Increasing concentration with increasing degradation rate

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mariaida
Posts: 4
Joined: Tue Nov 27, 2012 1:44 pm

Increasing concentration with increasing degradation rate

Post by mariaida » Thu Apr 18, 2013 6:56 am

Dear Jirka,

I + colleagues are experiencing something strange when we are trying to model pesticide transport for different types of substances with a model calibrated in terms of soil hydraulic parameters, and experimentally derived sorption and biodegradation data (for site-specific conditions). As part of the study, we are doing a sensivity analysis for the surficial degradation rate. Now, for some of the substances, an increased surficial (0-40 cm) degradation rate leads (with the same input flux) to an increased concentration at the filter level at c. 3 m depth. We cannot get our heads around this, and were wondering what the cause might be as we don't see any good physical explanation for it. We tried a number of things in order to check what might be the cause (including e.g. steady-state modeling + changing to uncalibrated, default soil parameters) but it was first when we increased the sorption of the same surficial material by two orders of magnitude that things changed to something more logical i.e. increased degradation --> decrease in concentration. Do you have any explanation for this? The strange thing is that this "problem" applies to substances with Kd all through the interval of our experimentally yielded values (high/low/mid). And, I could also mention that when testing the same reaction parameters on a simple, single-material, steady-state column model-run - we had no "problems" i.e. increasing degradation rate --> decreasing concentration.

Hope you can help us,

Best regards,
Maria

Jirka
Posts: 4762
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Increasing concentration with increasing degradation rat

Post by Jirka » Sat Apr 20, 2013 2:05 am

Maria,

I do not have any explanation until I would actually see the project. I would first look at the mass balance error (in the Mass Balance Information), and if this error is low, then the results are likely correct and there must be some explanation.

It is difficult to make any predictions when you have a two-layered system, and I do not know the sorption and degradation characteristics for these two layers. You should note that HYDRUS recognized degradation in the liquid and solid phases, and if you specify it only for one phase, then (obviously) degradation occurs only in this phase. If only a small percentage of solute is in this phase (that would depend on the distribution coefficient Kd) then degradation may not be important. Etc.

J.

mariaida
Posts: 4
Joined: Tue Nov 27, 2012 1:44 pm

Re: Increasing concentration with increasing degradation rat

Post by mariaida » Tue Apr 23, 2013 4:16 pm

Thank you very much for your rapid answer. We realised that we had some mass balance issues and are trying to resolve this by means of mesh refinement (mainly near the surface; according to you written instructions elsewhere in the forum and in formal hydrus-documents). So far, it seems to give the wanted effect, i.e. we are decreasing the mass balance error both for water and solute flux.

Maria

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