The vulnerability of groundwater to contamination is closely related to its age. Groundwaters that infiltrated prior to the Holocene have been documented in many aquifers and are widely assumed to be unaffected by modern contamination. However, the global prevalence of these ‘fossil’ groundwaters and their vulnerability to modern-era pollutants remain unclear. Here we analyse groundwater carbon isotope data (12C, 13C, 14C) from 6,455 wells around the globe. We show that fossil groundwaters comprise a large share (42–85%) of total aquifer storage in the upper 1 km of the crust, and the majority of waters pumped from wells deeper than 250 m. However, half of the wells in our study that are dominated by fossil groundwater also contain detectable levels of tritium, indicating the presence of much younger, decadal-age waters and suggesting that contemporary contaminants may be able to reach deep wells that tap fossil aquifers. We conclude that water quality risk should be considered along with sustainable use when managing fossil groundwater resources. Groundwater that predates the Holocene is commonly assumed to be unaffected by modern contamination. A global analysis of fossil groundwater suggests that modern contaminants are present in deep wells that tap fossil aquifers.

/pdf/global-aquifers-dominated-by-fossil-groundwaters-but-wells-443w7w2g7f.pdf

Global aquifers dominated by fossil groundwaters but wells vulnerable to modern contamination

The time that water takes to travel through the terrestrial hydrological cycle and the critical zone is of great interest in Earth system sciences with broad implications for water quality and quantity. Most water age studies to date have focused on individual compartments (or subdisciplines) of the hydrological cycle such as the unsaturated or saturated zone, vegetation, atmosphere, or rivers. However, recent studies have shown that processes at the interfaces between the hydrological compartments (e.g., soil-atmosphere or soil-groundwater) govern the age distribution of the water fluxes between these compartments and thus can greatly affect water travel times. The broad variation from complete to nearly absent mixing of water at these interfaces affects the water ages in the compartments. This is especially the case for the highly heterogeneous critical zone between the top of the vegetation and the bottom of the groundwater storage. Here, we review a wide variety of studies about water ages in the critical zone and provide (1) an overview of new prospects and challenges in the use of hydrological tracers to study water ages, (2) a discussion of the limiting assumptions linked to our lack of process understanding and methodological transfer of water age estimations to individual disciplines or compartments, and (3) a vision for how to improve future interdisciplinary efforts to better understand the feedbacks between the atmosphere, vegetation, soil, groundwater, and surface water that control water ages in the critical zone.

/pdf/the-demographics-of-water-a-review-of-water-ages-in-the-3pfmftib9k.pdf

The demographics of water: A review of water ages in the critical zone

Application of radon-222 to investigate groundwater discharge into small shallow lakes

[1] Studies involving dissolved conservative gases in groundwater have usually found a gas excess above atmospheric solubility equilibrium (excess air), but sometimes also gas depletion (degassing). Both excesses and deficits of dissolved gases in groundwater can be explained by interactions with trapped gas bubbles. Excess air is formed when trapped air dissolves under increased pressure, degassing may occur if biogenic gases exsolve or pressure decreases. The concept of closed-system equilibration between groundwater and trapped bubbles (CE-model) proved successful in modeling excess air. Here we show that this concept can also be used to model the loss of dissolved gases and compare its performance to alternative models assuming a diffusion-controlled degassing process. We find that for samples from laboratory tests and a shallow groundwater well that consistently exhibits gas undersaturation, the CE-model provides the best available, albeit not perfect, description of degassing.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2007WR006454

Modeling excess air and degassing in groundwater by equilibrium partitioning with a gas phase

Groundwater inflow to a shallow, poorly-mixed wetland estimated from a mass balance of radon

. Due to its high activities in groundwater, the radionuclide 222Rn is a sensitive natural tracer to detect and quantify groundwater inflow into lakes, provided the comparatively low activities in the lakes can be measured accurately. Here we present a simple method for radon measurements in the low-level range down to 3 Bq m−3, appropriate for groundwater-influenced lakes, together with a concept to derive inflow rates from the radon budget in lakes. The analytical method is based on a commercially available radon detector and combines the advantages of established procedures with regard to efficient sampling and sensitive analysis. Large volume (12 l) water samples are taken in the field and analyzed in the laboratory by equilibration with a closed air loop and alpha spectrometry of radon in the gas phase. After successful laboratory tests, the method has been applied to a small dredging lake without surface in- or outflow in order to estimate the groundwater contribution to the hydrological budget. The inflow rate calculated from a 222Rn balance for the lake is around 530 m³ per day, which is comparable to the results of previous studies. In addition to the inflow rate, the vertical and horizontal radon distribution in the lake provides information on the spatial distribution of groundwater inflow to the lake. The simple measurement and sampling technique encourages further use of radon to examine groundwater-lake water interaction.

