Case-study Mamer: Urban pressures by stormwater pollutants

 Introduction

 Motivation for the choice of the catchment

 Model choice and implementation

 Monitoring campaign

 Synoptic view of monitoring campaigns for the Mamer case-study

Introduction


Urban settings exert numerous pressures on surface waters through emissions from sanitation systems and impervious surfaces run-off but also because of the need to regulate and contain the discharge of rivers. With the upgrading of Waste Water Treatment Plants (WWTP) during the last decades, which mitigated emission of oxygen-depleting substances and nutrients, the focus shifted to the management of rain water and its specific pollutant loads. The hydraulic modeling of storm water runoff is common practice today but the quantification of pollutant emissions is still challenging and highly variable between sites and events. Furthermore, the fate of the pollutants in the receiving waters and their ecotoxicological effects are rarely addressed. This is especially true for substances that are carried in the solid phase like hydrophobic organic compounds (f. ex. PAH) and several metals (Pb, Cu, Zn). Their residence time in the receiving river is potentially much longer than that of water, via transient storage in sediments. The transfer of these pollutants to the food web is rarely assessed and hence impacts on the regional level are not known.

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Motivation for the choice of the catchment


Receiving waters in Luxembourg often have low natural flow as compared to WWTP outlet discharge. With high (treated) wastewater amounts the respect of emission thresholds can still lead to a significant impact on the immission situation. The same applies for storm water management infrastructure, the impact of overflows is higher with growing impervious surfaces connected to small discharge-weak rivers. The Mamer catchment is a typical case for Luxembourg, where in its upper catchment the WWTP of Mamer town contributes 50% of the base-flow of the receiving river. The impact of combined sewer overflows has clearly been identified with continuous oxygen measurements. Downstream the main WWTP the Mamer flows through a weakly urbanized catchment with significant groundwater contributions from the Luxembourg Sandstone. Hence the influence of the pollutant sources downstream can be followed and model predictions on sediment transport and intermittent storage be verified.

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Model choice and implementation


The model setup had to be adapted as compared to the Wark catchment as SWAT does not feature any module allowing for storm water routing simulation. Here, M3 applies a commonly used model by Luxembourgish engineering offices named KOSIM, a tanks in series gravitational model which can predict hydraulic loads of overflow events. The quality of the storm water has to be determined empirically via a dedicated sampling campaign and is introduced as event mean concentrations in the model. SWAT will nevertheless be employed to simulate hydrologic response of the catchment and the mobilization of sediments through soil erosion. Eroded soil loads will dilute stormwater sediments in river sediment budgets and are therefore an essential factor in the mitigation of the pollution.
The application of Aquatox will be much more extensive than in the Wark catchment, since in the Mamer food web toxicological effects will rely on internal toxicity of PAH and bioaccumulation of the latter will have to be modelled. In addition the sediment transport and transient storage within the river bed will also have to be simulated which calls for a segment in series modelling in the Mamer. The monitoring efforts need to step up accordingly.
The Mamer catchment will also be modelled with the Erftverband’s DatenFluss models as well as the DWA water quality model. The latter will be employed to simulate the fate of metals.

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Monitoring campaign

The monitoring efforts in the Mamer catchment pursue the following goals: a) monitoring of combined sewer overflow (CSO) quality, b) validation of the impact of CSO and stormwater on the river loads downstream the urban area, c) the propagation of pollution through the sediments downstream the urbanized area, d) the bioaccumulation of PAH and metals in invertebrate larvae. In addition metabolic parameters will be recorded to calibrate the biomasses in the Aquatox model.
The monitoring of CSO water quality will be monitored at the central retention basin of the WWTP Mamer with the aid of a triggered autosampler. The assumption is made that the loads recorded in this basin are representative for the sewer network.
The validation is performed on a second autosampler which monitors the river Mamer itself during storm events. The monitoring is supported by a multiparametric sonde covering turbidity and conductivity. This autosampler is located downstream the Mamer WWTP.
The propagation of pollution through the catchment is investigated via regular sediment net campaigns which collect suspended matter during base-flow. Here, the assumption is that the collected matter is re-suspended bottom material and washed of biofilms which should be representative for biota food substrate. As the sampling lasts over several days, the nets integrate the material stored in the segment. It should be more representative than discrete spot sampling of bottom sediments in a stretch. A series of 8 nets will be exposed on several occasions through the season with different distances to the source areas.
Finally the food web and bioaccumaltion model Aquatox needs validation for PAH internal concentration. This will be achieved via the sampling and extraction of invertebrates belonging to different feeding guilds on two spots in the river.    
Metabolism campaigns with the display of oxygen probes and the nutrient analyzer will complete the ecological description of the Mamer. It is accompanied by sediment budget monitoring on each campaign. Metabolism campaigns will be repeated several times through the season on 3 sites in the Mamer. The sites have been chosen according to the distance 
to the pollution source areas and the dominant light regime.

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Synoptic view of monitoring campaigns for the Mamer case-study

Campaign

Aim

Validation strategy

Location(s)

Parameters

Event-triggered autosampler

River

Capture stormwater runoff in flood waves

Verify predicted loads in receiving river

Mamer river downstream main urban area

PAH, Metals, SspM, POC, DOC, NH4

Event-triggered autosampler

Combined Sewer Overflow

Capture sewer overflow loads

Assess range of pollution by CSO

Central Retention Basin at WWTP Mamer

Id.

Suspended sediment nets

Monitoring low flow suspended matter quality downstream the source area

Sediment pollution level, transport and dilution downstream

8 suspended sediment nets in a longitudinal profile

PAH, metals, carbon, nitrogen, chlorophyll

River metabolism

Characterize functioning of river segments, provide references for Aqua-tox  & DWA-WQ

Verify evolution of metabolism metrics (biomass evolution) during the season

3 segments in the Mamer

Oxygen, nutrients

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