| | | 6.6 Combating eutrophication in the Seine river and the Seine Bight : scenarios analysis |
As practical examples of management options, various scenarios of phosphate removal were tested on the strongly eutophicated Marne river. The modelling strategy was to consider separately 9 sub-basins and the main branch, to which was coupled a model of the reservoir. After the model was validated through its ability to reproduce available water quality observations [], different realistic scenarios of future reduction of phosphorus load can be tested, in various hydrological conditions (dry and wet years). The effect of hydrological variations as well as the phosphorus loading from both point and diffuse sources can be explored.
Under dry hydrological conditions, eutrophication by phytoplankton remains a major problem for drinking water production, even after the implementation of drastic programmes of point source reduction down to 85% of the present loading, although already been reduced to 65% of the 1990 level. Diffuse phosphorus sources from agriculture is found to be important, especially because a high proportion of particulate phosphorus originating from soil erosion is exchangeable with the dissolved phase [][] and available for phytoplankton growth.
When the present conditions of diffuse and point sources were applied to the hydrological conditions of 1991, the driest in our data base, phytoplankton development reached values similar to those really observed for the year 1991, before the reduction in point sources (Figure 16). This result shows that, in hydrological conditions that are not extreme compared to 1976 and 2003 for example, the hydrosystem remains eutrophic, despite the reduction of phosphorus point sources by a factor of 3. Phytoplankton biomass can reach 80 µg Chl a l-1, as the phosphorus concentration is higher due to the weaker dilution of the point inputs by low water flow. These results tend to show that further or additional P treatment efforts are needed to prevent phytoplankton development.
Recently constructed wastewater treatment plants in the Seine basin (e.g. the Colombes treatment plant downstream from Paris), using advanced tertiary treatment technology are able to reduce the specific phosphorus load to less than 0.2 g P inhab-1d-1 []. A value of 0.15 g P inhab-1d-1 in the discharge of all wastewater treatment plants in the Marne would decrease the point load from the present one of 1260 kgP d-1 in 2000 to 200 kg P d-1. The lower P value has a significant effect on phytoplankton biomass, which remains however as high as 60 µg Chl a l-1 during the spring bloom (Figure 16).
It appears clearly from these results that action aimed solely at the point sources is not sufficient to completely prevent eutrophication. A further test by the model consisted in reducing diffuse sources of phosphorus. For that purpose, we reduced by a factor of 2 the phosphate concentrations in surface and groundwater runoff and considered an additional 50% riparian retention of suspended solids. In these conditions, the spring bloom falls below 40 µg Chl a l-1 (Figure 16).
Clearly, measures for reducing phosphorus emissions from WWTPs are not sufficient to completely control freshwater eutrophication but must be accompanied with reduction of diffuse sources linked to agricultural leaching and erosion.
The occurrence of episodic blooms of toxic dinoflagellates of the Seine Bight has led to investigate its relationships with human activity in the Seine watershed. The RIVERSTRAHLER model has been linked to SiAM-3D/ELISE, a model of coastal zone including the estuary; this latter model is a 3D hydrodynamic coupled to an ecological model of the Seine Bight allowing to reproduce the spatio-temporal variations of sediment transport, thermo-haline stratification and phytoplanktonic development in the plume of the Seine river [][][].
Once the chain of models have been validated by their ability to reproduce observed trends of interannual variations of nutrients delivered by the Seine, as well as the response of the marine biocenose in terms of diatoms and dinoflagellates development [], various scenarios of human activity in the watershed can be simulated with alternative policies of wastewater nitrogen and/or phosphorus treatment in terms of marine eutrophication reduction (Figure 17).
As the proportion of phosphorus point sources increases from the upstream to downstream in regards to diffuse sources, at short term a possible control actions able to reduce nutrient contamination at the coastal zone is the improvement of wastewater phosphorus treatment. An efficient reduction of the nitrate in term of eutrophication is more uncertain due to its dominant diffuse origin (especially from agricultural soils) and the time lag caused by nitrate accumulation in the aquifers [][]. We however explore the effect of phosphorus and/or nitrogen tertiary treatments of urban wastewater treatment plants, assuming that the diffuse sources of nutrients are constant, although, due to the time lag caused by nitrate accumulation in the unsaturated soil zone and in the aquifers, prospective models of ground- and surface water contamination in the Seine basin predicts further increase of nitrate concentration during the next decade, even for the hypothesis of a systematic improvement of farming practices [][].The scenarios are run under dry hydrological conditions, shown to be the most critical for the development of potentially harmful algae.
Whereas, a drastic reduction of the phosphorus load discharged with domestic and industrial effluents is technically feasible at the scale of the entire Seine watershed, nitrogen treatment of wastewater is technically more complex. The ordinary activated sludge process retains about 20% of the raw nitrogen load in the produced sludge but can be increased at 50%, by re-circulating a part of nitrified effluents at the head of the biological treatment for denitrification. A reduction of nitrogen from 70% to 90%, in wastewater is technically possible but at a much higher cost, by using methanol or ethanol as external electron donor for the denitrification.
Several hypotheses of phosphorus and/or nitrogen treatment of urban wastewater in the basin were therefore tested. For phosphorus, a generalized 90% reduction throughout the year was tested. For nitrogen, scenarios with 50%, reduction of point sources have been considered. A combined P and N reduction was then explored (Figure 17).
The phosphorus reduction of wastewater (P 90% trt) appears as a quite effective measure to reduce the potentially harmful algal blooms in the Seine Bight (Figure 17). Diatom maximum biomass is divided by a factor of 2, and that of flagellates by a factor of 10, phosphorus becoming the limiting factor. Treating phosphorus during the productive period only gives approximately the same result on flagellates as when the treatment is conducted the all year long []. In the Seine Bight, flagellate blooms especially occur in the river plume area directly under the influence of nutrient inputs. The Seine river flow and the tidal dynamics lead to a short resident time of freshwaters in that area. Therefore, winter nutrient inputs are rapidly exported into the English Channel, so that the phytoplankton production is mostly dependent on recent nutrient inputs. Regarding the reduction of nitrogen point sources, whereas it does not affect the diatom biomass, the flagellate biomass, limited by nitrogen with a 50% reduction, decrease by a factor of 2. Silica remains the limiting factor whatever the degree of nitrogen treatment of the wastewaters (Figure 17). Combining a 50% nitrogen reduction and a 90% phosphorus reduction of point sources in the watershed does not provide any significant improvement in terms of flagellate blooms compared to the sole phosphorus treatment. In fact, the nutrient limiting effect due to phosphorus for the P 90% trt scenario is greater than the one due to nitrogen in the N 50% trt scenario.
It is now recognized that increased phosphorus and nitrogen inputs cause silicon to be the main limiting factor for diatom production, favouring flagellate summer blooms which use the nitrogen and phosphorus left over after diatoms have depleted silica [277][][][]. The recent decreasing trend of phosphorus per capita load [][][] and the feasibility, at relatively low cost, of phosphorus treatment of wastewater, should in the nearby future allow a shift of the ecosystem from silica and nitrogen to a general phosphorus limitation, leading to a significant reduction of harmful algal blooms in the Seine Bight. However on one hand, the impact of nitrogen in excess over silica and phosphorus in surrounding marine areas is not fully understood, and on the other hand the future trend of phosphorus load from agriculture sources (fertilizers, farm effluents) is not known.
| | | 6.6 Combating eutrophication in the Seine river and the Seine Bight : scenarios analysis |