Uwe Morgenstern

Ultra low-level tritium measurement using electrolytic enrichment and LSC†

We describe an advanced methodology for low-level tritium measurement in regard to calibration, electrolytic tritium enrichment, liquid scintillation counting (LSC) measurement, and prevention of sample contamination. Details are given on enrichment parameters and electrode processes for optimisation of enrichment reproducibility and on optimisation of LSC stability. Intercomparison results demonstrate high accuracy of the tritium measurement system. The use of accurate tritium data for groundwater dating in the southern hemisphere is demonstrated with data from several groundwater systems of New Zealand. †Updated paper: originally presented on the IAEA International Symposium “Quality Assurance for Analytical Methods in Isotope Hydrology” (August 2004, Vienna).

New frontiers in glacier ice dating: Measurement of natural 32Si by AMS

Cosmogenic 32Si with a half-life of ca. 140 years is an excellent candidate to provide time information in the range 100–1000 years. Accelerator mass spectrometry (AMS) could greatly enhance its applicability as a natural clock due to the small sample size required for measurement. In this paper we describe the requirements for AMS measurement of natural samples, and we demonstrate the first 32Si AMS measurement for rainwater and glacial ice and snow. The results indicate that with AMS measurement of ca. 1 kg of water, a period of seven half-lives (ca.1000 years) can be covered. The 32Si result on Fox Glacier ice (New Zealand, 43°S) indicates an ice residence time on the bottom of the glacier of more than 800 years.

Solar Forcing Recorded by Aerosol Concentrations in Coastal Antarctic Glacier Ice, McMurdo Dry Valleys

Abstract Ice-core chemistry data from Victoria Lower Glacier, Antarctica, suggest, at least for the last 50 years, a direct influence of solar activity variations on the McMurdo Dry Valleys (MDV) climate system via controls on air-mass input from two competing environments: the East Antarctic ice sheet and the Ross Sea. During periods of increased solar activity, when total solar irradiance is relatively high, the MDV climate system appears to be dominated by air masses originating from the Ross Sea, leading to higher aerosol deposition. During reduced solar activity, the Antarctic interior seems to be the dominant air-mass source, leading to lower aerosol concentration in the ice-core record. We propose that the sensitivity of the MDV to variations in solar irradiance is caused by strong albedo differences between the ice-free MDV and the ice sheet.

River infiltration to a subtropical alluvial aquifer inferred using multiple environmental tracers

Chloride (Cl−), stable isotope ratios of water (δ18O and δ2H), sulfur hexafluoride (SF6), tritium (3H), carbon-14 (14C), noble gases (4He, Ne, and Ar), and hydrometry were used to characterize groundwater-surface water interactions, in particular infiltration rates, for the Lower Namoi River (New South Wales, Australia). The study period (four sampling campaigns between November 2009 and November 2011) represented the end of a decade-long drought followed by several high-flow events. The hydrometry showed that the river was generally losing to the alluvium, except when storm-derived floodwaves in the river channel generated bank recharge—discharge cycles. Using 3H/14C-derived estimates of groundwater mean residence time along the transect, infiltration rates ranged from 0.6 to 5 m yr−1. However, when using the peak transition age (a more realistic estimate of travel time in highly dispersive environments), the range in infiltration rate was larger (4–270 m yr−1). Both river water (highest δ2H, δ18O, SF6, 3H, and 14C) and an older groundwater source (lowest δ2H, δ18O, SF6, 3H, 14C, and highest 4He) were found in the riparian zone. This old groundwater end-member may represent leakage from an underlying confined aquifer (Great Artesian Basin). Environmental tracers may be used to estimate infiltration rates in this riparian environment but the presence of multiple sources of water and a high dispersion induced by frequent variations in the water table complicates their interpretation.

ENSO variability in the deuterium-excess record of a coastal Antarctic ice core from the McMurdo Dry Valleys, Victoria Land

Abstract The El Niño–Southern Oscillation (ENSO) signal in coastal Antarctic precipitation is evaluated using deuterium-excess data measured from an ice core located at Victoria Lower Glacier (VLG) Dome, McMurdo Dry Valleys. Recent studies suggest that interannual variations in the intensity and position of the Amundsen Sea low, a low-pressure centre that controls moisture flux in the West Antarctic sector, is modulated by the ENSO. Deuterium-excess values from the VLG ice core, which serve as a proxy for changes in regional moisture flux, exhibit oscillations of equivalent duration to those observed in the Southern Oscillation Index (SOI). Results of cross-spectral analyses show that temporal fluctuations in deuterium excess and the SOI covary and are coherent at ~4.9, 3.6, 3.0, 2.6, 2.4 and 2.0 year frequencies between 1950 and 2000. We ascribe this covariance to shifts in the source and transport pathway of precipitation that is deposited in coastal Victoria Land as a consequence of ENSO’s influence. High values of deuterium excess are consistent with increased meridional flow carrying warm, moist air southward across the Ross Sea when the low-pressure centre is positioned to the north of the Ross Ice Shelf (La Niña mode). Low deuterium-excess values, which reflect a more westerly to southerly flow across the West Antarctic ice sheet and Ross Ice Shelf leading to cooler and drier en-route conditions, occur when the low-pressure centre is positioned above the Amundsen Sea (El Niño mode).

