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

Abstract

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