Boron, Cl-, and Br- concentrations and boron isotopic compositions (Δ11B) were used to provide insight into the sources and water-sediment interactions of fossil water in a thick nonfractured aquitard system in Saskatchewan, Canada. The aquitard system consists of 80 m of late Pleistocene clay-rich till disconformably overlying 77 m of late Cretaceous marine clay. The clay is underlain by a regional sand aquifer. Variations in Cl- and Br- confirmed the presence of well-defined conservative mixing relationships between four types of fluids present in the aquitard system: (1) low Cl- water from the base of the fractured and oxidized till zone at 4 m depth; (2) water characterized by an elevated Cl- peak (190 mg/L) at about 12 m; (3) glacial meltwater present between 20 and 60 m depth; and (4) more saline water (Cl- up to 1000 mg/L) from the underlying Cretaceous clay and sand aquifer. Glacial meltwater has low Cl-, Δ11B(~20?), and Cl-/Br- ratios (50). The low Cl-/Br- ratio of the glacial meltwater is similar to that of modern inland precipitation, which suggests that the Br- budget in the atmosphere 20--30 ka B.P. was similar to that of today and that Br- enrichment in the atmosphere may be a natural process. The relatively low Cl-/Br- ratio (~150) and Δ11B values of the saline water in the sand aquifer (35 and 39?, respectively) and pore water in the Cretaceous clay (25.6 and 32.6?, respectively) arenot consistent with simple dilution of seawater but suggest dilution and modification of fifteenfold evaporated seawater with a low Cl-/Br- and a high Δ11B value (>39?). This evaporitic seawater may have been entrapped in the Cretaceous clay upon or after its deposition and subsequently flushed from the clay by fresher water. We suggest that the refreshening process modified the original brine into Na-HCO3 water type and extracted boron with Δ11B<39? from the adsorbed sites. In contrast, increase of Cl- in the upper section of the till (the 12 m peak) resulted in apparent depletion of dissolved boron and relative 11B enrichment, which can be explained by adsorption processes. The B-Δ11B relationships can thus be used to trace water-clay interactions in aquitard systems. ¿ 2001 American Geophysical Union