Avoidance response of sediment living amphipods to zinc pyrithione as a measure of sediment toxicity

An avoidance test was developed using non-cultured individuals of the sediment dwelling amphipod Monoporeia affinis. As test substance we used zinc pyrithione, an antifouling agent and a common shampoo ingredient. The toxicity to Daphnia and fish is well known but sediment toxicity of this very hydrophobic compound is less known. The preference of juvenile M. affinis was tested in jars, each including 12 petri dishes. In each replicate, half of the petri dishes contained sediment mixed with six concentrations ranging from 0 to 10microg zinc pyrithione per L sediment and half of the petri dishes contained the corresponding sediment-substance mixture plus an extra food addition. The amphipods significantly avoided petri dishes with the three highest concentrations of zinc pyrithione and the calculated EC(50) was 9.65microgL(-1) sediment. No difference in mortality was observed between concentrations. Using the avoidance behaviour in sediment toxicity testing is a simple and cost-effective screening for toxicants.     

Containment of groundwater pollution (methyl tertiary butyl ether and benzene) to protect a drinking-water production site in Belgium

The subsurface migration of methyl tertiary butyl ether (MTBE) and benzene towards a drinking-water production site in Belgium was monitored for 9 years. A large gasoline spill at a nearby fuel station had caused a 500-m long and 50-m-wide pollution plume of MTBE (10?30 mg/L) and benzene (2?10 mg/L). In order to prevent any intrusion of pollutants into the drinking-water supply, a conceptual model was used to design a pump-and-treat system that intercepted the gasoline-contaminated groundwater emanating from the spill. The contaminated soil in the spill zone was excavated. A numerical mass transport model was developed to evaluate the ongoing plume containment. The model describes the subsurface MTBE migration and was regularly updated, based on groundwater monitoring data and the measured mass of MTBE extracted with the pump-and-treat system. With continued interception pumping, the MTBE plume can be remediated in 14 years. Without it, MTBE and benzene concentrations up to 600 μg/L could have reached the drinking-water production site and the plume would persist for 9 years longer. Source zone treatment combined with plume interception pumping is a suitable risk-based remediation strategy for the containment of MTBE and benzene groundwater pollution.     

Establishing natural sediment reference conditions for metals and the legacy of long-range and local pollution on lakes in Europe

The intention of the European Water Framework Directive (WFD) and the national guidelines that implement the WFD is that present-day conditions and future management strategies are to be based on an understanding of reference conditions for the particular water body of interest. In the context of non-synthetic pollutants such as lead, mercury and cadmium, the criteria for a high ecological status are that “concentrations [are] within the range normally associated with undisturbed conditions”. How this normal range is to be defined is open to interpretation; for example, in Sweden reference conditions based on sediment records are defined as the conditions prior to modern industrialization, i.e. prior to the mid-1800’s. These pre-industrial reference conditions would correspond to sediments 15–30 cm depth. However, ‘reference conditions’ are not always synonymous with ‘natural background conditions’. Analyses of long sediment profiles from Swedish lakes and from a few other areas, however, have shown that pre-industrial pollution—at least with regard to lead—was extensive. Atmospheric lead pollution has its origin in antiquity, with a small, well-defined peak already during the Greek-Roman period 2,000 years ago. Sediments deposited 300–500 years in Sweden and Scotland, for example, show a dominance of pollution lead, and in some sediment records also cadmium and copper pollution was extensive. Thus, in order to characterize natural background concentrations of metals, long sediment profiles are needed to reach sediments unaffected by pollution (>3,000 years BP); this can correspond to sediments below 50 cm in some lakes, but in others sediments below 300 cm or more.     

Photometry and models of eight near-Earth asteroids

We present new observations and models of the shapes and rotational states of the eight near-Earth Asteroids (1580) Betulia, (1627) Ivar, (1980) Tezcatlipoca, (2100) Ra-Shalom, (3199) Nefertiti, (3908) Nyx, (4957) Brucemurray, and (5587) 1990 SB. We also outline some of their solar phase curves, corrected to common reference geometry with the models. Some of the targets may feature sizable global nonconvexities, but the observable solar phase angles were not sufficiently high for confirming these. None is likely to have a very densely cratered surface. We discuss the role of the intermediate topographic scale range in photometry, and surmise that this scale range is less important than large or small scale lengths.     

Freshwater forcing as a booster of thermohaline circulation

Making use of a simple two‐layer model, we analyze the impact of freshwater forcing on the thermohaline circulation. We consider the forward‐type circulation dominated by thermal forcing, implying that the freshwater forcing acts to reduce the density contrast associated with the equator‐to‐pole temperature contrast (prescribed in the model). The system is described by two variables: the depth of the upper layer ( H ) and the density contrast between the upper and lower layer (Δρ), which decreases with salinity contrast. The rate of poleward flow of light surface water and the diapycnal flow (i.e., upwelling) driven by widespread small‐scale mixing are both modeled in terms of H and Δρ. Steady states of thermohaline circulation are found when these two flows are equal. The representation of the diapycnal flow ( M_D ) is instrumental for the dynamics of the system. We present equally plausible examples of a physically based representation of M_D for which the thermohaline circulation either decreases or increases with density contrast. In the latter case, contrary to the traditional wisdom, the freshwater forcing amplifies the circulation and there exists a thermally dominated equilibrium for arbitrary intensity of freshwater forcing. Here, Stommel's famous feedback between circulation and salinity contrast is changed from a positive to a negative feedback. The interaction of such a freshwater boosted thermohaline circulation with the climate system is fundamentally different from what is commonly assumed, an issue which is briefly addressed.     

Fine sediment transport by tidal asymmetry in the high-concentrated Ems River: indications for a regime shift in response to channel deepening

This paper describes an analysis of the observed up-river transport of fine sediments in the Ems River, Germany/Netherlands, using a 1DV POINT MODEL, accounting for turbulence-induced flocculation and sediment-induced buoyancy destruction. From this analysis, it is inferred that the net up-river transport is mainly due to an asymmetry in vertical mixing, often referred to as internal tidal asymmetry. It is argued that the large stratification observed during ebb should be attributed to a profound interaction between turbulence-induced flocculation and sediment-induced buoyancy destruction, as a result of which the river became an efficient trap for fine suspended sediment. Moreover, an asymmetry in flocculation processes was found, such that during flood relative large flocs are transported at relative large flow velocity high in the water column, whereas during ebb, the larger flocs are transported at smaller velocities close to the bed??this asymmetry contributes to the large trapping mentioned above. The internal tidal asymmetry and asymmetry in flocculation processes are both driven by the pronounced asymmetry in flow velocities, with flood velocities almost twice the ebb values. It is further argued that this efficient trapping is the result of a continuous deepening of the river, and occurs when concentrations in the river become typically a few hundred mg/l; this was the case during the 1990 survey analyzed in this paper. We also speculate that a second regime shift did occur in the river when fluid mud layers become so thick that net transport rates are directly related to the asymmetry in flow velocity itself, probably still in conjunction with internal asymmetry as well. This would yield an efficient mechanism to transport large amounts of fine sediment far up-river, as currently observed.