PHREEQC output

Considering calcite equilibrium a pHc of 7.076 results, that is 0.376 pH units above the measured pH value of 6.7. The permitted deviation of 0.2 is exceeded. Since pH-pHc is negative, the water is calcite aggressive, i.e., it can still dissolve calcite and present a danger for pipe corrosion. Undersaturation can also be determined without calculation of the pHc, because within initial solution calculations in the PHREEQC output, calcite already shows a saturation index of -0.63 (= 23% saturation). 

The reaction rate is calculated (in statement 40) according to Equation (11.14), and we increase the duration to 106 years. The activation energy Ea (PARM (1)) and the pre-exponential constant A (PARM (2)) are obtained from the linear fit coefficients of Figure 11.3(b) see Equation (11.26). The results of both the phreeqc output and Equation (11.16) (Table 11.5) are shown in Figure 11.4. They are identical. 

The modeling can be started either via Calculations/Start or by the icon pocket calculator . A PHREEQC for Windows-progress window opens showing input, output and data set file as well as the calculation progress in line 4. DONE appears when the calculation is performed or terminated. By clicking on DONE, the progress window closes and the output folder opens. 

When considering the increment of 1 for pE and pH, i.e. 15 pH values 31 pE values, the output file would comprise 465 jobs, numbered from SOLUTION 1 to SOLUTION 465, each containing different pE- and pH values. In fact, there will be only 377 jobs since the SOLUTIONS with pE-pH values above or below the stability field of water are missing. The water constituents defined under SOLUTION (e.g. Fe, Ca, Cl, C, S, etc.) are alike in all 377 jobs. Opening this input file takes about 30 seconds. Because files larger than 32 k cannot be opened in the Windows environment of PHREEQC either they have to be divided into smaller files or they have to be started directly with phreeqc.exe in the DOS prompt (phreeqc Input-File-Name Output-File-Name Database name). 

To avoid looking for the predominant species in 377 output jobs manually after modeling, two means are offered At first, a SELECTEDOUTPUT (see also chapter 2.2.1.4) has to be defined in the PHREEQC master input file. Besides pE and pH, it will output all species of interest, for example all Fe species, in a. csv file. These species have to be specified explicitly under the sub key word molalities , e.g. Fe2+, Fe3+, FeOH+, etc. The BASIC-reproduction program inserts the 377 SOLUTION jobs before the key word SELECTED OUTPUT. Since the SOLUTIONS are not separated by an END, a SELECTED OUTPUT will be created out of all SOLUTIONS displaying for each of the 377 jobs a row with the columns pH, pE, m Fe2+ (concentration of Fe2+ in mol/L), m Fe3+, m FeOH+, etc. 

Using the minerals quartz and kaolinite in EQUILIBRIUM(PHASES causes a problem in PHREEQC regarding the elements Si and Al because they do not occur within the key word SOLUTION. Therefore you have to specify them in very small quantities in the solution (e.g. 1 pg/L). Furthermore, the sub key word -step divide 100 within the key word KINETICS is necessary. The output can be obtained most effectively using SELECTED OUTPUT.]... 

First of all both solutions, the acid mine water and the groundwater, are defined in the PHREEQC input file and mixed applying the keyword MIX. Then this solution is saved as solution 3 (SAVE SOLUTION) and the job is finished by END. A second job follows, which uses again SOLUTION 2 (groundwater) and SOLUTION 3 (1 1 diluted water) via the key word USE, once again mixes both solutions 1 1, and saves the result as SOLUTION 4, etc. SELECTED OUTPUT facilitates the further data processing in EXCEL by providing the pH values and the molalities of all uranium species. The key word itself has to be repeated for every job, as well as the definition of the desired parameters pH and molalities. 

Yet the file name (e.g. 3 uranium species pHdependent.csv) should only appear within the first SELECTEDOUTPUT block and not be repeated. This way PHREEQC writes the parameters of all modeling in one single SELECTED OUTPUT file. The headline, however, is repeated for every modeling ran. The sub key word reset false , also applied just once within the first SELECTED OUTPUT block, suppresses the standard output for all other modeling, which are written into the same file. 

The phreeqc input file is shown in Table 8.1. Note that we use sulfate for charge balance, because it is the most abundant ion. The default definition of alkalinity in phreeqc is as calcite , which is what our analysis shows, so we simply enter that number (153 mg L-1). Under the keyword REACTION, a suitable combination of moles of HCl and number of steps is found with some calculation and a certain amount of trial and error. We specify the file name of the output file to ensure we get an output file with the essential data arranged for copying to a spreadsheet. The results are shown in Figure 8.2. 

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