Which is the best method for knowing the yield of a well or aquifer?

Question

Since I am basically looking after all the water needs of a township housing more than 10,000 permanent people in Chennai, India, I need to assess the water yield to manage water supply systems.

Answer 1

Sanjay,
I wish I knew where to find a better source of information.

On a small scale, on farms, we drive points at the perimeter of the area and in the centre. We use rented pumps to draw water on all of them. We measure the volume at each (gallon pails per minute) and time when each draws air. (At that point the water table has been drawn down) We then time how long it takes before a point is productive again and for how long.
Sometimes it takes a fair bit of fuss to get the points at a working depth.
With this information we get a very rough estimates.
The most valuable comes after points draw dry. The recovery time, and following sustained volume gives a good sense of what our draw can be.

We've figured that 1/10 of that volume is pretty sustainable.

On some occasions we've been able to draw 100% of what we've measured. On some occasions only 3-5% are truly sustainable from that point season to season and year to year.


I know that doesn't help you much.

If you have a good sense of the geology of your region, you should be able to do some quick, back-of-napkin estimate.
From the area that you are to draw, identify the location and depth of the aquifer.
If you can identify the source region. (Usually those areas that are higher than yours - at least their aquifer is)
Given that region, and annual precipitation you should be able to get an overall idea. Of course only a small percentage of that is actually available... you can only draw what you can reach and sustain.
It is terribly complicated by the fact that aquifers don't always move in intuitive ways. Rock layers, soil makeup and density all play a role in where water goes and how much get evaporated.

I do wish I could be more helpful

Answer 2

Science Teacher gave a good low-tech answer. And acknowledged it is imperfect. But those were good rules of thumb and a way to go if you don't want to teach yourself (or hire out) a bit of hydrology.

The best (most accurate) would be a pump test. The longer the better.

Crudely, when you first start pumping a well, a very high rate can pumped because of water stored in the casing. Over a long time, a "cone of depression" develops around the well. At a constant pumping rate, the cone continues to change shape and get bigger, but slower and slower as weeks, months and years go by. (Years later, it would typically be effected more by new wells in the area than by its own expanding influence).

The tricky part is assess from short-term data (a few hours, days or weeks) what the next decade will hold. Aquifers vary in their thickness, extent, transmissivity, porosity, storativity, etc. Those parameters are teased out of pump test data and estimated from drilling logs. Any hydrology book (Freeze & Cherry or Todd) will discuss the theory and practice of constant-rate and constant-head pumping tests.

Probably the best single book is Groundwater and Wells by Fletcher Driscoll. Practice, theory, drilling, testing, developing, restoring wells etc.

Answer 3

I have dowsed wells,I never drilled them.Talking to an old timer putting in the well I dowsed.Once, they hit the water and developed the pocket,they added a chemical to stabilize the sides of the well pocket.Then, to determin the gallons per. minute that they could draw, before the level of the well dropped.They put in 8 inch casing,it was for the factory I worked for at the time.They then hooked up a 6 cylinder pump and pumped the well hard for 24 hour's,with this they came up with a figure of 289 gallons per minute,before the level would drop in the well.The old timer laughed,and said it was one of the largest wells he ever brought in.It all started about dowsing.Is it true or false,does it really work.The old timer seen it done before and knew it worked.On another part of the factory property was another well driller,I had dowsed over the area before,I knew water was there, but not the volume I found where the old timer was drilling.I approched the driller,asked about his 1 man rig and did he believe in dowsing.He said it was B.S.,and there is a river near.He said there is water all over here,I agreed and said the spot the old timer was drilling was bigger then where he was drilling.He said B.S.,so I made a gentlemens wager with him, that the old timers well would be bigger then his, when capped.He finished his well in less then 3 days.Both put in 8 inch casing,young guys well, 104 gallons per minute,before the level dropped.When I told him what the old timer said, he had brought in.In his face was disbeleif,he then went and talked to the old timer, and then left.I assume I made my point.I have dowsed wells in FL,UPSTATE,NY.No dry holes yet,going to try in TX. soon.We will see how that goes there.I don't know if this will help you.Its all I know about well volume.

Answer 4

I have encountered such agenda before on a television show, they used the satellite imaging to reveal the contour of the land and drive the pipes in into the lowest part of the land. I believe it was in Africa. And there it was, lot of water.

But for you, if you don't have any way in contacting any Bureau for Satellites, I suggest that you go and ask for the Geographic Map of your area of concern on your City Engineers.

Well, Good luck and hoping that you could find the suitable place.