Sensibilité à la pollution métallique de deux grands lacs africains (Tanganyika et Malawi)

Les lacs Tanganyika et Malawi sont, de par leur volume, les deux plus grands lacs africains. Ces réservoirs semblent pour l'instant épargnés par la pollution en éléments en trace. Il est toutefois crucial, en raison de leurs caractéristiques hydrologiques, de poser la question du temps de réponse de ces systèmes à une pollution chronique potentielle véhiculée par les affluents. Cet article simule ainsi cette réponse dans la fraction dissoute suite à l'introduction pendant 50 ans de polluant par tous les affluents. Cette démarche s'appuie sur un modèle hydrologique intégrant les trois compartiments des colonnes d'eau (épi-, méta- et hypolimnion) et sur la prise en compte de la réactivité des éléments dissous dans ces compartiments par l'intermédiaire du taux de rétention élémentaire. Ainsi quatre types d'éléments sont considérés, (i) le type Cl, non réactif, (ii) le type Si, réactif-nutritif, (iii) le type Mn et (iv) le type V tous deux réactifs sensibles aux conditions d'oxydo-réduction. La réactivité de l'élément, l'efficacité du mélange vertical ainsi que la position de l'oxycline dans la colonne d'eau conditionnent l'amplitude et la cinétique de réponse des systèmes ainsi que le temps de retour à la situation initiale après l'arrêt des apports polluants. Ces caractéristiques propres à l'élément et au lac influent sur le risque potentiel encouru par l'écosystème et l'homme. Ainsi la pollution affecte principalement les eaux de surface (types Cl et V), les réseaux trophiques (type Si), les eaux profondes (types Si et Mn) et le compartiment sédimentaire (types Mn et V).Lakes Tanganyika and Malawi are the largest African lakes as measured by volume. They constitute essential water and protein resources for the surrounding populations. These aquatic systems have become stressed due to high human population density, growth and associated activities. While eutrophication was apparent locally and organic pollutants were detected in fish and water, concentrations of several dissolved trace elements of potential concern corresponded to uncontaminated systems. However, due to their hydrological features, it was important to characterise the lake response time to chronic contamination loaded by the tributaries. This paper presents two simulations of this response, in the dissolved fraction, following 50 years of pollutant input by the tributaries. The first simulation corresponded to an annual pollutant input that was the same for both lakes, resulting in mean river input concentrations of 5.0 U L-1 and 3.7 U L 1 (where U is a weight or molar unit), respectively, for lakes Tanganyika and the Malawi. The second simulation corresponded to an annual input proportional to the lake volume, with mean river input concentrations of 5.0 U L 1 and 1.5 U L 1, respectively, for lakes Tanganyika and the Malawi. The polluted input was loaded by the dissolved fraction with the exception of Mn-type elements, which were carried by the particulate fraction. This approach was based on an annual hydrological model of three water column compartments (epi-, meta- and hypolimnion) of these meromictic lakes. In addition, the reactivity of dissolved elements in the water column was taken into consideration. The reactivity was characterised by the elemental retention rate that quantifies dissolved-particulate interactions linked to biological and physico-chemical processes. The reactivity of trace elements was assessed through their concentration distribution profile in the water column. Four element types were considered: the non-reactive elements characterised by homogenous concentrations in the water column (Cl-like); the micronutrient-type elements (Si-like) characterised by a strong positive concentration gradient below the thermocline; redox-sensitive elements (Mn-like) characterised by a strong positive concentration gradient below the oxycline and other redox-sensitive elements (V-like) characterised by a strong negative concentration gradient below the oxycline. Trace elements (F, Al, Fe, Mn, V, Ba, Sr, Mo, Cr, Ni, Co, Cu and Pb) in both lakes were associated with these element types but they did not necessarily belong to the same type in both lakes. Other elemental types likely occurred (e.g., carbonate type and Fe types) but they were not clearly identified. After 50 years, surface concentrations ranged from 0 to 1.15 U L 1 in Lake Tanganyika and from 0 to 2.40 U L 1 in Lake Malawi. The difference between the lakes was linked to the greater volume of Lake Tanganyika, mainly in its hypolimnion, and to the longer vertical water exchange time in Lake Tanganyika. For Cl-type elements the concentration response decreased for both lakes from the epi- to the hypolimnion with similar kinetics for the epi- and metalimnion and a delay for the hypolimnion. For Si-type elements the response decreased in Lake Malawi from the hypo- to the epilimnion and for Lake Tanganyika the maximal concentration was calculated in the metalimnion. The concentration range was higher in Lake Malawi than in Lake Tanganyika. For the Mn-type elements, the maximum concentration was calculated in the hypolimnion with a higher response in Lake Malawi. The metalimnetic water concentration of Lake Tanganyika increased slightly and epilimnetic and metalimnetic waters of Lake Malawi did not react. For V-type elements the epilimnetic waters were more sensitive to the increase, with a higher response for Lake Malawi. In Lake Malawi concentrations also increased in the metalimnion. Concentrations in the hypolimnetic zone of both lakes and metalimnetic zone in Lake Tanganyika remained zero. Depending on the element type and on the lake, the time required to return to initial conditions, when contaminant inputs stopped, varied from 30 to 7 300 years. In the epilimnetic zone of both lakes the intensity of reaction and the pollution persistence were higher for Cl-type elements. For Si-type elements, mainly in Lake Malawi, the vertical input from deep waters was sufficient to sustain productivity even after the input of pollutants was stopped. For these elements the dissolved contamination was mainly stored in deep waters. For Mn-type elements the contamination was also stored in deep waters with a relatively slow net transfer to the sedimentary compartment. V-type pollutants were transferred from the dissolved to the particulate phase in deep waters leading to a relatively rapid net transfer to the sediment. Once the pollutant was in the system and until its evacuation to the outlet or to sediment, the risk for the ecosystem and for the population was associated with its presence in the dissolved phase of the surface water. The risk was then higher for Cl- and V-type elements as well as for the Si-type elements that were introduced into the web food. For the Si- and Mn-type elements that were mainly stored in deep waters, the associated risk was linked to a breaking of the thermo-haline stratification or to a reinforcement of vertical mixing. For the V-type elements and also for the sedimentary fraction of the Mn-type elements, the risk was also associated with possible remobilization from the sediments due to physico-chemical changes at the water-sediment interface.Element reactivity, efficiency of the vertical mixing and the depth of the oxycline control the importance and the kinetic response. They also controlled the time to attain initial conditions once contaminant inputs were stopped. These features, relative to the element and to the lake, were key parameters in the assessment of the potential risk for both the ecosystem and people that rely on these lakes. Even if the elemental typology was the same for both lakes, elements can be considered a different type from one lake to another. Contamination from the same pollutant would then have different consequences, for instance regarding the associated risk. Computed hydrochemical budgets were simple but realistic, illustrating the behaviour of elements in the water column. Computation of this budget requires the knowledge of global water column fluxes, which have to be improved mainly for Lake Tanganyika. The element's reactivity was mainly linked to liquid-solid reactions. It would be interesting in future studies to characterise particulate phases and their reactivity and to introduce such processes in hydro-geochemical models. Computations of chronic contamination response indicate that for both lakes, due to the inertia of the hydrochemical system, the lack of lake water contamination does not imply a systematic lack of pollution in the tributaries. Once pollution is detected, it will be persistent. A global watershed monitoring program should be organised in the near future. Monitored parameters should be relevant to metallic and organic pollutants, as well as eutrophication

