The aim of this paper is to assess the correlation of groundwater level changes (or groundwater level anomalies (GWLA)) obtained from direct measurements in wells with groundwater storage anomalies (GWSA) calculated using Gravity Recovery and Climate Experiment (GRACE) products and Global Land Data Assimilation Systems (GLDAS) models across different climate zones, from temperate Poland to Arctic Sweden. We recognize that such validation studies are needed to increase the understanding of the spatio-temporal limits of remote sensing model applicability, not least in data-scarce sub-Arctic and Arctic environments where processes are complex due to the impacts of snow and (perma) frost. Results for temperate climates in Poland and southern Sweden show that, whereas one of the models (JPL_NOAH_GWSA) failed due to water balance term overestimation, the other model (CSR_CLM_GWSA) produced excellent results of monthly groundwater dynamics when compared with the observations in 387 groundwater wells in the region during 2003–2022 (cross-correlation coefficient of 0.8). However, for the sub-Arctic and Arctic northern Sweden, the model suitable for other regions failed to reproduce typical northern groundwater regimes (of the region’s 85 wells), where winter levels decrease due to the blocking effect of ground frost on groundwater recharge. This suggests, more generally, that conventional methods for deriving GWSA and its seasonality ceases to be reliable in the presence of considerably infiltration-blocking ground frost and permafrost (whereas snow storage modules perform well), which hence need further attention in future research. Regarding long-term groundwater level trends, remote sensing results for southern Sweden show increasing levels, in contrast with observed unchanged to decreasing (~10 mm/a) levels, which may not necessarily be due to errors in the remote sensing model but may rather emphasize impacts of anthropogenic pressures, which are higher near the observation wells that are often located in eskers used for water supply. For sub-Arctic and Arctic Sweden, the (relatively uncertain) trend of the remote sensing results nevertheless agrees reasonably well with the groundwater well observations that show increasing groundwater levels of up to ~14 mm/a, which, e.g., is consistent with reported trends of large Siberian river basins.

Per-and polyfluoroalkyl substances (PFASs) have gained importance since the 1950s and are integral components of most industrial and consumer products. The objective of this study is to critically review the distributions of two PFAS compounds, perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), in groundwaters worldwide, analyze their sources and identify current challenges. We also assess the data availability of PFOA and PFOS, and gaps in terms of geographical coverage, which is critical for global water resources management. PFOA in groundwater varied from below the level of quantification to 6570 μg L ^-1, and PFOS varied from below the level of quantification to 68,900 μg L ⁻¹ . A clear disparity is observed, with studies being concentrated in certain developed regions, whereas other regions experience a deficit due to a lack of awareness and access to advanced facilities. Developed countries in Europe and the United States show that contamination from PFOA and PFOS is from primary sources of PFAS substances, and nonindustrial sources are the major contributors in Asia. Given the contribution of groundwater to the drinking water supply, the legislation and guidelines on PFAS need to be regularly examined and updated. The need for systematic assessment of groundwater for PFAS contamination encompassing the least studied regions is emphasized. More focus is required on the transport of these substances from groundwater to humans, either through the drinking water cycle or through bioaccumulation in organic consumables, and on studying their impact on human health.

