Age determination and hydrochemistry of groundwater from the Ashley - Waimakariri Plains, Canterbury, New Zealand

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van der Raaij, R.W. 2011 Age determination and hydrochemistry of groundwater from the Ashley - Waimakariri Plains, Canterbury, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2011/02 68 p.

Abstract: Environment Canterbury has recently undertaken a study of groundwater chemistry in the Ashley Waimakariri plains area. The purpose of the study is to increase understanding of the variation of groundwater chemistry across the Ashley Waimakariri plains and the relationship of this variation to recharge source and groundwater age as well as their influence on water quality in spring-fed streams. Groundwater chemistry samples were taken from 82 wells along four transects. Two transects were parallel to groundwater flow and two were perpendicular to groundwater flow. A subset of 25 wells was sampled for various groundwater age-tracers (tritium, CFCs, SF6, 14C). In addition, previous age-tracer data from 53 wells were incorporated into the study. Groundwater ages varied across the study area and ranged from very young, in the order of a few years, to very old. The oldest groundwater had a radiocarbon age of almost 11000 years. South of the Eyre River, shallow groundwater is relatively old, due to the low permeability of the aquifer system in this area. Shallow groundwater north of the Eyre River had ambiguous age data and the groundwater age could not be resolved. Deeper groundwater is unequivocally old (> 50yrs) throughout the study area, with age increasing towards the coast. In the confined aquifer system groundwater is older (> 80 yrs), implying very limited offshore flow, except for a wedge of young groundwater adjacent to the Ashley River. Groundwater recharged from the Ashley River can be clearly identified by oxygen isotope (delta 18O) data and has delta 18O values more negative than -9.1‰. Groundwater over the rest of the study area, with delta 18O values between -8.5 and -9.1‰, appears to be a mix of recharge sources, including alpine rivers (Waimakariri, Ashley), foothill rivers (Eyre, Cust), and inland and coastal rain. Groundwater south of the Eyre River appears to have a higher contribution of river and inland rainfall than groundwater north of the Eyre River. The excess air content of the groundwater appears to be affected by recharge mechanism, and may be a useful tracer for recharge source identification in future studies. Prediction of groundwater age based on discriminant analysis of nine variables (well depth, electrical conductivity and the concentrations of the ions Na, K, Ca, Mg, HCO3, Cl and SO4) was mildly successful. Of 48 wells with sufficient data to make a prediction, the correct age classification was predicted as either the highest or second highest probability category for 28 bores (60%). Positive correlations between groundwater age and pH, conductivity, dissolved calcium, bicarbonate, silica and magnesium were observed, as expected due to prolonged water-rock interaction. No correlation between chloride and groundwater age was observed and variation in this parameter was attributed to different recharge sources, while a negative correlation b etween sulphate and groundwater age was linked to the groundwater redox state. (auth)