Determining the structure of a large tilted block between two major boundary faults in a continental rift (central Lake Baikal): a reflection seismic study

Between the major boundary faults of the central part of Lake Baikal (ie. the Ol’khon fault and the Primorsky fault), a structurally complex tilted area exists that is strongly influenced by the interaction between these two faults. This area, that is about 30 kilometer wide and a 100 kilometers long, consists of three main parts: Pri-Ol’khon, Ol’khon-island and the submerged Maloe More depression. It is believed that the area formed by the gradual propagation of the Primorsky fault in a southeast direction towards the Ol’khon fault.During the summer of 2001 a large amount of high resolution reflection seismic profiles were shot in Maloe More (>600 km), that could be used to get a better insight in the structural development of the area, and in the geometry of its different sub-blocks and basins. In a first stage we have investigated the morphology of the basement underneath the sedimentary cover, and we determined which structures were fault related and which not. Age constraints on the subsequent evolution came from the correlation of the sedimentary units in Maloe More with deposits on Ol’khon-island, and with data from the long BDP-cores in a nearby area (Academician Ridge).The depth of the basement gradually increases from the southwest towards the northeast, and its morphology is characterised by several ridge structures and faults that strike at high-angle to the main faults. Several of these ridges border basins that contain relatively old sediments (Miocene age; Unit A) later overlain by younger units. Therefore the main basement structures of the Maloe More area should be older than the general believed age for the southward propagation of the Primorsky fault (1 Ma according to earlier models). Moreover the occurrence of relatively thick deposits of unit A in the southwestern extremity of Maloe More and in Ol’khon-gate contradicts the idea that these parts of the area are the youngest, being submerged only recently.Instead, older (isolated) sedimentary traps and lacustrine environments must have existed in this area. Faulting in the younger sediments however shows that the presentday activity of the major boundary faults, still has a pronounced effect on the local structure between them. Some of the formed basins are still determined by displacements on the older structures.For this study we have tried to determine the evolution of the Maloe More area, based on its interpreted structure and the relation with overlying sedimentary deposits, and we have tried to link our observations with existing models for the development of the Primorsky and Ol’khon faults

