An isotopic study of dissolved inorganic carbon in the catchment of the Waimakariri River and deep ground water of the North Canterbury Plains, New Zealand

Abstract

Processes of evolution of total dissolved inorganic carbon (TDIC) and its isotopic composition have been followed by modelling and measurement in the catchment of the Waimakariri River and the ground water of the section of the Canterbury Plains recharged by infiltration from that river. Although the TDIC concentrations are low (maximum 1.2 mmol kg −1 water in this study), clear chemical—isotopic relationships emerge, which are consistent with the modelling approach. Total 14C concentrations in rivers and shallow ground water are linearly related to TDIC, with slope similar to the 14C concentration of atmospheric CO 2 during the sampling period. 13C concentrations of Waimakariri River and its tributaries reflect attainment of equilibrium between exchange with atmospheric CO 2 and direct addition of CO 2 to the water by decay of organic material. As river water infiltrates to the aquifers (fluvioglacial deposits of age several hundred thousand years, mainly greywacke), further TDIC accrues by residual decay of organic material of recent 14C concentration. Subsequent changes as the water moves to and through the deeper aquifers appear to be limited only to broadening of the age spectrum by natural flow processes, accompanied by radioactive decay of 14C. This allows age assessment of the tritium-free, artesian ground water presently drawn from the deep aquifer in near-coastal areas in and close to Christchurch. Based on the 14C evidence alone, mean residence times range from about 800 years just outside the confined zone to 2000–3000 years in the confined area. A separate age determination uses the 13C data to evaluate the contributions to TDIC of each deep water sample by the river and biogenic components at the time of recharge; agreement with the 14C residence times is achieved when reasonable pre-industrial 14C concentrations are assigned to these two components. The ages appear to be in conflict with positive tritium indications during the early 1970s and with shorter residence times evaluated from estimates of aquifer storage and withdrawals; this suggests that the flow regime has been altered by sharply increasing withdrawal during the past 4 decades, causing the appearance now of water from deeper levels which has remained underground for longer periods as a component of the undisturbed flow regime.