| Popis: |
The Zenne river in Belgium is a low discharge urban river (10 m³/s in average) crossing the city of Brussels over a distance of almost 20km with more than half of its Brussels reach culverted. Crossing Brussels, the river receives two treated wastewater treatment plant (WWTP) discharges: the first from the WWTP of Brussels South and the second, which is the main one, from the WWTP of Brussels North located just upstream of the outlet of the city border. The river is equipped with 3 YSI probes, one located upstream of Brussels city, the second at the outlet of the city and the third 8km downstream of the Brussels city border. They are measuring same diverse quality parameters such as dissolved oxygen (DO) and turbidity. All probes are providing continuous records with time frequencies equal or down to 5 minutes. Monthly manual measurement of quality parameters are also performed along the river by the regional public society of the Vlaamse Milieu Maatchappij (VMM). In this study, we will scrutinize all data together and interpret them in their spatial and temporal context in order to learn about the diversity of the dissolved oxygen problematic of such particular urban river. The analyze of dissolved oxygen long data series provided since 2011 downstream of Brussels city shows that the level of dissolved oxygen for that part of the river is dramatic for fish since dissolved oxygen concentrations daily average fall below 3 mg/L for a significant part of the year. Indeed, from 2011 to 2013, proportion of the year presenting such low concentration represented 44.1% of the year in 2011 where for 2012 and 2013 it represented respectively 16.2% and 31.6%. Most of the days where DO concentrations are as such low are observed to occur in summer when temperatures of the river are sufficiently high as shown in Figure 1. Temperature increase impairs the oxygen level by decreasing the oxygen saturation concentrations and by accelerating the respiration processes. Furthermore, the river warming impairing effect on oxygen levels is exacerbated by the very low discharge of the river. Indeed, in summer the average discharge of the river downstream of Brussels is only of 6.5m³/s. As shown in Figure 1, in summer, temperatures up to 17°C coupled to discharges down to 3 folds the lowest river water level induce oxygen concentrations dropping down to 3mg/L. Additionally, in spring and summer, photosynthesis is being limited in the Brussels river channel since it is culverted for the major part of the reach. All together, the warm conditions, the low discharge and the culverted configuration of the river lead, downstream of Brussels, to dramatic oxygen conditions for fish, for one third of the year in average. The major oxygen concentrations deteriorations occur during the crossing of the city. This is observed through the difference between the monthly oxygen measurements performed by the VMM at the inlet of Brussels City and at the measurement station located just upstream of the WWTP of Brussels North discharge. In average, the diminishing of the DO concentration between these two stations is 18% and can reach in summer the highest proportion of 65%. The WWTP of Brussels North discharges contribute actually to a significant increase of the oxygen level downstream of Brussels. Its contribution is for instance observed through the continuous oxygen monitoring downstream of Brussels. Indeed, the daily dissolved oxygen cycles are mainly driven by the succession of the photosynthesis and the respiration processes but also by the daily cycles of WWTP Brussels North discharges. From the VMM data, it can be deduced that the Brussels discharges represent in average 50% of the river discharge flowing out of Brussels and that the oxygen concentration of the WWTP is in average 7.6mg/L. Such oxygenated effluent discharges contribute to an average recovery of 60% of the Brussels impaired oxygen level and can reach in summer 200%. Oxygen data monitored since 2011 downstream of Brussels also allow us to identify the main sources of intermittent oxygen depletion in the river. First of them are the combined sewer overflows (CSO) occurring in the north part of the city [Le et al. 2013]. Analyzing the oxygen data series reveals also non negligible pollution discharges from urban effluent attributed to the WWTP of Brussels South. It appears that on only one year (November 2011 to October 2012) such pollution discharges occurred 22 times, each time following heavy rains and leading to severe oxygen depletion. The pollution induced is most probably due to activated sludge discharge since the monitored data show that no significant increase in water level is induced but significant increase of suspended matter (reflected by the turbidity records) accompanies the oxygen deficits. Less frequent but also noticeable, quality and hydrometric parameters monitored upstream of the Brussels city show that oxygen depletion can be induced by the transfer of an upstream floodwave. Indeed, upstream floodwave transit induces a river deposit-resuspension process affecting the oxygen concentration in the river. Continuous monitoring of oxygen can also lead to the evaluation of rate processes occurring in the river. By applying the Odum method (see for instance in Figure 2) on daily oxygen cycles monitored upstream of Brussels, the atmospheric reaeration coefficient and the respiration can be deduced and are shown to be respectively 10d-1 and 2.4mgO2/L.h in average in summer. Such knowledge is particularly valuable for the river quality modeling efforts. In conclusion, the data driven system analysis we performed allows the identification and the documentation of the oxygen status of the river and also the expected and unexpected sources and processes affecting it. The oxygen level in the Zenne river downstream of Brussels is observed to be dramatic for fish especially in summer when the river discharge is low, the temperatures are high and the photosynthesis is limited in the Brussels channel reach. Continuous data monitoring analysis allows deducing the atmospheric reaeration coefficient of the river. Also, significant contribution of the main Brussels WWTP is shown to lessen the impaired oxygen level observed, especially in summer. Data driven analysis additionally allows identifying significant sources of pollution such as the CSOs and the unexpectedly frequent WWTP activated sludge discharges. |
