In order to quantify how much groundwater is available, we need to figure out the position of a feature known as the water table. 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. The water table is the minimum depth that we would need to drill to ensure a consistent supply of groundwater.
In this article we want to take a look at the water table, a key feature that gets discussed a lot when we talk about the availability of groundwater. 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. However, we can see a steady decline in the depth of the water table if groundwater is consumed more rapidly than it is replenished.
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. Based on our interpretation we can see that the water table is sloping gently downhill, roughly parallel to the slope of the ground surface. 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. OK, let’s go and try to find the elevation of the water table in some nearby wells. The water table was 40 feet below the surface in the 1970s but is around 90 feet deep today. The top of the saturated zone is known as the water table.
We expect the water table to show some seasonal fluctuations and to rise during wet periods and fall during dryer months. The greater the water supply, the higher the elevation of the water table. However, as you can see from this outcrop, these rocks contain numerous fractures providing lots of pathways for water to enter the groundwater system. Where the pore spaces may are full of water is known as the saturated zone. From our measurements, we know the depths to the water table in each well. Water on the land surface may infiltrate through the soil and into the sediment and rock below.