Bernhard Wehrli

The Challenge of Micropollutants in Aquatic Systems

The increasing worldwide contamination of freshwater systems with thousands of industrial and natural chemical compounds is one of the key environmental problems facing humanity. Although most of these compounds are present at low concentrations, many of them raise considerable toxicological concerns, particularly when present as components of complex mixtures. Here we review three scientific challenges in addressing water-quality problems caused by such micropollutants. First, tools to assess the impact of these pollutants on aquatic life and human health must be further developed and refined. Second, cost-effective and appropriate remediation and water-treatment technologies must be explored and implemented. Third, usage and disposal strategies, coupled with the search for environmentally more benign products and processes, should aim to minimize introduction of critical pollutants into the aquatic environment.

The coordination chemistry of weathering: III. A generalization on the dissolution rates of minerals

A general rate law on the surface-controlled dissolution of oxides and silicates is discussed. Combining concepts of surface coordination chemistry with established models of lattice statistics and activated complex theory we propose a general rate law for the acid- and ligand-promoted dissolution of minerals: R = kxa,PjS where R is the proton- or ligand-promoted dissolution rate [moles m−2s−1], k stands for the appropriate rate constant [s−1], xa denotes the mole fraction of dissolution active sites [−], Pj represents the probability to find a specific site in the coordinative arrangement of the precursor complex [−] and S is the surface concentration of sites [mol m−2]. Surface complexes (surface chelates and metal proton complexes) are precursors in the rate-limiting detachment of a central metal ion from the surface into the solution. We develop a mechanistic model, which clarifies the pH-dependence of dissolution rates. First appropriate surface protonation isotherms are derived. The surface protonation equilibria of different minerals at constant ionic strength match a single Freundlich isotherm with a slope of ca. 0.2. This result explains the frequently reported fractional pH-dependence of dissolution rat`es. Based on the Bragg-Williams approximation a lattice statistical procedure is outlined which permits the calculation of the probability Pj of the precursor complex. The consistency of reported experimental dissolution rates and activation energy is tested and, as a consequence, different possibilities to correlate reaction rates (kinetic rate constants [s−1] are not reported) or apparent activation energies with thermodynamic data in the form of free linear energy relations are explored. We postulate that the site energy (Madelung energy) of the most stable lattice constituent (generally a metal cation at a surface site) or the ion formation energy of the solid—characteristic of the free energy needed to break the essential bonds in the lattice—are suitable free energy parameters to be correlated with the dissolution rates (log RH at pH 5).

Vanadyl in natural waters: Adsorption and hydrolysis promote oxygenation

The aquatic chemistry of vanadium is dominated by V(IV) and V(V). Their species VO2+, VO(OH)+ and H2VO−4, HVO2−4, respectively, occur primarily in natural waters. VO2+, as a very hard Lewis acid, has a strong tendency to coordinate with oxygen donor atoms and is thus capable of both forming strong complexes with soluble organic chelates and becoming specifically adsorbed to particles, especially hydrous oxides. Vanadates (V), like phosphates, also have a tendency to form, by ligand exchange, surface complexes with hydrous oxides. The V(IV)–V(V) couple is an interesting redox sensor because the redox transition occurs at EH-values typically often encountered at the sediment water interface; organic chelate formation may extend the redox boundary to EH-values of about +0.4V (pH 7–8), but in the presence of dissolved oxygen vanadate(V) prevails. Experimental data on the adsorption of VO2+ and of V(V) (HVO2−4, VO+2) on δ-Al2O3 and TiO2 (anatase) surfaces provide evidence for strong specific adsorption. The interaction of VO2+ with oxide surfaces is interpreted in terms of inner-sphere bidentate surface complexes with the surface central metal ions of the oxide VO(OM<)2; vanadate and VO2+ form monodentate surface species. The rate of oxidation of VO2+ by oxygen is significantly enhanced by hydrolysis or adsorption to hydrous oxide surfaces. The rate law, derived earlier (Wehrli and Stumm, 1988) shows a first order dependence on the concentration of VO(OH)+ in homogeneous solution or on the concentration of the surface complex of VO(OM<)2 in heterogeneous systems. A comparison with published data on Mn(II) and Fe(II) oxidation shows that coordinated OH-groups of solid surfaces are able, like soluble hydroxo complexes, to mediate the electron transfer from the metal ions to the O2-molecule.

