Transport of a reactive tracer in saturated alluvium described using a three-component cation-exchange model

A weakly sorbing cation, lithium, will be used as a reactive tracer in upcoming field tracer tests in the saturated alluvium south of Yucca Mountain, Nevada. One objective of the field tests is to determine how well field-scale reactive transport can be predicted using transport parameters derived from laboratory experiments. This paper describes several laboratory lithium batch sorption and column transport experiments that were conducted using ground water and alluvium obtained from the site of the planned field tests. In the batch experiments, isotherms were determined over 2.5 orders of magnitude of lithium concentrations, corresponding to the range expected in the field tests. In addition to measuring equilibrium lithium concentrations, concentrations of other cations, namely Na(+), K(+), and Ca(2+), were measured in the batch tests to determine Li(+)-exchangeable equilibria. This information was used in conjunction with alluvium cation exchange capacity measurements to parameterize a three-component cation-exchange model (EQUIL) that describes lithium sorption in the alluvium system. This model was then applied to interpret the transport behavior of lithium ion in saturated alluvium column tests conducted at three different lithium bromide injection concentrations. The concentrations were selected such that lithium ion either dominated, accounted for a little over half, or accounted for only a small fraction of the total cation equivalents in the injection solution. Although tracer breakthrough curves differed significantly under each of these conditions, with highly asymmetric responses occurring at the highest injection concentrations, the three-component cation-exchange model reproduced the observed transport behavior of lithium and the other cations in each case with a similar set of model parameters. In contrast, a linear K(d)-type sorption model could only match the lithium responses at the lowest injection concentration. The three-component model will be used to interpret the field tests, with the expectation that it will help refine estimates of effective flow porosity, particularly if the lithium response curves are asymmetric.