Nebraska’s Natural Resource District System: Collaborative Approaches to Adaptive Groundwater Quality Governance

Nonpoint source pollution of groundwater by nitrates from agricultural activity is a persistent problem for which developing effective policy approaches has proven difficult. There is little empirical information on forms of governance or regime attributes that effectively and sustainably address agricultural nonpoint source pollution of groundwater. Nebraska’s Natural Resource District (NRD) system is a rare example of a groundwater governance regime that is putting programmes in place that are likely to generate sustainable groundwater quality outcomes. We focus on three groundwater nitrate management programmes in the state that collectively represent the broader NRD system. The research shows that four elements of Nebraska’s groundwater governance regime are fundamental to its success in addressing groundwater nitrates: 1) the local nature of governance, which builds trust among stakeholders; 2) the significant authority granted to the local districts by the state, allowing for the development of locally tailored solutions; 3) the collaborative governance approach, which allows potential scale imbalances to be overcome; and 4) the taxing authority granted to NRDs, which enables them to fund locally tailored management solutions. We find that these aspects of the NRD system have created conditions that enable adaptive, collaborative governance that positions the state well to address emerging groundwater quality challenges. We present aspects of the governance regime that are generalisable to other American states as efforts to address nitrate pollution in groundwater increase

"Rondeel" beter voor burger, boer en beestje

Rondeel dóet niet aan maatschappelijk verantwoord ondernemen, Rondeel ís maatschappelijk verantwoord ondernemen, volgens zijn initiatiefnemers. Binnenkort kan iedereen kennis nemen van het concept. Op 8 april wordt de eerste Rondeel-houderij voor leghennen in Barneveld feestelijk geopend door Gerda Verburg, demissionair minister van LNV

The enabling and constraining connections between trust and digitalisation in incumbent value chains

Digitalisation is a disruptive socio-technical process that goes beyond digital technologies and their use within an organisation, and involves besides (in many cases radical) technological change, social, institutional and economic change. This creates uncertainties for value chain actors and the trust relationships between them. In this paper we aim to understand the connections between trust and digitalisation. We investigate how trust relations affect digitalisation, and how digitalisation affects trust relations among value chain actors, using the Dutch flower sector as a case study. Our findings show that the sector has a high level of interpersonal trust, but limited institutional trust, as the relationships between companies are highly competitive and transactional. In this context, limited trust hinders digitalisation in multiple and mutually reinforcing ways, inducing a vicious cycle whereby existing distrust or limited trust results in limited digitalisation, which in turn causes more distrust due to uncertainties around the digitalisation process, further increased by existing (technological) path dependencies. Hence there is a need for 1) awareness of mutually reinforcing (dis)trust dynamics and vicious (or virtuous) cycles in relation to digitalisation are needed; 2) higher levels of understanding of what digitalisation entails and 3) joint strategy building and foresighting in the value chain

What are innovation platforms?

Available in Chinese, English, Hindi, Thai and Vietnames

Sustainability transition pathways through ecological intensification: an assessment of vegetable food systems in Chile

Ecological intensification has been proposed as a promising lever for a transition towards more sustainable food systems. Various food systems exist that are based on ecological intensification and may have potential for a sustainability transition. Little is known, however, about their diversity and about how they perform against dominant systems in terms of the multiple societal goals. The aim of this study is to contribute to knowledge about sustainability transitions in food systems through an empirical analysis of vegetable food systems in Chile. The study (i) characterizes the diversity of vegetable food systems in Chile (ii) evaluates the food systems in terms of multiple societal goals, and (iii) assesses their potential for supporting sustainability transition pathways from the perspective of ecological intensification. Results indicate that among the five vegetable food system types, the agroecological and the small organic have potential to foster a sustainability transition. Nevertheless, these systems are small and localized, and scaling them requires actions to remove barriers in the relations with the agri-food regime and among themselves. The broader relevance of this analysis is that there needs to be awareness in research on transitions about the diversity of food systems present in countries and how they interact.</p

