Constraining spatial variability in groundwater recharge in an arid environment using carbon-14

Author: Cameron Wood

Wood, Cameron, 2015 Constraining spatial variability in groundwater recharge in an arid environment using carbon-14, Flinders University, School of the Environment

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Carbon-14 (14C) dating of groundwater has been widely used over the past 50 years to estimate apparent groundwater ages and investigate groundwater recharge. With a half-life of approximately 5730 years, 14C has proven particularly useful in arid environments, where low rainfall and low recharge rates can result in long residence times. However knowledge gaps relating to 14C interpretation still exist.

The first part of this thesis investigates the 14C activity of unsaturated zone gas. Unsaturated zone 14CO2 is typically assumed to be in equilibrium with atmospheric 14CO2 (ie. modern). A number of researchers have shown that this may not be the case, with significant implications for groundwater 14C interpretation. However little is known about how unsaturated zone 14C activities may vary spatially. Measurements of 14C in unsaturated zone gas were made at five sites across the arid Ti Tree Basin in central Australia. At all sites, a trend of decreasing 14C activity with depth in the unsaturated zone was observed. Variation in unsaturated zone thickness related to variation in the 14C activity above the watertable at each site (generally lower 14C for deeper unsaturated zones). Modelling of unsaturated zone CO2 production and transport showed that the dilution of unsaturated zone 14C related to production of ‘old’ CO2 in the unsaturated zone, most likely from mineral dissolution-precipitation fluxes. Where these processes are homogeneous, spatial variation in unsaturated zone thickness leads to spatial variation in the 14C activity of unsaturated zone gas above the watertable.

The second part of this thesis explores the influence of spatially variable 14C inputs and recharge on 14C activities in an aquifer. This was investigated through theoretical 2D groundwater flow and solute transport modelling, with a focus on low recharge environments (where diffusive transport is significant). Results show that recharge estimated from discrete point measurements of 14C may be wrong when recharge and 14C inputs vary spatially, and that vertical profiles of 14C in groundwater are needed to overcome this problem. Vertical profiles of 14C in groundwater in the Ti Tree Basin were then interpreted using 2D flow and transport models, with a spatially variable boundary condition for 14C. This revealed mountain front recharge, as well as recharge from ephemeral surface water to be important mechanisms.

The final part of this thesis synthesizes these findings into a 3D regional groundwater flow and solute transport model of the Ti Tree Basin. Carbon-14 activities were modelled directly, accounting for advection, diffusion and dispersion. The 14C boundary condition at the watertable was based on measurements of 14C near the watertable and unsaturated zone thickness (ie. lower 14C activities where unsaturated zones are thicker). Recharge rates were determined by calibrating to groundwater head and 14C activities. The results demonstrate the value of a distributed tracer data set (and understanding of its boundary condition) in resolving spatial variability in recharge. These findings will be relevant to other arid environments, and to other tracers which are transported to groundwater through the unsaturated zone.

Keywords: Groundwater, carbon-14, recharge, arid zone

Subject: Environmental Science thesis

Thesis type: Doctor of Philosophy
Completed: 2015
School: School of the Environment
Supervisor: Peter Cook