Investigating colloid-associated transport of cadmium and lead in a clayey soil under preferential flow conditions

Colloids have a high adsorption capacity and can be mobile under preferential flow, and so may facilitate heavy metal migration. Heavy metal migration with soil colloids in a clayey soil under preferential flow conditions was investigated through experiments. Adsorption tests were carried out to determine the adsorption of Cd2+ and Pb2+ to the clay and colloids. The preferential flow characteristics in the soil column were investigated by dye tracing tests. The mobility of soil colloids in the soil column was studied by breakthrough tests. Leaching tests of cadmium and lead with and without colloids were carried out conditions. The adsorption tests showed that soil colloids adsorbed more cadmium and lead than the silty clay. The dye tracing tests showed that moderate preferential flow in the soil column can be obtained by choosing clod-size distribution and dry density. The co-leaching test showed that the outflow of cadmium and lead was 1.49 and 33.88 times greater with colloids than without, respectively. The heavy metals adsorbed onto clay and the pore concentrations were both lower with colloids than without, indicating more heavy metals migrated downward with colloids. The migration of cadmium and lead was greatly enhanced by colloids under preferential flow conditions.

Experimental and numerical investigation of flow over a spillway bend with different combinations of permeable spur dikes

In this paper, the effects of different combinations of permeable spur dikes installed in the bend section of spillway on flow characteristics and energy dissipation rate were experimentally and numerically investigated. The results indicate that The permeable spur dikes installed in the spillway bend appreciably contributes to the improvement on the water surface uniformity, and the water surface uniformity can reach 90.13% with three permeable spur dikes installed in the bend. The permeable spur dike can lead to different degrees of decrease in the time-averaged longitudinal velocity in each zone of spillway bend. Different from previous study, no circulation zone is formed upstream and downstream of permeable spur dike due to the presence of permeable holes, and the flow upstream of permeable spur dikes could be divided into three distinctly different flow modes according to dye tracing. The presence of permeable spur dikes causes the concentration of TKE zone at concave bank of the spillway bend, except for TKE zone immediately next to the bottom plate. The TKE first increases and then decreases with the increase in the vertical distance from the bottom plate of the spillway bend, exhibiting a typical parabolic distribution. The energy dissipation rate in the spillway bend with permeable spur dike was calculated using a modified integral method, and the dissipation rate can reach as high as 21.08% with three spur dikes installed in the bend.

Dye Tracing and Concentration Mapping in Coastal Waters Using Unmanned Aerial Vehicles

Coastal water flows facilitate important nutrient exchanges between mangroves, seagrasses and coral reefs. However, due to the complex nature of tidal interactions, their spatiotemporal development can be difficult to trace via traditional field instrumentations. Unmanned aerial vehicles (UAVs) serve as ideal platforms from which to capture such dynamic responses. Here, we provide a UAV-based approach for tracing coastal water flows using object-based detection of dye plume extent coupled with a regression approach for mapping dye concentration. From hovering UAV images and nine subsequent flight surveys covering the duration of an ebbing tide in the Red Sea, our results show that dye plume extent can be mapped with low omission and commission errors when assessed against manual delineations. Our results also demonstrated that the interaction term of two UAV-derived indices may be employed to accurately map dye concentration (coefficient of determination = 0.96, root mean square error = 7.78 ppb), providing insights into vertical and horizontal transportation and dilution of materials in the water column. We showcase the capabilities of high-frequency UAV-derived data and demonstrate how field-based dye concentration measurements can be integrated with UAV data for future studies of coastal water flow dynamics.

Shape Effect of Polymetallic Nodules on Suction Forces and Flow Field During Seabed Hydraulic Collection

Turbulent suction pipe flow around a near-wall ellipsoid nodule, as basic research of polymetallic nodule hydraulic collection, is investigated numerically and experimentally in this paper. Seven ellipsoids with axe ratios (a/b) ranging from 1 to 2 are considered as nodule models to reveal the shape effect on the characteristics of suction forces and suction flow field. Methods of particle image velocimetry (PIV) and dye tracing were used to visualize the suction flow field. The results indicate that: (1) suction force coefficients increase with a/b; (2) the shape effect is more significant in the cases with smaller ratios of bottom clearance to semi-thickness of the ellipsoid (h/c); (3) the weak vortex shedding in suction flow results in small-amplitude fluctuations of time-history suction forces; (4) the detached-eddy simulation (DES) method based on shear stress transport (SST) model is validated to be accurate and feasible for predicting the suction forces and suction flow field. It is expected to provide references for the design of nodule pick-up devices and to help us further understand the mechanism of hydraulic collection.

Coupling geochemical and isotopic tracers (δ18O, δ2H and 87Sr/86Sr) to quantify groundwater mixing in carbonate environment

<p>Carbonate aquifers are known as a major source for drinking water in the Mediterranean region. Therefore, qualitative and quantitative estimation of the groundwater resource are crucial, especially in area with densely populated areas.</p><p>In this study, geochemical and isotopic tracers (δ<sup>18</sup>O, δ<sup>2</sup>H and <sup>87</sup>Sr/<sup>86</sup>Sr) have been used to discriminate different origins of groundwater and evaluate water-mass mixing in a Mediterranean carbonate environment. The case study, located in south-eastern France, offers an attractive geological context to explore strontium isotopic tracers: trias to cretaceous carbonate rocks, that have been eroded and locally resedimented in fluvial deposits during Oligocene extension and Quaternary periods. Moreover, many karst features act as sinking zones, generating fast infiltration. The 500 km² zone investigated has also a large range of elevation, from the sea to 1148m, giving an expected significant contrast in water isotopes. Monthly water samples have been collected in-situ for two years on ground and surface waters, in rivers, boreholes as well as karstic springs. Monthly rainwater samples provide the isotopic signal of the recharge at two distinct elevations and distance from the sea.</p><p>The hydrogeochemical signatures allow to discriminate the end-members of two main waterbodies as well as a slight marine influence on one borehole. Nitrates concentrations (NO<sub>3</sub><sup>-</sup>) highlight anthropogenic influence on groundwater resources. A multi-tracer approach taking in account water-rock interaction, mixing processes and recharge, coupled with hydrogeological conditions and dye-tracing tests, leads to a conceptual model of this complex hydrosystem.</p>

Invisible dye tracing – an efficient technique for study of groundwater channels

We studied excitation spectra of uranine fluorescence in eluates from charcoal traps saturated in cave waters and in reference uranine solutions and found that the most appropriate excitation wavelength is 295±5 nm, because it excites fluorescence of uranine many times stronger than fluorescence of natural fluorescent compounds in groundwater. This allows us to lower the detection limit of uranine fluorescence with one order of magnitude, even below the level of natural fluorescence of karst waters at integral UV excitation.