Chemical speciation modelling of groundwater in a shallow glacial sand aquifer part 1 General parameters

The aim of the work detailed in this report has been to gain a better understanding of the
speciation chemistry controlling the aqueous chemical forms of elements and compounds
normally present in groundwaters found at the BGS in situ migration experiment at Drigg,
Cumbria. This will form the basis of future modelling studies designed to interpret in situ
tracer experiments using 60Co in the presence of naturally occurring organic complexants.
Total element concentrations in relevant samples were obtained using Inductively Coupled
Plasma Optical Emission Spectroscopy. The aqueous speciation chemistry was modelled
using the geochemical code PHREEQE in conjunction with the UWIST thermodynamic
database. The natural chemical speciation of each sample was determined as was the sensitivity
to changes in chemical parameters such as pH and oxidation-reduction potential (Eh).
In addition, the possible solubility limiting phases were determined for elements of interest.
The results show that for all non-transition metal elements, the major aqueous species is the
free ion. However, for the transition metal elements, this is not the case. For manganese,
the carbonate species is by far the most abundant. This reflects the different complexing
abilities of transition elements as compared to other metals.
In the given system, carbonate equilibria are likely to control the calcium solubility. Calculation
predict that the precipitation of calcium carbonate (as calcite) from the aqueous phase
is unlikely. Hence, it can be concluded that coprecipitation of Ba2+ and sr2+ will not occur.
The speciation of aqueous iron is very sensitive to changes in pH and Eh. When the Eh is
changed by only 11 Om V (from +227m V to + 117m V) the dominant aqueous species is
changed from Fe(OHho to Fe2+. The total change in the concentration of Fe(OHho over
this range is an order of magnitude. Fe(OHho is an aqueous species of potential interest
because calculations suggest that species such as this may precipitate and coprecipitate with
radionuclide species (e.g. Co, Ni). The formation of iron hydroxide leads to an increase in
the number of surface sites available for sorption of charged species and a corresponding
reduction -in their aqueous concentration. There is also an increased probability Of the
formation of colloidal particles involving humic and fulvic substances.
Changes in pH and Eh are both possible during the tracer test, where waters of differing
compositions are likely to mix within the aquifer as a result of pumping. These will be
modelled and presented in part II, along with predictions for the speciation of various
radionuclides added to the groundwater.