Extreme Methane Emissions from a Swiss Hydropower Reservoir: Contribution from Bubbling Sediments

Methane emission pathways and their importance were quantified during a yearlong survey of a temperate hydropower reservoir. Measurements using gas traps indicated very high ebullition rates, but due to the stochastic nature of ebullition a mass balance approach was crucial to deduce system-wide methane sources and losses. Methane diffusion from the sediment was generally low and seasonally stable and did not account for the high concentration of dissolved methane measured in the reservoir discharge. A strong positive correlation between water temperature and the observed dissolved methane concentration enabled us to quantify the dissolved methane addition from bubble dissolution using a system-wide mass balance. Finally, knowing the contribution due to bubble dissolution, we used a bubble model to estimate bubble emission directly to the atmosphere. Our results indicated that the total methane emission from Lake Wohlen was on average >150 mg CH(4) m(-2) d(-1), which is the highest ever documented for a midlatitude reservoir. The substantial temperature-dependent methane emissions discovered in this 90-year-old reservoir indicate that temperate water bodies can be an important but overlooked methane source.

Benthic chamber and profiling landers in oceanography - A review of design, technical solutions and functioning

We review and evaluate the design and operation of twenty-seven known autonomous benthic chamber and profiling lander instruments. We have made a detailed comparison of the different existing lander designs and discuss the relative strengths and weaknesses of each. Every aspect of a lander deployment, from preparation and launch to recovery and sample treatment is presented and compared. It is our intention that this publication will make it easier for future lander builders to choose a design suitable for their needs and to avoid unnecessary mistakes.

Solute transfer across the sediment surface of a eutrophic lake: I. Porewater profiles from dialysis samplers

Porewater profiles often are used to identify and quantify important biogeochemical processes occurring in lake sediments. In this study, multiple porewater profiles were obtained from two eutrophic Swiss lakes using porewater equilibrators (peepers) in order to examine spatial and seasonal trends in biogeochemical processes. Variability in profile shapes and concentrations was small on spatial scales of a few meters, but the uncertainty in calculated diffusive fluxes across the sediment surface was, on average, 35%. Focusing of Fe and Mn oxides toward the lake center resulted in systematic increases in porewater concentrations and diffusive fluxes of Fe2+ and Mn2+ with increasing water depth; these fluxes are postulated to be regulated by the pH-dependent dissolution of reduced-metal phases. Despite higher concentrations of inorganic carbon, NH4+, Si and P in pelagic compared to littoral sites, diffusive fluxes of these substances across the sediment surface increased only slightly or not at all with increasing water depth. Porewater profiles did reveal temporal changes in Fe2+, Mn2+, Ca2+ and Mg2+ that were an indirect result of the large, seasonal changes in seston deposition, but no clear seasonal variations were found in diffusive fluxes of nutrients across the sediment surface. The intense mineralization occurring at the sediment surface was not reflected in the porewater profiles nor in the calculated diffusive fluxes. Calculated diffusive fluxes across the sediment surface resulted from decomposition occurring primarily in the top 5–7 cm of sediment. Diffusive fluxes from this subsurface mineralization were equal to the solute release from mineralization occurring at the sediment-water interface. Buried organic matter acts as a memory of previous lake conditons; it will require at least a decade before reductions in nutrient inputs to lakes fully reduce the diffusive fluxes into the lake from the buried reservoir of organic matter.

