Concentrations of stable isotopes of cesium and strontium in freshwaters in northern England and their effect on estimates of sorption coefficients ( Kd)

Abstract

Concentrations of stable isotopes of cesium and strontium were measured in twenty lakes in northern England. Stable Sr concentrations ranged from 8–214 μg L −1 and showed a good linear relationship to Ca + Mg concentrations. These observations are consistent with previous observations from waters with no major sources of strontium minerals in the catchment. Stable Cs concentrations were at least 3 orders of magnitude lower generally less than the 12 ng L −1 limit of detection. These data increase the number of stable Cs concentrations reported in the literature by a factor of 2. A critical analysis of literature data on stable Cs concentrations indicated a range of <12–60 ng L −1. Unlike the case for Sr, no relationship was found between Cs and its major competitor, potassium. An ion exchange model was used to derive equations for radiocaesium and radiostrontium sorption coefficients, K d , in the presence of both a competitive major ion and non-trace concentrations of the relevant stable isotope, i.e., stable isotope concentrations at which significant competition with the major ion occurs. Since the competition for sorption sites by stable Sr in natural freshwaters never exceeded 0.26% of the competition from Ca, the normal, simplifying assumption of trace Sr levels (i.e., concentrations which are too small to influence the chemistry) was confirmed as applicable in the ion exchange model of Sr sorption. This was also found to be true in laboratory studies when carrier was added with radioactive isotopes. The assumption of trace concentrations was also confirmed for stable Cs in natural waters. However, the very high selectivity coefficient of Cs, relative to potassium, resulted in the assumption being breached during the measurement of K d in the laboratory when stable carrier was present. K d s measured using non-carrier free isotopes were underestimated by up to a factor of 5. A very good relationship was found between log K d and the potassium concentration ( R 2 = 0.67 when corrections for the presence of stable carrier were made). The relative importance of other competing monovalent and divalent cations (Na +, K +, Mg 2+, Ca 2+) was investigated using modifications of the ion exchange definition of K d . In the natural waters we studied, Mg 2+ had an effect on the K d Sr equivalent to up to 30% of the Ca effect, whereas ammonia and potassium showed very little effect on the K d Sr. In our samples, Na influence never exceeded 50% of the Ca effect on K d Sr, but in estuarine and marine samples, the effect would obviously be greater. Calcium had no effect on K d Cs and Na + and NH 4 + had only a minor effect in our samples.