A yearlong field experimental campaign was conducted to reveal time scales over which antibiotic fluxes vary in the influent of a wastewater treatment plant (WTP). In particular, sampling was carried out to ascertain the amplitudes of monthly, daily and hourly fluctuations of several antibiotics.A total of 180 samples was collected at the entrance of a WTP in Lausanne, Switzerland. Sample concentrations were multiplied by flow rate to obtain monthly, daily and hourly mass fluxes of six antibiotics (trimethoprim, norfloxacin, ciprofloxacin, ofloxacin, clindamycin and metronidazole). Seasonality in mass fluxes was observed for all substances, with maximum values in winter being up to an order of magnitude higher than in summer. The hourly measurements of the mass flux of antibiotics were found to have a period of 12 h. This was due to peaks in toilet use in the morning and early evening. In particular, the morning peak in flushing coincided with high concentrations (and hence high mass fluxes) due to overnight accumulation of substances * corresponding author. Tel: +41 (0)21-693-8024.Email address: sylvain.coutu@epfl.ch (Sylvain Coutu) April 8, 2013 in urine. However, little variation was observed in the average daily flux. Preprint submitted to Science of the Total EnvironmentConsequently, fluctuations in mass fluxes of antibiotics were mainly evident at the monthly and hourly time scales, with little variation on the day-week time scale. These results can aid in optimizing removal strategies and future sampling campaigns focused on antibiotics in wastewater.

River inflows have a major influence on lake water quality due to their input of sediments, nutrients and contaminants. After leaving the river channel, river waters form a plume and interact with ambient lake waters. Strong hydrodynamic changes take place in the nearfield. To determine the nearfield hydro-sedimentary dynamics of the negatively-buoyant Rhône River plume in thermally stratified Lake Geneva, field campaigns were carried out at high and intermediate river discharge. High-resolution full-depth Acoustic Doppler Current Profiler (ADCP) transects were taken at 400, 800, 1,200, and 1,500 m from the river mouth, combined with profiles of temperature, turbidity and particle size distribution. These measurements provided, for the first time in a lake, detailed velocity fields across the full-plume cross-section in each transect. Furthermore, the unique combination of measurement techniques allowed the quantification of entrainment, Suspended Particulate Matter (SPM) concentrations and fluxes. They revealed that the negatively buoyant river inflow intruded into the metalimnion as a laterally unconfined interflow and continued flowing straight out in the streamwise direction, since currents in the lake were weak. At the same time, it mainly spread laterally due to entrainment of ambient water. The size of the interflow core and its velocity progressively decreased with distance from the mouth, as did SPM concentrations and volumes of particles (by a factor of 2–3 within 1,500 m), due to sediment settling and entrainment. The interflow momentum flux remained constant along the pathway, but interflow cross sections and discharge increased 2–3 times (within 1,500 m). The entrainment coefficient was >2 at 400 m and further increased non-linearly along the pathway. These values are much higher than those reported for laterally confined laboratory studies. The particle size distribution in the interflow was dominated by fine particles (<32 μm), which were transported up to 1,500 m from the mouth and most likely beyond, whereas larger particles (>62 μm) almost completely settled out before reaching that distance. The above processes occurred independent of river discharge; their intensity, however, changed with discharge.

Based on field investigations carried out in Lake Geneva during summer stratification in 2019, this study documents, for the first time, the rapid change of negatively buoyant river inflow dynamics caused by the passing of a coastal upwelling. Under calm conditions, the negatively buoyant Rhône River, entering at the eastern end of the lake, interacts with the lake density profile such that the river intrusion moves as an interflow in the thermocline layer straight out into the lake. A strong, large-scale spatially homogeneous wind that lasts for several days causes a downward thermocline tilt at the western end of the lake, coastal downwelling on the northern shore, and coastal upwelling on the southern shore. This cold-water upwelling progresses like a Kelvin wave around the lake after wind forcing ceases. When it arrives at the river inflow area, it homogenizes the lake water temperatures, and the river inflow transforms into an intrusion that spreads over the whole water column. Trapped by strong alongshore currents generated by the progressing coastal upwelling, the river plume is sharply deviated and flows along the shore in the nearshore zone, potentially bringing nutrients directly into the phototrophic near-surface layer. Following the passage of the coastal upwelling, the river inflow transforms back into an interflow. This change in inflow dynamics, which is documented for five strong wind events between June and September by combining in situ measurements, remote sensing, and numerical simulation, can be expected to occur in other large lakes with comparable wind-induced large-scale thermocline displacement.

No abstract

Two complementary measurement systems—built upon an autonomous floating craft and a tethered balloon—for lake research and monitoring are presented. The autonomous vehicle was assembled on a catamaran for stability, and is capable of handling a variety of instrumentation for in situ and near-surface measurements. The catamaran hulls, each equipped with a small electric motor, support rigid decks for arranging equipment. An electric generator provides full autonomy for about 8 h. The modular power supply and instrumentation data management systems are housed in two boxes, which enable rapid setup. Due to legal restrictions in Switzerland (where the craft is routinely used), the platform must be observed from an accompanying boat while in operation. Nevertheless, the control system permits fully autonomous operation, with motion controlled by speed settings and waypoints, as well as obstacle detection. On-board instrumentation is connected to a central hub for data storage, with real-time monitoring of measurements from the accompanying boat. Measurements from the floating platform are complemented by mesoscale imaging from an instrument package attached to a He-filled balloon. The aerial package records thermal and RGB imagery, and transmits it in real-time to a ground station. The balloon can be tethered to the autonomous catamaran or to the accompanying boat. Missions can be modified according to imagery and/or catamaran measurements. Illustrative results showing the surface thermal variations of Lake Geneva demonstrate the versatility of the combined floating platform/balloon imagery system setup for limnological investigations.