Well Drilling

[keithkarmann]

Hello everybody, just moved to a small mooban near Sukothai and I can see we are going to have problems with getting sufficient water. So I am considering getting a well drilling company in to drill for water. A friend in a neighbouring mooban has just had a company drill a bore hole on his property and found water at 90 meters. The first borehole went down to 120 meters and did not produce water and these boreholes can't be more than 30 meters apart. What I would like to know, what is the chances of the borehole that produces water, producing sufficient water bearing in mind that it is 90 meters deep and a neighbouring borehole at 120 meters has no water. If I got the contractors in to drill a bore hole it would need to produce, in the very dry season around 400 liters minimum a day. The cost for a bore hole with gauranteed water (but how much water?) is 100,000 Baht. 90 meters seems like a long way down and what size pump would it require to pump water to the surface that is 90 meters down? Thanks in advance. Keith

                       

[impulse]

I can't answer for the drilling, but for the pump size, you need more information.  The water zone may be at 90 meters, but depending on the aquifer pressure, the water level inside your well may actually come all the way to surface and flow with no pump at all (in which case, it's referred to as an Artesian well). 

 

To size the pump properly, you need to know what the pressure is in the aquifer, and how much that pressure varies with season and with the production rate.  For example, normal pressure you'd expect in an untapped aquifer at 90 meters is 90 x 3.28 x 0.433 = 128 PSI, which will support a column of fluid all the way to surface.  If the fluid level inside your well is at the surface, there is no need to set your pump much below the surface.  

 

But if 1000 of your neighbors are tapped into that same aquifer, the pressure may be nowhere near 128 PSI and you may have to set your pump as low as 90 meters.

 

Bottom line, pay for some knowledge from someone who knows the area, and has seen what happens when they drill in your area.  You will save money in the long run.  Not much is more frustrating than setting a pump in the wet season when nobody's tapping the aquifer, then finding out you set it too high when the dry season arrives.  You'll have to reset the pump deeper, AND probably need a bigger pump.  Ouch.

 

[owl sees all]

Hi Keith

 

Good questions indeed.

 

There are a few points in the post that's worth addressing.

 

Firstly the drilling depth. 120 metres, 90 metres. *Dry season matters little at those depths.

 

Underground there are lakes, rivers and strata of both permeable and impermeable ground. Water flows about/around and it generally goes deeper and deeper. However; a borehole at 100 metres could hit a lake and the underground surrounding pressures could force the water up the bore. Look up 'artesian well' on Wiki. In some cases the water will spurt above the ground.

 

*Now there is a conflict between the 'dry season' having little effect because the water table could rise a long way from your bore hole and increase pressure; forcing water in your bore-hole higher until equilibrium is attained. Rain, streams etc. might well run to a strata that is only a few metres down but will disappear in the 'dry season'.

 

Look at your surrounding topology (terrain). Think about if you were water where would you go. Check out other areas up to 2km away and see the way the land lies.

 

As for costing; 10k a bore hole is cheap, 100k is ludicrously expensive.

 

Delivery rates (extraction) should be in the order of 1 to 4 litres per second. So even at one litre per second it wouldn't take too long to get a week's supply into a decent sized tank.

 

Different ways of getting water out of the ground; Thai visa is a good place to start for info; especially on pumps.

 

I personally have drilled 6 bore holes (although one has had a cave-in rendering it unusable) and they all have different  characteristics from each other and through the year.

 

 

[impulse]

1 hour ago, owl sees all said:

Hi Keith

 

Good questions indeed.

 

There are a few points in the post that's worth addressing.

 

Firstly the drilling depth. 120 metres, 90 metres. *Dry season matters little at those depths.

 

Dry season matters at those depths only because other offsetting wells may be pumping to beat the band during dry season.  Unless you have a very strong recharge mechanism, the local pressure can drop by quite a bit when (for example) nearby farmers fire up their 1000 GPM pumps. 

 

But mostly, my point is (and you agree) that the pump size and set depth need to be designed for the entire yearly and even longer cycles of drought, and not just the water level on the day you punch the well.  The day your well's water level drops below your pump intake is a bad time to figure out you set it too shallow.  Because you may be waiting in a long line with others who have the same issue.

 

 

Edited April 11, 2018 by impulse

[owl sees all]

8 hours ago, impulse said:

The day your well's water level drops below your pump intake is a bad time to figure out you set it too shallow.  Because you may be waiting in a long line with others who have the same issue.

I found this out -  to my cost - a couple of years ago. We have 900+ oil palms and when I was designing the irrigation system I had a couple of test holes done at the top of the land which is 7 metres higher over 260 metres. Couldn't get the water, so decided to bore at the bottom (roughly) and lay a 40mm pipe up the hill, to fill another 6000 litre tank, which then ran down the hill. BBeing driven by a deep well pump (DWP).

 

The bore hole was 55 metres deep. and the water at the end of the wet season was at a depth 5 metres; which did not drop significantly at 3 litres a second extraction, over a two hour period on testing. I reckoned the dry season would lower the water table a bit so I set the DWP 16 metres.

 

Everything went swimmingly until March of following year. I set the DWP (running off a generator) and was watering about 100 trees. I had perfect equilibrium; the water going up the hill and into the tanks equaled the amount had being used for irrigation. I was going to pop back in two hours and switch the valves to do a different batch of palms.

 

When I did get back the generator had stopped. I checked it over and restarted. It stopped again under pressure. I spent a couple of hours investigating. I had burnt out the DWP. It took me ages to suss out the problem. The pump must have been going through phases of wet and dry; a real killer for a DWP. I checked the water table and it was at 10 metres but at that time of year the amount of water I was extracting couldn't be replenished quick enough.

 

Replaced the DWP and set it at 28 metres. No prob's since.

 

Just as an addendum; if the Mekong floods the water table, a week later, has risen about 2 metres. And we are 20K from the river.