The problem is with two As-F species, AsO3F-2 and HAsO3F-. The constants for these species look much too strong to me. They imply that they account for all of the arsenic, with the next species about 1e-10 smaller. The convergence problem is caused because under most initial choices of log activity F- and H2AsO4-2, these species cause all of the F to be calculated to be in these species as well and the two mass balance equations are the same on the left hand side within the numerical precision of the computer. Right hand sides are related to the total concentrations, and these differ. I would copy the definitions of these two species into your input file in a SOLUTION_SPECIES data block. Then you can either eliminate them by decreasing the log K's by 20, or you can decrease the log K's by perhaps 5 or 10, which will allow the program to converge. For the second case, the distribution of species will be effectively the same as the original log K's. The concentrations of the minor species will be larger, but with a choice of K's, they will still be small relative to the As-F species. I suspect there was a sign error or other tabulation error when they compiled the data at LLNL. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Michele Y Crouse To: David L Parkhurst/WRD/USGS/DOI@xxxx 11/23/01 06:30 cc: h2osoft@xxxxxxxx In-Reply-To: <OF6A7E98F1.E00262C9-ON85256B0D.004A26AA@xxxxxxxx> AM Subject: Re: PHREEQC version 2.3 ----- Forwarded by Michele Y Crouse/WRD/USGS/DOI on 11/23/01 08:29 a.m. ----- "RG&AJ Gerritse" <geoprocc@xxxxxx To: <mcrouse@xxxxxxxx> net.au> cc: In-Reply-To: <OF6A7E98F1.E00262C9-ON85256B0D.004A26AA@xxxxxxxx> Subject: Re: PHREEQC version 2.3 11/21/01 10:30 p.m. With the attached input file for a recent groundwater sample I get a a normal run with the WATEQ database, but with the LLNL database the program will not converge. Taking As or F out makes the LLNL run converge. What can the problem be? regards Robert Gerritse Geoprocc Pty Ltd Wembley Downs Western Australia (See attached file: Searl5.pqi)
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