Drawdown test schematic for a water well (C) Carson Dunlop Associates used with permissionb at InspectApediaHow Deep is the Well?
How to measure water well depth.
     

  • WELL DEPTH, HOW TO MEASURE - CONTENTS: How do we measure the depth of a water well? Use a string and weight to measure well depth. How much water is in the well? How long will the water well last? What is the well recovery rate? Well Flow Rate, Well Yield, & Water Quantity Explained - Problems & Repair Advice for wells.
  • POST a QUESTION or READ FAQs about the procedure to make the basic measurements of a water well: diameter, depth, water level, static head.
  • REFERENCES

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How to measure well depth & how to abandon a well that will no longer be used.

This article series describes how we measure the depth of a water well. We caution that 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, and even artesian wells is not answered just by measuring the depth of the well itself, but still we often need to know this number in order to know how to shock the well or how to prepare pipes, controls, and well pump choices or settings.

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How To Measure the Depth of a Water Well

The sketch at page top, courtesy of Carson Dunlop, outlines what happens during a well drawdown or well flow test procedure. Details are below.

First you have to find the well. Second you need to know what kind of well is installed: driven point, drilled, or hand dug well, for example.

Two Key Questions to Ask About the 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:

  1. Where is the well located? Is the well on the property being purchased?

  2. 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)

Watch out: Measurements like the well depth, well flow rate, well recovery rate are all useful, but taken by themselves some of these numbers can give a false reading about the basic question of how much water is in the well? What we really need to know is the total quantity of water that can be drawn from the well and the quality of that water: is it potable, hard (mineral laden), smelly, dirty, requiring treatment for any aesthetic or health-concern contaminant? See WELL FLOW RATE and see WELL FLOW TEST PROCEDURE.

How to Find the Depth of a Well - String and Weight Method

The Water Ace Pump Company offers this nice, simple well depth measurement procedure that works for drilled wells or hand dug wells. We have added some details.

Tie a small but heavy weight to the end of a piece of string (be sure there is enough string; some wells are very deep). Lower the weight into the well until it reaches the bottom.

Take up the slack and mark the string at ground level. Pull the weight out of the well and measure from the bottom of the weight to the ground level mark. This is the depth of your well.

For relatively shallow wells, 30' or less, we sometimes use a tape measure directly, but a weight on a string assures that the line used to make a measurement is absolutely straight, so more accurate.

Watch out: be careful putting anything into your well, that you don't send down a weight that gets hooked on pipes or valves in the well bottom. Use a small fishing weight or a rounded object and tie it carefully.

Watch out: this procedure is important for matching the well pump capacity to the physical depth of the well opening in the ground. But it does not tell us much about how much water is actually in the well. Except for artesian wells, the top of the water level in the well will virtually always be some depth lower than the top of the ground surface.

Reader Question: how do I measure the depth of the steel casing part of my well?

The current well casing is 6” steel and rusting, evidenced in the drinking water How do I determined the length of this casing? - J.D. 6/8/2013

Reply: string and magnet method; well camera method

My first preference in finding out details about a drilled well using a steel casing is to look in the building for notes about the well location, depth, and specifications; often the driller or plumber provides that data in writing and it may also be written on a wall or ceiling joist in a basement or crawl area where the well pipe enters the building, or at the water tank and well pump. I've also had success calling local well drillers to find out who drilled the well as usually the company keeps well drilling record.

But if you are left with no information and need to know just the depth of the steel well casing (as opposed to the total well depth) try this procedure, working carefully to avoid dropping anything into the well or hanging up your cord and weight-magnet.

  • Pick up a strong alnico magnet, tie it to a strong but light nylon rope,.
  • Lower the magnet lower into the well center until you are sure you are below the casing, or to well bottom.

Watch out to avoid hanging the magnet up on the well pump, pipe connections, wiring. I like a round or cone-shaped magnet that is less likely to get caught up, and one that is just an inch or so in diameter;

  • Pull the magnet up slowly until you feel when magnet first grips the steel well casing
  • Mark rope at well top (I used a magic marker)
  • Pull the magnet and rope out of the well completel.
  • Measure the rope length between the casing-top-mark and the magnet.
  • This approch may take a couple of tries.

Watch out: Don't drop stuff on the well.

An alternative is to hire a well service company who offer an inspection camera service. Such cameras are lowered into the well to inspect for casing cracks, rust holes, damage;

Matching Well Pump Capacity to Well Depth

Water Ace continues:

Subtract five feet from the depth of your well. This number should not exceed the maximum rated depth for your pump. If it does, it will greatly hinder or prevent the proper operation of the pump.

FYI the five foot deduction from the physical well depth used to estimate pump capacitu is assuming that the water pick-up end of your in-well piping will be held about five feet off of the physical bottom of the well. That's because placing the water pick-up too close to the very bottom of the well risks picking up silt and debris, clogging the foot valve, piping, and water pump

Well Abandonment / Bore-hole Sealing Procedures for Wells Taken out of Service

The steps you need to take to seal off a water well or to de-commission a well - this topic has been moved and expanded and is now found at WELL ABANDONMENT PROCEDURE

 

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 REPAIR GUIDE 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, WELL FLOW TEST PROCEDURE, 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.

 

Continue reading at WELL LOCATION, HOW TO FIND or select a topic from the More Reading links shown below.

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WELL DEPTH, HOW TO MEASURE at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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