The Detritic Compartment in a Posidonia oceanica Meadow: Litter Features, Decomposition Rates, and Mineral Stocks

Abstract. The ecosystem associated to the Mediterranean seagrass Posidonia oceanica shows a clear distinction in two subcompartments regarding turnover time: aboveground and belowground. Aboveground parts (leaves) are highly dynamic, and most of the leaf material is decomposed or exported in less than one year, representing a net loss of nutrients. In contrast, belowground biomass (roots and rhizomes) has a turnover time of the order of centuries, with a consequent accumulation of organic matter in the sediment. The accumulation rates for the single elements rank in the order C > N > P. This ecosystem may be considered as a sink for biogenic elements.     

Improved chromatographic analysis of N:N ratios in ammonium or nitrate for isotope addition experiments

Estimating nitrogen transformation rates in aquatic ecosystems by isotope dilution techniques is simplified by directly measuring nitrogen isotopic ratios for NH₄⁺ in the water using high performance cation exchange liquid chromatography (HPLC). Modifications of HPLC conditions and implementation of a median-area method for retention time determination improved and linearized a previously reported sigmoid relationship between the retention time shift (RT_shift) of the NH₄⁺ peak and the ratio of [¹⁵NH₄⁺]: [Total NH₄⁺] in seawater fortified with ¹⁵NH₄⁺. Increasing the temperature of the HPLC column from 47 to 85 °C increased mobile phase buffer flow rate relative to column back pressure, decreased the retention time for NH₄⁺, and allowed the buffer pH to be optimized relative to the pK of NH₄⁺. The use of median-area rather than maximum-height to define the retention time of NH₄⁺ further improved the linearity (r > 0.995) of the relationship between the ratio [¹⁵NH₄⁺]: [Total NH₄⁺] and RT_shift over the range of isotope ratios. Reduction of NO₃⁻ to NH₄⁺ by adding zinc dust to acidified (pH 2) seawater or lakewater samples, followed by pH neutralization, and subsequent analysis of NH₄⁺ isotope ratios by HPLC, extended application of the method to isotope dilution experiments with NO₃⁻. Advantages of this direct-injection method over mass-measurement approaches traditionally used for isotope dilution experiments include small sample size and minimal sample preparation.     

Origin of groundwater salinity and hydrogeochemical processes in a confined coastal aquifer: Case of the Rhône delta (Southern France)

The Rhône delta, South of France (Camargue, 750 km²) is a coastal saline wetland located along the Mediterranean Sea. The confined aquifer of this delta shows high values of electrical conductivity rising from the north (4 mS/cm) to the shoreline (58 mS/cm). This work aims to identify the origin of groundwater salinity and the geochemical processes occurring in this coastal confined aquifer according to the degree of salinity. A natural tracing approach is considered using monthly sampling in 8 piezometers for chemical and isotopic analyses (¹⁸O, ²H, ¹³C_TDIC). Ionic and isotopic ratios demonstrate that strong salinities are due to a simple mixing between Mediterranean seawater and freshwater; seawater contribution reaches up to 98% at 8 km from the shoreline. Seawater intrusion induces a particular groundwater chemistry which varies with the degree of seawater contribution: (1) In the less saline part of the aquifer (seawater contribution <20%), the intrusion induces an increase of Na⁺ in groundwater leading to Ca²⁺/Na⁺ exchange processes. The δ¹³C_TDIC analyses show that matrix exchange processes most likely occur for the less saline samples. (2) In the saline part of the aquifer (seawater contribution >20%), the intrusion induces SO₄ reduction which is confirmed by highly depleted δ¹³C_TDIC values (up to −19‰). The δ¹³C_TDIC also reveals that methanogenesis processes may occur in the most reductive part of the aquifer. Due to SO₄ reduction, the intrusion induces a shift in carbonate equilibrium leading to supersaturation with respect to dolomite and/or magnesian calcite. Thus carbonate precipitation may occur in the area strongly influenced by seawater.     

The mechanism and kinetics of DTPA-promoted dissolution of barite

The dissolution rate of natural barite, BaSO₄, was measured in solutions of DTPA (diethylene triamine penta-acetic acid) to investigate the mechanism of ligand-promoted dissolution using a strong chelating agent. Experiments were carried out over a range of DTPA concentrations 0.5–0.0001 M solutions, at room temperature (22 °C), as well as a range of temperatures, 22–80 °C at 1 atm. The dissolution rate is inversely related to the DTPA concentration in solution. A more dilute DTPA solution is shown to be more efficient as a solvent in terms of the approach to the equilibrium saturation value for the dissolution of Ba²⁺. An analysis of the temperature dependence of the dissolution rate at high pH by the determination of activation energies indicates that the reaction is probably controlled by the pre-exponential term in the rate constant. This indicates that reaction frequency mostly controls differences in reactivity and suggests an explanation for the results in terms of stearic hindrance due to adsorbed DTPA molecules at the barite surface. The effect of DTPA on the solvation of the Ba²⁺ ion may also influence the dissolution rate.     

Snow and frost: implications for spatiotemporal infiltration patterns – a review

Vast regions of the northern hemisphere are exposed to snowfall and seasonal frost. This has large effects on spatiotemporal distribution of infiltration and groundwater recharge processes as well as on the fate of pollutants. Therefore, snow and frost need to be central inherent elements of risk assessment and management schemes. However, snow and frost are often neglected or treated summarily or in a simplistic way by groundwater modellers. Snow deposition is uneven, and the snow is likely to sublimate, be redistributed and partly melt during the winter influencing the mass and spatial distribution of snow storage available for infiltration, the presence of ice layers within and under the snowpack and, therefore, also the spatial distribution of depths and permeability of the soil frost. In steep terrain, snowmelt may travel downhill tens of metres in hours along snow layers. The permeability of frozen soil is mainly influenced by soil type, its water and organic matter content, and the timing of the first snow in relation to the timing of sub‐zero temperatures. The aim with this paper is to review the literature on snow and frost processes, modelling approaches with the purpose to visualize and emphasize the need to include these processes when modelling, managing and predicting groundwater recharge for areas exposed to seasonal snow and frost. Copyright © 2015 John Wiley & Sons, Ltd.     

Impact of human activities on stream flow in the Biliu River basin,China

The obvious decline in stream flow to the Biliu River reservoir over the period 1990–2005 has raised increasing concerns. Climate change and human activities, which mainly include land use changes, hydraulic constructions and artificial water consumption, are considered to be the most likely reasons for the decline in stream flow. This study centres on a detailed analysis of the runoff response to changes in human activities. Using a distributed hydrological model, (Soil and Water Assessment Tool), we simulated runoffs under different human activity and climate scenarios to understand how each scenario impacts stream flow. The results show that artificial water consumption correlates with the precipitation (wet, normal and dry) of the year in question and is responsible for most of the decrease in runoff during each period and for each different wetness year. A Fuzzy Inference Model is also used to find the relationship between the precipitation and artificial water consumption for different years, as well as to make inferences regarding the future average impact on runoff. Land use changes in the past have increased the runoff by only a small amount, while another middle reservoir (Yunshi) has been responsible for a decrease in runoff since operation began in 2001. We generalized the characteristics of the human activities to predict future runoff using climate change scenarios. The future annual flow will increase by approximately 10% from 2011 to 2030 under normal human activities and future climate change scenarios, as indicated by climate scenarios with a particularly wet year in the next 20 years. This study could serve as a framework to analyse and predict the potential impacts of changes both in the climate and human activities on runoff, which can be used to inform the decision making on the river basin planning and management. Copyright © 2012 John Wiley & Sons, Ltd.