An aquifer is an underground layer of rock and sand that contains water. It is usually composed of sand, gravel, or permeable rock which lies upon a layer of clay or other impermeable material.
The low impermeability layer is also called an aquiclude. In the following diagram, there is an aquiclude above and an aquiclude under the aquifer. (Ignore the other words, they’re just……, French.)
In unconsolidated sediments (silts, sands and gravels), groundwater is stored in the pore spaces between loose grains of sediment.
|The diagram across describes an aquifer that is open to the atmosphere through the soil pores. The horizontal blue line demarks the boundary between the saturated and unsaturated zones. In the saturated zone, all pore spaces are filled with water. In the unsaturated zone, there’s air in the pores which connects to the atmosphere. In this type of aquifer, the pressure above the water table is atmospheric.
An unconfined aquifer is a subterranean layer of water found below porous layers of earth. Unconfined aquifers are more easily polluted than confined aquifers because liquid contaminants and/or water-born contaminants can seep into them unimpeded.
A confined aquifer is a subterranean layer of water that is bound above and below by dense layers of rock or clay. The water table is separated from the atmosphere by the impermeable layer and is under pressure. This type of aquifer is sometimes called an artesian aquifer.
If one were to insert vertical tubes, the water would rise in the tubes. If one were to insert tubes in many locations, the water would rise in each tube to an elevation determined by the pressure at that location. If the number of tubes were to be increased infinitely and the elevation points connected, they would describe a surface. We call it Piezometric Surface (Piezo in Greek is pressure). Another name is Potentiometric Surface.
In the diagram below, you see a contaminated, unconfined aquifer, resting above a clean, confined aquifer. The contaminants are illustrated in brown and yellow: LNPLs are substances lighter than water, like oil, they float; DNAPL are substances heavier than water (like chlorinated solvents), they sink.
Another unique situation is when a confining unit (such as clay) is found within the unsaturated zone and water is trapped on this forming a saturated zone above the main water table. This is termed a perched aquifer.
In case of contamination, the perched groundwater would be the first to get impacted. When drilling to perched aquifers, we must recognize their nature and not pierce the confining units.
In the investigation of contaminated aquifers, we exercise caution not to breach the integrity of confining layers. Where installation of wells into a confined aquifer is called for, we screen only under the confining layer. In the diagram below, the top of the confined aquifer is indicated by the groundwater level. There are five feet (not shown) of confining clay bed just above it. The well is sitting inside a hole, where the screen is totally immersed in the aquifer. The annular space above the screen is sealed with impervious bentonite (clay). We seal the annular space tight inside the confining layer to prevent passage of substances between the two aquifers.