Geotextile in a water well to prevent sand intrusion.

[isslandboy]

Hello,

I have a question regarding the use of geotextile in a well.

We have a hand dug 30 inch X 14 foot well very near the coast, (50 meters) the soil is almost pure sand.

We are getting a lot of sand in the filter (5 micron), so we are wondering if we were to use geotextile membrane if this would help alleviate the problem, and if so should it be placed above the rock or below directly on the soil surface.
In the wet season the water in the well comes back after being depleted at a rate of about 3 feet an hour, or about 100 gallons per hour, in the dry season it is about half of that amount.

Respectfully, Greg

 

[1503-44]

5 microns seems like it would pass a geotextile filter. And if it did not, then you would just get the same clogging problem in the geotex, but it would be down in the well. Sounds easier to trap it in a surface filter.

If you want to to try a down hole cure, it is more common to pack gravel, gushed gravel and larger sand grains into the well itself to filter those smaller particles.

Another potential cure might be to reduce the well pumpout flow rate. You may be washing those particles from the well surrounding soils into the well by having a flow rate that is just simply too high..



--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[swazimatt]

I agree with 1503-44 you may be "sucking" too hard. if possible you could increase the well size with filter fabric on the ouside of the excavation with washed stone between that an the pump, effectively increasing the flow area from the soil. If you just use a filter fabric, what gets stuck will ultimately clog the fabric - which is why it would be placed against the sand as this creates the maximum area of filter fabric, reducing the time for it to get fully clogged.

Another alternative is to have a settlement tank as your first tank to catch the smaller particles and let them settle (possibly even include floculant

 

[Valvecrazy]

I never tried any filter sock or membrane that didn't clog. Sand makes a good media for a filter. I would cap the bottom end of a piece of 6" well casing with .020 slots. Sticking the capped end as deep in the well as possible, I would fill the rest of the well with gravel pack material that is as small or smaller than the sand, just slightly larger than the .020 slots. The smaller sand will wedge in the larger sand and make a media filter out of the well itself.

 

[LittleInch]

What material is the well bore made out of?

If you hand dug a 14ft deep well only 30 inches in diameter you much have some small people....

Or took a huge risk of collapse?

"so should it be placed above the rock or below directly on the soil surface." Eh?? This doesn't make any sense I'm afraid. What does it refer to and what has it got to do with the well? Can you sketch out what you have at the moment and also what flow rate you have or how you use it - sounds a bit like you empty the well then let it fill back up?

Like others are saying 5 microns looks very much like a very fine filter. Maybe you need a cyclone type instead and send the waste back to the sea.

Also that close to the coast are you not getting brackish water at best?



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[isslandboy]

Great info guys, thanks a bunch. Your comments certainly made me think of the situation from a different perspective.

 

[isslandboy]

In response to LittleInch.

The well casing is a 30 diameter double wall corrugated plastic pipe. A hole about 3 feet deep is dug initially, the pipe is then placed in that hole and the smallest guy on the crew gets inside and digs out the sand (it is bucketed to the top of the pipe and discarded), as he digs out the sand the pipe slowly keeps sinking lower and lower, no risk of collapse.

Above the rock (I should have said gravel) or below refers to where to place the membrane, directly on the bottom of the well on the sand, or back fill with gravel and then place the membrane, what I was thinking is if the membrane was above the gravel, when the pumping action stops any sand the had collected on the membrane would fall back down amongst the gravel (sort of self flushing). I hope that makes more sense.

In the rainy season the well does not deplete, but in the dry season we will only get about 3 feet of water max height in the well (about 100 gallons)when we irrigate we can use up the total volume of water in the well. We have 2-2100 l tanks for a buffer. In the dry season it will recharge at about 25 gallons per hour. We are using a simple sump pump to move the water to the tanks.

The water is sweet, no treatment other than filtering.

AS stated by you and others 5 microns is pretty fine, so I'm thinking of trying some coarser filters, we used to get about 3 months to a filter, but the last time it only lasted about a month, so not really sure what happened.

 

[LittleInch]

Ah, Ok, that makes more sense.

So does all the water simply emerge from the sand layer at the bottom of the well?

How do you get the water out? With a hose and "suck" it up or a submersible pump?

You might be better off sinking an internal 24" perforated pipe or maybe a cage say 4 or 5 ft high and pumping out from that with a cover of your geotextile.

Then just pull it out every now and then and replace the geotextile?

If you're going for the alternative I would have initially the rocks / gravel and then the geotex just held in there, but able to be pulled and removed. If it gets clogged then your well is dead unless you remove all the rocks / gravel above the geo tex.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[isslandboy]

LittleInch

Yes the water just permeates up from the water table.

We suck it up with a common sump pump,

We are afraid to go much deeper as the locals tell us we will hit brackish water (no one can tell us a what level or who it has ever happened to, they have also told us the level of the well will go up and down with the tide, I have spent many hours monitoring this and the tide has no effect on our well) and as this is our only source of water we are a bit afraid to push the envelope.

I think what I may try is to make a "hoop" out of some 1/2 inch plastic electrical tubing that fits snugly in the 30 inch pipe, sew the geotextile onto the "hoop" with some hooks attached so that I can pull it out as needed and replace the geotextile. Will also consider a smaller pump (currently 3/4 horse) so as to draw from the well a bit more slowly.

Thanks for the feedback it has helped immensely!

 

[bimr]

The well appears to involve a lot or work.

Why aren't you using a tradition driven well point system?

 

[isslandboy]

In response to bimr,

The well gives us extra storage, and I'm not sure if the system you are talking about would give us enough flow. Thanks for the question, it did get me thinking about using well point system to determine at what depth one would encounter brackish water.

 

[GeoEnvGuy]

Is there an issue with putting in filter sand and gravel a the base of the well in 3 inch lifts by hand? I wouldn't put geotextile right next to the sump pump as it might clog the intake to the pump. If you do want the geotextile around the intake you could form the geotextile into a sock and attach it, when it clogs you can yank the pump and hose off or replace the geotextile.

 

[isslandboy]

GeoEnvy Guy

They soil around us is pretty much pure sand, we did put some rock in, but it is larger 1 to 2 inch chunks, we also made a "sock" for the pump. When we get into late dry season we will have the guys come and clean out the excess sand that has accumulated (we have to wait so the water level and recharging of the well slows down. At that time we are going to implement some of the suggestions from this group.

Remove the geotex from the bottom of the well next to the sand.
Install a layer (6 inches?) of 1/4 inch crushed rock, tamped as well as possible.
Re-install the 1 to 2 inch rock
Build a removable frome to mount the geotex, install next to the larger rock.
Install the pump with a geotex sock (possible doubled up.

Thoughts or suggestions?