Groundwater Measurement in Wells

Hydrologic Field Methods November 3, 2005

Instructor: Jerry Fairley,

Groundwater Measurement in Wells

Introduction

One of the most fundamental skills in groundwater hydrology is the ability to accurately measure and interpret the depth to groundwater in an observation well or piezometer. Measuring the elevation of the potentiometric surface is a routine task for any groundwater professional; however, the difference between a professional and the summer help is that a professional is capable of understanding the resulting measurements. In order to reliably interpret groundwater measurements, you must be familiar with a few basic ideas.

Basic Concepts

It is commonly believed that “water runs downhill,” although this is not always true for groundwater. Instead, water flows from regions of higher potential to regions of lower potential. Hydrogeologists measure the driving force for groundwater flow in terms of “head” [L]. Head is a combination of gravitational potential and pressure potential, and is reported as an equivalent height of a column of water:

where H [L] is the hydraulic head, z [L] is the elevation of the measurement point above some datum, P [M/LT2] is the pressure at the measurement point, ρ [M/L3] is the density of water, and g [L/T2] is the acceleration of gravity. Head is measured by determining the height to which water will rise in a well open to the aquifer; it is therefore common practice to report head in terms of elevation above sea level. When a well has a significant length (or “screened interval”) open to the aquifer, the water level in the well is an average of head for the entire screened interval. If the well has only a very limited screened interval (approximating a point) the well is known as a “piezometer”, and the water level measured is the head at the open point (see Figure 1). Wells are generally open only at the screen- the bottoms are assumed to be sealed (although this may not be true in practice, particularly in hard rock, where screening may be omitted).

Figure 1: Monitoring wells and piezometers

So why would a groundwater professional prefer data from a piezometer, rather than another type of well? If there is a vertical component to the groundwater flow, there must be vertical differences in head (remember, water flows from high to low head). It is possible to determine the vertical direction of flow if two or more piezometers are available at the same location (but open to different depths). If head measurements from regular wells are all that is available you may be able to tell the horizontal direction of flow, but not whether vertical flow is occurring. By measuring head (water elevations) in the monitoring wells 1 and 4 (MW-1 and MW-4), shown in Figure 2, it is clear that groundwater is flowing from MW-1 towards MW-4, because the head is lower at MW-4 than at MW-1. However, only by noticing the difference in head between the two piezometers, MW-2 and MW-3, would a groundwater professional know that water in the aquifer is actually flowing upward, as well as left to right.

Figure 2: Measuring vertical and horizontal gradients

Most people are familiar with the idea of a “water table”. Hydrogeologists say water table aquifers are “unconfined”, because the head in the aquifer is at (approximately) the same elevation as the water table. Often, however, an aquifer may be “confined” by an overlying layer of low-permeability material. Head in a confined aquifer is at a higher elevation than the top of the aquifer. In a confined aquifer, the level of head forms an imaginary surface above the aquifer known as the “potentiometric surface”. In an unconfined aquifer the water table is equal in elevation to the head in the aquifer; therefore, the water table is the same as the potentiometric surface (Figure 3).

Figure 3: Unconfined and confined aquifers

Vertical and horizontal gradients often become important when working in multiple aquifer systems, because you may want to know if water is flowing from a deeper aquifer to a shallower aquifer, or the other way around. More importantly, if you measure head in several wells, and the wells are screened in different aquifers, you may not be able to make sense of the data (Figure 4). For this reason, always make sure you know what intervals are screened in the wells you are taking measurements in, and be sure you understand how the screened intervals relate to the local geology and hydrologic units!

Figure 4: A multi-aquifer system

Questions

· What is the difference between an ordinary well and a piezometer? What is the difference between a monitoring well and a piezometer? Why are the differences important?

· What is the difference between a water table and a potentiometric surface?

· In a multiple aquifer system, is it possible for groundwater to flow one direction in one aquifer, and a different direction in another aquifer? If no, why not? If yes, how would you detect it?

· What is the difference between a confined aquifer and an unconfined aquifer?

· What is the minimum number of wells you would need to detect a vertical gradient? What is the minimum number of wells you would need to detect a horizontal gradient? What is the minimum number of wells you would need to determine the horizontal direction of flow?

· What is head? How is it measured? What are its units?

· A drinking glass of water is placed so that its bottom is 5280 feet above sea level. The water in the glass is four inches deep. What is the head two inches from the bottom of the glass? Three inches from the bottom? One inch?

Assignment

The table below describes construction details for wells at the University of Idaho Groundwater Research Site (UIGRS). The chart is reproduced from Nimmer (1998):

On the following page you will find a map (also from Nimmer, 1998) of the UIGRS showing the locations of the wells noted above. Using the tape and e-tape, measure the elevation of the water surface in the wells at the research site.

To Turn In:

A three page report on your findings, typewritten and formatted as a letter report for a paying client (see attached example), including a brief summary of what you did, why you did it, and how many independent aquifer units you believe exist at the UIGRS. Page two should be a table summarizing your data, and the third page will be the map of the UIGRS (map provided on following page).

References

Nimmer, R.E., 1998. Groundwater Tracer Studies in Columbia River Basalt, M.S. Thesis, University of Idaho Department of Geological Sciences.