Migration of acidic groundwater seepage from uranium-tailings impoundments, 1. Field study and conceptual hydrogeochemical model

In this first paper of a series, the results of a study at a non-operational tailings site are presented and are used to construct a general conceptual model for seepage migration from uranium-tailings impoundments. Many parts of the model are applicable to other types of tailings and to acid drainage in general.At the field site, the impoundment lies over a portion of a glaciofluvial sand aquifer. Tailings seepage drains downward into the aquifer and then migrates laterally away. Results of the field study indicate the seepage can be divided into three geochemical zones: (1) the inner core, which is essentially unaltered, acidic seepage from the tailings; (2) the neutralization zone, in which inner-core water is neutralized and aqueous concentrations decrease significantly; and (3) the outer zone, which contains both neutralized water from the neutralization zone and pH-neutral process water from the uranium milling operation. Yearly comparisons from 1979 to 1984 indicate the neutralization zone and inner core are migrating downgradient at a rate of about 1 meter/year, which is about 1/440 of the groundwater velocity. The mechanisms that produce the retardation and the decreases in aqueous concentrations are part of the conceptual model.The main features of the conceptual model are solid-liquid interactions, particularly mineral precipitation-dissolution, and buffering effects of dominant aqueous species. The important minerals undergoing precipitation-dissolution are the calcite-siderite solid solution, gypsum, Al-OH minerals, and Fe-OH minerals. “Cell and streamtube” calculations are used to evaluate the general trends in aqueous concentrations and to assist in explaining observed migration rates. Co-precipitation with the above minerals apparently accounts for decreases in other major, minor, and metal solutes. Because of the large amount of mineral precipitation and co-precipitation, variations in ²H and ¹⁸O were observed over a flow distance of several meters, while ¹³C remained essentially constant.