Submarine groundwater discharge (SGD) is widely acknowledged as a key driver of environmental change in tropical island coral reefs. Previous work has addressed SGD and groundwater-reef interactions at isolated submarine springs; however, there are still many outstanding questions about the mechanisms and distribution of groundwater discharge to reefs. To understand how groundwater migrates to reefs, a series of offshore ²²²Rn (radon) and submarine electrical resistivity (ER) surveys were performed on the tropical volcanic island of Mo’orea, French Polynesia. These surveys suggest that fresher water underlies the fringing reef, apparently confined by a <1-m-thick low-permeability layer referred to as a reef flat plate. Reef flat plates have been documented elsewhere in tropical reefs as thin, laterally continuous limestone units that form through the super-saturation of calcium carbonate in the overlying marine waters. In other tropical reefs, the reef flat plate is underlain by a highly permeable karstic limestone formation, but the submarine reef geology on Mo’orea is still uncertain. Numerical modeling of two-dimensional reef transects and SGD quantifications, based on water budget and radon/salinity mass balance, support the confining nature of the reef flat plates and indicate important implications for SGD impacts to tropical reefs. Except where incised by streams or local springs, reef flat plates may route SGD to lagoons or to the reef crest 100s of meters offshore. Because groundwater can transport pollutants, nutrients, and low pH waters, the reef flat plate may play an important role in the spatial patterns of reef ecology and coastal acidification.

AbstractCurrently, the use of unconventional food sources in the inclusion of fish diets is cause of great interest. The growth performance of African catfish (Clarias gariepinus) was evaluated, with the inclusion of duckweed meal (Lemna perpusilla) in four levels (0, 6, 12 and 18 %) in the diet. African catfish with an average weight of 1.27 ± 0.03 g, were distributed in a completely randomized design, with 16 experimental cages (four replicates/treatment). The fingerlings were fed for 48 days of experimentation. An analysis of variance and a Duncan´s test were performed. No significant differences were found (p&lt; 0.05) for the first two levels evaluated, but with the rest there were differences, as the percentage of inclusion of the duckweed meal in the ration increased. Final weight was decreasing, as well as absolute growth rate, increase in daily weight, feed conversion ratio and food efficiency. Survival throughout the experiment was between 72 y 65 % in all treatments. The inclusion of duckweed meal in the diet did not affect the parameters of water quality. It was concluded that the L. perpusillase meal can be included up to 12 % in diets for African catfish fingerlings, without affecting the growth performance.ResumenEl uso de alimentos no convencionales en las dietas para peces es actualmente de gran interés. Se evaluó el comportamiento productivo de alevines de bagre africano (Clarias gariepinus), con la inclusión de la harina de la lenteja de agua (Lemna perpusilla) en cuatro niveles (0, 6, 12 y 18 %) en la formulación de la dieta. Se utilizaron 240 animales, con un peso promedio de 1,27 ± 0,03 g, distribuidos en un diseño completamente al azar, con 15 por réplicas (cuatro réplicas/tratamiento). Se realizó un análisis de varianza y una prueba de medias por Duncan. Los alevines fueron alimentados durante 48 días, no se encontraron diferencias significativas (p&lt; 0,05) para los dos primeros niveles evaluados, y si con los restantes, en la medida que aumentaba el porcentaje de inclusión de harina de la lenteja de agua en la ración. El peso final fue descendiendo, así como el crecimiento absoluto, incremento en peso diario, factor de conversión alimenticia y eficiencia alimentaria. La supervivencia durante todo el experimento estuvo entre 72 y 65 %. La inclusión de este alimento en la dieta no afectó los parámetros de calidad del agua. Se concluyó que la harina de L. perpusilla se puede incluir hasta 12 % en dietas para alevines de bagre africano, sin afectar los indicadores bio-productivos.ResumoO uso de insumos alimentares não convencionais na inclusão de dietas para peixes é atualmente de grande interesse. Foi avaliado o comportamento produtivo de alevinos de jundiá (Clarias gariepinus), com a inclusão de farinha de lentilha-d'água (Lemna perpusilla) em quatro níveis (0, 6, 12 e 18 %) na formulação da dieta. Foram utilizados 240 animais, com peso médio de 1.27 ± 0.03 g, distribuídos em delineamento inteiramente casualizado, com 15 por repetições (quatro repetições/tratamento). Foi realizada uma análise de variância e um teste de médias de Duncan. Aos 48 dias de cultivo experimental, não foram encontradas diferenças significativas (p&lt; 0,05) para os dois primeiros níveis avaliados, e se com os demais, conforme aumentou a porcentagem de inclusão de farinha de lentilha-d'água na ração. O peso final foi diminuindo, assim como o crescimento absoluto, aumento do peso diário, fator de conversão alimentar e eficiência alimentar. A sobrevivência durante todo o experimento foi entre 72 e 65 %. A inclusão desse alimento na dieta não afetou os parâmetros de qualidade da água. Concluiu-se que o farelo de L. perpusilla pode ser incluído em até 12 % em dietas para alevinos bagrefrenses, sem afetar os indicadores de bioprodução.

