Search

Denitrification in groundwaters is a key nitrate (NO3-) attenuation process, where leached NO3- can be reduced to gaseous forms of nitrogen (N2). In this study, calculation of the concentration of excess N2 is applied to the New Zealand context to quantify the extent of denitrification occurring in groundwater systems under natural flow conditions. The concentration of dissolved atmospheric N2, according to the recharge conditions of the water, can be established by the measurement of two noble gases that are also part of the atmosphere. This enables differentiation of the excess N2 gas produced via denitrification reactions from atmospherically derived dissolved N2 gas. The excess N2 method was applied to ten shallow piezometers in the Lake Taupo catchment, in combination with other denitrification proxies: δ18O and δ15N isotopes of NO3-; identification of microbial denitrifying genes; and redox conditions. Eight of the ten sites had measurable excess N2. Excess N2 was detected at sites where δ18O and δ15N did not identify denitrification and at some sites that were oxic or in a mixed redox state. This is because the redox classification only indicates the potential for denitrification to occur, unlike the excess N2 method (which detects the actual accumulated denitrification product), and the dual nitrate isotope method is limited by the potential variation in source signatures and the nitrate itself being consumed. [ABSTRACT FROM AUTHOR]

Springs in the Christchurch region were sampled and geochemically analysed with the aim of understanding their sources and subsurface flowpaths. As well as the chemistry and water stable isotopes, a range of age tracers including tritium, chlorofluorocarbon, sulphur hexafluoride and Halon-1301 concentrations were examined. The chemical compositions and stable isotopes showed that the waters are dominated by seepage from the Waimakariri River with variable additions of infiltrated rainfall. Tritium and the gas concentrations showed that the four springs emerging on the western side of Christchurch are young (2 to 14 years), and likely sourced from the first confined aquifer (Aquifer 1). The two eastern Christchurch springs are very old (>180 and 150 years) and are likely to represent leakage from deeper aquifers. [ABSTRACT FROM AUTHOR]

We recently discovered a new groundwater age tracer, Halon-1301, which can be used to date groundwater recharged after the 1970s. In a previous study, we showed that Halon-1301 reliably inferred groundwater age at the majority of groundwater sites studied. At those sites, ages inferred from Halon-1301 agreed with those inferred from SF6 and tritium, two reliable widely applied groundwater age tracers. A few samples, however, showed reduced concentrations of Halon-1301, preventing meaningful age interpretation from its concentration. These reduced concentrations were likely a result of degradation or retardation of Halon- 1301 in the aquifer. However, we could not provide full evidence for this due to the limited number of groundwater samples analysed (18 in total). In this study, we assess the potential of Halon-1301 as a groundwater age tracer for a larger dataset of groundwater samples under specific groundwater conditions, including highly anoxic young groundwater which can significantly degrade Halon-1301, to gain more information on the magnitude of occurrence and the causes of reduced Halon-1301 concentrations. In this study, we analysed 302 groundwater samples for Halon-1301, SF6, tritium and the CFCs CFC-11, CFC-12 and CFC-113. Comparison of age information inferred from the concentrations of these tracers allows assessment of the performance of Halon-1301 compared to other well established and widely used age tracers. The samples are taken from different groundwater environments in New Zealand and include anoxic and oxic waters with mean residence times ranging from < 2 years to over 150 years (tritium-free). The majority of assessed samples have reduced or elevated concentrations of CFCs, which makes it impossible to infer a reliable age using the CFCs for these samples. Halon-1301, however, reliably infers ages for CFCcontaminated waters. Three other groundwater samples were found to have elevated SF6 concentrations (contaminated). Again, at these SF6-contaminated sites, ages inferred from Halon-1301 agree with ages inferred from tritium. A few samples (14 sites) exhibit reduced concentrations of Halon- 1301, which result in elevated inferred Halon-1301 ages in comparison to those inferred from SF6, tritium and/or CFC- 113. Assessment of the groundwater environment at these sites gives further insight into the potential causes of Halon- 1301 reduction in groundwater. Overall, Halon-1301 gives age information that matches ages inferred from SF6 and/or tritium for the majority (97 %) of the assessed groundwater sites. These findings suggest that Halon-1301 is a reasonably reliable groundwater age tracer, and is in particular significantly more reliable than the CFCs, which may have contamination and degradation problems. Halon-1301 thus has potential to become a useful groundwater age tracer where SF6 and the CFCs are compromised, and where additional independent tracers are needed to constrain complex mixing models. [ABSTRACT FROM AUTHOR]