Numerical simulation of transient groundwater age distributions assisting land and water management in the Middle Wairarapa Valley, New Zealand

This study used numerical models to simulate transient groundwater age distributions using a time-marching Laplace transform Galerkin (TMLTG) technique. First, the TMLTG technique was applied to simple box models configured to match idealized lumped parameter models (LPMs). Even for simple box models, time-varying recharge can generate groundwater age distributions with highly irregular shapes that vary over time in response to individual recharge events. Notably, the transient numerical simulations showed that the breakthrough and mean ages are younger than in the steady flow case, and that this difference is greater for sporadic recharge time series than for more regular recharge time series. Second, the TMLTG technique was applied to a transient numerical model of the 270 km2 Middle Wairarapa Valley, New Zealand. To our knowledge this study is the first application of the TMLTG technique to a real-world example, made possible by the dataset of tritium measurements that exists for the Wairarapa Valley. Results from a transient mean age simulation shows variation from a few days to over a decade in either temporal or spatial dimensions. Temporal variations of mean age are dependent on seasonal climate and groundwater abstraction. Results also demonstrated important differences between the transient age distributions derived from the TMLTG technique compared to the much simpler steady-state LPMs that are frequently applied to interpret age tracer data. Finally, results had direct application to land and water management, for example for identification of land areas where age distributions vary seasonally, affecting the security of groundwater supplies used for drinking water. This article is protected by copyright. All rights reserved.

Potential groundwater age tracer found: Halon‐1301 (CF3Br), as previously identified as CFC‐13 (CF3Cl)

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.

Vertical stratification of redox conditions, denitrification and recharge in shallow groundwater on a volcanic hillslope containing relict organic matter

Natural denitrification in groundwater systems has been recognised as an ecosystem service that reduces the impact of agriculturally-derived nitrate inputs to surface waters. Identification of this ecosystem service within the landscape would permit spatially differentiated land management and legislation. However, spatial variation in groundwater redox conditions poses a significant challenge to such a concept. To gain understanding of the small-scale mosaic of biogeochemical and hydrological controls on denitrification, we established a well field consisting of 11 multilevel well (MLW) clusters on a hillslope containing relict organic matter buried by volcanic deposits 1.8 ka before present. Based on site-specific redox classification thresholds, vertical redox gradients and denitrification potentials were detected at 7 of the 11 sites. Palaeosols or woody debris, which had previously been identified in laboratory experiments as resident electron donors fuelling denitrification, were visually recognisable at 4 of the 7 MLW sites with vertical redox gradients. Moderately enhanced groundwater dissolved organic carbon (DOC) concentrations occurred where resident electron donors were evident. DOC concentrations were lower where anoxic and nitrate-depleted groundwater was found but with an absence of resident electron donors. In these instances, it was assumed that nitrate reduction had occurred somewhere upgradient of the sampled well screen along the lateral groundwater flow path, with the proximate electron donor (DOC) largely consumed in the process, since no evidence was found for denitrification being fuelled by inorganic electron donors. Due to high variability in the isotopic signature of nitrate in oxidised groundwater, the nitrate dual isotope method did not yield firm evidence for denitrification. However, realistic vertical patterns were obtained using the excess N2 method. Tritium-based age dating revealed that oxic conditions were restricted to young groundwater (mean residence time ≤ 3 y), while anoxic conditions were observed across a wider age range (3-25 y).

Contrasting Groundwater and Streamflow Ages at the Maimai Watershed

Understanding the links between groundwater age, runoff generation processes, and their effects on stream water transit time (TT) is a major research challenge. Here we present new tritium age dating and hydrogeological characterization from 40 bedrock wells drilled at the intensively studied Maimai Experimental Watershed in New Zealand. We investigated the extent, dynamics, and age of the groundwater in a 4.5 ha headwater catchment over a 400 day period. In particular, we explored the controls of bedrock structure on aquifer dynamics, the aquifer flow domain and its influence on time-varying stream water TT. We show that low permeability hillslope bedrock minimizes deep recharge, thereby regulating groundwater age, stream water MTT, and surface water-groundwater interaction. Two distinct hydrologic units can represent catchment storage: shallow young soil storage and deep much older bedrock groundwater. Groundwater ages near discharge zones were up to 23 years compared to soil water ages that ranged between 0.1 and 0.5 years. This difference in age for the two main storages resulted in contrasting seasonal stream water TT response. During the 8 month wet season, stream water TT was young and stable while stream water TT in the slightly drier summer season was highly dynamic. These qualitative field observations are a process exemplar that support the Berghuijs and Kirchner (2017) quantitative descriptions of preferential release of young streamflow; and for the Maimai catchment, support the notion that most groundwater is exchanged only slowly with the surface and is therefore relatively old.

Groundwater Subsidy From Headwaters to Their Parent Water Watershed: A Combined Field‐Modeling Approach

Headwater groundwater subsidy, defined here as out of catchment groundwater flow contribution from a headwater catchment to its larger parent watershed (i.e. higher-order stream), can influence the water quality and quantity of regional water resources. But the integrated flow and transport modeling approaches currently being implemented to quantify this subsidy are limited by an absence of critical field observations, such as water table dynamics and groundwater age that are required to test such models. Here, we couple tracer (and hydrometric) observations from the wellstudied 4.5 ha M8 headwater catchment in the Maimai experimental watershed with a new semianalytical free-surface integrated flow and transport model. Our main research goal is to quantify the magnitude, age and flowpaths of the headwaters groundwater subsidies at the Maimai experimental watershed. Additionally, we explore through virtual experiments, the effects of watershed slope, watershed active thickness, and recharge rate on the age, flowpath and magnitude of out of catchment headwaters groundwater subsidies versus within-catchment (or local) groundwater flow contributions. Our results show that more than 50% of groundwater recharged in the Maimai headwaters subsidizes their parent watershed. The relative proportion of headwaters groundwater subsidies is inversely proportional to recharge rate and/or directly proportional to slope angle. Our results also show that the age of the headwaters groundwater subsidies is more than 500 years, almost 9 times older than the age of within-catchment groundwater flow contributions. These findings highlight the need to consider headwaters groundwater subsidies in groundwater management area considerations.