On the isotopic composition of dissolved inorganic carbon in rivers and shallow groundwater; a diagrammatic approach to process identification and a more realistic model of the open system.

Abstract

Rivers and shallow groundwater are deep groundwater precursors. Their dissolved inorganic carbon content (DIC) and its isotopic composition are end members in the evolution of these properties in confined situations, and are therefore essential information when applying carbon isotopes as tracers of groundwater processes and determining aquifer residence times using (super 14) C. During studies of regional aquifer systems in New Zealand, a simple model has been developed to explain the isotopic compositions of DIC encountered in rivers and shallow groundwater. The model format incorporates a diagrammatic approach, providing a framework for tracing the subsequent evolution of DIC in both precipitation-and river-recharged aquifers under closed conditions. DIC concentration of rivers continuously adjusts toward chemical and isotopic equilibrium between direct addition of CO (sub 2) to the water (via plant respiration and decay of dead organic material) and exchange of CO (sub 2) across the river-air interface. In the shallow groundwater situation, the gaseous reservoir is soil CO (sub 2) , generally at significantly higher partial pressure. In both cases, calcite dissolution or other processes may be an additional source of DIC directly added to the bicarbonate and dissolved CO (sub 2) components; while these may add or remove DIC, steady-state isotopic concentrations are considered to be determined only by the dynamic balance between directly added CO (sub 2) and gas exchange. This model allows the calculation of steady states, using selectable parameters in river or groundwater situations. These appear as straight lines in (super 13) C or (super 14) C vs. 1/DIC, or total (super 14) C vs. DIC plots, into which the experimental data can be inserted for interpretation. In the case of (super 14) C, the steady-state balance is very often complicated by the presence of an old component in the directly added DIC; the understanding achieved via the (super 13) C patterns is helpful in recognizing this. Data from four contrasting aquifer systems in New Zealand. The success of the approach has depended crucially on DIC concentrations measured very accurately on the isotope samples, rather than separate chemical analyses.