REACTIVE TRANSPORT OF 14C THROUGH A CARBONATE AQUIFER: IMPLICATIONS FOR CONTAMINANT MIGRATION

In many contaminant transport models, ¹⁴C is treated conservatively, and is assumed to move at the same velocity as groundwater.  However, this work and previous studies show that ¹⁴C moving through a carbonate aquifer can experience significant retardation. In this study, batch experiments were conducted to help understand the mechanisms that control ¹⁴C retardation, and improve reactive transport models.

Groundwater ¹⁴C is typically present as part of the bicarbonate anion, which tends not to sorb at naturally occurring groundwater pHs.  Even in the absence of sorption, there are several processes that may retard ¹⁴C.  These include matrix diffusion, precipitation/dissolution, and isotopic exchange with aquifer carbonate minerals. Batch experiments using ¹⁴C, high purity calcite, and dolomite were conducted to evaluate these processes. Minerals used in the experiments were analyzed via BET and SEM.  The experimental variables included mineral saturation state, particle age, and particle size, all of which affected ¹⁴C uptake.  Experiments suggest that ¹⁴C is very reactive with carbonate minerals, and that ¹⁴C activity is lost via a non-equilibrium uptake process that cannot be described by an isotherm.

A conceptual model of ¹⁴C uptake that captures the non-equilibrium retardation behavior was developed, incorporating exchange/sorption, exchange/recrystallization, and solid-phase diffusion reactions.  Carbon-14 uptake was modeled numerically using a continuous distribution of mass-transfer rate coefficients.  The numerical model describes a process where the sorbent mineral surface is divided into compartments, and a unique reaction rate is applied to each, rather than treating the mineral surface as a homogeneous, single reaction rate mass sink.  The set of reaction rates is calculated with a continuous gamma probability density function.  Mass-transfer rates varied from relatively fast (10^-2 h^-1) to very slow (10^-15 h^-1), depending on the experimental variables.