> Thank you for your response and offer to look at the file. This file contains data from the most upstream site (solution 1) and the most downstream site (solution 2). I thank you for any assistance you can give me. I realize WATEQ4F and PHREEQC databases do not contain Cr. I haven't decided what to do about Cr yet but I left it in the file anyway. The only harm it seems to cause is that I get a warning message telling me it is not found in the database. > Jim asked why I chose inverse modeling instead of solution/solubility. As I told him, results of the inverse modeling examples in the PHREEQC manual were easier for me (a biology background not a chemistry one) to interpret and understand in relation to the questions I am trying to answer. If another model is better suited to identifying precipitation/dissolution of metals, or will give me the same information in a different way, I am more than willing to listen to your advice. In fact, I have been looking at the solution species results, but I am not sure how to take the output (molality, activity, saturation indices and solubility constants) and say, for example, Cd precipitates at these sites in the river, Zn precipitates at these sites, etc. I took a quick look at the speciation model results. I normally just look at calcite, CO2, and gypsum for starters. Iron and aluminum look a little high, dissolved concentrations should be used in speciation modeling. Other trace metals probably don't form pure solids but precipitate in solid solutions or sorb on sediments, so the SI's don't indicate a lot. As for inverse modeling, I would stick to the majors, Ca, Mg, K, Na, C, SO4, and Cl. Your input file doesn't have any sources or sinks for elements (-phases in INVERSE_MODELING), but then it's probably hard to guess what would be reacting. In a stream, you may form some calcite and you will exchange CO2 with the atmosphere. After that, my guess is that the concentrations change mainly because of inflow of water of different compositions. So unless you have those compositions, from which you could estimate some mixing fractions, there's not a lot you can do by modeling. For starters, I'd look at the changes in concentrations and try to figure out whatever you can. Looks like Na and Cl go up and Ca, Mg, and SO4 go down. You need to decide whether this is real or an artifact of sampling and flow conditions. If it is real, then you need to devise a hypothesis that explains the changes. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225
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