Groundwater Resource Monitoring
Groundwater is a vital resource that presently accounts for about one third of all water usage in New
Zealand. Study of isotopic tracers is giving us a powerful tool to manage groundwater in sustainable ways, and to protect sources from contamination.
Tritium is a rare and naturally occurring radioactive isotope of hydrogen with a half-life of 12.4 years. Tritium water dating is based on the radioactive decay of tritium. The concentration of tritium in groundwater reflects the length of time since rainwater entered the subsurface, therefore representing the age of the sample. Water dating has been hailed internationally as a breakthrough in managing aquifer systems and in detecting early signs of deteriorating water quality. The ability to accurately date young groundwater adds considerable value to groundwater resource or quality surveys.
The GNS Science water dating laboratory has partnered with numerous research programmes and government and industry projects around the world. One such example is our on-going partnership with local authorities across New Zealand to determine groundwater age as a parameter for drinking water security. Deeper groundwater sources can overcome the most common problems that arise from microbiological contamination of surface water, shallow groundwater and spring water. The Drinking-water Standards for New Zealand specifies that if the fraction of water with age less than one year is less than 0.005% of the water present in the aquifer, it is unlikely that contamination from recent sources of pollution will be a problem. We measure samples collected by councils from around the country for regular monitoring of drinking water contamination, including the fraction of water less than one year old. Our ability to detect very low levels of tritium is essential for precision determination of water age.
The GNS Science water dating lab is currently recognised by the IAEA (International Atomic Energy Agency) as the most precise tritium laboratory world-wide. Every year, we analyse hundreds of groundwater sample from around the world for a wide range of research programmes and industrial applications in groundwater resource, climate change, oceanography, geothermal dynamics and hydro-dam security.
Radon-222: an effective way to understand interactions between groundwater and surface water
Isotopic tracers reveal a great deal about recharge areas, flow paths, aquifer structure and impact of human activities on water quality, which provide a scientific basis for effective management of water allocation, water quality monitoring, prevention of contamination and water-resource planning.
One example is a recent pilot project to measure the concentration of radon-222 gas in river water as a more effective way to understand interactions between groundwater and surface water. Often times these two resources are strongly interconnected and the quality and quantity of one directly affects the other. Knowing where these exchanges occur, and how much water is being exchanged, can help significantly in managing the quality of our fresh water bodies.
Radon is a soluble colourless, gaseous, unstable isotope produced by the decay of radium. It has a half-life of 3.8 days and emits an alpha particle. Radon is abundant in groundwater but has almost negligible concentrations in surface water due to rapid radon loss to the atmosphere through degassing. This contrast in radon concentrations between groundwater and surface water enables radon to be an ideal tracer to measure groundwater-surface water interaction. Surface waters that have elevated concentrations of radon indicate a location where groundwater is discharging into the surface water.
In a Hutt River study, we sampled river water at 500-800 m intervals over a 16kms reach. We then measured the alpha particle radiation in the sample, which relates directly to radon concentration. This method enabled us to identify where groundwater is discharging into the river as well as where river water may be recharging into the aquifer system. Recently, a higher sensitivity radon measurement procedure has been developed at GNS Science allowing us to identify the radon concentration of surface waters with very little radon and with higher accuracy.
The entire reach of the Mangatainoka River, approximately 70 km long, was also surveyed over a two day period using radon. Radon identified the locations of groundwater discharge and recharge. The results of this study will aid in the development of nutrient flow models being developed for the area.
Radon sampling and analysis is a rapid, cost-effective method for screening long river reaches to identify local groundwater inflows. The liquid scintillation method used in these pilot studies has the advantage over other methods of being able to measure large numbers of samples at once. In addition, this method provides another tool to study the transport of nutrients from farms to streams and rivers.
Scientists at GNS Science use isotopes for better understanding of the source, fate, and future loads of nitrogen
The global environment is increasingly ‘saturated’ in nitrogen, due to increasing industrial and transport emissions and the intensification of agriculture. Excess nitrogen cascades through the environment, and enters ground and surface water as nitrate. Nitrate contaminates groundwater resources, and causes eutrophication in streams, lakes and estuaries.
Scientists at GNS Science provide Australasia’s only capability using naturally occurring isotopes to track the source and fate of nitrate, NO3. Within the NO3 molecule, N isotope ratios allow us to trace the source of nitrogen, while O isotope ratios allow us to identify the source of oxygen, in either H2O or, in the case of some fertilisers or atmospheric sources, atmospheric O2. Once NO3 is formed, de-nitrification and other biogeochemical processes affect isotope ratios, allowing the fate of NO3 to be tracked. For most water samples <100 mL, scientists convert NO3 into N2O while preserving its isotopic composition for measurement on our mass spectrometers.
Our science is working toward identifying successful mitigation of nitrate contaminants, improving the nitrogen efficiency of New Zealand agriculture. By combining this knowledge with precise groundwater ages from our tritium laboratory, we calculate future trajectories of nitrate contamination in lakes and other waters. Our work enables options for mitigating the contamination and eutrophication of valuable freshwater to be assessed for policy decisions.
For more information, please contact:
Stew Cameron, Head of Department, Hydrogeology, GNS Science