We recently discovered a new groundwater age tracer, Halon-1301, which can be used to date groundwater recharged after the 1970s. In a previous study, we showed that Halon-1301 reliably inferred groundwater age at the majority of studied groundwater sites. At those sites, ages inferred from Halon-1301 agreed with those inferred from SF6 and tritium, two reliable widely applied groundwater age tracers. A few samples, however, showed reduced concentrations of Halon-1301, preventing meaningful age interpretation from its concentration. These reduced concentrations were likely a result of degradation or retardation of Halon-1301 in the aquifer. However, we couldn't provide full evidence for this due to the limited number of analysed groundwater samples (18 in total). In this study, we assess the potential of Halon-1301 as a groundwater age tracer for a larger dataset of groundwater samples under specific groundwater conditions, including highly anoxic young groundwater which can significantly degrade Halon-1301, to gain more information on the magnitude of occurrence and the causes of reduced Halon-1301 concentrations. In this study, we analysed 302 groundwater samples for Halon-1301, SF6, tritium and the CFCs CFC-11, CFC-12 and CFC-113. Comparison of age information inferred from the concentrations of these tracers allows assessment of the performance of Halon-1301 compared to other well established and widely used age tracers. The samples are taken from different groundwater environments in New Zealand and include anoxic and oxic waters with mean residence times ranging from < 2 years to over 150 years (tritium-free). The majority of assessed samples have reduced or elevated concentrations of CFCs, which makes it impossible to infer a reliable age using the CFCs for these samples. Halon-1301, however, reliably infers ages for CFC-contaminated waters. Three other groundwater samples were found to have elevated SF6 concentrations (contaminated). Again, at these SF6 contaminated sites, ages inferred from Halon-1301 agree with ages inferred from tritium. A few samples (14 sites) exhibit reduced concentrations of Halon-1301, which result in elevated inferred Halon-1301 ages in comparison to those inferred from SF6, tritium and/or CFC-113. Assessment of the groundwater environment at these sites gives further insight into the potential causes of Halon-1301 reduction in groundwater. Overall, Halon-1301 gives age information that matches ages inferred from SF6 and/or tritium for the majority (97 %) of the assessed groundwater sites. These findings suggest that Halon-1301 is a reasonably reliable groundwater age tracer, and is in particular significantly more reliable than the CFCs, which may have contamination and degradation problems. Halon-1301 thus has potential to become a useful groundwater age tracer where SF6 and the CFCs are compromised, and where additional independent tracers are needed to constrain complex mixing models. [ABSTRACT FROM AUTHOR]

Groundwater dating using anthropogenic and natural tracer substances is a powerful tool for understanding groundwater dynamics for improved management of groundwater resources. Due to limitations in individual dating methods, often multiple tracers are used to reduce ambiguities. It is commonly accepted that there is a need for further complementary age tracers, in addition to current ones (e.g., tritium, SF6, and CFCs). We propose a potential new groundwater age tracer, Halon-1301 (CF3Br), which can easily be determined using gas chromatography with an attached electron capture detector (GC/ECD) developed by Busenberg and Plummer (2008). Its peak was noted by Busenberg and Plummer (2008), but they believed it to be CFC-13 (CF3Cl) at that time. We performed rigorous tests on gases containing or excluding Halon-1301 and CFC-13 and modern water samples and concluded that the two compounds have extremely similar retention times. Additionally, we found that the ECD response of CFC-13 is far too low to be detected in groundwater or air using standard volumes and sampling techniques. However, the peak areas and concentrations Busenberg and Plummer (2008) reported are in line with what would be expected for Halon-1301. Thus, we are confident that the peak formerly identified as CFC-13 is actually Halon-1301. Busenberg agrees with our findings. We further suggest that Halon-1301 has potential as a (complementary) age tracer, due to its established atmospheric history, and could hypothetically be used to date groundwater recharged in the 1970s or onward. We discuss known relevant properties, such as solubility and stability of Halon-1301 in the context of how these effect its potential application as a groundwater age tracer. Some open questions remain concerning how conservative Halon-1301 is-is it subject to degradation, retardation, and/or local contamination in groundwater. We are confident that Halon-1301 possesses important tracer relevant properties, but further work is required to fully assess its applicability and reliability as a groundwater age tracer in different groundwater environments. [ABSTRACT FROM AUTHOR]

Nitrate concentrations exceeding Ministry of Health potable limits (11.3 mg/L nitrate-N) have been a problem for Waimea Plains groundwater for a number of years. This work uses nitrogen isotopes to identify the input sources of the nitrate. The results in relation to nitrate contours have revealed two kinds of nitrate contamination in Waimea Plains groundwater - diffuse contamination in the eastern plains area (in the vicinity and south of Hope) attributed to the combined effects of the use of inorganic fertilisers and manures for market gardening and other land uses, and point source contamination attributed to a large piggery to the south of Hope. Once nitrate is introduced to a groundwater system it can take many years for it to be flushed out. Tritium measurements in wells are interpreted to give mean residence times, and the spread of residence times around the mean, for groundwater in different parts of the plains. Mean ages are youngest in the area south of Hope, where nitrate concentrations are highest, and increase to the south, west and north. The age distributions have been used to produce a nitrate input history for the Upper and Lower Confined Aquifers by simulating the nitrate measurements in the various wells. The timing of the derived nitrate input history shows that both the diffuse sources and the point source were present from the 1940s, which is anecdotally the time from which there were increased nitrate sources on the plains. The large piggery was closed in the mid-1980s. Unfortunately, major sources of nitrate (including the piggery) were located on the main groundwater recharge zone of the plains in the past, leading to contamination of the Upper and Lower Confined Aquifers. The contamination travelled gradually northwards, affecting wells on the scale of decades. Input of nitrate to the groundwater has been decreasing since about 1988 due to closure of the piggery. The resulting decrease in nitrate concentrations is now also gradually travelling northward. Groundwater to the south and west already had relatively low nitrate because of river and/or rainfall recharge with low nitrate concentrations. Improved monitoring and practices (e.g., best management practices and nutrient budgeting) need to be encouraged among market gardeners and other land users, taking special account of the location of the groundwater recharge areas around and south of Hope. [ABSTRACT FROM AUTHOR]