Wednesday, 13 July 2016

Groundwater finds its way to surface water – but where?

Heather Martindale sampling
in the Hutt River.
Heather Martindale (GNS Science Water Dating Laboratory Snr Technican) recently graduated with a Masters in Environmental Management from Massey University in Palmerston North. Her project looked at “The use of radon and complementary hydrochemistry tracers for the identification of groundwater – surface water interaction in New Zealand”.

Knowing the interaction between groundwater and surface water is an essential part of understanding the movement of harmful nutrients in groundwater to surface water.

Why is Radon used as a tracer to find out where groundwater flows into surface water?

Radon-222, a colourless gas with a half-life of 3.8 days, is generated by the decay of uranium present in rocks and soil and is abundant in groundwater. While, radon has a very low concentration in surface water (rivers, streams) due to degassing (water releasing the gas to the atmosphere), the concentrations are slightly higher at, and/or immediately downstream of groundwater leakage to surface water. This concentration difference makes radon an ideal tracer to identify locations of groundwater leakage to surface water.
              
Heather’s Master’s project involved the collection of water samples at 500m – 1000m intervals in a 14km stretch of the Hutt River. She mixed the samples with a scintillation cocktail. When radon radioactively decays, it emits energy in the form of an alpha particle. This energy is absorbed by the scintillation cocktail and then released as a measurable light pulse. Heather’s study clearly demonstrated the usefulness of radon as a tool for identifying groundwater leakage into surface water, as shown in Fig. below.

Shows the measured radon concentrations in the Hutt River, with green (no radon) indicating areas of  no groundwater discharging into the river and red (high radon) indicating areas of high groundwater discharge into the river.
 For further reading, Heather’s full thesis is published here.