Downward movement of pollution

[Max1]

Hi all,

I'm trying to model the progress of a contamination (molybdate) with soil depth over a time span of ~47 years. I use different SOLUTIONS key data blocks to represent different parts of the soil. Recent measurements from the field can be used as the influx of the contamination in PHREEQC. There is also a part with kinetics: this would reduce the amount of sulphate in my model through reduction by organic matter. This is at a depth of -5 meters. 
However, I don't understand the graph I get from this: the amount of molybdate seems to increase in the layer containting organic matter, rather than decrease. I expect that my molybdate would progress downward with time, hence, the front should be the same size, but deeper. Or am I making logical errors? I attached my input file and the graph.

Thanks in advance. 

[dlparkhurst]

First, I would recommend that you use TOT("Mo") for the dissolved Mo, rather than a single Mo species MoO4-2, although in this case it probably does not matter. Also, SURF("Mo", "Hfo") will give you the total Mo sorbed, which in this case does make a difference.

I can't tell you exactly what is going on because you have a lot of cells and long runs. I would probably simplify to a smaller column until you understand the chemistry. However, the basic situation is that you have a relatively large pool of sorbed Mo and a relatively small pool of dissolved Mo. Small changes in the sorbed pool result in large changes in the dissolved pool. There is a lot of structure in the pH profile.

 Plot shows 100 and 200 shifts.You can plot directly from PHREEQC using USER_GRAPH. The graph was created with the following:

USER_GRAPH 1
    -headings               Dist TOT(Mo) Sorb(Mo) pH
    -axis_titles            "Depth" "mg/kgw" "pH"
    -initial_solutions      false
    -connect_simulations    false
    -plot_concentration_vs  x
  -start
10 GRAPH_X DIST
20 GRAPH_Y  TOT("Mo")*159.9576*1000, SURF("Mo", "Hfo")*159.9576*1000
30 GRAPH_SY -LA("H+")
  -end

[Max1]

Thanks mr. Parkhurst. I will try to construct a simplified model. However, I don't understand "Small changes in the sorbed pool result in large changes in the dissolved pool"..? Because of the relative size of the pools on each other?
When I model these graphs, I can clearly see the increase of TOT(Mo) and Sorb(Mo) at the depth with organic material. So, the reduction of sulfate leads to an increase in molybdate?

[Max1]

I simplified my model, making it have less cells and less shifts. Total time simulated is still ~47 years. The graph for molybdate show the same pattern. The concentration seems to increase when it reaches the depth were sulfate is being reduced... Is it because the pH level drops in these conditions, and molybdate then desorbs, increasing molybdate concentrations?

[dlparkhurst]

I am probably not telling you anything new, but you are advecting Mo through the first zone and it is accumulating as sorbed Mo at the interface. The sorbed accumulation coincides with a peak in dissolved Mo.

As for the small and large changes, I was looking at 15 m where there is a definite peak in dissolved Mo that comes from a the sorbed Mo pool, which is has only a very slight relative decrease.

If you are generating sulfide and have iron in the system, you should probably be considering a FeS or FeS2 phase. I am sure saturation indices for these phases will be quite large and they do form in sulfate reducing environments.

[Max1]

A theory proposed was the formation of molybdenite, MoS, in these sulfate reducing conditions, as an explanation of the lowered concentrations of sulphate and molybdate at a depth of 5 - 10 meter. I cannot really find literature that underpins this theory. Fe2+ reacting with Sulfide seems to be a more logical conclusion.

I uploaded the modeled Mo concentrations against the actual Mo that it is in the soil now (based on x measurements which have been used for the different zones). I have to think more about the chemistry of sorbed Mo and Molybdate in solution, before I assume new explanations.