For the quantitative evaluation of the impact of mining on a groundwater system, it is necessary to constrain the hydrogeological and mechanical properties. However, the in situ estimation of the mechanical properties of rock such as compressibility and porosity, is often difficult. Additionally, determining the hydraulic properties such as hydraulic conductivity, of rock by conventional methods is often expensive. The response of the groundwater level to external loading such as Earth tides and barometric pressure, couples the hydrogeological and mechanical processes of rocks, thus providing a way to infer these properties in the field. This study compared aquifer parameters inferred from tidal and barometric responses with those inferred from conventional hydraulic tests and rock mechanics tests in three groundwater monitoring wells at a site in China. The results show that the hydraulic conductivity inferred from a tidal response is similar to that of a pumping test. The compressibility values calculated for the three wells are all higher than those determined by experiment, and the porosity values calculated are all lower than those determined by experiment, but the differences between the calculated and experimentally measured values are lower than one order of magnitude. Considering the costs and convenience of the water-level response method, this method is a good choice for obtaining the properties of an aquifer, especially those in areas of tectonic activity and those affected by anthropogenic perturbations.

‘Blue water’ is the portion of freshwater flowing through rivers and the subsurface (groundwater) that is available for human consumption. ‘Green water’ is the portion stored in the unsaturated soil and vegetation canopy that is available only indirectly. Security of blue and green water resources is assessed over the Dorudzan Dam watershed in southern Iran. Precipitation and temperature data from 22 models of the Coupled Model Intercomparison Project Phase 5 are transiently downscaled at five climatic stations under three Representative Concentration Pathway (RCP) scenarios. The Soil and Water Assessment Tool (SWAT) is used to simulate and quantify blue and green water components over the region at the present time and under climate-change conditions. Climate-change study indicates that precipitation decreases (13–17%) and temperature increases (1.7–3. 3 °C) under the three RCPs, leading to substantial dam-inflow reduction. Evapotranspiration will increase while soil-water content will decrease, further intensifying green-water scarcity and vulnerability. Water use from the Kor River is sustainable at present, but future climate change will raise some ecological hotspots. Groundwater exploitation is currently unsustainable in all aquifers of the study area and climate change will further decrease the available groundwater, leading to intensification of the water crisis. Assessment of inter-annual security under climate change indicates that maximum scarcities of green water and surface blue water occur during spring and summer, and subsurface blue water (groundwater) maxima occur throughout the year. Thus, climate change threatens the future security of water resources in this arid watershed, requiring different management strategies for sustainability.

Groundwater flow and the geochemical evolution of a freshwater lens in an aquifer on a Pacific atoll were investigated by hydrogeochemical surveys. Sulfur hexafluoride measurements showed that deeper groundwater and groundwater at the periphery of the lens are older, consistent with a downward and outward groundwater flow scheme. This is the typical flow scheme on Pacific atolls where a Holocene–Pleistocene unconformity restricts the shape of the freshwater lens. Enrichment of Mg/Ca in the groundwater is another indicator of a longer residence time, because contact between the groundwater and the carbonate sediments composing the aquifer leads to the release of Mg from high-Mg calcite and the precipitation of Ca as low-Mg calcite. Groundwater quality was also affected by anthropogenic nitrogen loading and aboveground organic matter, which were altered by denitrification and sulfate reduction in the aquifer, especially in the older groundwater. The chemical composition of the groundwater in the center of the island, where saline water is up-coning, implies that freshwater recharge dilutes the older saline water, which as time passes will eventually be replaced by newly recharged freshwater.