Active destabilisation of gas hydrate accumulations in Lake Baikal by tectonically induced fluid-flow

Multi-channel seismic profiling and deep drilling have evidenced the presence of gas hydrates in Lake Baikal, Siberia. They occur in the deep basins around the large Selenga River Delta. The presence of the hydrates is evident on seismic records by virtue of a distinct high-amplitude, reversed-polarity, cross-cutting BSR. Locally, however, the BSR shows a very anomalous behaviour. In the vicinity of some of the main, active, intra-basin faults, its depth strongly fluctuates, with undulations (positive as well as negative compared to background positions) and vertical displacements of several hundreds of ms TWT. Locally, the BSR is even entirely disrupted by vertical ‘chimneys’ that reach up to the lake bottom.High-resolution deep-tow side-scan sonar mosaics over one of such areas of deformed and disrupted BSR show a cluster of morphological irregularities on the lake floor, in contrast to areas above a regular BSR where the lake floor is absolutely regular and flat. Four large irregularities - aligned parallel to the fault – were discovered, one of them coinciding with one of the ‘chimneys’. They were mapped in detail by bathymetric sounding and proved to be either elevations (mud volcanoes ?) or depressions (craters) at the lake floor. Echosounding has also shown venting associated with these features, which is evidenced by an acoustically non-transparent plume, reaching 10-25 m above the bottom (in other places in a similar context plumes were observed of > 200 m of height). CTD-profiling, which shows very little change in bottom-water temperature at the venting sites, suggests that the plumes represent cold seeps.Heat-flow values measured over the area show a good correlation with changes in BSR depth: values vary between 50-60 mW/m² to 80-90 mW/m². In the craters, heat-flow values are highest, but they do not exceed 165 mW/m². Our observations suggest that the Baikal hydrates are locally - along particular segments (about 15 km long) of active faults - destabilizing by tectonically controlled upward flow of fluid and heat, and that this results in active venting of gasses and/or fluids at the lake floor

An inventory of hydrate-related gas seeps in Lake Baikal

Lake Baikal in Siberia, one of the world’s largest rift lakes, is known to be the only fresh-water lake with gas hydrates in the subsurface. Seismic data clearly image the base of the hydrate layer in the Central and Southern Baikal Basins, at both sides of the Selenga delta. Gas seeps and short-lived mud volcanoes were discovered in Lake Baikal’s South Basin at places where the gas hydrate layer shows anomalous thickness variations, which was attributed to localized heat flow anomalies along active fault segments. The gas seeps were interpreted as the result of localized destabilization of gas hydrates by injected thermal water along fault segments; it is probably the inevitable consequence of gas hydrate accumulation in an active rift basin.New data from Lake Baikal’s Central basin, acquired during the summer of 2002, show that the four seeps in the South Basin are not isolated cases. More seeps were discovered, all situated in Baikal’s hydrate accumulation area, all near active fault segments, and all associated with anomalous thickness variations of the underlying hydrate layer. This poster gives an overview of the occurrence of gas seeps in Lake Baikal in relation with thickness anomalies of the gas hydrate layer. The gas seeps in Lake Baikal were found in three areas: 1. Posolsky fault area. Four methane seeps were encountered in the footwall of a small antithetic fault of the Posolsky fault zone. The seeps occur as blow-out craters and conical mud volcanoes. 2. Olkhon fault splay area. Two gas seeps, St.Petersburg and Novosibirsk, were discovered in 2002 at the footwall of a splay of the large Olkhon border fault. The seeps appear to be conical mud volcanoes. 3. The Kukuyu canyon area. The Kukuyu canyon is a large, probably fault-related, canyon, at the northern slope of the Selenga delta. Gas seeps are documented on side scan sonar data and subbottom acoustic profiles.The newly discovered seeps support the interpretation that gas seeps and mud volcanoes in Lake Baikal are caused by the localized dissociation of gas hydrates by thermal input at the base of the hydrate layer. Seepage is probably intense but short-lived, and sometimes accompanied by mud extrusion at the lake floor. They are a rare example where hydrates are the source for intense methane venting, and not vice versa