The first step in geochemical speciation modelling is defining the mineral phases, sorptive phases and occurrence of solid solutions of a material that describe the solubility control for the respective major, minor and trace elements. In modelling it is crucial that as many elements as possible are considered simultaneously to capture mutual interactions. The test results of the pH dependence test (EN 14429, EN 14997, ISO 21268-4 or EPA 1313) are used to define the available contents (maximum released amount over the entire pH range from 1 – 14), the mineral assemblage including solid solutions, the reactive clay, hydrated iron oxide, particulate and dissolved organic matter amounts, total inorganic carbon (reflects carbonate content) and redox status (pH+pe), which together define the Geochemical Speciation Fingerprint (CSF). The selection of phases is calibrated against the measurements, such that a minimum deviation remains between model description and measurement. It occurs that a gap between model description and measurement cannot be filled. This can be the result of missing thermodynamic data for potential mineral phases or in some cases slow kinetics. Through analogy in behaviour sometimes gaps can be filled by assuming the same constant by just replacing the element (e.g. metal silicates).
The resulting CSF can be used to predict release as a function of pH at low liquid to solid ratio (L/S in L/kg; relevant indicator for porewater) and compare the first fraction of the percolation test (EN 14405, ISO 21268-3 or EPA 1314) with this prediction. Recently, a low L/S option has been applied to EPA 1313 to allow modelling a few pH conditions around the own pH of the material at a condition, which is closer to porewater conditions in the field.
The CSF of the material can subsequently be used as starting point for reactive transport modelling. First addressing the laboratory test procedures (percolation and monolith leach test) and thus verifying the prediction capabilities against actual measurements and then in a next step predicting an actual field exposure scenario taking assumed exposure conditions into account (e.g. infiltration, preferential flow, atmosphere exposure with carbonation and oxidation occurring). This approach allows to define a source term for subsequent environmental impact assessment (see section …).
Examples test data
Geochemical modelling
Source term definition
