CaCO3 Scaling in Wells 1

[sewerratt]

Any ideas why I now have screens, columns and shafts scaling in a raw water supply well? When the initial water testing was done less than five years ago, none of the parameters were out of the ordinary. Is it possible for groundwater chemistry to change that quickly? These are 400-600ft wells with water age (according to the experts) in the 10k yr range. Pumping rates have not been excessive. If the water composition hasnt changed significantly, what are some things that can cause CaCO3 to start precipitating? Stray currents? Mixing with treated potable water? (the pump/shaft is water lubed from a potable source since there was not enough pressure topside from the raw water discharge, but it is not a large amount). Testing on the exact composition of the precipitate (initial determination is a carbonate complex, predominately CaCO3 with some iron and MgCO3) is being conducted now.

Any thoughts would be appreciated.

 

[thewellguy]

Yes, it is possible for water quality to change or time. You stated that the pumping rate have not been excessive. What does that mean. Also this is not uncommon for the scale to develop. My best guess it that it is due to the water levels in the wells changing when the pump is on and off. This gives the water a chance to mix with air and this could cause the scale to form.

 

[bimr]

The reason that carbonate incrustation usually occurs is that the water level in the well has decreased. If you have an unconfined aquifer, the water table is drawn down to produce the necessary gradient to cause water to flow into the well. The hydrostatic pressure is decreased in the water-bearing formation, with the greatest change being at the well. Because of the reduction in pressure, some of the carbon dioxide has been released, which causes an increase in pH, and carbonate scaling. Have you compared the pH of the water at the present time to the past? Have you compared the water level now to the past?

 

[sewerratt]

The aquifers are confined. The static water level is about 100 ft below grade. The pumping-rate-excessive comment was to indicate we haven't been drawing more water out of it than we always have of other wells in the area, meaning pulling so much water that we were getting 'different' water than we initially sampled. I guess I was not so clear. Anyway, we have multiple screens in two different aquifers, with one aquifer at 350-450 and the other from 500-600 ft deep. Water sampling from a test well at the site during design showed similar pHs and mineral contents in the two aquifers. Some of our consultants are saying it is the mixing of different pHs from the two aquifers that is causing the precipitate, but the initial sampling, and current field sampling doesnt bear that out. pH is different, but not dramatically so. We're doing further testing. The CO2 coming out of solution due to pressure change is interesting. We'll investigate that further.

Thanks for the input. Keep it coming

 

[thewellguy]

How deep is the pump set? What is the non-pumping and pumping levels? Is the water level low enough that the top screen is being exposed and the water is cascading into the water below and airating the water?

 

[sewerratt]

Top screen is near 300 ft. Pump is around 200. Static level is around 100. At the 1000gpm flow, drawdown is about 80 ft. Water is not cascading. Screens are all below the pump. The pressure in the two aquifers is different, but I don't have those numbers.

Thanks again

 

[bimr]

Calcium carbonate can be carried in solution in proportion to the amount of dissolved carbon dioxide in the water. The ability of the water to hold carbon dioxide in solution varies with pressure. The higher the pressure, the higher the concentration of carbon dioxide. So if you are doing anything that reduces the pressure, then you will causing the precipitation of carbonate.

One would think that if the well screen was fouled, you may be releasing the pressure across the screen.

 

[bimr]

I should have also added that pumping a well in a confined aquifer allows a similiar reduction in pressure and resulting precipitation.