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, it is an extremely useful measurement that 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 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.  The following 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.

Estimating Transmissivity

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 become 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 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 (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 the 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.

References

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

Thomas Ballard

Thomas E. Ballard, aka “The Groundwater Guy” is a consulting hydrogeologist with over 35 years experience. He is a registered Professional Geologist in California and Tennessee and Certified Hydrogeologist in California. His work focuses mainly on water resources development for small water districts and groundwater contamination issues.

This Post Has 14 Comments

  1. Randall Roberts

    So 200/40=4.9? In my day, that would have been 5, even with a slide rule. Just wondering where the 0.1 went.

    1. Thomas Ballard

      Fixed now – I’m not sure how that happened, but thanks for catching my error.

  2. mario doromal

    based on the sample well maximum pumping rate of 250gpm what would be my water pump rating? thanks

    1. Thomas Ballard

      Specific Capacity is based on the drawdown and pumping rate, with drawdown being the difference between the non-pumping water level and the pumping water level, so if you had a drawdown of 20 feet and were pumping at 250 gallons per minute, your specific capacity would be 250 divided by 20 or 12.5. Just substitute your drawdown for the example and you will come upon with your specific capacity. If you are looking for specifics on your pump efficiency, that is a whole other calculation and we would need a bit more information. If you email me at tballard@sehydrogeology.com, I can give you the specific calculations for wire-to-water pump efficiency.

  3. M. Abdelhadi

    How do I calculate the available drawdown?

    1. Thomas Ballard

      Drawdown is the difference between the static (non-pumping) water level and the water level during pumping. Available drawdown is theoretically the difference between the bottom of the well and the static water level – so, in effect, how much you could pump the water level down before the well goes dry. Typically, we will try to set the pump intake at 70% of available drawdown unless we have pumping test data that indicates we can go shallower.

  4. DONIES MULUMBWA

    What is drainable volume for each aquifer in the hydrostratigraphy and how do you calculate it? I do ask this question because the total amount of groundwater to pumped was calculated from the addition of drainable volume quantity plus the recharge. I would like to understand what drainable volume amounts of water mean in an aquifer?

    1. Thomas Ballard

      Drainable volumes of water in an aquifer are going to be the specific yield (the value you are looking for), which is essentially equal to the effective porosity. Effective porosity is calculated by the following formula: ((weight of saturated sample – weight of sample after gravity drainage)/(weight of saturated sample – weight of air-dried sample)) x (void volume/total volume) x 100%. This is will give you effective porosity as a percentage of the total volume of rock and that is essentially the same as your specific yield.

  5. Dave

    What is your opinion of the reliability of using air-lift tests to calculate specific capacity (SC) and estimating transmissivty (T) during well development immediately after drilling? Regulators typically require, as they should, the assumption that the pumping water level is equal to the depth of the air line, but that cannot be determined with this test method. Therefore, SC would be calculated at a lower value, thus affecting the T estimate. Would you assume a high turbulence with this method, thus affecting SC even more?

    Are you aware of any research regarding this?

    Thanks

    1. Thomas Ballard

      Field experience indicates that airlift testing is really only good for order of magnitude estimates of pumping rates and specific capacity. My experience is that airlift tends to overestimate yields in a well, but your experience may vary. What is true is that actual pumping tests and initial airlift results rarely align mand that is mostly because of the turbulence, as well as the fluid properties of air vs water. I think that estimating transmissivity via specific capacity should be used with caution and not be considered as accurate as a pumping test. Even pumping tests are subject to a lot of variables that cannot be controlled, so it is important to understand those results that get calculated to 10 decimal places are not really that precise.

  6. Carl

    When setting up a spec for an under reamed gravel pack well, what is a good range for the specific capacity? We have a spec that calls out “the minimum specific capacity shall be 20.0 gpm/ft for 1-hour drawdown at 500 gpm or a minimum of 80% well efficiency.” I was thinking that may be a high number.

    1. Thomas Ballard

      Specific capacity is really going to depend on a lot on aquifer properties – that is why we do not spec out an actual specific capacity number in the drilling specifications we put together. In some aquifers, a specific capacity of 5 may be as good as you can get because that is all the aquifer will support. Within reason, well efficiency can impact SC, but there are limits based on aquifer properties – mainly transmissivity. You can have a 100% efficient well, but only get 10-20 gpm if you have a really “tight’ aquifer. Well design comes into play, as well. The bottom line is the hydrogeology really comes into play and you cannot specify an SC without knowing your aquifer properties first. I’ve seen some projects that specify that a new well produce a certain gpm – something that is just not realistic in most cases. If I was a driller, I would never sign a contract that required a specific yield. In answer to your question, 20 is a fairly high SC, but not impossible to achieve in the right circumstances. For a 500 gpm pumping rate, that would really translate to 25 feet of drawdown which is certainly realistic if the aquifer will support a 500 gpm pumping rate. I hope that is not an obtuse answer, but I go back to being opposed to to requiring an actual specific capacity number in the drilling specs.

  7. Odell Ward

    If specific capacity can be estimated as Transmissivity / 1500 for an unconfined aquifer, where does the 15oo come from?

    1. Thomas Ballard

      Don’t forget that this method to estimate transmissivity is an approximation. There are a lot of factors that influence transmissivity calculations during a real pumping test, but for most purposes, an approximation is good enough. The specific capacity method provides a quick and dirty method to give a transmissivity without going through the expense and time of a pumping test. That said, the 1500 is essentially a constant based on using typical values in the Jacob straight-line formula, which is itself is a simplified version of the Theis equation. You can find more details on page 1021 of the second edition of Groundwater and Wells by Fletcher Driscoll.

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