In an earlier blog post, I discussed the sources of drinking water and their availability. The sources are surface water, groundwater under the direct influence of surface water and groundwater. Today, I would like to talk more in depth about groundwater.
When working with groundwater, water source protection is extremely important. Unlike surface water, once groundwater is contaminated, it can take anywhere from days to millennia before the damage can be reversed. First, the contaminant must be completely removed; then the groundwater must be recharged.
Groundwater is drawn from aquifers. There are two types of aquifers an operator needs to be aware of – an unconfined aquifer and a confined aquifer. An unconfined aquifer is an aquifer that is restricted by an impervious layer on the bottom but not on top. Confined aquifers are those that are covered (confined) by an impermeable or semi-permeable layer of rock on both the top and bottom. A confined aquifer is also called an artesian aquifer.
The movement of groundwater can be changed and affected in many ways. When a well is placed in a water-bearing layer of the earth’s crust and pumped, water will flow toward the center of the well. In a water-table aquifer, this movement will create a depression in the water table called the cone of depression. The shape and size of the cone is dependent on the relationship between the pumping rate and the rate of water movement.
If any contamination is in the area, it will be drawn into the cone of depression. This makes the placement of your well even more important. The water table is the average depth of groundwater over a selected area. The zone of influence reaches from the farthest points of the cone of depression.
Specific capacity is the short-term sustainable discharge in GPM divided by the drawdown yielding the discharge in feet (GPM/ft). Drillers measure specific capacity and report it to the state. The math example below illustrates how specific capacity is related to drawdown.
Well #25 yields 235 gpm at a drawdown of 90 feet. What is the specific capacity?
235 gpm ÷ 90 feet = 2.6 gpm/ft.
The protection of surface water and groundwater sources helps us all by protecting our drinking water supply, lowering the cost of treatment to produce drinking water and protecting the plants and animals in our environment.