Salt loads in the Colorado River Basin are a primary water quality concern. Natural groundwater brine discharge to the Dolores River where it passes through the collapsed salt anticline of the Paradox Valley in western Colorado (USA) is a significant source of salt to the Colorado River. An airborne electromagnetic survey of Paradox Valley has provided insights into the three-dimensional distribution of brine in the surficial aquifer. A combination of stochastic and deterministic resistivity inversions was used to interpret the top of the freshwater–brine interface and to qualitatively describe the vertical salinity gradients across the interface. Low-resistivity regions indicative of brine occur near the land surface where brine discharges to the Dolores River and increase in depth several kilometers up-gradient along the axis of the valley. The most conductive parts of the brine plume are found in the areas below and adjacent to the river, suggesting that the brine becomes shallower and more concentrated as it reaches its natural discharge location. A significant freshwater lens overlying the brine west of the Dolores River is spatially correlated to the intermittent West Paradox Creek and agricultural irrigation. Below this lens, the transition from freshwater to brine appears to occur abruptly over a few meters and correlates to available well information. However, away from these regions and particularly with distance from the river, the freshwater–brine interface appears to be more diffuse.

A three-dimensional variable-density finite element model was developed to study the combined effects of overabstraction and seawater intrusion in the Pingtung Plain coastal aquifer system in Taiwan. The model was generated in different layers to represent the three aquifers and two aquitards. Twenty-five multilayer pumping wells were assigned to abstract the groundwater, in addition to 95 observation wells to monitor the groundwater level. The analysis was carried out for a period of 8 years (2008–2015 inclusive). Hydraulic head, soil permeability, and precipitation were assigned as input data together with the pumping records in different layers of the aquifer. The developed numerical model was calibrated against the observed head archives and the calibrated model was used to predict the inland encroachment of seawater in different layers of the aquifer. The effects of pumping rate, sea-level rise, and relocation of wells on seawater intrusion were examined. The results show that all layers of the aquifer system are affected by seawater intrusion; however, the lengths of inland encroachment in the top and bottom aquifers are greater compared with the middle layer. This is the first large-scale finite-element model of the Pingtung Plain, which can be used by decision-makers for sustainable management of groundwater resources and cognizance of seawater intrusion in coastal aquifers.

Bank filtration (BF) is an established indirect water-treatment technology. The quality of water gained via BF depends on the subsurface capture zone, the mixing ratio (river water versus ambient groundwater), spatial and temporal distribution of subsurface travel times, and subsurface temperature patterns. Surface-water infiltration into the adjacent aquifer is determined by the local hydraulic gradient and riverbed permeability, which could be altered by natural clogging, scouring and artificial decolmation processes. The seasonal behaviour of a BF system in Germany, and its development during and about 6 months after decolmation (canal reconstruction), was observed with a long-term monitoring programme. To quantify the spatial and temporal variation in the BF system, a transient flow and heat transport model was implemented and two model scenarios, ‘with’ and ‘without’ canal reconstruction, were generated. Overall, the simulated water heads and temperatures matched those observed. Increased hydraulic connection between the canal and aquifer caused by the canal reconstruction led to an increase of ~23% in the already high share of BF water abstracted by the nearby waterworks. Subsurface travel-time distribution substantially shifted towards shorter travel times. Flow paths with travel times <200 days increased by ~10% and those with <300 days by 15%. Generally, the periodic temperature signal, and the summer and winter temperature extrema, increased and penetrated deeper into the aquifer. The joint hydrological and thermal effects caused by the canal reconstruction might increase the potential of biodegradable compounds to further penetrate into the aquifer, also by potentially affecting the redox zonation in the aquifer.

Groundwater distribution and flow dynamics were studied in a small watershed located in the discontinuous permafrost zone near Umiujaq in Nunavik (Québec), Canada, to assess the seasonal variations and perform a quantitative analysis of the water cycle in a subarctic watershed. Due to the complexity of the subsurface geology within the watershed, an integrated investigation was instrumental to provide a detailed understanding of the hydrogeological context as a basis for the water balance. Based on this water balance, for the two studied hydrological years of 2015 and 2016, the average values are 828 mm for precipitation, 337 mm for evapotranspiration, 46 mm for snow sublimation, 263 mm for runoff, 183 mm for groundwater exchange (losses with other aquifers outside the watershed), and 0 mm for change in water storage. Although these values likely have significant uncertainty and spatial variability, this water balance is shown to be plausible. It was also found that permafrost influences surface water and groundwater interaction, even if located in low-permeability sediments. It is expected that permafrost degradation will likely increase stream baseflow, especially in winter.

Three main stages in the development of groundwater pollution assessment since the 1970s are described. The first steps involved aquifer vulnerability assessment. In the second stage (from the late 1980s), three methodological approaches to risk assessment were developed. The latest stage (from the 1990s) has involved new technologies and approaches. At present, all three stages coexist, and their advantages and disadvantages are discussed. Experience highlights the need to account for the social vulnerability in risk assessment, particularly with respect to large cities in developing countries. Assessing groundwater pollution risk through an integrated approach appears to be the greatest challenge.