It will be impossible to tell you what reactions are present without knowing what compounds are present.

Are you using bleach as your chlorine source? Or are you using something else?

How much arsenic is in the water before and after treatment.
Water pH before treatment? What is your ion exchange medium?

thanks alice...for your response.

Yes I am injecting a solution of bleach.
The arsenic level changes but I think the highest one tested was 0.1ppm and the IE system was sized for 10x that amount. I always get 0 (undetected) levels in the treated water.
The pH is around 7.2 and the resin its a Purolite A300 (1 cu. feet)

Okay, initially, the reaction between bleach and Arsenic is:

H3AsO3 + NaClO --> H2AsO4- + Na+ + Cl- + H+

Just looking at the equation above, you might assume that one molecule of bleach plus is all it takes to oxidize one molecule of arsenic. However, it doesn't quite work that way. 1:1 ratio will only oxidize 70% of your arsenic. You need at least 3 molecules of bleach to 1 molecule of arsenic to get the reaction to go to completion, oxidizing all of your arsenic.

You appear to have that covered by using an excess of bleach - that's a good safety measure as it ensures that even if your arsenic were to rise as high as 0.7 ppm you would still oxidize all of it.

Now, as to where your chlorine is going, I failed to ask a pertinent questions, which is: What are you using to regenerate your resin?

If I assume you are using NaOH(caustic soda), this places your resin in an OH form, which will grab any negatively charged ions it sees, including Cl-, explaining the disappearance of your chlorine. If you are regenerating with something else, a different explanation will be required.

Thanks again.

I use the cheap salt for regeneration. The motrin green bag (~$5/50lbs). I think that would make it NaCl.

So, the oxidation takes place in the retention tank, right? and the chlorine I am testing is the Cl- from your equation above? (I use a total chlorine test kit).

Also is it good or bad to let that chlorine reach the resin? I say that because it seems like the previous owner had a BB20 filter between the tank and the IE system and I think he was removing the chlorine that way.

One last thing I didn't mention because I though it was irrelevant: Mixed in the bleach solution I add Soda Ash (Na2CO3) to make sure my water is neutral. My well pH is only 5.5.

You got saddled with some nice water, didn't you?

A total chlorine kit should measure all forms of chlorine in your water, not just the free chlorine(Cl-). The sodium thiosulfate reagent reduces (basically, breaks down) the chlorinated compounds down to free chlorine and then you measure free chlorine.

Okay, a couple more questions:

Do you have a more complete water analysis?

When you drop reagent in your sample what happens, precisely? Does it start clear and stay that way? Does it flash pink then go to clear?

Tell me how you go about taking your sample, how long between sampling and testing?

pH after retention tank?
pH after IE?

Yes. Wait until you hear about the level of the radon on my water. it is out of the chart. So the last stage in my water treatment system is an aeration. I also have a softener just before injecting (Soda Ash + chlorine) solution.

I inherited the system but yes I did full blown water analysis before buying the house. I copied the pertinent results underneath

First I wanted the local water treatment company who designed the system to keep maintaining it but I was not satisfied by their service. None of the technicians the send knew actually how the system works and forget about giving me any explanation. so I decided to take over myself. I found out they were charging me double the price for water testing using the same independent lab I am using now (for Arsenic).

Anyway here are my responses.

After retention tank:

There is a spigot special for sampling water at that point. first I make sure the water has been sitting in the tank for while (15min at least), I open the spigot let the water run (~15sec.) to empty the pipe and get the one in the tank. Then I test the sample immediately. the water turns yellow at the contact of the first reagent drop.
PH 7-7.5

After the IE: Same procedure. The water doesn't change color at all. Same pH.

Please don't forget my questions in the previous message.
thanks very much

++++++++++++++++++++++++++++++
Calcium, MG/L 13.7
copper, MG/L 0.036
Iron, MG/L 0.1
Magnesium, MG/L 4.8
Manganese, MG/L 0.01
Silica as SiO2, MG/L 16.7
Alkalinity, MG/L 24
Chloride, MG/L 25.1
Hardness, Total, MG/L 54
Nitrate as N, MG/L 0.78
pH, PH AT 25C 5.5
Sulfate, MG/L 8.5
Turbidity, NTU 1.12

Thank you for the information - just needed to ensure all bases were covered. You are sampling correctly - doing a better job than most.

First, your active chlorine, as ClO- is attaching to your resin, which is why it is disappearing after the EI.

You also asked if the bleach should be removed prior to IE. Yes, it should. Bleach will cause de-crosslinking of the resin components. You resin in comprised of Gel polystyrene crosslinked with divinylbenzene. Break the crosslinking and you no longer have nice plastic beads - they begin to get soft and break apart. It takes time to cause a problem, but it will happen.

A quick fix would be to install a cartridge GAC filter between your retention tank and your EI to adsorb the bleach. A longer-term solution would be a fixed bed GAC filter, appropriately sized.

Thank you aliceinwonderland.