PHREEQC-2 20 years
6.1.8 Plausibility of the Reactive Transport Results in the Pure Continuum System
conduit nodes and the magnitude of the exchange coefficients, deviations would occur in location and amount of dissolved calcite and kaolinite as well as precipitated goethite and gypsum.
6.1.8 Plausibility of the Reactive Transport Results in the Pure Continuum System
In this subsection, plausibility of the reactive transport results for the different scenarios in the pure continuum system is demonstrated.
Effect of pyrite oxidation on pH. The comparison of the modelling results for scenarios 1-3 show that the simulated pH-profiles are very similar in both time and space. This is illustrated in Fig. 19 for a simulation time of 20 years. The same applies to the mass of pyrite that is oxidised in the first tailing cell (~ 155 mols for all three scenarios, Table 7). Table 7 lists the locations and quantities of mass turnover by mineral dissolution/precipitation for all scenarios after 20 years.
In contrast, scenario 4 draws a different picture, which is explained by the absence of the major oxidising agent ferric iron in the recharge solution (compare Chapter 6.1.7 and Table 7). Here as little as 12 mols of pyrite are mobilised from the first cell (tailing) after 20 years. Consequently, the pH values in both the tailing and aquifer in this scenario do not drop as low as in the other scenarios with time. In the tailing, the pH drops from 7 to 1.9 for scenarios 1-3 while for scenario 4, it decreases only to 3.9. In the aquifer, the pH decreases from about 9.7 to 6.1 for scenarios 1-3 and for scenario 4, it decreases to only 8.9. Those differences in temporal pH development are illustrated in Fig. 20 and Fig. 21 for scenarios 1 and 4, respectively. Fig. 20 shows that after 1 year, the pH established in scenario 1 at around 10 m in the tailing and that the front progressed to the end of the tailing. This is due to that the tailing was flushed with a velocity of 10 m yr-1. After 2 years, the pH nearly established at the beginning of the aquifer (at around 20 m) while the front moved already 20 m further downgradient. After 5 years, the effect of dispersivity (of 1 m) can more extensively be seen in Fig. 20 since the pH front already progressed 80 m downgradient. After the aquifer is flushed with one pore volume, i.e., after 10 years, the pH remains constant at the downstream end of the aquifer for all scenarios. This is depicted in Fig. 20 and Fig. 21 for scenarios 1 and 4, respectively.
Fig. 19: Simulated pH profiles of scenarios 1-4 for a continuum system after 20 years.
Fig. 20: Simulated temporal pH profiles of scenario 1 for a continuum system.
1.5 4.5 7.5 10.5
0 20 40 60 80 100
Distance downgradient (m)
pH
Scenario 1 Scenario 2 Scenario 3 Scenario 4
Scenario 4
Scenario 3 Scenario 1 Scenario 2
20 years
1.5 4.5 7.5 10.5
0 20 40 60 80 100
Distance downgradient (m)
pH 0 yr
1 yr 2 yrs 5 yrs 10 yrs 15 yrs 20 yrs
15 and 20 yrs 0 yr
1 yr
2 yrs 5 yrs
10 yrs
Scenario 1
Fig. 21: Simulated temporal pH profiles of scenario 4 for a continuum system.
Dissolution of calcite and kaolinite. After a simulation time of 20 years, scenarios 1-3 show a dissolution of calcite of approximately 4500 mols from the first 5 aquifer cells (Table 7), which corresponds to a complete depletion of calcite in the first 4 cells. As soon as a cell is completely depleted in calcite, kaolinite starts to dissolve. In scenarios 1-2, approximately 400 mols of kaolinite are dissolved after 20 years in the first aquifer cell. This leads to some concentration peaks of aluminium and silicon within this first aquifer cell. Scenarios 1-2 show aluminium and silicon concentrations of 6.34x10-3 and 5.71x10-3 mol L-1, respectively (Fig.
14b). Note that the increased concentrations of aluminium and silicon in the adjacent calcite-depleted cells rather result from physical transport (from the first cell of the aquifer where kaolinite dissolves) and not through the dissolution of kaolinite within these cells. This is due to reaching saturation of the aquifer solution with aluminium and silicon by receiving these components from previous cells.
