. We want to know where the underground water is going? [the direction of flow]

2. We know that water will flow from an area of high potential energy ( 위치에너지 ) to low potential energy. [from high pressure to low pressure ( 유체 압력 )]

3. The pressure is measured by looking at how high the water rises in a tube. [piezometer]

4. The height of the water is related to the pressure at the bottom of the tube.

5. The pressure can be mapped as lines of equipotential.

6. Water will flow perpendicular (90o) to the lines of equipotential.

7. We can draw diagrams of the flow of the water. [flow nets]

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Techniques for measuring water levels in wells, standpipes and stilling-tubes

A Watershed Dynamics Tutorial

© John F. HermanceMarch 29, 2003

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How do we “see” what is happening underground?

http://techalive.mtu.edu/meec/module06/WellDrilling.htm

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Measuring the height of the water table, or what it wants to be.

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Selected Instruments

(From: Fig. 2.28; Davis & DeWiest, 1966)

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Chalked tape

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“Bubbler”

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Electrical contact

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Selected Instruments

(From: Fig. 2.28; Davis & DeWiest, 1966)

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Sonic (Sound Pulse)

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Pressure transducer(Vented or non-vented)

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Mechanical float &strip-chart recorder

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Because of turbulence in a discharging well, one may monitor water levels in a “stilling tube”. (Also used for measuringsurface elevations of flowing streams.)

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The Piezometer: A hydrogeologist’s best friend.

Applications

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A piezometer provides a means to sample the hydraulic head at a point in the saturated zone.

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Think of a piezometer as a long “insertion probe” to measure the pressure (pressure head) at a point.

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The difference in hydraulic head provides a clue for the consequent direction of flow.

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Consider unconfined flow.

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A piezometer in operation: The elevation of water in the piezometer provides a measure of h(P).

A piezometer measures the hydraulic head at a point.

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Example: Potentiometric surfaces (h-lines) for unconfined flow.

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To what level does water rise in a vertical slotted (screened) well?

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Questions comments?

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To what level does water rise in a vertical, continuously-cased well,open (screened) at the bottom?

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Questions comments?

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Close-up Views.

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Slotted, or Continuously-Screened Well.

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Continuously-Cased Well.

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To what level does water rise in a slanted, slotted (screened) well?

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Questions comments?

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An interesting aside is the direction of flow in the well.

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To what level does water rise in a slanted, continuously-cased well,open (screened) at the bottom?

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Questions comments?

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Close-up Views.

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Slotted, or Continuously-Screened Well.

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Continuously-Cased Well.

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Let’s revisit our numerical model.

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A Close-up View.

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Utility of Piezometers.

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People want to understand the flow of groundwater.

There is a problem.The aquifer can be complex. The wa

ter can move in 3 dimensions.

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Flow NetsThe flow of the groundwater can be moveme

nt in 3 dimensions. This means, for steady state, ie no change i

n the amount of stored water

This solution is based on boundary conditions, and in the transient case, on initial conditions.

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Let us look at the two-dimensional, steady-state case.

Maybe there is an easier way of working this out.

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A graphical technique can be used to find the solution to the GW flow equation for many situations. This technique involves the construction of a flow net.

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A flow net is the set of equipotential lines (constant head) and the associated flow lines (lines along which groundwater moves) for a particular set of boundary conditions. For a given GW flow equation and a give

n value of K, the boundary conditions completely determine the solution, and therefore a flow net.

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Calculations from Flow Nets:It is possible to make many

good, quantitative predictions from flow nets. In fact, at one time flow nest were the major tool used for solving the GW flow equation.

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Example of a Numerical Model.

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Flow-net in color.

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Note: No flow directions.

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Example of flow direction.

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A Close-up View.

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Utility of Piezometers.

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• Not always a correlation between elevation of watertable and depth of water in well.• “Nested” wells can provide local flow direction.

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Some Examples of Regional Flow(Source: Fetter, 2001)

A Watershed Dynamics Tutorial

© John F. HermanceApril 07, 2003

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(Source: Fig. 7.1; Fetter, 2001; after Hubbert, 1941.)

70(Source: Fig. 7.34; Fetter, 2001.)

71(Source: Fig. 7.4; Fetter, 2001.)

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It may be easy to draw a flow net for simple systems but usually the systems can be very complex.

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Show the video “trans”

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I recommend that you rely on using two things in combination:

1. Use sample wells to find out the actual underground flows at various points.

2. Use computer software to model the underground flow.

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The US Geological Survey has software at http://water.usgs.gov/software/lists/groundwater

For instance MODFLOWMODular three-dimensional finite-difference

ground-water FLOW model--2000 updated version