Isotope Tracking of Surface Water Groundwater Interaction in the Beninese Part of the Iullemeden Aquifer System

The Kandi basin is located in northeast Benin (West Africa). This study is focused on the estimation of water fluxes exchanged between the river Niger (and its tributaries) and the transboundary Iullemeden Aquifer System. In that framework, an innovative approach based on the application of the Bayesian Mixing Model (MixSIAR) analysis on water isotopes (oxygen-18, deuterium and tritium) was performed. Moreover, to assess the relevance of the model outputs, Pearson’s correlation and Principal Component Analysis (PCA) have been done. A complex relationship between surface water and groundwater has been found. Sixty percent (60%) of groundwater samples are made of more than 70% river water and rainwater;while 31.25% of surface water samples are made of about 84% groundwater. To safeguard sustainable water resources for the well-being of the local communities, surface water and groundwater must be managed as a unique component in the Kandi basin.

Linking bacterial and archaeal community dynamics to related hydrological,geochemical and environmental characteristics between surface water and groundwater in a karstic estuary

Subterranean estuaries(STEs)are characterized by the mixing of terrestrial fresh groundwater and seawater in coastal aquifers.Although microorganisms are important components of coastal groundwater ecosystems and play critical roles in biogeochemical transformations in STEs,limited information is available about how their community dynamics interact with hydrological,geochemical and environmental characteristics in STEs.Here,we studied bacterial and archaeal diversities and distributions with 16S rRNA-based Illumina MiSeq sequencing technology between surface water and groundwater in a karstic STE.Principal-coordinate analysis found that the bacterial and archaeal communities in the areas where algal blooms occurred were significantly separated from those in other stations without algal bloom occurrence.Canonical correspondence analysis showed that nutrients and salinity can explain the patterns of bacterial and archaeal community dynamics.The results suggest that hydrological,geochemical and environmental characteristics between surface water and groundwater likely control the bacterial and archaeal diversities and distributions in STEs.Furthermore,we found that some key species can utilize terrestrial pollutants such as nitrate and ammonia in STEs,indicating that these species(e.g.,Nitrosopumilus maritimus,Limnohabitans parvus and Simplicispira limi)may be excellent candidates for in situ degradation/remediation of coastal groundwater contaminations concerned with the nitrate and ammonia.Overall,this study reveals the coupling relationship between the microbial communities and hydrochemical environments in STEs,and provides a perspective of in situ degradation/remediation for coastal groundwater quality management.

Field scale interaction and nutrient exchange between surface water and shallow groundwater in the Baiyang Lake region,North China Plain

Fertilizer input for agricultural food production, as well as the discharge of domestic and industrial water pollutants, increases pressures on locally scarce and vulnerable water resources in the North China Plain. In order to：（a） understand pollutant exchange between surface water and groundwater,（b） quantify nutrient loadings, and（c） identify major nutrient removal pathways by using qualitative and quantitative methods, including the geochemical model PHREEQC） a one-year study at a wheat（Triticum aestivum L.） and maize（Zea mays L.） double cropping system in the Baiyang Lake area in Hebei Province, China, was undertaken. The study showed a high influence of low-quality surface water on the shallow aquifer. Major inflowing pollutants into the aquifer were ammonium and nitrate via inflow from the adjacent Fu River（up to 29.8 mg/L NH4-N and 6.8 mg/L NO3-N）, as well as nitrate via vertical transport from the field surface（up to 134.8 mg/L NO3-N in soil water）. Results from a conceptual model show an excess nitrogen input of about 320 kg/ha/a. Nevertheless,both nitrogen species were only detected at low concentrations in shallow groundwater,averaging at 3.6 mg/L NH4-N and 1.8 mg/L NO3-N. Measurement results supported by PHREEQC-modeling indicated cation exchange, denitrification, and anaerobic ammonium oxidation coupled with partial denitrification as major nitrogen removal pathways. Despite the current removal capacity, the excessive nitrogen fertilization may pose a future threat to groundwater quality. Surface water quality improvements are therefore recommended in conjunction with simultaneous monitoring of nitrate in the aquifer, and reduced agricultural N-inputs should be considered.

Fully integrated modeling of surface water and groundwater in coastal areas

This paper presents an improved model based on a three-dimensional non-hydrostatic wave model NHWAVE to simulate the interactions between the surface water and the groundwater affected by tides or waves in coastal areas. With the model, both the surface water flow and the groundwater flow are calculated based on the well-balanced Volume-averaged Reynolds-averaged Navier-Stokes equations. The spatially varying porosity and hydraulic conductivity are used to identifiy the domains for the surface water and the groundwater. The model is calibrated and validated using a wide range of laboratory measurements reported in the literature, involving the tide propagation through a sandy embankment, the tide-induced groundwater table fluctuation in a sandy beach, and the wave setup in a sloping sandy beach. The interactions between the surface water and the groundwater are analyzed and the influencing factors on the groundwater flow are discussed. The phase lag between the surface water elevation and the groundwater table fluctuation is mainly influenced by the hydraulic conductivity of the porous media. The wave attenuation in the groundwater is proportional to its propagation distance. The computed wave setup elevationin the permeable beach is smaller than in the impermeable beach. It is shown that the fully integrated model is capable of simulating the interactions between the surface water and the groundwater affected by tides or waves in coastal areas. It can be an efficient tool to study the tide and wave dynamics in the permeable sandy beach.

Environmental Isotopes and Cl/Br Ratios Evidences for Delineating Arsenic Mobilization in Aquifer System of the Jianghan Plain,Central China

Environment isotopes(δ18O andδ2H)and Cl/Br ratios in surface water and groundwater are combined to investigate arsenic mobilization in aquifer system of the Jianghan Plain.The groundwater has relatively high arsenic concentrations,ranging from 3.6 to 1055.3μg/L with an average of 102.2μg/L,which exceeds China’s drinking water standard(10μg/L).The arsenic content of surface water samples is quite low with the range of 6.0–14.3μg/L,averaging 9.5μg/L.δ18O andδ2H values for surface water and groundwater samples plot close to the local meteoric water line(LMWL),reflecting their meteoric origin;a subset of the samples(shallow wells,10 m)shows a shift to LMWL,commensurate with mixing with surface water and evaporation.The correlations betweenδ18O values and Cl concentration and Cl/Br ratios as well as arsenic concentration demonstrated that surface water and groundwater interactions,including active exchange between river/pond water and groundwater and vertical infiltration from agricultural and aquacultural soils,were dominated processes affecting arsenic mobilization in shallow groundwater system and lateral recharge was the main process controlling arsenic behavior in deep groundwater system.The results of this study will be beneficial to understanding the causes of arsenic mobilization in Jianghan groundwaters at different depths.