A general reactive transport modeling framework for simulating and interpreting groundwater¹⁴C age and δ¹³C

Abstract

A reactive transport modeling framework is presented that allows simultaneous assessment of groundwater flow, water quality evolution including d 13 C, and 14 C activity or ''age''. Through application of this framework, simulated 14 C activities can be directly compared with measured 14 C activities. This bypasses the need for interpretation of a 14 C age prior to flow simulation through factoring out processes other than radioactive decay, which typically involves simplifying assumptions regarding spatial and temporal variability in reactions, flow, and mixing. The utility of the approach is demonstrated for an aquifer system with spatially variable carbonate mineral distribution, multiple organic carbon sources, and transient boundary conditions for 14 C activity in the recharge water. In this case, the simulated 14 C age was shown to be relatively insensitive to isotopic fractionation during DOC oxidation and variations in assumed DOC degradation behavior. We demonstrate that the model allows quantitative testing of hypotheses regarding controls on groundwater age and water quality evolution for all three carbon isotopes. The approach also facilitates incorporation of multiple environmental tracers and combination with parameter optimization techniques.

Physical and Chemical Controls on the Simultaneous Occurrence of Young and Old Groundwater Inferred From Multiple Age Tracers

“…Groundwater apparent age determination with 14 C relies on the well-defined radioactive decay from an initial 14 C activity at the time groundwater is recharged, which is usually assumed to be 100 pmC (Kalin, 2000). However, in natural systems, rock-water interactions can have a large influence on 14 C (Clark & Fritz, 1997;Salmon et al, 2015). A number of correction techniques that account for the sample's δ 13 C signature allow corrections to be made for 14 C activities resulting from chemical reactions (Han & Plummer, 2013;Ingerson & Pearson, 1964;Mook et al, 1974).…”

“…The selected model setup represents an unconfined aquifer, 4-km long and 200-m deep, loosely based on aquifer geometries observed at the Pilbara site by Cook et al (2017). A 2-D model geometry was selected over a full 3-D model for the sake of higher computational efficiency while not significantly compromising the general findings as per previous studies (D'Affonseca et al, 2011;Ekwurzel et al, 1994;Ma et al, 2014;Salmon et al, 2015;Solomon & Sudicky, 1991;Zhang et al, 2013). Groundwater flow was simulated with MODFLOW-NWT (Niswonger et al, 2011) and multispecies environmental tracer transport simulations were performed with MT3DMS-USGS (v1; Bedekar et al, 2016).…”

“…A spatially uniform recharge distribution is the basic assumption of several age dating methods (Cook & Böhlke, 2000) and also a commonly made assumption in numerical modeling studies (e.g., Salmon et al, 2015;Stauffer et al, 2002;Weissmann et al, 2002). However, in most aquifer settings, recharge varies spatially across a catchment due to variations in soil and vegetation cover (Cook et al, 1998), topographic relief (McGuire et al, 2005), localized recharge sources such as surface waters, wetlands or mountain front (Fulton et al, 2012;Siade et al, 2015;Wilson & Guan, 2004), and the variability of vadose zone processes (Scanlon et al, 2002;Wood et al, 2015Wood et al, , 2017.…”

“…These processes lower the 14 C activity in aquifers and thus cause an overestimation of groundwater ages (Clark & Fritz, 1997;Cook et al, 2017;Han & Plummer, 2013;McCallum et al, 2017;Salmon et al, 2015). In the previous studies undertaken in the Pilbara region, Cook et al (2017) corrected samples using the Pearson Model (Ingerson & Pearson, 1964), whereas McCallum et al (2017) used the Mook Model (Han & Plummer, 2013).…”

“…Additionally, chemical processes may cause erroneous interpretation of the true groundwater age for one or more of the analyzed tracers. For 14 C, these processes include dissolution of old carbonates and fractionation due to reactions including ion exchange and oxidation-reduction (Clark & Fritz, 1997;Salmon et al, 2015). CFCs are also prone to sorption and degradation under specific conditions (Bauer et al, 2001;Cook & Solomon, 1995;Hinsby et al, 2007), which can influence tracer concentrations and create bias in groundwater age dating.…”