Water quality during flooding

C.2 Water quality during flooding

The concentration of contaminants in the suspended particulate matter (SPM) of a river depends on the load of the SPM, the amount of contaminants that adsorb to the sediment particles, and the amount of contaminants that are present as carbon, mineral and/or tar particles in the SPM. The SPM quality is therefore first and foremost dependent on the (relative) contribution of the different sources of SPM and contaminants. According to Blok et al. (2006), the substances in the area Island of Dordrecht (in Dutch Eiland van Dordrecht) that exceed the Water Framework Directive (WFD) objectives are the nutrients phosphorus and nitrogen. The Maximum Permissible Concentrations for total phosphate and total nitrogen are

0.15 and 2.2 mg/l, respectively.

Concentrations of heavy metals and agriculture pesticides diuron and isoproturon are lower than the (MPC) for these substances in surface water. However, Muller and Ruigrok (2007) reported in the WFD plan for the area Alblasserwaard, which is located to the North of Dordrecht, copper and zinc as substances problems that exceed the MPC at specific locations (3.8 and 40 µg/l for copper and zinc, respectively). Unfortunately, no data on SPM quality for the study area was available. Therefore and on the basis of previous studies, an expert judgment is done in this section about the quality of the SPM that is settled down after the flooding.

Dissolved contaminants introduced in the river system, the redistribution of previously deposited sediment-bound contaminants, and the relative contribution of relatively clean ‘fresh’ sediment are controlling factors for the SPM quality in the River Rhine. During periods of low flow, SPM concentrations are low, but a lot of sources still introduce contaminants in the river system. Therefore, SPM quality can

be poor. During periods of high river flow, SPM concentrations increase and SPM quality can improve. Asselman (2003), and Van der Heijdt and Zwolsman (1997) attributed this to dilution of the SPM with clean sediment material from the surrounding area. Data of sediment-associated heavy metal concentrations obtained at Lobith analysed by Asselman (2003) showed an improvement in the SPM quality with increasing discharge ( Figure C.10 ). However, this analysis does not include many data with discharges higher than 6,000 m3/s. The sediment quality in floodplains confirms this relation: the quality of sediments at lower parts of the floodplain is much worst than at higher parts, even when the sediment quality is corrected for organic matter.

An increase of the SPM concentrations is simulated in the present study: 50 an 100 mg/l. The highest measured SPM concentration at Papendrecht according the available data was 79 mg/l, and the average concentration was around 40 mg/l. However, we do not know about the quality of the SPM or sediment at the study area in order to be able to describe the SPM as clean. Thonon (2006) showed a decrease in the zinc concentrations in suspended matter at Lobith during the period of 1988-2004. In the Rhine, direct discharges of contaminants have been forced back in the last years. This and the ongoing deposition of sediments that have buried heavily polluted older sediments, led nowadays to a topmost, clean, layer of sediment which will be eroded before the older polluted layer becomes prone to erosion. Considering this, we expect an improvement in the SPM quality with respect to concentrations of copper and zinc during high discharge situations.

Figure C.10 SPM-associated heavy metal concentrations (‘concentratie’) versus Rhine discharge

(‘afvoer’) at Lobith (Asselman, 2003)

In the case of nutrients, we have to make a distinction between nutrients that absorbed to SPM and nutrients that do not. Van Vliet (2006) studied quality of the River Meuse at Eijsden during the floods of 1993, 1995 and 2003 and observed decreases in concentrations of dissolved compounds, like chloride, ammonium and nitrite, due to increased dilution under higher flow conditions. In the case of the latter and although the nitrate concentration was also affected by increased dilution, the concentration responses during the three investigated floods of 1993, 1995 and 2003 were relatively constant. Van Vliet attributed this to the increased nitrate supply by soil leaching and overland flow during high flow conditions, which counterbalances the effect of the increased dilution. Peaks in the total concentration of phosphate can be expected high flow conditions, which can be explained by the increased amount of the SPM under higher discharges, the high adsorption capacity of SPM for phosphate, and continuous high emissions of phosphate from agriculture.

The grain size distribution is an important parameter for the distribution and composition of contaminants inside the river channel as most contaminants are adsorbed to the clay and organic matter fraction. At low flow velocities, only a low amount of mainly fine material will be transported in the river, while at higher flow velocities also coarser material can be distributed. The effects of the grain size distribution can also be found back with the deposition of contaminants on the floodplain section or inundated area. The contribution of fine material (clay fraction) increases with increasing distance to the river channel. Coarse material will settle down close to the river channel when the water flow velocity is not high enough for transport. Middelkoop (2000) determined this effect in the Rhine-Meuse delta ( Figure C.11 ). In this study, no distinction was done between different fractions of SPM. However, we expect an increase of the clay and organic matter fraction as the distance from the inundation area to the river channel also increases (as further from the river channel as higher the metal concentration), together with an increase in the metal concentration.

Figure C.11 Spatial distribution of clay, organic matter and heavy metal content with increasing distance to the river channel, during the December 1993 flood of the river Meuse near Bern (Middelkoop, 2000)

References

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