In order to understand something of the ground-water regime in which a well is completed. A basic understanding of some of the terms and principles used in hydrogeology and in the water well industry is required. Below are the definitions of some of the common terms.
Definitions of an Aquifer
- Aquifer A saturated, permeable geologic unit that will yield water to wells at sufficient rates so that the well can serve as a practical source of water. Aquifers may be either unconfined, confined or semi-confined.
- Unconfined Aquifer (water table) An unconfined or water table aquifer is a partially saturated geologic formation whose partially saturated geologic formation whose upper boundary is formed by the water table whose surface is under atmospheric pressure.
The level of the water in a well at rest in an unconfined aquifer is at the level of the water table. - Confined Aquifer (Artesian) A confined or artesian aquifer is a completely saturated geologic formation that is contained by impervious boundaries and that is under pressure greater than atmospheric pressure.
The water level in a well at rest in an artesian aquifer stands above the top of the aquifer.
Where the water level rises above the ground surface a flowing condition will exist. - Semi-confined Aquifer A semi-confined or leaky aquifer is an artesian aquifer whose confining layers are semi-pervious and have a low but measurable permeability. Pumping a well changes the hydraulic gradients within an aquifer system causing the transmission or flow of water through the semi-pervious layers towards the well.
- Static Water Level The static water level in a well is the level of the water in the well casing when no water is being pumped from the aquifer. The depth is measured from ground surface or a known measuring point to the water level in the well casing. The static water level can be above ground level (flowing well).
- Pumping Level The pumping level of a well is the water level in the well casing when pumping is in progress. The depth is measured from ground surface or a known measuring point to the water in the well casing.
- Drawdown The drawdown in a well is a measure of the decline in the water level when pumping is in progress. It is the difference between the static water level and the pumping level at any point in time.
- Cone of Influence When water is pumped from a well a depression is produced in the water table or potentiometric surface (imaginary surface formed from the water levels in wells penetrating an artesian aquifer). The depression is called the cone of influence of a well. The cones of influence of wells in the same aquifer may overlap. The effect of one well on the water level in another is called well interference.
- Specific Gravity The specific gravity of a well is a measure of the number of litres per second per metre of drawdown in a well. The specific gravity can be used to estimate the drawdown produced at different pumping rates. For example, a very efficient well delivering 21.0 L/s with 3.0 m of drawdown would have a specific capacity of 7.0 L/s/m. At 14.0 L/s the drawdown in the well would be 2 m.
Calculating the drawdown for pumping rates greater than those actually experienced is possible when wells are completed in overburden aquifers, but should not be attempted for wells in rock aquifers. - Perennial Yield The perennial yield or safe yield of a well is the amount of water that can be withdrawn annually without producing an undesired effect such as: depletion, interference, and land subsidence.
