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NC State Soil Science: SSC 470/570 Wetland Soils: Unit 4 - Topic 2 - Lesson 2

Hydrology, Techniques Used in Hydrology - Wells and Water Table Measurements

 

  • 1. Discussion

Discussion

A. Principle Points

1. A well is a slotted tube in the soil that allows water to enter it (Fig. 4). The top of the water level in a well will be called the "water table" in this class. It can also be called the "phreatic surface" or the "apparent water table".

2. Wells can be made from PVC pipe by drilling holes in the pipe. Alternatively, "well-screen" can be purchased which is basically a PVC pipe with thin slits cut into it.

3. The bottom of the well can be capped with a cone-shaped tip or other plug to keep excessive amount of soil from entering.

4. Wells should be used when the depth to the water table is the main point of interest. Wells do not show the direction of water movement.

5. For wetland hydrology determinations, a well extending approximately 100 cm into the soil should be appropriate.

6. Water levels should be measured at least weekly during the wet portion of the growing season. Daily measurements may be needed if the water level is expected to drop to a level below 30 cm from the surface during a 14 day period.

Remember: The depth to the water table must be reported as the depth from the soil surface, not the depth from the top of the well pipe.

7. Measurements of water table depth can be made with a tape measure or electrical devices that "beep" when they contact water.

B. Installation of Wells

1. Wells should be installed as shown in Fig. 4. The screen should be surrounded by sand to filter fine soil particles out of the water before they can plug up the well-screen.

2. Soil mounds and bentonite seals must be used to prevent surface water from running into the space between the soil and the well pipe.

3. Well pipe must be capped to prevent water and animals from entering the pipe. Caps should fit loosely so as not to entrap air in the pipe, and prevent water from entering the well. Small holes or "weep holes" should be drilled in the pipe or cap to prevent air entrapment.

4. One well will work fine in soil whose horizons have similar textural classes, such that no perched water tables will be present.

5. In soils with perched water tables, one well should be installed above and slightly into the perching layer. If deeper measurements are needed, a second well should be installed below the perching layer such that no portion of the screen is above the perching layer. Bentonite should be used to seal the well below the perching layer. More information on installation techniques is given in the handout.

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