Objectives.After this lab, you should be able to measure, apply and calculate hydraulic head, pressure and pressure head, elevation, and their interactions. You should be able to make measurements of water levels in the field and use information about wells to calculate head and head gradient.

Overview. Come prepared for being outdoors on the day of the lab. This lab will be conducted at the OSU Motor Pool, which has a number of wells that have been used for monitoring of gasoline contamination. Your TA will give you a brief introduction and orientation to the site, and instructions for the equipment.

Hand in. Hand in your lab report containing answers to all questions. You may hand-write your answers, or complete them with a computer (which will be somewhat tedious because of the equations). Neatness, writing, and professionalism in the hand-ins are worth 10% of every lab grade.

Field measurements – OSU Motor Pool

  1. In 1991, a gasoline spill was discovered at the OSU Motor Pool, resulting in the installation of a number of monitoring wells. We will be measuring water levels in those wells. A data sheet is provided for your use, as well as two copies of a site map. Your TA may instruct you to make modifications to the wells on the map (add, subtract, or change wells in the question).

a)MW-10. What is the total depth of this well from the top of the casing (TOC)? You will need to use a tape measure to measure this.

b)Measure and record the depth to water in this well. Enter these data in Table 1.

c)Obtain the well log for MW-7, print it out and hand it in with the lab. Well logs may be obtained over the internet by going to and searching by the criteria available. The most common ways to search are by land location or owner name. However, since we know the exact well we are looking for, we know its start card number (see table).

d)Enter the depth of this well and the screened interval into Table 1 (in meters!) What type of material is this well screened in (the screen is the interval over which water can flow into the well)?

e)Find three other well logs in the OSU/Corvallis area, using the website listed previously (key search criteria in bold): (1) a geotechnical hole drilled at Reser Stadium (formerly Parker Stadium) and completed on 05/15/1997 (company name = Oregon State University); (2) a domestic well owned by A. Johnson, drilled in 1973, and having land location 34-11S-5W (Section-Township-Range); (3) a domestic well owned by D. Duncan drilled in 1962, and having land location 34-11S-5W. All wells are in Benton County and all are shallower than 50 ft. Assuming (quite reasonably) that the shallow subsurface geology at these locations is similar to the Motor Pool, what is the approximate thickness of the geologic unit in which the Motor Pool wells are screened? What type of material likely lies below this geologic unit?

f)Compare the depth to water in the Johnson and Duncan wells to the depth to water at the Motor Pool wells. In addition to horizontal flow, what direction might groundwater be flowing in addition to horizontal, and why?

g)At MW-7, complete the following questions. Use sea level as your datum. Do your calculations in metric units, and use gauge pressure (i.e., neglect atmospheric pressure). See pgs 46-47 in your textbook for diagram and equations.

  1. What are the elevation, pressure head, pressure, and total head at the water table?
  2. What is the relationship between total head and the water table elevation in this well?

h)Calculate the water table elevation at all wells and enter these onto your map.

i)Are there any water levels that appear to be anomalous, or that do not fit with the rest of the data in any way? If so, is there any reason that this/these wells may have different water levels than the rest?

j)Draw a contour map of the water table at the Motor Pool, ignoring any water levels you feel are anomalous. Choose a contour interval such that you have 4-6 contours on the map.

k)If you assume that flow is predominantly horizontal (not necessarily true), what direction is groundwater flowing at the Motor Pool? (Remember water flows from high head to low head, and perpendicular to contour lines)

l)Consult a map of Corvallis (google maps will be fine). What surface water body might this groundwater flow into?

m)What is the horizontal head gradient (dh/dx) from motor-pool to the water body, approximately?(Use google to determine elevation of water body, and assume that the water body represents the water surface elevation at that location)

Exercise

  1. The following field notes were taken at a nest of piezometers installed side-by-side at a single site. Again, refer to pgs 46-47 in your textbook for an example.

Piezometer / a / b / c
Elevation at surface (m) / 450 / 450 / 450
Depth of piezometer (m) / 150 / 100 / 50
Depth to water (m) / 27 / 47 / 36

Let A, B, and C refer to the points of measurement of piezometers a, b, and c (i.e., the points where a, b and c are open to the aquifer).

  1. Draw a cross-section to scale showing the land surface, the piezometers, and points A, B, and C. Indicate on your drawing that z is positive upward.
  2. Calculate the hydraulic head (i.e., total head, or “head”) at A, B, and C, in m.
  3. Calculate the pressure head at A, B, and C, in m.
  4. Calculate the pressure at A, B, and C in kPa.
  5. Calculate the elevation head of A, B, and C.
  6. What direction is flow in the region between A and B? (hint: think outside the box. Under what conditions would you observe a difference in water level of 20m for wells that are side-by-side?)
  7. What direction is flow in the region between B and C? (see previous hint)
  8. What might cause the flow pattern you’ve observed in this example?

Roy Haggerty, Dept. of Geosciences, Oregon State University, 2013.

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Date:______

Well / Start Card #[1] / Diameter (in) / Depth to bottom of well (m) / Screened Interval (m) / Elevation, TOC (m) / Depth to Water (m) / Static Water Level (m)
MW-10 / 4 / 73.718
MW-2 / 23323 / 2 / 4.11 / 0.91-3.96 / 73.317
MW-3 / 23322 / 2 / 4.11 / 0.91-3.96 / 73.417
MW-5 / 23320 / 2 / 3.81 / 2.13-3.66 / 73.567
MW-7 / 23318 / 2 / 73.097
MW-8 / 23446 / 4 / 6.86 / 2.13-6.71 / 73.359
MW-9 / 23335 / 2 / 4.57 / 1.52-4.57 / 73.371

Table 1: Water level data at OSU Motor Pool. Depth to bottom of well is depth from the top of the casing (TOC). Elevation at TOC is meters above sea level. Depth to water (DTW) is depth downward from TOC. Static water level (SWL) should be calculated in meters above sea level.

Notes:

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[1] If the start card number is known, this is an easy way to find a well in a well log.