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This article describes how we measure the amount of water available and the water delivery rate ability of various types of drinking water sources like wells, cisterns, dug wells, drilled wells, artesian wells and well and water pump equipment. The sketch at page top, courtesy of Carson Dunlop, outlines what happens during a well drawdown or well flow test procedure. Details are below.

Readers of this document should also see Water Tank Types and before assuming that a water problem is due to the well itself, see Water pump and pressure tank repair diagnosis & cost an specific case which offers an example of diagnosis of loss of water pressure, loss of water, and analyzes the actual repair cost.

In a companion article, How to Test Well Water Quantity, we describe both valid and questionable ways people measure well yield, and we offer some simple steps any home owner or home buyer can take to check the adequacy of water pressure and water quantity at a building.

© 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

How Much Water is Actually In the Well and How Much Water Can Be Drawn Out of the Well for How Long? Well Flow Rates or Well Recovery Rates Explained

Two Key Questions to Ask About the Well When Buying a Property With a Private Water Well

If you are purchasing a property served by a private well of any sort, here are the critical questions to ask about the well itself:

• Where is the well located? Is the well on the property being purchased?
  
  
• What kind of well is it: hand dug well, driven point well, drilled well, spring, stream, (these water source alternatives have implications for sanitation and water quantity as well as safety)

Three Key Questions to Ask About the Water Provided from a Well

There are three basic questions that must be asked about a private water supply provided from a well. It's helpful to state them since otherwise a property buyer may receive only answers to some of these questions, all of which are critical:

1. Water Quantity from the Well: how much water, in gallons or liters, can we draw out of the well before we run out, and how fast can we take it out? We discuss these questions in this document after this introduction.
   
   
2. Quality of water from the Well: what's in the water? Is it sanitary? Are there contaminants that are a health threat? That's water potability. But further, are there other water contaminants that are not a health threat but which are aesthetic or even functional concerns such as color, sediment, water hardness, or water odors? You can see that a simple bacteria test to "pass" or "fail" a water well won't address most of these questions. See WATER TESTING GUIDE for a guide to selecting the appropriate tests to perform when purchasing or maintaining a property with a private water well.
   
   
3. Water Supply Functionality: is there a good useable flow rate coming out of the water supply piping at every fixture? The answers to this question usually describe the condition of the piping and well pump, not the condition of the well itself, though in some cases deliberate flow restricters may have been installed at a building which is served by a well that has a very limited water quantity. SeeWATER PRESSURE LOSS DIAGNOSIS GUIDE for help diagnosing poor water pressure and poor water flow.

Static Head: Well Water Starting Quantity - What is the Static Head of Water in the Well?

Calculating the gallons of water per foot of well casing

We have about 1.5 gallons of water per foot of depth of a well when we're using a standard residential 6" well casing. The height of water column inside the well and available to the pump is less than the total well depth. Except in artesian walls the water column does not extend from the well bottom to the top of the ground.

Static head location in a well

In this sketch, distance (h) is the "static head" which is the total volume of water available to the pump. The static head in a drilled well extends from the very bottom of the pump (since water can't jump up to the pump) upwards to the highest point that water reaches inside the well casing when the well has rested and reached its normal maximum height.

Well water quantity calculation at pump startup

• (a) air in top of the well casing: water rises only to a height somewhere below the very top of the well.
• (h) static head: explained above
• (c) well bottom clearance: our well pump or foot valve (if the pump is not in the well) was placed 5' off of the well bottom © in the sketch, a distance to avoid drawing mud into the pump
• (d) total well depth: measured from the top of the ground to the bottom of the well

Static head water quantity (h) = Total well depth (d) - Air (a) - Clearance at bottom (c)

Calculating the volume of water or gallons of water stored in water piping

In some circumstances such as deciding how much water to flush out of a pipe for certain water tests, it is useful to know the volume of water required to fill well piping or water piping.

For long runs of well piping there may be a significant volume of water in the piping itself. Using 600' of plastic well piping as an example, we need simply to calculate the volume of a cylinder (the inside of a water pipe) into cubic inches per foot.

