Groundwater age determination along a transect across the Central Plains, Canterbury, New Zealand

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van der Raaij, R.W. 2012 Groundwater age determination along a transect across the Central Plains, Canterbury, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2012/16 56 p.

Abstract: Following a study into the variation in chemistry along two transects located on the Central Plains area of Canterbury, 18 wells were sampled for age-tracers (tritium, CFC-11, CFC-12, SF6, 14C) in 2011 along the NW-SE trending transect which lies parallel to the general groundwater flow of the area. In addition, age-tracer data from various other wells located along or nearby the transect were available from past sampling campaigns conducted between 1970 and 2011. Groundwater ages vary across the study area and range from very young, in the order of a few years, to very old. The oldest groundwater had a radiocarbon age of 3000 to 4400 years. Groundwater ages increase both with depth and with distance along the transect. Deeper groundwater (i.e. >150m) throughout the study area is unequivocally old with ages of at least 100 years based on tritium. In the confined aquifer system groundwater is also old, implying upwelling of older water and limited offshore flow. Shallow wells which are predominantly land surface recharged have younger groundwater but show considerable variation in the age to depth relationship possibly due to a variety of reasons including the heterogeneity of the aquifer material, variable depths of the unsaturated zone and variable contributions of rainfall and irrigation to recharge. Excess air concentrations may be controlled by fluctuations in water level, with higher concentrations in areas with deep unsaturated zones where changes in water level are relatively large. CFC and SF6 data, while generally showing younger model groundwater ages than tritium, are useful for overcoming ambiguity in age data derived from tritium in areas where the groundwater is relatively young (i.e.