The original version of this article unfortunately contained mistakes.

The affiliation of Daniel Strasser is hereby corrected to: Department of Geotechnical Engineering, Federal Waterways Engineering and Research Institute (BAW), Kussmaulstraße 17, 76187 Karlsruhe, Germany

In order to evaluate the behavior of a freshwater lens in the uplifted atoll island of Minami-Daito, Japan, groundwater level and electrical conductivity were simultaneously measured in six monitoring wells. The monitoring allowed determination of the position and variability of the top and bottom positions of the freshwater lens. The freshwater lens periodically oscillates with phase lags (delay time) every 3 h following sea tides. Recharge from local rainfall temporarily increases the volume of the freshwater lens but is disturbed by the low-permeability muddy sediments deposited on the central lowland of the island. Changes in the groundwater levels correlate well with rainfall, after first removing semi-diurnal, diurnal, and long-term components of the sea tides from the groundwater level data using a multiple regression analysis. Changes in the water electrical conductivity provide information on the temporal and spatial fluctuation of the freshwater lens. The monitoring scheme for this freshwater lens could be applied to other uplifted atolls, contributing to better evaluation of the potable groundwater resources and to making freshwater use sustainable on other islands.

Groundwater is used intensively in Asian mega-deltas yet the processes by which groundwater is replenished in these deltaic systems remain inadequately understood. Drawing insight from hourly monitoring of groundwater levels and rainfall in two contrasting settings, comprising permeable surficial deposits of Holocene age and Plio-Pleistocene terrace deposits, together with longer-term, lower-frequency records of groundwater levels, river stage, and rainfall from the Bengal Basin, conceptual models of recharge processes in these two depositional environments are developed. The representivity of these conceptual models across the Bengal Basin in Bangladesh is explored by way of statistical cluster analysis of groundwater-level time series data. Observational records reveal that both diffuse and focused recharge processes occur in Holocene deposits, whereas recharge in Plio-Pleistocene deposits is dominated by indirect leakage from river channels where incision has enabled a direct hydraulic connection between river channels and the Plio-Pleistocene aquifer underlying surficial clays. Seasonal cycles of recharge and discharge including the onset of dry-season groundwater-fed irrigation are well characterised by compiled observational records. Groundwater depletion, evident from declining groundwater levels with a diminished seasonality, is pronounced in Plio-Pleistocene environments where direct recharge is inhibited by the surficial clays. In contrast, intensive shallow groundwater abstraction in Holocene environments can enhance direct and indirect recharge via a more permeable surface geology. The vital contributions of indirect recharge of shallow groundwater identified in both depositional settings in the Bengal Basin highlight the critical limitation of using models that exclude this process in the estimation of groundwater recharge in Asian mega-deltas.

Seawater intrusion (SWI) has triggered an accelerating process of freshwater contamination and significantly affected the soil fertility and local groundwater supply in the coastal area of Jiaozhou Bay, Shandong Province, China. This study establishes a three-dimensional numerical model based on SEAWAT code to simulate transient regional SWI to coastal aquifers in Dagu River Basin (DRB) adjacent to Jiaozhou Bay. The hydrogeological parameters in the SEAWAT model are calibrated and validated with the observed data of groundwater level and chloride (Cl⁻) concentration from 1 January 2010 to 1 June 2018. Also, global sensitivity analysis is used to evaluate the impacts of hydrogeological parameters (aquifer hydraulic conductivity and specific yield), along with sources/sinks consisting of recharge from precipitation infiltration and groundwater abstraction, on the SWI model. The sensitivity analysis results indicate that the progression of SWI is sensitive to the groundwater recharge from precipitation infiltration and the groundwater abstraction in the study area, while the hydraulic conductivity is of secondary importance. Furthermore, the baseline SWI model is applied to predict the extent of SWI under different scenarios considering the possible future precipitation and groundwater abstraction. It is shown that increased recharge and reduced groundwater abstraction could effectively lessen the extent of future SWI. As a case study, the research efforts on the regional SWI model are of critical importance for investigating the occurrence of SWI, identifying the factors most influential on the SWI process, and providing useful predictive information for SWI management in the DRB aquifer of Jiaozhou Bay.