In contrast to scenarios 1-3, in the scenario with less extensive pyrite oxidation (scenario 4) significant less calcite (~ 59 mols) is consumed and after 20 years, dissolution has not progressed past the first aquifer cell. Under the pH-conditions of this scenario, only about 2 mols of kaolinite are dissolved from the first aquifer cell (Table 7). After one pore volume, the pH remains almost constant across the aquifer cells in this scenario, which also results in constant aluminium and silicon concentrations of 1.07 x10-5 mol L-1 throughout the aquifer and with time.
1.5 4.5 7.5 10.5
0 20 40 60 80 100
Distance downgradient (m)
pH 0 yr
1 yr 2 yrs 5 yrs 10 yrs 15 yrs 20 yrs
15 and 20 yrs
0 yr 1 yr
2 yrs 5 yrs 10 yrs
Scenario 4
Table 7: Dissolved and precipitated amounts and fractions of the different mineral phases of scenarios 1-4 for a continuum system after 20 years.
Scenario 1 2 3 4
cell amount fraction cell amount fraction cell amount fraction cell amount fraction
(mols) (%) (mols) (%) (mols) (%) (mols) (%)
Pyrite dissolv. t 1 155.2 t 1 155.2 t 1 155.2 t 1 12.4
precip. t 20 13.5 aq 3-5 124.8 100.0 aq 3-5 124.8 100.0 aq 1 0.5 aq 1,2,6-80 1.5×10-2 1.2×10-2 aq 1,2,6-80 1.6×10-2 1.3×10-2
aq all 124.8 100.0 aq all 124.8 100.0
pre/dis 8.7 80.4 80.4 4.3
Calcite dissolv. aq 1-5 4514.9 99.5 aq 1-5 4546.9 99.5 aq 1-5 4546.9 99.5 aq 1 58.9 aq 6-80 22.3 0.5 aq 6-80 21.1 0.5 aq 6-80 21.4 0.5
aq all 4537.2 100.0 aq all 4568.0 100.0 aq all 4568.3 100.0
Kaolinite dissolv. aq 1 396.7 aq 1 405.9 aq 1 1.9
precip. aq 2-5 386.1 aq 2-5 398.6
prec/dis 97.3 98.2
Goethite precip. aq 2-5 1739.2 99.9 aq 3-5 1747.7 100.0 aq 3-5 1747.6 100.0 aq 1-2 10.6 100.0 aq 19-80 1.6 0.1 aq 12-80 0.2 0.0 aq 12-80 0.2 1.1×10-2 aq 3-80 6.8×10-6 6.4×10-5
aq all 1740.8 100.0 aq all 1747.9 100.0 aq all 1747.8 100.0 aq all 10.6 100.0 Gypsum precip. aq 4-5 996.2 100.0 aq 4-5 1004.2 100.0 aq 4-5 1004.5 100.0
aq 6-7 9.4×10-3 9.4×10-4 aq 6-9 2.2×10-3 2.2×10-4 aq 6-9 2.2×10-3 2.2×10-4 aq all 996.3 100.0 aq all 1004.2 100.0 aq all 1004.5 100.0
Where t=tailing, aq=aquifer and all=all cells.
Precipitation of kaolinite. Like its dissolution, the precipitation of kaolinite depends strongly on the pH. Therefore, the increased aluminium and silicon concentrations from calcite-depleted cells will precipitate readily in form of kaolinite in cells where a non-acidic environment is predominant. For scenarios 1-2, this implies that, as soon as the dissolved kaolinite from a calcite-depleted cell enters a calcite buffered environment, it will precipitate. The difference between scenario 1 and 2 is that in scenario 2, all minerals in the base case can dissolve in the tailing and aquifer regions (compare Table 5). Table 7 shows that in these two scenarios, up to 98 % of the dissolved kaolinite from the first aquifer cell precipitates in the adjacent 2-3 cells of the aquifer (see also Fig. 22).
