Aurélie Noret

Geochemical and isotopic mass balances of kettle lakes in southern Quebec (Canada) as tools to document variations in groundwater quantity and quality

Given increasing anthropogenic and climatic pressures on water resources, groundwater and surface water need to be better managed and preserved. As these two water stocks can be connected to each other, their evolutions are linked and need to be considered as such. However, interactions between lakes and groundwater are not well understood and, most of the time, are not taken into account. Therefore, establishing a comprehensive approach to quantify groundwater and lakes’ hydrogeochemical interactions in various settings is of foremost importance for assessing the sensitivity of lakes to groundwater evolution. In this study, small kettle lakes set in fluvioglacial deposits and that are most likely well connected to shallow unconfined aquifers are specifically targeted. Geochemistry and isotopic results highlight that groundwater flux to the lakes is generally the dominant parameter of the lake water budget. The 222Rn results in particular suggest that 38% of the studied lakes have a high proportion of groundwater in their balances. It appears that the different tracers are complementary: geochemistry is influenced by groundwater inflows, reflecting its quality and the local geology, whereas water stable isotopes correspond directly to the volumetric lake water budget, and both of these tracers are impacted by in-lake processes. Moreover, the third tracer considered, 222Rn, highlights the location of groundwater inputs in space and time. Finally, the studied kettle lakes are characterized by a short to medium flushing time by groundwater. As a result, these lakes can be highly sensitive to environmental and climate changes affecting groundwater.

Interactions between groundwater and seasonally ice-covered lakes: using water stable isotopes and radon-222 multi-layer mass balance models

Interactions between lakes and groundwater are of increasing concern for freshwater environmental management but are often poorly characterized. Groundwater inflow to lakes, even at low rates, has proven to be a key in both lake nutrient balances and in determining lake vulnerability to pollution. Although difficult to measure using standard hydrometric methods, significant insight into groundwater–lake interactions has been acquired by studies applying geochemical tracers. However, the use of simple steady-state, well-mixed models, and the lack of characterization of lake spatiotemporal variability remain important sources of uncertainty, preventing the characterization of the entire lake hydrological cycle, particularly during ice-covered periods. In this study, a small groundwater-connected lake was monitored to determine the annual dynamics of the natural tracers, water stable isotopes and radon-222, through the implementation of a comprehensive sampling strategy. A multilayer mass balance model was found outperform a well-mixed, one-layer model in terms of quantifying groundwater fluxes and their temporal evolution, as well as characterizing vertical differences. Water stable isotopes and radon-222 were found to provide complementary information on the lake water budget. Radon-222 has a short response time, and highlights rapid and transient increases in groundwater inflow, but requires a thorough characterization of groundwater radon-222 activity. Water stable isotopes follow the hydrological cycle of the lake closely and highlight periods when the lake budget is dominated by evaporation versus groundwater inflow, but continuous monitoring of local meteorological parameters is required. Careful compilation of tracer evolution throughout the water column and over the entire year is also very informative. The developed models, which are suitable for detailed, site-specific studies, allow the quantification of groundwater inflow and internal dynamics during both ice-free and ice-covered periods, providing an improved tool for understanding the annual water cycle of lakes.

Influence of the balance of the intertropical front on seasonal variations of the isotopic composition in rainfall at Kisiba Masoko (Rungwe Volcanic Province, SW, Tanzania)

ABSTRACT Tropical rainfall isotopic composition results from complex processes. The climatological and environmental variability in East Africa increases this complexity. Long rainfall isotope datasets are needed to fill the lack of observations in this region. At Kisiba Masoko, Tanzania, rainfall and rain isotopic composition have been monitored during 6 years. Mean year profiles allow to analyse the seasonal variations. The mean annual rainfall is 2099 mm with a rain-weighted mean composition of −3.2 ‰ for δ18O and −11.7 ‰ for δ2H. The results are consistent with available data although they present their own specificity. Thus, if the local meteoric water line is δ2H = 8.6 δ18O + 14.8, two seasonal lines are observed. The seasonality of the isotopic composition in rain and deuterium excess has been compared with precipitating air masses backtracking trajectories to characterize a simple scheme of vapour histories. The three major oceanic sources have two moisture signatures with their own trajectory histories: one originated from the tropical Indian Ocean at the beginning of the rainy season and one from the Austral Ocean at its end. The presented isotopic seasonality depends on the balance of the intertropical front and provides a useful dataset to improve the knowledge about local processes.

A Newly Designed Analytical Line to Examine Fluid Inclusion Isotopic Compositions in a Variety of Carbonate Samples

δ 18 O and δD of fluid inclusions in carbonates provide insights into temperatures and fluid chemical compositions prevailing during the carbonate precipitation, however, various analytical restrictions limit a wider application of this proxy. This paper presents a new fluid inclusions isotopic analytical line coupled to an online cavity ring‐down spectrometer that increased the analytical productivity up to 10 carbonate samples per working day. This efficiency allowed for the first time to assess the reliability a large set of water samples with size ranging from 0.1 to 1 µL. Good reproducibility (±0.5‰ for δ 18 O and ± 2‰ δD; 1σ) is obtained for water quantity superior or equal to 0.3 μL and no evidence of memory effect is found. The line is further tested using two types of natural carbonates: (1) modern speleothems samples from caves for which δ 18 O and δD values of drip water were measured and (2) diagenetic carbonates for which the δ 18 O of the parent water were independently back‐calculated from carbonate clumped isotope Δ 47 measurements. Speleothem fluid inclusion values despite falling close to the Global Meteoritic Water Line are not always representative of the isotopic composition of the parent drip water. Results on diagenetic cements show that the δ 18 O water values measured in fluid inclusions agree, within 1%, with the δ 18 O water independently derived from Δ 47 measurements. Overall, this study confirms the reliability and accuracy of the developed analytical line for carbonate fluid inclusion analyses with a good reproducibility obtained for water quantity above 0.3 μL.