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Notes from the Groundwater Guy by Thomas E. Ballard, P.G., C.H.G. Southeast Hydrogeology, PLLC Using Specific Capacity to Monitor Well Performance Because it usually requires very little effort to collect the data necessary for the calculation of Specific Capacity (SC), it is an extremely useful measurement which can be used to identify declines in well perform- ance, which allows for planning an optimal well rehabilitation schedule. The specific capacity of a well is the pumping rate (gpm=gallons per minute) (Q), divided by the drawdown in feet (s). Specific Capacity can also be used to 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.Transmissivity is the rate water is transmitted through an aquifer under a unit width and a unit hydraulic gradient. The following equation is used to calculate specific capacity: SC = Q/s Note: SC = Specific Capacity (gpm/foot) Q = discharge (gpm) s = drawdown (feet) Typically, a well should run continuously for at least 24 hours at a constant yield before recording the drawdown to allow drawdown to stabilize. 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 compari- son for all future values. As time goes by, the specific capacity will decline as plug- ging 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 lev- els (static and pumping) should be collected monthly to provide early detection of potential well problems. Rehabilitation work should be scheduled when a well's spe- cific 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 exam- ple, a well with 40 feet of drawdown is pumping 200 gpm has a specific capacity of 4.9. If the well has an available drawdown of 50 feet, the maximum pumping rate would be 4.9 times 50 feet, or 245 gpm. While this is considered an estimate and more accurate calculations can be developed with a field test, it is a useful approximation for under- standing the pumping limits of the well. Estimating Transmissivity: The initial specific capacity value can also be used to estimate the transmissivity (T) of the aquifer. Transmissivity 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: T = 1500 * Q/s (for an unconfined aquifer) T = 2000 * Q/s (for a confined aquifer) Note: T = Transmissivity (gpd/ft=gallons per day, per foot) Q/s = Specific Capacity (gpm/foot) 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 in performance to be tracked so an optimal schedule for well rehabilitation can be estab- lished. The specific capacity of the well can be tracked, along with other important measurements, using a series of key performance indicators (KPI) which allow easy recognition of well performance trends through a series of time graphs. A decline in specific capacity occurs when the well's screen, filter pack, or the for- mation 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 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 processes, 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. Tom michele@worldwidedrillingresource.com 34 JANUARY 2018 WorldWide Drilling Resource ®