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Mapping groundwater discharge into rivers using radon

Funder: Horizons Regional Council and GWR research programme  Duration: 2014- present
Contact: Heather Martindale GWR Research Programme

Objectives

Understanding how surface waters and groundwaters interact is an integral component of managing the influence of nutrient inputs to water quality. The aim of this research was to establish the potential of radon for measuring groundwater and river water interaction in two North Island, New Zealand gravel bed rivers. Test studies were carried out in the Hutt and Mangatainoka Rivers in the lower North Island.

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Description

Radon is a soluble colourless, gaseous, unstable isotope with a half-life of 3.8 days. It is generated as part of the uranium decay series. Uranium is present in almost all rocks and soils, resulting in the release of radon from uranium bearing minerals in groundwater. Radon is abundant in groundwaters but has almost negligible concentrations in surface waters due to rapid radon loss to the atmosphere through degassing. This contrast in radon concentrations between groundwaters and surface waters enables radon to be an ideal tracer to measure groundwater-surface water interaction. Surface waters which have elevated concentrations of radon indicate a location where groundwater is discharging into the surface water.

We collected 25 mL radon grab samples at 500-1000 m intervals along a 14 km stretch of the Hutt River in April 2014 and January 2015 and across the entire length, approximately 70 km, of the Mangatainoka River under low flow conditions during February 2015. The collected samples were then analysed for radon measurement at the GNS water dating laboratory using a Low Level Liquid Scintillation Counter.

In addition to the radon sampling, extensive flow gauging surveys were undertaken in both the Hutt and Mangatainoka Rivers, with flow measured at 22 sites in the Hutt River and at 29 sites in the Mangatainoka River.

Findings

Groundwater discharge locations were identified in both the Hutt and Mangatainoka Rivers using radon (Fig. 1 and 2).

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.

Figure 1

Figure 1: Measured radon concentrations in the Hutt River at low flow on 10 January 2015. The colour denotes radon concentrations in BqL-1, where concentrations over approximately 0.5 BqL-1 indicate groundwater discharge.

Fig Radon concentration along the Mangatainola River

Figure 2: Measured radon concentrations in the Mangatainoka River at low flow in February 2015. The colour denotes radon concentrations in BqL-1 where concentrations over approximately 0.5 BqL-1 indicate groundwater discharge.

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