Groundwater surface water conceptual flow from environmental tracers Pukekohe and Bombay. Summary
Author:U Morgenstern, M Moreau, M A Coble, K Johnson, D B Townsend, GNS Science
Source:GNS Science | Auckland Council Research and Evaluation Unit, RIMU | Auckland Plan, Strategy and Research Department
Groundwater and surface water conceptual flow from environmental tracer signatures in the Pukekohe and Bombay area. Summary
Prepared by the Auckland Council, Auckland Plan, Strategy and Research Department.
High nitrate concentrations in groundwater from the Pukekohe/Bombay basalt aquifers and springfed streams have been recognised since the 1970s. Concentrations exceed the Maximum Acceptable Value (MAV) for drinking water and the National Policy Statement for Freshwater Management 2020 (NPS-FM) national bottom line (NBL) for nitrate toxicity in rivers.
The purpose of this study was to improve understanding of conceptual groundwater flow, its connection with surface water, nitrate pathways, the potential risk of nitrate contamination in deeper aquifers, future nitrate loads, and connections within the aquifer system. This will improve understanding of land use effects on water quality over time. It will also ensure any future nutrient management tools and regional plan provisions for the Pukekohe/Bombay area are evidence based.
Streams in the Franklin area (Hingaia, Ngakoroa, Oira Whangapouri, Whangamaire, Mauku and Waitangi) were studied. Flow gauging and surface water samples were collected down the length of each steam in February and April 2022 (low-flow conditions). Groundwater samples were collected in January and March 2021. Groundwater mean residence times (MRT) were established based on concentration of age tracers (radon, tritium). A range of water hydrochemistry variables, isotopes, and atmospheric gas concentrations were also analysed.
Summary of GNS Science report, 2022/63