Moreau, M.; Morgenstern, U.; van der Raaij, R.W.; Horrox, J. 2021 West Coast groundwater dynamics and hydrochemical evolution as inferred from regional water age and chemistry tracer data. Lower Hutt, N.Z.: GNS Science. GNS Science report 2021/16. 69 p.; doi: 10.21420/60DD-SQ44
Abstract
This study aims to holistically describe the flow sources, pathways and lag times of water through the rivers and aquifers of the West Coast catchments. This information is required to ground truth and improve groundwater flow models and management tools. Such improvements will help to prevent degradation of rivers and aquifers from land-use activities that impact on cultural, recreational and economical values, as well as on drinking water supplies and quality. This study is mainly funded through the MBIE Endeavour programme ‘Te Whakaheke o Te Wai’ and complemented by the MBIE Envirolink fund (C05X2005). Groundwater chemistry and age tracer data were assembled from a range of sources: (i) GNS’s national datasets (National Groundwater Monitoring Programme, National Tracer Survey), (ii) regional datasets (State Of the Environment monitoring and specific site investigations), (iii) existing data from the Water Dating laboratory and (iv) water samples collected in March 2020 as part of this study. The aggregated dataset consists of 100 sites (including 97 bores, two springs and one river) and spans the period November 1998 to March 2020. The environmental tracer data (age, isotopes, temperature, gas concentration and chemistry) was combined and interpreted using graphical analysis (e.g. Piper diagram) spatial analysis and multivariate statistics. These analyses inform characterisation of groundwater flow and hydrochemical processes in the West Coast aquifers. Short (less than 10 years) residence times are typical for groundwater resources in the region. Three geographically distinct hydrochemical signatures are consistent with topography and geological distinct areas: pristine, alpine foothills (cluster A); impacted coastal or fluvial (clusters B1 and B2) areas; and a dilute valley with indication of land-use impact (clusters C1, C2 and C3). West Coast rivers and streams are hydraulically well connected to adjacent Holocene gravel aquifers. These aquifers are mostly oxic in nature. There is does not appear to be a relationship between well depth and groundwater age, indicating absence of confined or disconnected groundwater condition throughout large parts of the region’s aquifers. In the Grey Valley, groundwaters have tracer signatures indicative of recharge from local rain and rivers. This area has the largest groundwater demand and abstraction for the region (Mourot and White 2020). Time series groundwater age data for the valley was not sensitive enough to identify impact of abstraction on groundwater circulation from increased abstraction between 2012 and 2017 (WCRC 2014). It is expected that significant change in water allocation may be rapidly reflected in future water age monitoring data due to the short residence time of water in this system (auth)
