Arsenic adsorption 1

[Bajanbuilder]

Hello all

A site which used to be an old orchard is producing soil samples with 100 ppm arsenic concentrations. Let us assume this is "available" arsenic, as in, if possible it will enter water solution.

What would be an effective calculation method to show this "available" arsenic's solubility in groundwater?

Thank you for your time.

 

[greenone]

Can't you collect a water sample and get an actual, instead of a calculated concentration? How deep does the contamination penetrate? What kinds of soils? All these variables will affect the outcome.
Or are you concerned with runoff, rather than groundwater impact?

 

[Bajanbuilder]

Yes I am more concerned with runoff, i should have stated that instead of saying groundwater. This is part of the analysis in that the town is looking for a calculated method to show how much of this "available" arsenic,(no concern for arsenite or arsenate, will enter solution.

Basically a proof of the amount of arsenic not bound to soil (available) 100ppm which "would" enter solution.

 

[Bajanbuilder]

Let me add that I am aware that arsenic occurs in various forms before it undergoes desorption. This can be triggered by the reduction of Fe 3 to Fe 2 oxides in the soil for example.

My problem is that a general solubility calculation is required for this situation, provided only with an available arsenic conc of 100 ppm for an old orchard soil.
I am not sure what form of arsenic to use or how to structure this calculation.

Any suggestions on how to look at this are appreciated.

 

[fattdad]

Total concentration of arsenic is no indication of what may become available to either ground or surface water. The exercize can only be misleading.

Using the TCLP (toxicity characteristic leaching proceedure), the maximum level would be 1/20th of the total concentration of ANY total metal concentration. Add to this the soil's natural CEC (cation exchange capacity) and you have too may unanswered variables to provide anything meaningful.

As a relavent aside, do you have any idea of natural background arsenic levels in the area? Here in Central Virginia, natural arsenic levels can often be elevated owing to the parent rock mineralogy (i.e., the arsenic is so locked into the soil minerology that it never went anywhere).

Good luck, just trying to help.

f-d

¡papá gordo ain’t no madre flaca!

 

[soiledup]

Fattdad - I agree totally. Background levels crucial to arguing this case away. If it's naturally occuring why try to remediate - excuse spelling I'm in rush

 

[Bajanbuilder]

Thank you both for your contributions. Fattdad I'm glad you outlined in that way, as I have been at a loss really for this. The general theme was that town inspectors were concerned about arsenic entering runoff and wanted us to come up with a simple proof using a concentration in soil, to produce a concentration in water. It seemed they were expecting something chemical, and I have never dealt with this before.

 

[Bajanbuilder]

Here is some additional background

Lead Arsenate was a popular insecticide during the first half of the 20th century because of its low toxicity to plants and great effectiveness for controlling insect pests. The most common use was for control of codling moth on commercial apple orchards. (THIS IS MY CASE) Repeated applications of lead arsenate over time cause lead and arsenic to accumulate in soil. Soil lead and arsenic concentrations vary considerably in former orchard sites because applicators using hand-held sprayers applied lead arsenate individually to trees. Higher concentrations appear between the former tree sites. Lead arsenate was banned on most food crops in the U.S. in 1988 and all food crops in 1991.

 

[fattdad]

do a TCLP for arsenic. It'll take a week or so (there's a 4 day rotation in acedic acid) and you'll know how much of the arsenic is potentially mobile. Even knowingg that, there is a huge leap of faith in calculating just how much may enter the surface water supply.

If you have a 1-in rain event and you get 1/3-in runoff how would you determine the arsenic flux? No clue. Afterall, there have likely been many 1-in rain events and some level of arsenic (potentially mobile forms) remain.

Not sure if I'm helping, but I do appreciate the problem you are facing. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[Bajanbuilder]

Thank you for your comments. What I have done for now, is produce a list of solubility product calculations for all possible arsenates since these are the partially soluble forms of arsenic, whereas the acidic forms and the arsenites would be too mobile for such a long term evaluation.

I tabulated these and showed how much in ppm of each would dissolve in a litre of water ie mg/L

This is the best I could come up with. eg calcium magnesium lead and iron 3 arsenates.

 

[fattdad]

Don't these solubility calculations require some "contact time" assumption? How would you possibly handle that in dealing with surface water runoff?

¡papá gordo ain’t no madre flaca!

 

[Bajanbuilder]

Yes you are right. What I said in the writeup is that to use the Ksp values for the solubility calculations, the concentrations I find for each of the partially soluble arsenates will be maximum values for saturated solutions. ie Infinite contact time. For smaller time periods or events, the amount dissolved would be significantly less than these concentrations since as you say, the contact time with the passing water would be shorter.

This is all I could come up with to produce a bracket of values which inspectors could use. Is this method totally ludicrous? Like you I am questioning this entire undertaking but Im sure you would understand that town simply wants to know.

 

[fattdad]

If the town really wants to know it would be better to make a reasonable assumption then to use a maximum saturation value, which is something we know not to be correct.

Not trying to be blunt, but this is the peril of being too conservative.

f-d

¡papá gordo ain’t no madre flaca!

 

[Bajanbuilder]

Thank you, what you have said is the most sensible thing to do. I will have to think on this to come up with a pratical knockdown. I do not have detailed runoff information for the area as a result I am finding my way without a torch.

 

[fattdad]

runoff typically follows the form Q=cia, where "c" is a coefficient that depends on the travel distance and surface character, "i" is the rainfall intensity and "a" is the watershed area. This forumua is called the "rational method." The term "c" is dependent on the "time of concentration, which relates to the longest travel path of the most-distant drop of rainfall. If the measurement exceeds 200 ft you'd usually figure that the rest of the run would be in channel flow. So a part of your time of concentration may be in sheet flow and another part in channel flow. There are widely published nomographs that help you with this. I may even have a publication at my web page:

http://www.fatt-dad.com

(look under "hydrology").

For surface water flow, the contact time would have to be something less than the time of concentration (that is if you are only considering surface water adsorption).

Just something to think about.

f-d

¡papá gordo ain’t no madre flaca!

 

[fattdad]

I MADE A MISTAKE!!!

The term "i" is a function of the time of concentration. "c" is a direct chart take-off. Please look at my link, go to hydrology and click on the link for the "rational method" If you print it out, you'll have something to go on. Sorry for the oversight. Maybe I posted too late, sigh. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[Bajanbuilder]

No, you didn't post too late. After having a look at your site, which is great btw, and a look at some of my old textbooks I managed quite well. Thank you for your efforts and time!