Ground-Water Supply
When ground-water wells are being pumped, the water level in the well bore and in the aquifer itself is lowered. This increases the gradient of the water table surrounding the well and induces ground water to flow into the well. The amount of water that can be extracted from a well is a function of the hydraulic conductivity of the water bearing rocks and fractures and the amount of allowable drawdown (essentially the depth from the top of the static water level in the well to where the pump intake is set). Pump tests (time/drawdown tests) are used to determine the sustainable yield for water wells and to calculate hydraulic properties of the aquifer.
Step-Drawdown Test
The general procedure for new municipal or commercial wells is to start with a step-drawdown test to estimate the maximum sustainable pumping rate. The yield provided by the well driller is used as a mid point and pumping is started at a rate below the drillers yield and increased in time increments (usually between one and two hours) until the pump rate appears too high (the well starts to “go dry” if the drawdown in well reaches the elevation of the pump intake). In the test illustrated below, 50 gallons per minute was chosen as the maximum allowable yield for this well.
When ground-water wells are being pumped, the water level in the well bore and in the aquifer itself is lowered. This increases the gradient of the water table surrounding the well and induces ground water to flow into the well. The amount of water that can be extracted from a well is a function of the hydraulic conductivity of the water bearing rocks and fractures and the amount of allowable drawdown (essentially the depth from the top of the static water level in the well to where the pump intake is set). Pump tests (time/drawdown tests) are used to determine the sustainable yield for water wells and to calculate hydraulic properties of the aquifer.
Step-Drawdown Test
The general procedure for new municipal or commercial wells is to start with a step-drawdown test to estimate the maximum sustainable pumping rate. The yield provided by the well driller is used as a mid point and pumping is started at a rate below the drillers yield and increased in time increments (usually between one and two hours) until the pump rate appears too high (the well starts to “go dry” if the drawdown in well reaches the elevation of the pump intake). In the test illustrated below, 50 gallons per minute was chosen as the maximum allowable yield for this well.
Time-Drawdown Test
The well is continuously pumped at the design rate for some period of time (one to three days) and the water level in the well (drawdown) is measured at regular intervals throughout the test. The results may confirm the sustainable pumping rate and will give an indication of amount of available drawdown. Several properties of the aquifer may be calculated if the water levels in nearby wells were monitored as part of the test. The transmissivity and storativity of the aquifer can be used to predict the possible drawdowns in neighboring wells as a result of pumping the test well. The figure below shows the drawdown and recovery after pumping was halted. The next figure shows the calculation of aquifer properties (transmissivity (T) and storativity (S) for a nearby observation well.
The well is continuously pumped at the design rate for some period of time (one to three days) and the water level in the well (drawdown) is measured at regular intervals throughout the test. The results may confirm the sustainable pumping rate and will give an indication of amount of available drawdown. Several properties of the aquifer may be calculated if the water levels in nearby wells were monitored as part of the test. The transmissivity and storativity of the aquifer can be used to predict the possible drawdowns in neighboring wells as a result of pumping the test well. The figure below shows the drawdown and recovery after pumping was halted. The next figure shows the calculation of aquifer properties (transmissivity (T) and storativity (S) for a nearby observation well.
Cone of Depression
All pumping wells produce a disturbance in the water table (or potentiometric surface) due to the lowering of the water table at the well. Geologic and hydraulic features control the rate, direction, and severity of this disruption throughout the aquifer. If several wells are pumping in the same area they may in effect be competing for water and produce a compound cone of depression from the overlapping drawdowns. Very detailed studies and multiple pump tests may be needed to determine the extent and shape of the cone of depression. This information may also be useful for delineating “wellhead protection zones”. Most wells in the central Appalachians draw water from fractures in the bedrock so there is a large directional difference in transmissivity because the fractures tend to be more open in one direction (usually parallel to the trend of the fold axis). The plot shown below shows the expected range of drawdowns surrounding a pumping well under the very unlikely assumption of no recharge to the aquifer for a year.
All pumping wells produce a disturbance in the water table (or potentiometric surface) due to the lowering of the water table at the well. Geologic and hydraulic features control the rate, direction, and severity of this disruption throughout the aquifer. If several wells are pumping in the same area they may in effect be competing for water and produce a compound cone of depression from the overlapping drawdowns. Very detailed studies and multiple pump tests may be needed to determine the extent and shape of the cone of depression. This information may also be useful for delineating “wellhead protection zones”. Most wells in the central Appalachians draw water from fractures in the bedrock so there is a large directional difference in transmissivity because the fractures tend to be more open in one direction (usually parallel to the trend of the fold axis). The plot shown below shows the expected range of drawdowns surrounding a pumping well under the very unlikely assumption of no recharge to the aquifer for a year.