Consequently, the aqueous concentrations of aluminium and silicon in these cells decrease to a concentration of 8.56 x10-7 mol L-1, which represents the solubility of kaolinite at this pH.
Fig. 22: Simulated kaolinite profiles of scenarios 1, 2 and 3 for a continuum system after 20 years.
Goethite and gypsum precipitation. Fig. 23 depicts that in a scenario with more extensive pyrite oxidation (1-3), goethite mainly precipitates in the first 2-5 aquifer cells after 20 years (up to 1748 mols, Table 7). For scenarios 1-3, gypsum shows very little variation with respect to the location and the amount of precipitation (up to 1005 mols). Most of the precipitated amount occurs only over 2 cells in the first portion of the aquifer (Fig. 24 and Table 7).
0 0.7 1.4
0 20 40 60 80 100
Distance downgradient (m) Kaolinite (mol L-1 )
Scenario 1 Scenario 2 Scenario 4 Scenario 1
Scenario 2
20 years
Fig. 23: Simulated goethite profiles of scenarios 1-4 for a continuum system after 20 years.
Fig. 24: Simulated gypsum profiles of scenarios 1-4 for a continuum system after 20 years.
0 0.4 0.8
0 20 40 60 80 100
Distance downgradient (m) Gypsum (mol L-1 )
Scenario 1 Scenario 2 Scenario 3 Scenario 4
0 0.4 0.8
22 24 26
20 years 0
0.4 0.8
10 14 18 22 26
Distance downgradient (m) Goethite (mol L-1 )
Scenario 1 Scenario 2 Scenario 3 Scenario 4 0
0.004 0.008
18 20 22
Scenario 4
Scenarios 2 and 3
Scenario 1 20 years
At high pH-values and respective low iron concentrations in scenario 4, very little goethite precipitates (~11 mols). Gypsum does not precipitate at all in this scenario, since only little calcite is required to buffer the acidic solution.
Therefore, the solubility of gypsum is not exceeded.
Influence of pyrite precipitation in the aquifer. In all scenarios where pyrite is promoted to precipitate in the aquifer (scenarios 2-4) but especially in those with more extensive pyrite oxidation (scenarios 2 and 3), sulphite is precipitated as pyrite. In scenarios 2 and 3, the largest fractions of pyrite precipitation (about 80.4 % of the amount dissolved further upstream) occur in the aquifer cells 3-5 (Table 7 and Fig. 25). In scenario 4, where pyrite oxidation is less extensive, only about 4 % of the dissolved amount of pyrite in the first cells precipitate subsequently in the first aquifer cell after a simulation time of 20 years.
Fig. 25: Simulated pyrite profiles of scenarios 1-4 for a continuum system after 20 years.
In the scenario where pyrite precipitation is oppressed in the aquifer (scenario 1), sulphite remains in solution. As sulphite inhibits the oxidation of Fe2+ to Fe3+ by means of sulphate ions required for goethite precipitation, the concentrations of Fe(II) and sulphate in cells with calcite are higher for scenario 1 in comparison with scenarios 2 and 3. The equilibrated Fe(II) concentration in the aquifer of scenario 1 is about 8.16 x10-4 mol L-1 higher than in scenarios 2 and 3 (Fig. 26).
The same applies to the equilibrated sulphate concentration, the difference is 2x10-4 mol L-1 ( Fig. 27) in that case. As a result of these elevated dissolved Fe(II) and sulphate, the redox potential within the aquifer is lower in scenario 1 compared to scenarios 2 and 3 (~ 0.9, see Fig. 28).
Fig. 26: Simulated Fe(II) profiles of scenarios 1-4 for a continuum system after 20 years.
Fig. 27: Simulated sulphate profiles of scenarios 1-4 for a continuum system after 20 years.
0 0.01 0.02
0 20 40 60 80 100
Distance downgradient (m) Fe(II) (mol L-1 )
Scenario 1 Scenario 2 Scenario 3 Scenario 4
0 0.00004 0.00008
0 50 100
Scenario 4