Predicting the Movement of Groundwater & the Behavior of Wells

Instructor: Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

Groundwater is the source of fresh water for many people. In this lesson, we will learn about the different types of wells and how to predict where groundwater will move.

Well Water

When people are looking for a property to rent or buy, it is common for them to check where the water out of the tap comes from. It can either be water provided by the city or well water. Actually, in many cases, the city gets the water to distribute to the community from well water! Why does this even matter? Well, what if there is an animal farm or gas station upstream from your water well? There could be contaminants entering your well! Let's learn how to determine which way ground water flows from a groundwater contour map.

Groundwater Contour Map

A groundwater contour map is one whereby points of equal groundwater levels are connected with a line. Monitoring wells are dug, and the depths to the water in these wells are measured. Diagram 1 shows groundwater levels at each monitoring well.


Diagram 1. Levels in feet above sea level
wells

Using this data, we draw contour lines between points of equal groundwater elevation. Diagram 2 shows the contour lines drawn.


Diagram 2
contour_num

Now, we draw a cross-section on this map, which will show us how the groundwater will flow along the cross-section.


Cross-section from A to B
cross_section

We can think of the way the groundwater flows based on which way a marble would move if we placed it at point A and let it go. You can see that it would roll downhill toward point B.

If you wanted to avoid drawing a cross-section, you could draw little arrows perpendicular to the contour lines all pointing toward the lower elevations. Diagram 3 shows the arrows. This method can give you a good idea of which way the water flows over the entire region.


Diagram 3. Arrows point to direction of flow at their location.
arrows

Using your artistic ability, you can draw trend lines showing the general direction of water flow over the entire area. This is shown in Diagram 4.


Diagram 4. Directions of regional water flow
mass_flow

If we were thinking of developing this land for housing and industrial purposes, we would want to put the industrial areas on the south side of the map so that if there is any contamination from the industries, it would flow away from the residential areas.

Now let's look at how rock strata affect groundwater flow and well behavior. Rock strata are the layers of rock under the ground.

Well Water Behavior

The water table is the top of the rock layer where ground water exists. The speed of the water flow underground depends on the relative elevation of the water table and the material the water flows through. The larger the spaces between the grains of sediment and rock, the faster the water can flow. The range of groundwater flow is from inches per day to many feet per day.

It is common in hilly and mountainous areas to have streams. Some streams flow constantly, and others only flow when there has been significant rainfall. A gaining stream flows year-round with or without significant precipitation. This happens because the stream intersects the water table. A losing stream is one that is above the water table, which is why it is generally dry unless there is significant precipitation.

An aquifer is a mass of groundwater in an area. Sometimes the aquifer has an impermeable layer over it. Impermeable means that groundwater cannot get through a rock layer easily, if at all. It, in essence, traps the groundwater. This is a confined aquifer. An artesian well is a well that pushes water toward the surface without the use of a pump. The layers of rock outlined in red in Diagram 5 are the impermeable layers. The sandstone layer is the confined aquifer.


Diagram 5. Artesian well
aq

Sometimes, if the pressure is great enough, and if the well is not too deep, the water will gush out at the surface.


Artesian well
art_well

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