Numerical modeling of field scale application of ground silicate minerals for groundwater pH buffering

Recently, it was shown that ground silicate minerals are a potentially suitable pH buffering material for contaminated soils undergoing acidification, for example due to acid mine leachate and coal pile runoff infiltration, as well as during the degradation of chlorinated ethenes (Lacroix et al., 2012, Doi: 10.1007/s11270-011-1058-4). Compared to traditional buffering methods, such as the circulation of an alkaline solution, silicate minerals are appealing because they are long-term sources of buffering capacity. In this work, the applicability of ground silicate minerals to a realistic field scale scenario was examined, and possible solutions to deliver the silicates in the contaminated area were explored. To this end, a reactive transport model was developed using PHAST to study particle filtration and dissolution, pH evolution and the effect of soil and groundwater geochemistry. The model accounts for particle advection and dispersion, deep-bed filtration, porosity and hydraulic conductivity changes associated with deposition and mobilization. The deep-bed filtration coefficients vary with the flow rate and the composition of the pore-solution, ionic strength and, in particular, pH. Experimental data taken from the literature were used to calibrate and validate the deep-bed filtration model and the relationships that describe porosity and hydraulic conductivity variations. A satisfactory comparison was found in most situations. A two-dimensional model was setup to study the delivery and spreading of silicate minerals in a hypothetical contaminated site. Different injection scenarios were tested. It was found that the injection flow rate and well configuration strongly affect the distribution of silicates and therefore the buffering efficiency. In general, it was observed that the distance between two injection wells should not exceed 25 m to ensure a sufficiently homogeneous distribution of the substrate. It was further observed that the optimal size of the injected particles is around 5 μm. Since ground minerals have a rather large reactive surface area, relatively small amounts of silicate minerals are needed to guarantee sufficient buffering in most situations. For example, to degrade 40 mM of TCE to ethene in 100 d, 10 g (kg soil)^-1 of silicates are sufficient. With this amount, the variation of the soil porosity is less than 2%, and the associated hydraulic conductivity change predicted is also small. In summary, the numerical experiments performed confirmed that the injection of silicate minerals can be a viable strategy to provide pH buffering capacity to soils.