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The neighbouring countries of Latvia and Lithuania should use common water assessment methodologies and measures for water management in the transboundary rivers according to EU Water Framework Directive, establishing a framework for community action in the field of water policy. However, there are no national and local legislative acts describing how to harmonise water policy and activities for good water status at the cross-border level. This study intends to propose practical measures for management of chemical pollution risk in the cross-border region by mixing zones. The transboundary basin of the Venta River was used as a case study. After initial assessment of water quality status on both sides (Latvian and Lithuanian) in Venta River Basin District, 12 hot spots of pollution were selected for evaluation of mixing zones. Chemical analysis of cadmium, lead, nickel, chromium, copper, zinc, arsenic, phenols, formaldehyde, mercury, benzene as well as total nitrogen and total phosphorus in the water samples from the selected sites was performed. The modelling, using the selected Discharge Test, ensured evaluation of mixing zone dimensions. Such joint investigations might benefit the evaluation of possibilities to integrate mixing zones to the overall transboundary water body for harmonisation of water quality management. [ABSTRACT FROM AUTHOR]

Abstract: In acidified aluminium (Al) rich freshwater positively charged Al-species (Ali) are the key toxic components due to the accumulation in fish gills. As a countermeasure, liming is used to increase the pH and reduce the concentration of Ali-species; in particular low molecular mass (LMM) Ali-species by hydrolyses. However, very toxic high molecular mass (HMM) Al polymers can form in the unstable mixing zone immediately after liming. In the present work gill reactivity of LMM and HMM Al-species was studied under controlled conditions in eight channel–tank mixing zone systems in the field where Atlantic salmon (Salmo salar L.) kept in cages were exposed to defined mixing zone water. Mixing zones were created by continually liming acid river water (pH 5.0–5.7) high in LMM Ali to pH 6.0 and 6.4, respectively. Transformation processes affecting the Al-speciation as a function of time after liming were documented by in situ hollow fibre ultrafiltration interfaced with ion chromatography, while the Al accumulation in fish gills was used as bioindicator. For fish exposed to mixing zone water immediately after liming (1 min) the Al accumulation in gills (μg Al g−1) was higher (factor of 2) than for fish exposed to acid water prior to liming, due to the formation of gill reactive HMM Ali-species. The Al accumulation in gills followed a first order kinetic expression reaching steady-state conditions after 24-h exposures. The deposition rate of Al in gills (μg Al g−1 h−1) correlated with the water concentrations of HMM Ali (R 2 =0.80) in the mixing zones, and for LMM Ali in the acid water (R 2 =0.92). Due to the transient nature of HMM Ali the deposition rate of Al decreased from the point of liming with a factor of 10 downstream the channel–tank system (i.e. 100 min after liming). The concentration of gill accumulated Al was higher (factor of 3) immediately after high level liming (pH 6.4) than following low level liming (pH 6.0). However, high level liming was more efficient in detoxifying Al in downstream waters. Furthermore, the bioavailability of a given LMM Ali concentration as well as bioreactivity following liming was dependent on the TOC and silicon concentration in the acid water. Increased concentration of TOC (1.5 to 4.3 mg l−1) and silicon (0.3 to 1.0 mg l−1) reduced the gill reactivity of Ali by approximately 50%. [Copyright &y& Elsevier]

A new three-dimensional Laser-Induced Fluorescence (3DLIF) system was applied to study the mixing of a produced water outfall discharge typical of that used in offshore production operations. The outfall consists of a pipeline terminating in a multiport diffuser discharging into water about 12 m deep. Scaled laboratory experiments were done using conditions based on measured oceanographic conditions near the site. The current was perpendicular to the diffuser, and the receiving water was linearly stratified. The model was undistorted and the jet densimetric Froude number was the same as in the prototype. This scaling results in correct modeling of the source volume, momentum, and buoyancy fluxes, and equality of other line source dimensionless parameters. The experiments were conducted in a constant refractive-index environment which enabled detailed three-dimensional concentration measurements in the near field mixing zone to be obtained despite the density variations in the flow field. The mixing processes were found to be complex. The jets discharging upstream were quickly swept downstream, where they began merging with the downstream jets. Lateral mixing caused the concentration profiles to quickly become laterally homogeneous. Initially, dilution increased rapidly with distance from the diffuser, but farther away the rate of increase of dilution slowed as the turbulence became affected by the ambient density stratification. The dilution eventually becomes approximately constant; the location where this occurs defines the end of the near field, where mixing is primarily due to turbulence and other processes induced by the discharge itself. The end of the near field is marked by collapse of the self-induced turbulence under the influence of the ambient stratification.The results were compared to predictions of commonly used mathematical models of marine mixing zones: RSB, UM3, and CORMIX. The best predictions of the near field dilution and the length of the near field were by RSB. UM3 gave good predictions of the near field dilution, but CORMIX overestimated the dilution and rise height substantially. It is believed that this overestimation by CORMIX is due to its approximation of the diffuser as a row of vertically discharging jets of equivalent total momentum flux; this is clearly a poor approximation to the diffuser conditions modeled here.3DLIF is an exciting new technique that can give unprecedented insight into the complex hydrodynamics and processes occurring in buoyancy-modified mixing zones and to improvement of near field mathematical models. Experiments on other discharge configurations are continuing. [Copyright &y& Elsevier]

Groundwater and associated nutrients discharge from the beachface to the Delaware Estuary at Cape Henlopen, Delaware, and appear to contribute significantly to the ecological structure of the adjacent intertidal and subtidal benthic communities. The cross-sectional distributions of salinity and nutrient concentrations at one seepage site indicate that there are two distinct groundwater masses that mix with seawater in the sandy beachface during discharge. The dissolved nutrient concentrations in the beachface at this site are substantially higher than those found in the adjacent estuarine surface water. Nutrient concentrations and distribution in the beachface water reflect: (1) the discharge of nutrient-rich upland water; (2) mixing between nutrient-rich groundwater and estuarine water; and (3) diagenetic recycling of estuarine organic material in the beachface mixing zone. Simple mixing and hydrological models are used to determine the relative magnitude of upland and diagenetic contributions and to estimate absolute nutrient discharges across the beachface. Nutrient fluxes during the summer at this site are sufficient to support carbon fixation rates of 4–17 mol C/m/year along the beachface. These nutrient fluxes are substantially higher than can be supported by the upland discharge alone. This suggests that beachfaces serve as traps and reservoirs of estuarine particulate matter and associated nutrient elements. During the summer, when samples were collected at the Cape Henlopen site, nutrient concentrations in the estuary are low and the remineralization of particles in the beachface may contribute significantly to the productivity of the dense plant and animal communities found adjacent to the beachface. [Copyright &y& Elsevier]