/pdf/tracing-and-quantifying-groundwater-inflow-into-lakes-using-3cyghbyi8k.pdf

Tracing and quantifying groundwater inflow into lakes using a simple method for radon-222 analysis

Localising and quantifying groundwater inflow into lakes using high-precision 222Rn profiles

Factors controlling terrigenic SF6 in young groundwater of the Odenwald region (Germany)

. We present CTD-measurements from two shallow meromictic mining lakes. The lakes, which differ in size and depth, show completely different seasonal mixing patterns in their mixolimnia. However, the measurements document the occurrence of similar seasonal convective mixing in discrete layers within their monimolimnia. This mixing is induced by double diffusion and can be identified by the characteristic step-like structure of the temperature and electrical conductivity profiles. The steps develop in the upper part of the monimolimnion, when in autumn cooling mixolimnion temperatures have dropped below temperatures of the underlying monimolimnion. The density gradient across the chemocline due to solutes overcompensates the destabilizing temperature gradient, and moreover, keeps the vertical transport close to molecular level. In conclusion, preconditions for double diffusive effects are given on a seasonal basis. At in general high local stabilities N2 in the monimolimnia of 10−4–10−2s−2, the stability ratio Rρ was in the range of 1–20. This quantitatively indicates that double diffusion can become visible. Between 1 and 6 sequent steps, with sizes between 1 dm and 1 m, were visually identified in the CTD-profiles. In the lower monimolimnion of the deeper lake, the steps systematically emerge at a time delay of more than half a year, which matches with the progression of the mixolimnetic temperature changes into the monimolimnion. In none of the lakes, the chemocline interface is degraded by these processes. However, double diffusive convection is essential for the redistribution of solutes in the inner parts of the monimolimnion at longer time scales, which is crucial for the assessment of the ecologic development of such lakes.

/pdf/evidence-for-double-diffusion-in-temperate-meromictic-lakes-3pm0qnjycy.pdf

Evidence for double diffusion in temperate meromictic lakes

Papers published in Hydrology and Earth System Sciences Discussions are under open-access review for the journal Hydrology and Earth System Sciences Abstract Due to its high activities in groundwater, the radionuclide 222 Rn is a sensitive natural tracer to detect and quantify groundwater inflow into lakes, provided the comparatively low activities in the lakes can be measured accurately. Here we present a simple method for radon measurements in the low-level range down to 3 Bq m −3 , appropriate 5 for groundwater-influenced lakes, together with a concept to derive inflow rates from the radon budget in lakes. The analytical method is based on a commercially available radon detector and combines the advantages of established procedures with regard to efficient sampling and sensitive analysis. Large volume (12 l) water samples are taken in the field and analyzed in the laboratory by equilibration with a closed air loop 10 and alpha spectrometry of radon in the gas phase. After successful laboratory tests, the method has been applied to a small dredging lake without surface in-or outflow in order to estimate the groundwater contribution to the hydrological budget. The inflow rate calculated from a 222 Rn balance for the lake is around 530 m 3 per day, which is comparable to the results of previous studies. In addition to the inflow rate, the 15 vertical and horizontal radon distribution in the lake provides information on the spatial distribution of groundwater inflow to the lake. The simple measurement and sampling technique encourages further use of radon to examine groundwater-lake interaction.

C. von Rohden

Papers

Tracing and quantifying groundwater inflow into lakes using a simple method for radon-222 analysis

Localising and quantifying groundwater inflow into lakes using high-precision 222Rn profiles

Factors controlling terrigenic SF6 in young groundwater of the Odenwald region (Germany)

Evidence for double diffusion in temperate meromictic lakes

Tracing and quantifying groundwater inflow into lakes using radon-222