Hydraulic head and groundwater age data are effective in building understanding of groundwater systems. Yet their joint role in detecting and characterising low-permeability geological structures, i.e. hydrogeologic barriers such as faults and dykes, has not been widely studied. Here, numerical flow and transport models, using MODFLOW-NWT and MT3D-USGS, were developed with different hydrogeologic barrier configurations in a hypothetical aquifer. Computed hydraulic head and groundwater age distributions were compared to those without a barrier. The conjoint use of these datasets helps in detecting vertically-oriented barriers. Two forms of recharge were compared: (1) applied across the entire aquifer surface (uniform), and (2) applied to the upstream part of the aquifer (upgradient). The hydraulic head distribution is significantly impacted by a barrier that penetrates the aquifer’s full vertical thickness. This barrier also perturbs the groundwater age distribution when upgradient recharge prevails; however, with uniform recharge, groundwater age is not successful in detecting the barrier. When a barrier is buried, such as by younger sediment, hydraulic head data also do not clearly identify the barrier. Groundwater age data could, on the other hand, prove to be useful if sampled at depth-specific intervals. These results are important for the detection and characterisation of hydrogeologic barriers, which may play a significant role in the compartmentalisation of groundwater flow, spring dynamics, and drawdown and recovery associated with groundwater extraction.

Research into the unsaturated zone and groundwater recharge can greatly improve understanding of hydrological processes and assist in sustainable groundwater management. Groundwater recharge of the Ljubljana Field aquifer, a coarse-gravel porous aquifer in Slovenia, was estimated with reference to soil characteristics, outflow data from a weighing lysimeter, and water-table fluctuation. The specific yield of the upper unsaturated zone determined from soil characteristics was 0.141 for the top soil layer (0–0.35 m), between 0.042 and 0.066 for the layer below the top soil (0.35–1.3 m), and between 0.239 and 0.219 for the underlying upper coarse layer. During long dry periods, especially in combination with times of high plant-water requirements, only substantial precipitation events directly contribute to considerable groundwater recharge, as ‘substantial precipitation’ is defined as those rainfall events that fill storage and exceed retention capacity of the upper soil layer. Lysimeter measurements show that 50% of the precipitation is lost by evapotranspiration and the other 50% contributes to groundwater recharge. Most infiltrated water was stored for a short time in the unsaturated zone and did not result in a significant discharge from the lysimeter. Average specific yield, calculated using the water-table fluctuation method, was 0.144. The nature of the gravely unsaturated zone is that once the retention buffer of the soil is exceeded, the water front travels through relatively quickly, which can be seen as an advantage for recharge or a disadvantage for prevention of groundwater pollution.

Submarine groundwater discharge (SGD) as a major source of alkalinity has rarely been studied in Jiaozhou Bay, China. The presented study used radon (²²²Rn) and radium isotopes to investigate SGD and its influence on alkalinity and nutrient inputs into the bay. Time-series observations of ²²²Rn were used to quantify groundwater dynamics over tidal time scales and the results showed that the SGD rates at point-scale were 0–67.2 (mean: 17.8) cm/day and 0–43.6 (mean: 12.3) cm/day in wet and dry seasons, respectively. Using radium mass balance models, the SGD in the whole bay was estimated to be (1.29–2.60) × 10⁷ m³/day in wet season and (5.81–6.83) × 10⁶ m³/day in dry season. Thus, both sets of results indicated higher SGD fluxes in wet season than in dry season. Such a seasonal variation pattern suggests a rapid response to local precipitation. The alkalinity fluxes associated with SGD were generally greater than those from the local rivers. Among the nutrient sources, SGD contributed about 63, 24 and 37% of total dissolved inorganic nitrogen, reactive phosphorus and silicate inputs, respectively. These results demonstrated that groundwater seepage is a major factor driving alkalinity and nutrients (especially dissolved inorganic nitrogen) into Jiaozhou Bay. SGD may have an important influence on the budgets of elements (C, N, P) and ecological environments in coastal waters.

The Lez aquifer in southern France comprises low-porosity karstified limestones and provides drinking water for ~400,000 inhabitants. Population growth and climate change have increased the stress on the water resources. In order to provide long-term protection and to optimize the water supply, the hydrogeology of the Lez aquifer must be better characterized. This study focused particularly on the St-Clément major fault zone (12 km long with a 500-m normal throw) which was structurally characterized using accurate geological mapping of the area, outcrops analysis and geophysics tools. The research highlights and explains the close relationship between the fault and the karstic occurrences. Moreover, tracer tests and piezometric head variations in boreholes have shown (1) strong interconnection between the observed karstic formations and (2) the major role of St-Clément fault on mass and pressure transfers in the aquifer. At the reservoir scale, the other major faults of the Lez aquifer, such as Corconne-Matelles or Gourg Noir faults, have shown some common morphologic and dynamic characteristics, and suggest a similar hydrogeological functioning. This study then extends this model to a larger scale. It proposes that, in aquifers of low-porosity carbonates, fault zones control the development of the main karstic network which, in turn, controls the main groundwater flows. Thus, faults should be reconsidered in order to improve the vulnerability studies and the quality of karstic aquifer modelling. Therefore, this report can contribute to protecting the groundwater resource, improving yields and optimizing groundwater supply exploitation in this type of aquifer.