9/27/04

I. Well construction

Darcy's law tells us that we can determine directions of groundwater flow if we know the distribution of heads (which allows us to determine grad h). Heads are measured in the field by determining the depth to water in some type of well. Slides and quicktime movies were shown to illustrate the most common methods of constructing wells (cable tool, rotary drilling, auger drilling, direct push/geoprobe) and samples of PVC and stainless steel well "screen" materials were shown. Note that the powerpoint show provided on the web site for the course text also shows a number of well construction techniques. Well development was also discussed briefly.

To see quicktime movies, follow the links below. (You'll need QUICKTIME or other software that can play a ".AVI" file installed to view these.)

auger drilling

geoprobe

inertial (Waterra) pump

II. Interpretation of water level measurements

Since water levels and heads can never be measured at every point in an aquifer system, we need to interpolate measurements from a finite number of wells. Common types of wells include "water table wells" and "piezometers". Water table wells should be screened across the water table in order to provide an accurate measurement of the location of this physical surface along which any one of the following (equivalent) conditions holds:

p = po (pressure is equal to atmospheric pressure)

p/rwg = 0 (the pressure head is zero)

h = z (the hydraulic head is equal to the elevation head)

Piezometers measure head at some point in the subsurface and are ideally constructed with short screens to allow a "point" measurement. A set of piezometers screened at the same elevation (z) will allow measurement of water levels corresponding to a potentiometric surface. Unlike the water table, this is not a physical surface in the earth, but rather the "imaginary" surface corresponding to the "topography" of heads within the horizontal plane to which the piezometers are open. A potentiometric surface for an aquifer can also be defined by a set of piezometers open at different elevations in the aquifer as long as the equipotentials in the aquifer are essentially vertical (no vertical gradient, so head does not change with z within the aquifer).

Example 3.10 in the text illustrates the process involved in determining the vertical gradient from a pair of wells in a nest. Note the the answers in the text have too many significant digits in most cases. Also note that there is an implicit assumption of horizontal flow in the sand, since the gradient is calculated using the thickness of the till unit (13 m) rather than the difference between the elevations at the bottoms of the wells (14 m.)

Examples of water level cross-sections were discussed (see figure 3.15 in the text and handout provided in class).

Examples of water level maps were discussed and an example of an error in constructing one of these using wells open to different elevations (or open over long intervals) was illustrated using the handout provided in class.