Spatial Heterogeneity of Methane Ebullition in a Large Tropical Reservoir

Tropical reservoirs have been identified as important methane (CH(4)) sources to the atmosphere, primarily through turbine and downstream degassing. However, the importance of ebullition (gas bubbling) remains unclear. We hypothesized that ebullition is a disproportionately large CH(4) source from reservoirs with dendritic littoral zones because of ebullition hot spots occurring where rivers supply allochthonous organic material. We explored this hypothesis in Lake Kariba (Zambia/Zimbabwe; surface area >5000 km(2)) by surveying ebullition in bays with and without river inputs using an echosounder and traditional surface chambers. The two techniques yielded similar results, and revealed substantially higher fluxes in river deltas (∼10(3) mg CH(4) m(-2) d(-1)) compared to nonriver bays (<100 mg CH(4) m(-2) d(-1)). Hydroacoustic measurements resolved at 5 m intervals showed that flux events varied over several orders of magnitude (up to 10(5) mg CH(4) m(-2) d(-1)), and also identified strong differences in ebullition frequency. Both factors contributed to emission differences between all sites. A CH(4) mass balance for the deepest basin of Lake Kariba indicated that hot spot ebullition was the largest atmospheric emission pathway, suggesting that future greenhouse gas budgets for tropical reservoirs should include a spatially well-resolved analysis of ebullition hot spots.

Varve formation since 1885 and high-resolution varve analyses in hypertrophic Baldeggersee (Switzerland)

According to their microstratigraphical composition the laminated sediments of Baldeggersee are true varves. Two varve time-series from freeze cores taken in the deepest part (66 m) of Baldeggersee have been analysed for annual and seasonal layers. The varve time-series covers the period of 1885 to 1993.The two freeze-cores from Baldeggersee can be accurately correlated by means of distinct layers (e.g. marker varves, turbidites), lithological units, and varve measurements. The Baldeggersee varve chronology has been checked by independent high-resolution137Cs dating. Mobility of cesium has not been detected in Baldeggersee. A highly positive correlation between phosphorus concentration in the lake water and median grain size of calcite crystals has been observed and allows, together with the results of geochemical analyses, to hindcast the trophic state in Baldeggersee for periods where no limnological data are available.The main features of the Baldeggersee varve time-series can be summarized as follows: before 1885 there are packets of 5–10 varves interrupted by massive, homogeneous marl beds, indicating oscillating hypolimnetic oxygen levels until the lake ultimately became anoxic in 1885. Between 1885 and 1905 varves are thin, with a high carbonate content. Between 1905 and 1910 the thickness of varves increased steadily, and an increase in calcite grain size suggests a major step in eutrophication. Median grain size values increase again at the end of the 1940s, whereas the thickness of the seasonal layers decreased between the early 1940s and the early 1960s. At the onset of the 1960s, during the time of highest epilimnetic phosphorus concentrations in Baldeggersee, the total phosphorus concentrations in the sediments as well as the varve thickness and the total accumulation rates all increased. At the beginning of the 1980s the median grain size decreased substantially as a result of lower phosphorus concentrations in Baldeggersee.

Measurement of denitrification in sediments with the 15N isotope pairing technique

Due to increasing nitrogen concentrations in surface waters ([6][1]) and eutrophication of coastal waters ([14][2], [26][3], [51][4]), the quantification of denitrification rates in sediments of lakes, rivers, and estuaries has gained importance. Different methods for measuring denitrification have

Sources and sinks of nitrous oxide (N2O) in deep lakes

As reported from marine systems, we found that also in15 prealpine lakes N2O concentrations werestrongly correlated with O2 concentrations. Inoxic waters below the mixed surface layer, N2Oconcentrations usually increased with decreasingO2 concentrations. N2O is produced in oxicepilimnia, in oxic hypolimnia and at oxic-anoxicboundaries, either in the water or at the sediment-waterinterface. It is consumed, however, incompletely anoxic layers. Anoxic water layers weretherefore N2O undersaturated. All studied lakeswere sources for atmospheric N2O, including thosewith anoxic, N2O undersaturated hypolimnia.However, compared to agriculture, lakes seem not tocontribute significantly to atmospheric N2Oemissions.