Because it usually requires very little effort to collect the data necessary for the calculation of Specific Capacity, it is an extremely useful measurement which can be used to identify declines in well performance, which allows for planning an optimal well rehabilitation schedule. The Specific Capacity of a well is the pumping rate (gpm) (Q) divided by the drawdown in feet (s). Specific Capacity can also be used provide the design pumping rate or maximum yield for the well and to estimate the transmissivity of the surrounding formations penetrated by the well screens. Thefollowing equation is used to calculate Specific Capacity:
SC = Q/s
Note: SC = Specific Capacity (gpm/ft); Q = discharge (gpm); s = drawdown (ft)
In the figure at the left, the well has a drawdown of 40 feet. If the well was pumping at 200 gpm, the Specific Capacity would be 200 gpm divided by 40 feet of drawdown to give a Specific Capacity of 5.
Typically, a well should run continuously for at least 24 hours at a constant yield before recording the drawdown to allow drawdown to stabilize (Driscoll, 1986). Ideally, to provide the best comparison of Specific Capacity measurements over time and a comparison to the original test results, the same continuous pumping time frame should be used for each subsequent test.
The Specific Capacity obtained just after a well is drilled and properly developed is typically the highest value that will be produced and is the baseline for comparison for all future values. As time goes by, the Specific Capacity will decline as plugging of the well’s perforations or filter pack occurs or as static water levels change. Specific Capacity testing should be performed at least semi-annually and water levels (static and pumping) should be collected monthly to provide early detection of potential well problems. Rehabilitation work should be scheduled when a well’s Specific Capacity drops by 15% or more.
Estimating Maximum Pumping Rate
The maximum pumping rate of a well can be estimated using the initial Specific Capacity. The maximum pumping rate is calculated as the Specific Capacity times the maximum available drawdown. For example, with a well that has 40 feet of drawdown and is pumping 200 gpm has a Specific Capacity of 5. If the well has an available drawdown of 50 feet, the maximum pumping rate would be 5 times 50 feet, or 250 gpm. While this is considered an estimate and more accurate calculations can be developed with a field test, it is a useful approximation for understanding the pumping limits of the well.
The initial Specific Capacity value can also be used to estimate the transmissivity (T) of the aquifer. Transmissivity is the rate water is transmitted through an aquifer under a unit width and a unit hydraulic gradient. It equals the aquifer’s hydraulic conductivity (K) times the aquifer thickness (b). The higher the transmissivity, the greater the capability of the aquifer to move water and the lower the drawdown in the well. The following equations can be used to estimate transmissivity per (Driscoll, 1986):
T = 1500 * Q/s (for an unconfined aquifer)
T = 2000 * Q/s (for a confined aquifer)
Note: T = Transmissivity ( gpd/ft); Q/s = Specific Capacity (gpm/ft)
A new well will start to lose Specific Capacity as soon as it starts pumping. The rate of decline will vary from well to well, a good record tracking system will allow declines performance to be tracked so that an optimal schedule for well rehabilitation can be established. The Specific Capacity of the well can be tracked, along with other important measurements, using a series of Key Performance Indicators (KPI) that allow easy recognition of well performance trends through a series of time series graphs.
A decline in Specific Capacity occurs when the well’s screen, filter pack, or the formation adjacent to the well becomes plugged from physical processes (sand bridges, silt, and clay particles), chemical processes (mineral incrustations), or biological processes (biofouling). Rehabilitation is performed to remove these blockages and restore the well’s Specific Capacity and improve the well’s efficiency. There are both short-term and long-term declines in Specific Capacity over the normal life span of a well. Short-term declines are caused by the plugging as a result of one or more physical, chemical or biological process and may be partially reversed with well rehabilitation efforts. However, the causes of the plugging can generally not be removed completely resulting in a chronic, long-term decline in Specific Capacity over the life of the well until Specific Capacity reaches the point where a replacement well is needed (see graph).
Other Factors Affecting Specific Capacity
When turbulent flow occurs in the well, the specific capacity declines as the discharge rate is increased (Sterrett, 2007). It’s important to assess the laminar and turbulent flow components to determine optimal pumping rates and pump depth. This can be accomplished through the use of a step-drawdown test. The relative proportion of laminar and turbulent flow occurring at each step can be derived from the pumping test data using the Jacob model.
There is also evidence that Specific Capacity can be affected by changes in temperature of the groundwater. This effect is most notable in wells that are located in an aquifer which is directly under the influence of surface water and experiences seasonal temperature changes.
Driscoll, F.G., 1986, Groundwater and Wells, Second Edition: Johnson Filtration Systems Inc.
Roscoe Moss Company, 1990, Handbook of Ground Water Development: John Wiley & Sons, Inc.
Sterrett, R.J., 2007, Groundwater and Wells, Third Edition: Johnson Screens