Short answer: Rehabilitate a well when its specific capacity (gallons per minute per foot of drawdown) drops about 10% below its established baseline. That is the latest you should act, not the earliest. By the time you can feel a problem at the tap, plugging has usually been under way for years, and some of it may already be permanent.
If you manage municipal wells, an irrigation account, or a facility that cannot afford to run dry, this is the one number worth tracking above all others. Here is why, how to measure it, and what happens when you wait.
Most well owners decide it is time for a rehabilitation the same way most people decide it is time to see a doctor: something hurts. Flow is down, the pump is running longer to fill the tank, the drawdown is deeper than anyone remembers. That is when the phone rings.
The trouble is that a well does not announce plugging while it is happening. It happens quietly, out in the formation, in pore spaces you cannot see with a camera. A well can lose a meaningful share of its ability to move water before a single operator notices anything at the surface, because the pump keeps pulling the water level down further and further to squeeze out the same gallons. The symptom you eventually feel at the tap is the last chapter of the story, not the first.
That is why the timing question matters so much. The right time to rehabilitate a well is not "when it fails." It is when a number you are already able to measure starts to slide.
Specific capacity is the well's pumping rate divided by the drawdown it takes to achieve that rate:
Specific capacity (gpm/ft) = pumping rate (gpm) ÷ drawdown (ft)
If a well pumps 500 gpm and the water level drops 25 feet to do it, specific capacity is 20 gpm/ft. Track it over time and it becomes the single best top-level measure of well health, because it captures how easily the formation gives up water. When specific capacity declines, the well is having to be drawn down further to deliver the same water, which is the physical signature of a plugging formation.
Here is the plain-English version we use with clients: specific capacity is your well's gas mileage. It tells you how many gallons a minute you get for every foot we have to pull the water level down. When that number starts sliding, the well is getting clogged and working harder to give you the same water, even if you have not felt it yet. We watch it the way a doctor watches blood pressure: a small, steady change is the early warning that lets us fix things cheaply, before it becomes a crisis.
Our position, and we are firm on it: the very latest a rehab should be triggered is when specific capacity drops about 10% from the well's established baseline.
You will see other thresholds cited in the industry. Some guidance says 15%, some says 25%. The more conservative your trigger, the more longevity you preserve, and here is the reasoning behind the number.
Plugging begins long before you would ever notice a flow problem. Even at 10%, you are not early. You are catching the problem at the earliest point where you can prove it to a client with data, which is a very different thing from catching it at the earliest point it exists. We would rather intervene early and cheaply than late and permanently.
That last word is the important one.
Plugging is not a one-time event. It snowballs.
There are typically three distnct causes of well plugging, which can occur seperately or can feed off each other. We call them the plugging trifecta:
Each of these reduces porosity, and lost porosity is lost recharge. But the part most owners never hear is this: left long enough, plugging becomes effectively irreversible. Deposits harden and lock into the formation where no tool or chemical can reach them. From that point on, every rehabilitation is less effective and shorter-lived than the one before.
Plot specific capacity over twenty years of a typical well's life and you can see it on the chart. Each rehab event lifts the curve, and the decline resumes from there. But the later recoveries reach lower peaks and fade faster. That is what compounding plugging looks like, and it is the single strongest argument for a regular maintenance program instead of crisis-mode rescues.
Specific capacity is the metric you want, but plenty of the early signals show up in ordinary operating data if you know how to read them. The catch is that how a failing well looks depends entirely on how the pump is controlled.
Fixed-speed pumps. A fixed-speed pump spins at one constant rate no matter what. As the well plugs and cannot feed it, flow drops. Counterintuitively, power consumption drops too, because a pump only really does work when it is moving water. A plugging fixed-speed well looks like a slow fade: declining flow, declining power, deeper drawdown.
