Well performance is influenced by several factors, including the nature of the ground from which the ground water is being extracted, the nature of the well screen used in the well (if any), the diameter of the well, the pumping rate, etc. We often think of a well as having unlimited capacity, and in relative terms, there are some wells that approach this performance. In reality, though, all wells have a finite production capacity, and this capability can be measured a number of ways. Among the most accurate ways to measure well production is the pump test; pumping water out of the well, at varying flow rates, and measuring how the water level in the well is affected over time. Figure 1 shows a drawdown chart created from such a test.
Besides offering an accurate way to determine well performance, pumping is a good way to develop a well. By pumping at increasing flow rates, over a period of several hours, to a rate slightly faster than the pumping requirement of the job, foreign matter left in the formation and filter pack around the well screen from the drilling operation is removed, so the well will produce good clear water. In Figure 1, the operator's notes about water quality in the right column show the well clearing during the pump test.
Another caution regarding well performance tests: Make sure you test the well for a long enough period of time for the performance to stabilize at each pumping rate. I know of a particular hard-rock residential well in my area that was tested at a steady flow rate for four hours and appeared to produce enough water to meet the customer's expectations. Once the homeowner moved in and started using the well, its production dropped to a fraction of the tested rate. The contractor suspected pump problems, but when a new pump was installed - at 1,100 feet - it performed the same as the first pump.
It turned out there was a large underground cavity acting as a reservoir, feeding the well. The actual well production rate was only about 1 gpm, but when left idle for several weeks, the reservoir filled up, giving the appearance of a 5 gpm well for four hours. Had the contractor performance tested the well longer than just four hours when it was first drilled, he could have saved the labor of installing two pumps, the cost of which he ate. This is an example of the knowledge base of local conditions a water well contractor develops over time which allows him to do excellent work for his clients, and to avoid some of the pitfalls we all encounter along the way.
Well Curves
Just as the performance of a pump can be described using a pump curve in terms of feet of water and gallons of flow, the performance of a well can be defined using the same terms. When measuring the total performance of a well, a slight departure from the technique used above is in order. In the above example, the contractor knew that the customer wanted a maximum of 40 gpm from the water well system, so he tested to 50 gpm. If your job is to test the total performance of a well, it is necessary to test the well to its limits.This is done by pumping at various increasing flow rates until the well can no longer support the pumping rate. Additionally, it is necessary to stop at each flow rate long enough for the water level in the well to stabilize before moving to the next flow rate. If you have a large enough pump, a point will be reached where the water level does not stabilize, but keeps dropping, at which point, you have exceeded the production capability of the well. Figure 2 is an example of what a well curve looks like. You can see from the curve that at 50 gpm, the water level in the well continues to draw down, indicating that the production capacity of the well has been exceeded.
The operating point of a particular pump system with a particular well is determined by adding the drawdown of the well, plus the static level to the total head pressure required for the rest of the system. If you have a 30/50 pressure switch on your system, add the 50 psi in terms of feet of head or 115.5 feet of head (2.31 x 50 psi) to the drawdown and static level, giving you a curve like that in Figure 3. Now you can overlay different pump curves and come up with a good idea of the actual performance in your well offered by each potential pump.
Taking the time to pump test a new well, develop a well curve, and keep the results in your job file can go a long way toward providing each of your customers with a good system up front, and in keeping them happy through the years. Next month, we will begin a two-part series on solar pumping. 'Til then ….
ND