Groundwater is one of the most important freshwater resources for ecosystems and mankind. Because of its fundamental role in the Earth's water and energy cycles, groundwater has been declared an essential climate variable by GCOS, the Global Climate Observing System. Similar to other subsurface states and fluxes, groundwater is difficult to monitor at the global scale, with sufficient spatial coverage and over climate-relevant time scales. The Global Gravity-based Groundwater Product (G3P) is a global observation-based data set of large-scale groundwater storage variations. G3P capitalizes on the unique capability of GRACE and GRACE-FO satellite gravimetry as the only remote sensing technology to monitor subsurface mass variations. In a mass balance approach, satellite-based, in situ observation-based and model-based water storage variations of snow water equivalent, root-zone soil moisture, glacier mass, and surface water storage are subtracted from GRACE/-FO terrestrial water storage anomalies to result in monthly variations of groundwater storage. For this combination, the individual compartmental storage data are spatially filtered to be consistent with the spatial resolution of terrestrial water storage from satellite gravimetry. The G3P data set presented here covers the period 2002 to 2023 with monthly resolution on a 0.5° global grid and includes propagated uncertainty information. We describe the details of the G3P data processing chain and of each contributing data stream, provide examples of spatial and temporal groundwater storage variations represented by the G3P data set, and present exemplary evaluation results against in situ groundwater observations for three large aquifer systems. G3P is a prototype for an operational global groundwater service, under development as a cross-cutting extension of the existing portfolio of the Copernicus Climate Change Service C3S. The G3P data set is available via GFZ Data Services at https://doi.org/10.5880/G3P.2024.001 (Güntner et al., 2024).

Groundwater, as the largest storage of liquid freshwater on Earth and a major component of the water cycle, is critical for supporting surface water flow, ecosystems as well as water and food security. Human dependence on groundwater has increased sharply over recent decades, contributing to wealth creation and poverty alleviation. However, anthropogenic activities, including groundwater abstraction and climate change, have modified groundwater dynamics, leading to groundwater level (GWL) changes. We analysed 20 year time series of GWLs in 47 countries distributed across a range of climatic, geographic, hydrogeological and socioeconomic contexts worldwide. Using different indicators based on three time series analysis methods, we show that almost one third of the GWLs trends are declining—thus reflecting overexploitation of groundwater—while GWLs are rising in 18% of wells—not always indicating a recovery but also the consequence of human impact on the environment. Globally, GWLs show a mix of declining and rising trends, however, at regional scale, patterns and hotspots are evident. We also explored the associated impacts of GWL change on humans and ecosystems, by reviewing 28 case study examples. We show that both rising and falling GWLs have substantial impacts on water and food security, ecosystems, infrastructure and socioeconomic wellbeing. Our findings underscore an urgent need for expanding monitoring programmes, protecting groundwater, and defining acceptable impacts to determine sustainable groundwater usage and minimise impacts of GWL change.

While geogenic arsenic (As) contamination of aquifers have been intensively investigated across the world, the mobilization and transport of As from anthropogenic sources have received less scientific attention, despite emerging evidence of poor performance of widely used risk assessment models. In this study we hypothesize that such poor model performance is largely due to insufficient attention to heterogeneous subsurface properties, including the hydraulic conductivity K and the solid-liquid partition (K_d), as well as neglect of laboratory-to-field scaling effects. Our multi-method investigation includes i) inverse transport modelling, ii) in-situ measurements of As concentrations in paired samples of soil and groundwater, and iii) batch equilibrium experiments combined with (iv) geochemical modelling. As case study we use a unique 20-year series of spatially distributed monitoring data, capturing an expanding As plume in a Chromated Copper Arsenate (CCA)-contaminated anoxic aquifer in southern Sweden. The in-situ results showed a high variability in local K_d values of As (1 to 107 L kg^-1), implying that over-reliance of data from only one or few locations can lead to interpretations that are inconsistent with field-scale As transport. However, the geometric mean of the local K_d values (14.4 L kg^-1) showed high consistency with the independently estimated field-scale "effective K_d" derived from inverse transport modelling (13.6 L kg^-1). This provides empirical evidence for the relevance of using geometric averaging when estimating large-scale "effective K_d" values from local measurements within highly heterogenous, isotropic aquifers. Overall, the considered As plume is prolonged by about 0.7 m year^-1, now starting to extend beyond the borders of the industrial source area, a problem likely shared with many of the world's As-polluted sites. In this context, geochemical modelling assessments, as presented here, provided a unique understanding of the processes governing As retention, including local variability in, e.g., Fe/Al-(hydr)oxides contents, redox potential and pH.