Fine white sand from deep well

My sister-in-law lives on a ridge in northern CA. She has a 255' deep well with a submersible pump pumping into a 10,000 gallon storage tank. Depth of pump and depth of water unknown. The well has produced plenty of excellent water with no problems for 10 years. She recently began seeing a very fine white sand in her teapot, and, eventually, everywhere else in the system. The bottom of the storage tank, she says, is "the most gorgeous white beach you'd ever want to see."

Locals say it's time to "clean the well," but neither of us knows what they mean or how to do it. It's apparently a common issue; they refer to her area as "the sand pit".

Any expert advice on this?

Remove the Sand from the Tank

She should start out by cleaning all of the sand out of the 10,000 gallon tank. It may be that the sand has built up over 10 years and is now at the outlet of the tank.

A 10,000 gallon tank will settle out a lot of sand. There is a way to improve the settling performance of the tank by a factor of 5 or more. If it comes to that, I can tell you how to do it, but it is too much to describe if you don't need or want to do it.

Excellent suggestion; I'll pass it on to her today, and try to find out more specifics about the system from her husband. If that's the case, a simple standpipe at the tank outlet might solve the immediate problem while they find a way to get rid of the sand. Some sort of swimming pool vacuum system comes to mind. Whatever they use, I get the impression the stuff is so fine it will get stirred up pretty easily.

You can clean it out but I'm sure they want to save the water. If the water isn't valuable it could be pumped out and cleaned by getting into the tank.

You mignt be able to circulate it through a swimmming pool filter. Take the water off the bottom with a pipe to collect the sand, put it through the filter, and reintroduce it at the other end of the tank.

You may have to backwash the filter several times.

Other temporary filters are possible if it comes to that.

Tank arrangement can settle the sand

I have calculated the settling rates and you can almost certainly settle out anything larger than 50 microns in a 10,000 gallon tank if they are using typical residential amounts of water.

I don't know the dimensions of the tank, but I assumed 8 ft deep and 167 square feet, 10 degree C water, and quartz or granite sand.

The inlet should be as far away as possible from the outlet, and should be baffled in a way that will minimize the turbulence. A distribution pipe that distributes the water across the width of the tank at very low velocity is the way to minimize the turbulence.

An internal pipe can be used to take the water from the current inlet to the farthest point in the tank.

You can also set up a collector to collect from near the surface. I once designed a floating collector that took the water from about 3" below the surface. That eliminates the floating stuff that gets in the tank and keeps the inlet away from the sand that settles to the bottom.

Thanks, Bob... My guess would be they're using more than typical amounts. It's a rural setting, she's got a large garden area she waters every day, plus some farm animals, a couple of teenage daughters, etc. They also have a couple of filters, but she didn't know what their ratings were. I'll try to talk to my BiL tonight and get some better info. Maybe I can get them to send me a sample, but I gather she's very anxious to get this resolved.

That part about cleaning the well sounds a little fishie to me. I haven't heard of a well that can be cleaned out of sand that didn't simply pump sand all the time.

In my opinion this well has always pumped sand and is like Bob said. Just getting to the outlet of the tank.

bob...

"I gather she's very anxious to get this resolved."

The way to get the most benefit immediately is to:

1. Separate the inlet and outlet pipes, running the inlet pipe as far away as possible from the outlet pipe. Use a large pipe at the discharge of the inlet flow to minimize velocity and turbulence. The last 10 ft could be 3" PVC with DWV fittings. Point it toward the wall about 1 ft away from the wall. Make sure there are no low spots in that pipe so it won't collect sand.

2. Remove the sand from the tank, especially near the outlet.

3. Raise the outlet off bottom as much as practical without giving up too much reservoir volume.

After you do all that work in the tank, try to let it settle a bit before you withdraw water. It will take a few hours to settle the finest material.

If you can't get the sand out now, the most important parts will be to separate the inlet from the outlet, put the inlet far away from the outlet, and raise the outlet above the sand in the tank.

$900 to clean a well...

...and apparently it's fairly common in this area. I offered the "tank is filling up" theory, but it turns out they do clean the tank regularly, the outlet is raised above the bottom of the tank, etc. It's a concrete, cylindrical tank.

The well is actually 220 feet deep, and the (15 year-old) pump is at 210 feet. I suggested they try raising the pump, but that's not a DIY job, and after talking to the local well experts, the well cleaning job was scheduled for this weekend, I think. The theory seems to be that the well itself is filling up with the stuff, so the volume of sand delivered by the pump is rising to a high-enough level that it stays in suspension long enough to be noticeable in the house.

The "sand" is apparently extremely fine. You can draw a clear bottle of water and after a day or so you can see a fine film on the bottom, but it takes a very long time for the stuff to settle out. I'm going to see if they can send me a sample of it.

mike

I drilled a well years ago that had sand that fine. It wasn't silt but it was the finest sand I had ever seen. I used a double 200 gauze screen which is the finest screen you can find. This stuff went right through like it wasn't there. It only happened when the pump first turned on, then it was fine through the rest of the cycle no matter how long it lasted. You could put this sand in the palm of your hand, let it dry then blow it away and it wouldn't hit the ground before it went out of sight.

bob...

Is the inlet separated from the outlet? A 10,000 gallon tanks should settle out a lot of fine material. If they use 1000 gallons per day, the average time in the tank is 10 days.

But if the inlet and outlet are near each other, there is no settling time.

The effectiveness of a settling tank is proportional to the plan-view area of the tank. It is also important to avoid circulation in the tank so the inlet water doesn't immediately "short circuit" to the outlet of the tank.

Still don't know the details of the tank. The well guys are coming this week sometime; she'll try to collect a sample of the sand for me if they pump up a bunch of it. I think her intellectual-curiosity meter is pegged on zero at this point -- she just wants her clean water back.

Epilogue

The well-cleaning guy came and did his thing. Sister-in-law repors that a lot of sand and really disgusting stuff came out, but the bottom line is that she now has plenty of clean water again. She will send me a sample of her "fine white sand" which she now describes as looking like flour. FWIW, a lot of wells in the area are running dry, although hers is apparently in fine shape.