In this lesson we want to take a close look at the water table, a key feature that gets discussed a lot when we talk about the availability of groundwater.
Our learning objective is to use some simple models to define what the water table represents
and take you on a short field trip to explain how the position of the water table varies relative to the land surface.
Water on the land surface may infiltrate through the soil and into the sediment and rock below.
This is groundwater.
This water is moving slowly downslope, eventually returning to the ocean to complete the hydrologic cycle.
For more about the hydrologic cycle, see our video on Porosity and Permeabilty.
In order to quantify how much groundwater is available, we need to figure out the position of a feature known as the water table.
As water infiltrates into the ground
it fills up connected spaces in sediment or rock formed by fractures or small gaps between grains known as pores.
We are going to add colored water to this gravel-filled beaker to illustrate how the position of the water table is dependent upon which pore spaces are filled with water.
Where the pore spaces may are full of water is known as the saturated zone.
Closer to the surface, the spaces are empty or are only partially filled. These earth materials are in the unsaturated zone.
The top of the saturated zone is known as the water table.
OK, let’s go and try to find the elevation of the water table in some nearby wells.
This hillside slopes down to a small pond that you can see through the trees.
There are two wells here, one is visible; the other is hidden in the shadows further up the slope.
The relative positions of the wells are illustrated on this cross section.
If we look around we can find outcrops of bedrock not far from the wells.
The bedrock here is a type of metamorphic rock.
Normally we wouldn’t think of metamorphic rock as an ideal groundwater source
as these types of rocks lack interconnected pore spaces.
However, as you can see from this outcrop, these rocks contain numerous fractures providing lots of pathways for water to enter the groundwater system.
We are going to demonstrate how to measure the water level in the wells starting with Well #1.
Shining a flashlight down the well shows the reflection off the top of the water.
The water level meter is essentially a measuring tape attached to a probe that will make a beeping sound when it touches the water.
The probe is carefully lowered down the well until . .
Now we measure the depth by reading the tape.
We will do the same again for Well #2.
This time the water level was 3.39 meters below the edge of the casing.
So let’s look to see what all this means.
The two wells are about 50 meters apart with well #2 about 2 meters higher in elevation.
From our measurements, we know the depths to the water table in each well.
Based on our interpretation we can see that the water table is sloping gently downhill, roughly parallel to the slope of the ground surface.
Groundwater is flowing downslope toward the small creek below Well #1.
The water table is the minimum depth that we would need to drill to ensure a consistent supply of groundwater.
The greater the water supply, the higher the elevation of the water table.
We expect the water table to show some seasonal fluctuations and to rise during wet periods and fall during dryer months.
For example, this graph illustrates how water levels changed in a well in eastern North Carolina.
Note that in 2015 the water table was typically within a few feet of the surface but it dropped to nearly 8 feet deep during October before bouncing back due to winter precipitation.
However, we can see a steady decline in the depth of the water table if groundwater is consumed more rapidly than it is replenished.
This graph illustrates ground water levels over the course of more than 40 years for a well in western Kansas.
This well is located above the High Plains aquifer which experiences relatively low precipitation
and heavy agricultural groundwater consumption.
The water table was 40 feet below the surface in the 1970s but is around 90 feet deep today.
You can find out information about groundwater levels in wells in your state by visiting the US Geological Survey’s Groundwater Watch site.
We had two learning objectives for this lesson. How confident are you that you could complete these tasks.