VFD pumps. A variable frequency drive is trying to hit a target, usually a flow or pressure setpoint. As the well plugs, the drive does what you told it to: it ramps up speed to hold the number. So instead of flow fading, you see the drive straining, drawing down deeper and consuming more power than a fixed-speed pump would under the same plugging.
Same disease, opposite power trends. A drop in power on a fixed-speed well and a rise in power on a VFD well can be pointing at the exact same problem. If you are watching SCADA for early warning, know which kind of pump you are looking at before you interpret anything.
Other signals worth logging:
None of these is proof on its own. All of them are reasons to run a proper test and get a specific capacity number you can compare to baseline.
The metric lives or dies on the quality of the pump test behind it. A few rules keep the trend honest:
Start from a rested well. Specific capacity requires a true static water level. If you start pumping before the well has recovered from its last run, your static level is a lie, your drawdown is a lie, and your specific capacity is a lie. Measure the water level as it recovers and wait for it to stabilize, then record that same rest period every time.
Test at the same rate every time. Specific capacity sags at higher pumping rates because well loss grows with the square of the rate. A number taken at 300 gpm and a number taken at 600 gpm are not comparable. Pick the well's real day-to-day operating rate, use it on every test, and always report the rate next to the number: "specific capacity = 22 gpm/ft at 500 gpm."
Keep the conditions consistent. Same method, same measurement reference point (top of casing), same neighboring wells offline. If well #3 down the road was off during your baseline three years ago, it should be off today.
Respect shallow wells. In a deep well with a big drawdown, a foot of static-level wobble is noise. In a shallow well with a small drawdown, that same foot is a screaming false signal. The shallower the well, the more carefully you need to control the test.
Log it against the date. The magic is in the trend. One specific capacity reading tells you almost nothing. Five years of readings, captured the same way, tell you exactly when to act.
A declining specific capacity points at the well. But on an older well, it is frequently not the only thing going on, and a good project manager rules out two other culprits before committing to a treatment.
Is it the pump? A pump is a manufactured machine with an expected service life of roughly eight to fifteen years. Mechanical failure shows a different fingerprint than plugging: the pump falls short even though drawdown, static recovery, and neighboring wells all look normal. If specific capacity at the rate you can achieve is still healthy but the pump cannot make its numbers, look mechanical.
Is it the aquifer? If the whole basin's water table has dropped from years of over-pumping, no amount of cleaning one well will bring back water that is not there. Check regional monitoring data and, most usefully, look at neighboring wells. Same aquifer, healthy neighbors, sick well: the well is the problem. Same aquifer, everyone declining together: the aquifer is the problem, and you should say so plainly even though it is not the answer that sells a rehab.
On a twenty-year-old well it is often all three at once: a tired pump, a basin that has dropped, and a plugged screen, each taking its slice of the lost capacity. The job is not to crown one winner. It is to apportion the loss well enough to know what a rehab can honestly recover.
One more thing to settle before the work starts: what does "success" mean?
Success is never just how much you improved a metric on day one. It is that improvement multiplied by how long it lasts. Plugging starts coming back almost immediately after any rehab, because the chemistry, biology, and physics that plugged the well the first time never signed a peace treaty. So the real goal is to restore as much porosity as possible, creating a buffer of open pore space well beyond what the pump strictly needs, so the well can keep feeding the pump for years before plugging eats through it again.
A method that produces a dramatic day-one number but only cleans a thin skin at the borehole face may plug right back up. A method that penetrates into the filter pack and formation might show a similar bump and hold it far longer. Longevity is the tiebreaker.
And set expectations honestly: if a well once produced water, a rehab can generally get it back. If a well never had the water, rehabilitation is not designed to create it.
Early and cheap beats late and irreversible. That is the whole philosophy in one sentence.
Want to see what a rehab is worth on your well in dollars, not just gpm/ft? The free AirBurst Return-on-Rehab estimator at ror.airbursttechnology.com carries specific capacity, efficiency, and energy figures through to payback and net present value.