After closure of the uranium mine at Königstein (G),
the acid mine drainage will enter the aquifer above the mine and flow towards the Elbe River
(Bain et al., 2001, JCH 52, 109). We like to predict how quickly the heavy metals and uranium
advance and whether they are retarded with respect to water flow, vi =
vH2O / R.
Traditionally, retardation is calculated from R = 1 + q/c, where q
is sorbed and c is solute concentration (mol/L). However, for transport the flushing
factors given by (∂qi /
∂cj)i ≠ j
should be considered (the slope of the isotherm if j = i),
rather than the distribution coefficient Kd = q/c
as explained in Appelo and Postma, 2005.
In the Königstein case the distribution coefficients differ markedly from the flushing factors,
shown for Zn and U(6) as function of pH in the graph.
The distribution coefficient of Zn is determined by ion exchange at low pH, increases
at intermediate pH's by sorption on iron-oxyhydroxides, and dwindles again at high pH
through aqueous complexation. The distribution coefficient of U(6) shows the same pattern
but is zero at low pH since the element is not exchanged.
The slope (∂qU /
∂cU)i ≠ U
is higher than Kd, indicating a concave isotherm for U(6)
(at a fixed pH). Consequently, the U(6) front will be sharpening when concentrations decrease.
Concentrations were calculated along a flowline from the mine to the Elbe River and show
the retardation effects:
Sulfate shows almost conservative behavior and Zn is retarded by 2.1. The concentration of U(6) shows an increase as if a heap of snow is pushed up in front of a snowplough. The typical shape is related to the pH variation at the front. Uranium is sorbed at neutral pH, but as the pH decreases to below 4 in the acid mine water, U is desorbed again, and the decreasing concentration gives a sharp front as we noted above.
A different picture is predicted for the aquifer, where the confining layers contain pyrite that reduces U(6) to U(4). The latter forms insoluble compounds like coffinite (USiO4), which generates an infinite retardation for U and produced the natural enrichment in the sediment over time. The animated gif shows U transport in a rectangular cross-section of the aquifer, 50 * 2000 m from the Königstein mine to the Elbe river. Acid mine water enters the section from the lower left corner and is accompanied by an equal flow of natural water from the upper left. The water leaves the section in the middle right. The lower right quarter of the section, from the mine until the outflow cells, contains pyrite. At the front the snowplough of U concentrations appears, while part of U is lost towards the reduced zone where coffinite precipitates. Together, the two effects lead to the development of a typical concentration pattern, shaped like a mushroom from which the hat is blown off gradually.
The aquifer was discretized in 40 * 100 cells and modeled with PHT3D version 2 and PHAST. Both models give essentially the same concentration patterns, see the review by Appelo and Rolle (2010), 700 kB pdf.
Are you curious to see what happens with the U concentrations after 20 years? And, what U concentrations could
enter the Elbe river?
It is easiest to try it out with PHAST, using files
u2.trans.dat and u2.chem.dat.
Originally, an MT3D/PHREEQC combination developed by Alain Dimier at ANDRA (F) was used for
modeling this case, see Appelo and Postma, Chpt. 11, and
Appelo and Dimier, 2004; 87 kB pdf.