SS groundwater well screen 1

[6240]

I have some people that want to install impressed current cathodic protection for groundwater monitoring wells with stainless steel screens. However, the monitoring wells are supposed to monitor for metals. I think that many of the metals in the groundwater will be plated on the anode or cathode, which will eliminate (decrease) the concentration of the metals (if any in the GW) and defeats the purpose of moniotring. I am for drilling new wells with PVC screens. Please, explain if my chemistry/physics is totally off in this case.

 

[mcguire]

You are correct. Not only is cathodic protection unnecessary for the stainless, the applied potential will distort monitoring efforts. Skip the cathodic protection.

 

[BarryMartin]

You may have a good point. If you have an impressed current system installed, you will deposit materials onto cathodic sites. The question remains: How significantly will your monitoring results be effected?

I do not understand the equipment arrangement. I do not know what is being cathodically protected (i.e., in proposed system). I am assuming that you have a metallic well casing (probably carbon steel) that is the primary material to be protected. If the outside of the casing is being protected, then an internal screen is not going to get much current and not cause a significant problem.

If the well casing is a well coated carbon steel (i.e., with a non-conductive coating like paint), then the amount of current required for protection of holidays is probably minor, perhaps 1~3% of total surface. This would result in minimal effect on the accuracy of your monitoring.

If the stainless steel screen were electrically insulated from the primary circuit being protected, then you would not deposit any metals electrochemically. A dielectric (i.e., electrically insulating material like micarta, rubber, etc) could be used.

 

[jre]

(New Zealand)

I have been making and installing shallow water bores for some years using s/s screen over all PVC components and suction pipe (down to 6.5m max). These are normally uncased but I have used 100 or 150mm PVC casing where needed. MY experience is that as soon as you introduce a dissimilar metal, either corrosion will occure, or metallic elements in the water (ie iron) will be attracted to the screen and may eventually block it.

 

[6240]

I want to thank the people that tried to answer my question. However, I did not get the info I needed. My personal problem with the set up is that I have never heard anybody install cathodic protection for the stainless steel (SS 304) casing and screen of a groundwater monitoring well. I hoped that may be somebody with experience (cathodic protection) could tell me if this was done before.
I will try to explain the set up in more details. The installation has many groundwater monitoring wells most of which were installed in 1988 to 1990. Some of the wells have only stainless steel casing and screen. Others have combination of PVC and stainless steel casing and stainless steel screen. The installation claims that they get Chromium in their groundwater because the stainless steel in corroding (not because they have contamination). So, they make up the solution of catodic protection of the stainless steel to prevent further corrosion of the stainless steel casing and screen. The design has one cathod inside of the well for cathodic protection of the screen, and another outside the well to take care of the casing (again I have never seen cathodic protection with two cathodes). Now, in order to have proper contacts with the casing, a wire has to be soldered (welded) to the satinless steel casing. How exactly they are going to lower a welding device down the well to about 50-60 feet (some of the wells have PVC near the surface), start it, and weld, is different story (very vague).
The most important point for me it is engineering feasible to have cathodic protection at 300 to 600 ft below surface (that's how deep the monitoring wells are), and is it possible that such design could change the chemistry of the groundwater which is monitored. I know about cathodic protection from the USTs (underground storage tanks) and ASTs (aboveground storage tanks), and their piping but I have never heard of such protection for groundwater wells.
Thanks again.