Dug well showing rocky sides (C) Daniel Friedman Alan Starkman Hand Dug Water Well Depth
When is the Well Deep Enough?
     


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This article series offers advice for Hand Dug Water Wells and the sanitation and maintenance concerns with this water supply type. This article describes the process of digging a well to provide usable water and the steps taken to make the well safe and sanitary.

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The Dug Well at 11 Meters Deep - Deciding When Deep is Deep-Enough

Dug well showing rocky sides (C) Daniel Friedman Alan Starkman Alvin Starkman M.A., LL.B., Casa Machaya, Oaxaca Bed and Breakfast.

This article describes the process of digging a well to provide usable water and the steps taken to make the well safe and sanitary. We include both technical advice and a description of the practical problems that one must encounter and overcome in providing usable water in an area where public water supply is absent or limited.

[Click to enlarge any image]

Readers should also review Hand Dug Wells what are they, can they be sanitary and safe?

Fermín lived up to billing, in short order digging down to over 11 meters. [In the U.S. and Canada most dug wells stop at about 10 meters or around 30 feet deep.]

It took over an hour every morning to pump out the water from the night before.  Then Fermín disappeared.  He just didn’t show up one Monday morning, and didn’t answer his cellular for a few days.  Eventually he did respond, and advised that he’d return in two weeks.  Five weeks later he was back, advising that we’d probably have to go another three meters down.

After three or four days of digging, Fermín told me that he would not be returning.  He said that it was now taking him about two hours in the morning to pump out the water before he could begin digging, and that there was about five meters of water to be removed.

Our well photo (left) shows the rocky sides thorough which Fermín had been chiseling.

To Fermín that signified that the well would produce sufficient water for our requirements and that there was no need to dig further.  To me that meant we had ourselves an honest pozero who could have continued (he didn’t have another job pending, since my friend had altered his plans regarding using Fermín in the foreseeable future), but let us know it wasn’t necessary for him to continue.

As is often the custom with trades in Oaxaca, a brief “discussion” ensured about how much we owed him (measuring his progress by lowering a stone attached to a rope to the bottom of the well, and then using a tape measure), followed by the friendly departure of Fermín & Son.

How Much Water Will our Dug Well Deliver?

Large water storage tank (C) Daniel Friedman

Question: how many liters of water can the well produce in a 24-hour period

I'm at about 10 meters depth, well diameter about 1.4 meters.  At 4 pm when the digger stops for the day (by hand hammering through rock with a mallet and chisel), he drains the water. 

At 9 am the next day the well has 1.6 meters of water in it. I intend to complete digging further into the dry season.  However, based on the above data, how many liters of water can the well produce in a 24 hour period? 

By way of update, the pozero returned at about 11.15 meters, and said we'd need another three meters or so.  Then a week later he said he's finished, that in the morning there is 5 meters of water and it takes two hours to pump it out, so for sure we don't need more.

Our photo (left) shows a plastic water storage tank under construction in San Miguel de Allende, Mexico. Where a well flow rate is limited, water storage is often necessary to provide good water flow to the building served.

Answer: well water quantity is a rather easy calculation, if you have the data and know what the measurements mean

Most code compliance and health officials want to see 3-5 gallons per minute. You've got about 0.6 gpm. We would try to keep good potable water piping out of contact with other non potable piping - the concern is that events like loss of pump or pressure can back-contaminate between the two. Also, a submersible pump will be the most powerful way to lift water - depending on how high you need to raise it you might want that.

An accurate answer to th question of how much water is in a well needs to address both the static head or volume of water in a well when it is at rest and not in use (see Static Head of Water in the Well), as well as the flow rate or yield: the rate at which water flows into the well opening (see WELL YIELD DEFINITION). This is also called the well recovery rate. Together these tell us how much water is actually available from a well -the WELL QUANTITY TOTAL.

At WELL FLOW RATE we provide a detailed calculation of how much water we expect this dug well to yield, measured in gallons per day, per hour, or per minute of water flow. Excerpts from that article are just below:

Well Flow Rate = gph

The Well Flow Rate for a water well of any type is normally expressed in gallons per hour or gph that water enters the well from surrounding soils. We calculate a well's flow rate measured in gph by dividing the quantity of water in the well (we calculated that just above, right?) by the number of hours it took for that water to enter the well.

Well Flow Rate = Gallons / Hours - or gph, gallons per hour or water flow rate into the well, provided that no one is taking water out of the well during this same interval.

Mr. Starkman made it easy by telling us that the well was pumped dry at 4PM. He measured the water depth and well diameter and kept track of how much time had passed (17 hours).

From our well volume formulas above we know that starting with zero water, after 17 hours the well contained 650 gallons of water.

For this example, 650gallons / 17hours = 38 gallons per hour (gph) - this is the well flow rate for a 17 hour period for this particular well.

Converting the well flow rate in gph to flow rate in gallons per minute is important in matching the well pump rate to the well's safe yield. (See Define Safe Yield for a Well).

Well Flow Rategpm = Well Flow Rategph / 60

For our dug well example, 38 gph / 60 = 0.63 gpm - this is the measured well flow rate in gallons per minute.

In this case that's a weak, marginal well flow rate - just over half a gallon per minute. In the U.S. most building or health departments who must approve a private well water supply when issuing a final certificate of occupancy for new construction want to see 3 to 5 gallons per minute or 3-5 gpm.

Watch out: if you install a pump whose pumping rate exceeds the well yield or flow rate (see WELL YIELD DEFINITION), the pump may run dry and be damaged. The risk of pump damage is greater in a well that has a small static head (see Static Head of Water in the Well) or in conditions under which the pump is left running for long periods so that the static head is likely to be exhausted. If you have this risk or this problem on a well, see the advice on protecting pumps given at WELL PIPING TAIL PIECE.

Pumps & Controls: Moving Water from the Dug Well to Point of Use

Types of water pumps (C) Carson Dunlop AssociatesI have to work through decisions regarding pumping directly into our 17,000 liter cistern, or first into a smaller cistern and then up to the main one (in case we can drink it with or without treatment I wouldn't want to mix it with the unsanitary municipal water), or have "Y" tubing so I can divert into a smaller cistern whenever I want OR send it directly to the main cistern, etc.

Note on pumps: for pushing water from a dug well up a steep hill to point of use, you will probably want a pump with good lift capacity, such as a submersible unit, combined with a water pressure tank or additional water storage higher on the property.

At left our sketch of a types of well water pumps is courtesy of Carson Dunlop Associates. The drawing shows the key differences between a one line jet pump, two line jet pump, and a submersible water pump.

See WATER PUMP LIFE EXPECTANCY for a discussion of types of well pumps.

Also see WATER TREATMENT EQUIPMENT CHOICES for alternative methods of assuring that water from a dug well remains sanitary and potable, and see WATER PUMP LIFE EXPECTANCY for choices on methods for moving water from a dug well to storage tanks or to the point of use.

 

Continue reading at WELL ARCH: BUILDING or select a topic from the More Reading links shown below.

Or see HAND DUG WELL PROCEDURE article start

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DUG WELL DEPTH: HOW DEEP IS ENOUGH? at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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