PITCHCARE CLASSIFIEDS TECHNICAL
What is a water table?
Understanding the terminology used to describe sub-surface water will help explain why water tables rise and fall
Most people know that the “water table” has something to do with ground water. The word table provides an image of a flat surface, like a tabletop, and it is commonly assumed that, when a well is drilled, it strikes water once it reaches below the water table.
There is also a general understanding that, in times of serious drought, water table levels may drop and wells may run dry. Understanding the terminology used to describe sub-surface water can help explain why water tables may rise and fall.
Ground water is sub-surface water, but not all sub-surface water is ground water. Having an appreciation of the types of sub-surface water and knowledge of local geology can explain why some water table levels may vary by tens of feet and others, in the same area, may only change by inches and some hardly at all.
The upper surface of ground water is the water table. Below this surface, all the pore spaces and cracks in sediments and rocks are completely filled (saturated) with water. These saturated layers, known as the saturated zone (or the phreatic zone), are where ground water occurs. Strictly speaking, only water found in the saturated zone is ground water.
In the top layers of soil, unconsolidated sediments or bedrock, pore spaces may not be completely filled with water. Some may contain water, some air, and some may only be partly filled with water. This is known as the unsaturated zone (also called the zone of aeration or the vadose zone). After heavy rainfall, this zone may be almost saturated, while during a long dry spell, it may become almost dry. Precipitation infiltrates downwards through the unsaturated zone. This infiltrating water is
What is an aquifer?
An aquifer is a body of saturated rock through which water can easily move. Aquifers must be both permeable and porous and include such rock types as sandstone, conglomerate, fractured limestone and unconsolidated sand and gravel. Fractured volcanic rocks such as columnar basalts also make good aquifers. The rubble zones between volcanic flows are generally both porous and permeable and make excellent aquifers.
In order for a well to be productive, it must be drilled into an aquifer. Rocks, such as granite and schist, are generally poor aquifers because they have a very low porosity. However, if these rocks are highly fractured, they make good aquifers.
A well is a hole drilled into the ground to penetrate an aquifer. Normally such water must be pumped to the surface. If water is pumped from a well faster than it is replenished, the water table is lowered and the well may go dry. When water is pumped from a well, the water
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known as soil water when it is still shallow enough to be used by plants, and as vadose water when it is below root level, but still unsaturated. With further infiltration, however, excess water will eventually reach the water table.
The vertical distance from the ground surface to the water table varies from place to place - it may be a few feet, or several hundred feet. Generally, the water table is deeper beneath hills and shallower beneath valleys. It is hardly ever flat!
In any one place, the water table usually rises with increased recharge from precipitation and declines in response to seasonally dry weather, drought, or excessive pumping of ground water. If, however, the water table is hundreds of feet down, it may take years for the infiltrating water to reach the saturated zone and there will be no seasonal change in water table levels. If ground water is “confined” by overlying impermeable rock formations, the well water levels represent a pressure level and NOT a water table level.
The spaces between soil or sediment particles and cracks in solid rock are called voids or pores. Each sediment and rock type has differences in porosity, (the amount of water a rock formation can hold). Porosity is expressed as the ratio of pore space to solid material per unit volume. For example, saturated sand may have 30% pore space to 70% solid material, whilst fractured granite may have 1% pore space to 99% solid rock. The sand is, therefore, more
table is generally lowered into a cone of depression at the well. Groundwater normally flows down the slope of the water table towards the well.
Is an aquifer an underground river?
No. Almost all aquifers are not rivers. Since water moves slowly through pore spaces in an aquifer’s rock or sediment, the only life-forms that could enjoy floating in such a ‘river’ would be bacteria or viruses which are small enough to fit through the pore spaces. True underground rivers are found only in cavernous rock formations where the rock surrounding cracks or fractures has been dissolved away to leave open channels through which water can move very rapidly, like a river.
Ground water has to squeeze through pore spaces of rock and sediment to move through an aquifer (the porosity of such aquifers make them good filters for natural purification). Because it takes effort to force water through tiny pores, ground water loses energy as it flows, leading to a decrease in hydraulic head in
porous than the fractured granite.
Imagine a cubic foot of granite and a cubic foot of sand with porosity of 1% and 30%. Now add water to each. The granite will “fill up” first because there is less pore space. If it were a real aquifer, the water table level in the granite would rise faster. Similarly, because there is less storage than in the sand, the fractured granite water table would decline more rapidly in response to pumping or drought.
Ground water is always on the move, although usually very slowly. The discharge (or outflow) of water from aquifers occurs as part of the natural movement of water in the hydrologic system. Water table levels in aquifers therefore represent the combined effects of rates of recharge and rates of discharge. If pumping of aquifers takes place in excess of recharge then resource use will eventually not be sustainable. Careful monitoring of water levels in wells can show how water table levels change, and well data, with water levels and dates of the measurement are very important for ground water management.
For any well data, it is very important to know exactly which rock formations the well penetrates. There can be more than one aquifer beneath the surface! Water table information, in addition to other information about geology, precipitation and pumping rates are of great value when assessing ground water potential.
© American Ground Water Trust.
www.agwt.com
the direction of flow. Larger pore spaces usually have higher permeability, produce less energy loss, and therefore allow water to move more rapidly. For this reason, ground water can move rapidly over large distances in aquifers whose pore spaces are large (like the lower Portneuf River aquifer) or where porosity arises from interconnected fractures.
Ground water moves very rapidly in fractured rock aquifers like those in volcanic rock. In such cases, the spread of contaminants can be difficult or impossible to prevent.
What does an aquifer look like?
Every aquifer is unique, although some are more generic than others. The boundaries of an aquifer are usually gradational into other aquifers, so that an aquifer can be part of an aquifer system. The top of an unconfined aquifer is the water table. A confined aquifer has at least one aquitard (a bed of low permeability) at its top and, if it is stacked with others, an aquitard at its base.
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