The volume of a cylinder V = pi x r² x h

where pi = 3.1416, r = cylinder radius (1/2 the diameter) and h = the cylinder height or length of pipe in our case and
G = the volume of water in gallons = 0.004329 gallons per cubic inch

There is more water in long piping runs than one would have guessed.

To translate cubic inches of water inside of a pipe, 1 cu.in. is about 0.004329 gallons

• 1/2" internal diameter piping contains 2.35 cubic inches or (2.35 x 0.004329) = 0.01 gallons per linear foot:
  .5 / 2 = r = .25", and h = 12" per foot,
  V = (.25)² x 3.1416 x 12 = 2.36 cu.in. per foot. Multiplying this by 0.004329 we obtain
  G = 0.01 per linear foot of pipe
• 3/4" internal diameter piping contains (.75)² x 3.1416 x 12 x 0.004329 = 0.02 gallons per linear foot
• 1" internal diameter piping contains (1)² c 3.1416 x 12 x 0.004329 = 0.04 gallons per linear foot

Well Yield: Well Water Delivery Rate - the Well Recovery Rate or Well Flow Rate

Well Recovery Rate is the rate at which water runs into the well from the rock fissures and openings into the lower portion of the well below the steel casing, while we're pumping water out of the well. Some other terms for well recovery rate include well yield, well flow rate, and well water quantity. Since the "recovery rate" of a well describes the rate at which water runs into the well, a well recovery rate also defines the rate at which water can be pumped out of a well without pumping the well down so far that the pump "runs dry".

Typical numbers for well recovery rates (if measured honestly over a 24-hour period) run from a fraction of a gallon per minute (a terribly poor well recovery or flow rate) to 3 gallons a minute of water flow (not great but useable) to 5 gallons per minute (just fine for residential use) to more than 10 gpm (a great well recovery rate for residential use).

The well flow rate or recovery rate is not equal to the well pumping rate: that is, most water pumps can pump water out of a well faster than water runs in unless the well has a great recovery rate. For wells with modest recovery rates of say 2-3 gpm, some well installers or plumbers design the pump so that it cannot pump faster than this rate, thus avoiding pumping the well dry and possibly damaging the water pump itself.

So you could pump water out of a well very fast, say at 10 or even 15 gpm. But if the well recovery rate is less than the well pumping rate, you're going to run out of water. How soon you run out of water depends on how much water was in the well casing when you started pumping, and also on the well recovery rate. We explain this in more detail just below.

We offer a more detailed (and more confusing) equation used to calculate the details of a well recovery rate in our discussion at Drilled Wells - steel casings. But it's easier to simply pull water out of a well at a given rate and see how long we can do so. That's about what a well driller does to determine the effective well flow rate when a new well is drilled. Pulling water out of the well (using a variable-rate pump running at a rate set by the well test professional) integrates all of the different rock fissure flow rates into a single quantity of water.

Definition of the Total Amount of Water Available From a Water Well

People sometimes confuse things by describing what we call the well 'flow rate" as the "water quantity" available from a well. They're different. You could have a great flow rate - 20 gallons per minute, but if it the water will only run at that rate for five minutes before you run out, the well has a very poor water quantity (5 minutes x 20 gpm = 100 gallons of water).

So watch out for errors or deliberate misrepresentation about well capacity when buying a property. A true well flow rate is not what we can measure in the building over five minutes, it's the ability of a well to deliver a sustained water flow rate over a longer period, usually measured over 24-hours. When a local health department or building department approve the flow rate of a water well, that rate should have been measured by a plumber or well driller and should represent something more than a five minute test. The standard period over which a well flow rate must be measured varies among communities. Find out what the standard is for your area.

The amount of water that can be pumped out of a well at any given time is limited by the size of the static head and the well recovery or well flow rate, and of course by the pump rate the gallons per minute that the pump itself can or is set to deliver.

Well pumps are usually intended to pump water out of a well slowly enough that the pump and well don't run dry. Some pump systems have fittings that recycle the very last water in the well through the pump, ceasing delivery of it to the building, to protect the pump from overheating.

