Project overview

This project is part of the Smart Aquifer Characterisation (SAC) Programme, which aims to develop innovative methods to characterise groundwater systems in a cost-effective and rapid way. Substances that are part of the natural atmosphere on a global scale and enter the subsurface with rain water and have time-dependent concentrations can be used as age tracers for groundwater dating. However, they occur in extremely low concentrations and are thus difficult and expensive to analyse. This project aims to find faster ways to measure existing age tracers, and to find new age tracers that can be measured in a cost-effective way.

Background

For sustainable management of groundwater resources and watersheds in terms of water quantity and quality, and for validation of groundwater transport models, it is important to understand time scales (groundwater age) and related rates of recharge and flow, fate of contaminants, and lag time between land-use changes and changes in water quality in receiving water bodies. Without knowing the ages of groundwater, it is nearly impossible to quantify these processes. Tritium is the most robust age tracer but is extremely difficult and slow to measure—it takes two months to complete an analysis due to the low environmental levels of tritium. Gases like SF6 and CFCs can complement groundwater dating and help identify groundwater recharge processes, however, more than one gas tracer is required, and as CFCs are being phased out, they are now less efficient. Figure 1 shows the time-variable age tracer concentrations used in New Zealand. Hydrochemistry as an additional age proxy has been demonstrated only for water weeks to months old.

Tritium, CFC, SF6, and Halon-1301 input concentrations for New Zealand rain

Figure 1: Tritium, CFC, SF6, and Halon-1301 input concentrations for New Zealand rain.

Tritium

With its dating range of 0 – 100 years, tritium is the standard groundwater dating tool. Tritium is the ‘ideal’ tracer for water because chemically it is hydrogen and therefore is part of the water molecule. Tritium behaves conservatively—it decays only through radioactive decay and is not influenced by hydrochemical or biological reactions. The ‘tritium clock’ starts when the water infiltrates into the ground, therefore the tritium age includes travel time through the unsaturated zone.

Wider use of tritium is, however, limited due to extremely low natural tritium concentrations in rain, with ratios of tritium/total hydrogen of c. 10-18, which require lengthy analytical procedures. Using pre-concentration of tritium, followed by radiometric decay counting, we have established at GNS the most sensitive and accurate tritium lab in the world (Morgenstern and Taylor, 2009), with precision suitable for groundwater dating in the Southern Hemisphere. The use of mass spectrometry for tritium analysis through its decay product 3He has no advantage over the radiometric technique, as it requires long waiting times and extremely expensive analytical procedures.

Accelerator mass spectrometry (AMS) is also an extremely sensitive isotope analysis technique, with much reduced analysis times through direct counting of the abundant radio-isotopes, as opposed to waiting for them to decay and measuring their radiation or decay products. If AMS could be applied to measure environmental tritium concentrations, sample throughput could be increased 10-fold. With the relevant research groups in Germany and the United States who have previously worked on tritium AMS, we have explored if AMS has potential for low-level tritium measurement. The conclusion of their experiments with ‘hot’ tritium samples and our extrapolation to low-level measurement is that the sensitivity of the current AMS technique for tritium is 5 – 7 orders of magnitude too low, and it is not possible to measure tritium with AMS at the low concentrations that are required for the dating of water. Therefore GNS’ current technique for tritium analysis, despite being lengthy, will continue to be the most sensitive method for tritium analysis necessary for water dating for the foreseeable future.

Halon 1301

The use of gas tracers as complementary dating tools requires multiple gas tracers, due to limitations in individual techniques, with different shapes of input functions and robustness to degradation and local contamination. 3He and 85Kr techniques are available but extremely expensive and therefore limited in their use. More economic methods include sulphur hexafluoride (SF6), a stable compound in the atmosphere and in groundwater of primarily anthropogenic origin, and chlorofluorocarbons (CFCs) (Fig. 1). Steadily increasing concentrations of SF6 in the atmosphere since 1980 provide a unique age signal for waters recharged since then. In contrast, CFC concentrations have declined, after their use was phased out due to their contribution to stratospheric ozone depletion, and in addition with CFC degradation in anoxic groundwater and CFC contamination in many groundwater systems now make the CFCs a less efficient age tracer, with a non-unique age interpretation. Another age tracer is required! Within this SAC research programme we searched for and discovered a new age tracer, Halon-1301 (Beyer et al., 2014), and demonstrated that it is suitable for groundwater dating (Beyer et al., 2015). Halon-1301 is used as refrigerant gas and fire suppressant agent and its concentration in the atmosphere is still increasing (Fig. 1), making it a suitable age tracer. It is extremely economic, as it can be measured simultaneously with SF6 at very little additional cost.

Hydrochemistry as age proxies

The hydrochemistry of groundwater is influenced by time-dependent processes, and therefore may be useable as an indicator of groundwater age. Hydrochemical parameters can be measured at low cost and are spatially and temporally widely available through national and regional groundwater monitoring programmes. However, their use is challenging, as groundwater chemistry can be influenced by many factors in addition to residence time, such as abundance and type of minerals and microbes, climatic conditions, weathering, redox reactions, hydrolysis, precipitation, sorption, complexation and ion exchange reactions. In this SAC programme we show that a number of mineral weathering products such as Si, Na, Ca and TDS are suitable as complementary age proxies over suitable timeframes (years to decades) to reduce uncertainty in age information (Beyer et al., 2016).

Summary

With the most sensitive tritium lab world-wide, enabling for the required high precision, and using the new complementary age tracer Halon-1301, together with SF6, and progress in the use of hydrochemical parameters as age proxies, GNS is well equipped to maintain its leading position in the improved management of water resources through age-tracer-validated groundwater flow and transport models in years to come (e.g. Stewart and Morgenstern, 2016, Toews et al., 2016).

Data

The Halon sampling locations are available in the SMART Mapviewer.

References:

Beyer, M., R. van der Raaij, U. Morgenstern, and B. Jackson (2014) Potential groundwater age tracer found: Halon-1301 (CF3Br), as previously identified as CFC-13 (CF3Cl), Water Resour. Res., 50.

Beyer, M., van der Raaij, R., Morgenstern, U., and Jackson, B. (2015) Assessment of Halon-1301 as a groundwater age tracer, Hydrol. Earth Syst. Sci., 19, 2775-2789.

Beyer, M., Jackson, B., Daughney, C., Morgenstern, U., and Norton, K. (2016) Use of hydrochemistry as a groundwater age tracer, Accepted for Journal of Hydrology

Morgenstern, U. and Taylor C.B. (2009) Ultra Low-level tritium measurement using electrolytic enrichment and LSC, Isotopes in Environmental and Health Studies Vol. 45, No. 2: 96-117

Stewart, M.K., Morgenstern, U. 2016: Importance of tritium-based transit times in hydrological systems. WIREs Water 3, 145–154. doi: 10.1002/wat2.1134

Toews, M. W., Daughney, C. J., Cornaton, F. J., Morgenstern, U., Evison, R.D., Jackson, B.M., Petrus, K., and Mzila, D, (2016) Numerical simulation of transient groundwater age distributions assisting land and water management in the Middle Wairarapa Valley, New Zealand Water Resour. Res., revision submitted