The use of radon and complementary hydrochemistry tracers for the identification of groundwater - surface water interaction in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Environmental Management at the Institute of Agriculture and Environment, Massey University, Palmerston North, New Zealand
Understanding how surface waters and groundwaters interact is an integral component of managing the influence of nutrient inputs to water quality. Knowledge of the potential nutrient loads from discharging groundwater is essential for meeting the bottom line nutrient concentrations in surface waters. Radon-222 is an emerging tracer for measuring groundwater-surface water interaction which has been underexploited in New Zealand. The aim of this research was to establish the potential of using radon for measuring groundwater and river water interaction in the New Zealand environment.
Low and high resolution radon surveys were carried out in two gravel-bed rivers, the Hutt and Mangatainoka Rivers, in lower North Island of New Zealand. To provide accurate measurements of radon concentrations in surface waters containing very little radon, the development of a cost and time effective, simple and reproducible high sensitivity radon measurement method was investigated. Furthermore, the study aimed to assess the potential of using radon measurements in combination with concurrent stream flow gauging and other hydrochemistry data for providing more detailed information on groundwater and river water interaction processes.
Radon measurements were found very helpful to identify groundwater discharge and recharge locations in both the Hutt and Mangatainoka Rivers. Furthermore, a high sensitivity radon analysis method was developed with a lower limit of detection of 0.006 BqL-1, a vast improvement on the direct count method, and offering practical advancements over previously published methods. This high sensitivity method was used to establish radon concentration thresholds to identify locations of groundwater discharge, potential groundwater recharge and hyporheic exchange in NZ gravel-bed rivers.
In both studied rivers the groundwater discharge and potential recharge patterns identified by radon were not always matched by the concurrent flow gauging surveys, highlighting the ambiguity surrounding the use of concurrent flow gauging in gravel-bed rivers for mapping river gains and losses. In some sections of the studied rivers the concurrent flow gauging data indicated areas of groundwater recharge or discharge where the radon data showed the opposite process to be occurring. This has led to the conclusion that underflow beneath the gravels and other parafluvial exchange processes can cause the interpretation of concurrent flow gauging results to be misleading. Flow gauging combined with radon sampling gives a more conclusive picture of the groundwater and river water interaction processes in the gravel-bed rivers.