For these reasons, we've occasionally found clients dissatisfied with their well after they install a new, more powerful water pump. The owners install a more powerful pump to increase water pressure in the home, but the effect may be also to draw water out of the well faster than ever before, thereby disclosing a marginal well flow rate that they had not understood.

For this reason it's a dangerous simplification to simply assert "we can put on a bigger pump" when water flow rate is poor in a building. See WATER PRESSURE LOSS DIAGNOSIS GUIDE for more diagnosis of bad water pressure.

Remember that water quantity (how much water we can obtain) is not the same thing as water pressure (how fast water comes out of the tap). Water quantity comes from what the well can deliver. Water pressure is the amount of force with which the water pump can push water into the building piping and fixtures. Higher water pressure does give us more gallons per minute flow but that's describing a condition at the plumbing fixture. It's not measuring how much water the well can deliver.

A Poor Flow Rate Well Might Seem to Work Acceptably

If our well has a huge static head, say 300 gallons of water, and considering that at most buildings, certainly at residential properties, most water usage occurs in two big surges, in the morning and in the evening (giving the well time to recover between), the well could have a terrible recovery rate, say 1/2 gallon a minute or less, but we might never notice it in the building. We're always running off of the "reserve" or static head.

But over time, as minerals and debris clog those rock fissures that feed water into our well, and if we started with just a small recovery rate of less than a gallon, our well may not continue to deliver the water quantity we need.

A Good Flow Rate or Good Well Recovery Rate is Best

A well with a good recovery rate, flowing at say 5 gpm or more, is more likely to continue to give good service over time, and we might get by with a small static head if the flow rate is good enough.

These are the parameters that a well driller is considering when they decide how deep to go in drilling and how much well flow rate is going to be acceptable.

Well Flow Rates are Not a Simple Number, and Need to Be Measured Over Time

Because ground water typically flows into a drilled well through multiple rock fissures or other underground passages, and because these passages are at different depths, the actual total flow rate into a well is made up of flow from multiple individual openings. Each of these may have its own characteristic flow rate and also flow duration. For example a fissure may flow at a high rate for 20 minutes and then drop to a slow rate or even stop entirely.

This is why the flow rate at a new well is typically measured over a long period, say 24 hours. If you measure the flow rate at a well for just a few minutes, you can have no idea of the well's actual ability to deliver water over any sustained time of usage.

Well Flow Rates Can Change

• Someone drilling a new well nearby can tap into the same water table causing it to drop
  
  
• Local blasting can change the level of a local water table. Local blasting can also change water quality. The well at one of our properties gave seemingly limitless quantity from a shallow 29' deep drilled well from about 1920 up to about 1999. Then the town was improving the nearby roadway and had to do some blasting. The well quantity did not change but suddenly wells along this section of roadway had red silt in their water - it has remained a problem for some home owners in the area.
  
  
• Seasonal water level variations: Local ground water table levels are often lower at different seasons (less in dry weather)
  
  
• Permanent water level shifts & Global Warming: Local ground water tables may drop permanently. In some areas of Florida so much water has been pumped from below ground that salt water has begun to intrude into the aquifer. Changing sea levels due to global warming can be expected to affect coastal drinking water wells by raising the level of salty water.

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• Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.

Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

WELLS CISTERNS & SPRINGS
  Basement Wells
  Cisterns
  Drilled Wells - steel casings
    Drilled Well with Submersible Pump
    Shallow Well with One Line Jet Pump
    Well with Two Line Jet Pump
  Driven Point Wells
  How Much Water is In the Well?
  How to Test Well Water Quantity
  How to Get More Water From a Well
  Hand Dug Wells
  Springs as Water Supply
  Well Pits
  WATER PRESSURE LOSS
  WATER TANK TYPES
  WELL CLEARANCE DISTANCES
  WELL LIFE EXPECTANCY
  WELL PIPING CHECK VALVES
  WELL PIPING FOOT VALVES
  WELL PUMP TYPES & LIFE EXPECTANCY
  WELL PUMP PRIMING PROCEDURE
WELL CHLORINATION SHOCKING PROCEDURE

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