A 40-year record of carbon-14 and tritium in the Christchurch groundwater system, New Zealand: Dating of young samples with carbon-14

Abstract

Knowledge of the sources and flowpaths of water in the Christchurch groundwater system will be vital to future management of the system. To gain such knowledge, oxygen-18 (18O), tritium (3H), carbon-14 (14C), and chemical concentrations have been measured on deep and shallow groundwaters since 1970. 18O measurements show that seepage from the Waimakariri River is the dominant source of the groundwater. Early 3H measurements (in the 1970s) showed non-zero concentrations in the deep groundwaters, but these were discounted at the time as due to “at most a few percent of very young water”. However, reinterpretation in light of the 14C ages in this work has revealed much younger ages for the deep waters than was previously believed, with average ages of 38 years in 1971, 71 years in 1976, 98 years in 1985, greater than 120 years in 1986, and greater than 150 years in 1993–1994. Because the Waimakariri River was identified as the major source of the deep groundwater, the river’s 14C concentration between 1986 and 2006 was modelled by combining the records of its two carbon sources (biogenic carbon and atmospheric CO2). This allowed the initial 14C concentrations of the groundwaters to be unequivocally determined and their mean 14C ages estimated using the same flow model as was applied to the 3H measurements. The resulting mean 14C ages are in the range 5–1500 years. The long sequence of measurements reveals that the mean ages of the deep Christchurch groundwater have changed markedly during the study. The pre-exploitation rate of turnover of water in the system is not known, but was probably quite slow. By the 1970s, ages in the deep system (Aquifers 4 and 5) had become relatively young right across Christchurch (with mean ages of 60–70 years) indicating mainly lateral inflow of young Waimakariri River water because of groundwater abstraction. Mean ages measured since have gradually increased showing increasing upflow of much older water from depth – this water has 10–15% rainfall recharge and is sourced from the inland plains region. There is now (in 2006) a steep gradient in age from west to east across Christchurch (from 300 years to 1400 years) showing that a large body of much older, deeper water is stored on the seaward side of the system where the deep aquifers are blind. This body will yield good quality water for many years, but eventually it is likely to be replaced or bypassed by younger (a few hundred years old), Waimakariri River-dominated but surface recharge-bearing, water from inland.