Organic micropollutants discharged by combined sewer overflows – Characterisation of pollutant sources and stormwater-related processes

Accepted Manuscript Organic micropollutants discharged by combined sewer overflows – Characterisation of pollutant sources and stormwater-related processes Marie A. Launay, Ulrich Dittmer, Heidrun Steinmetz PII:

S0043-1354(16)30582-6

DOI:

10.1016/j.watres.2016.07.068

Reference:

WR 12263

To appear in:

Water Research

Received Date: 12 April 2016 Revised Date:

12 July 2016

Accepted Date: 27 July 2016

Please cite this article as: Launay, M.A., Dittmer, U., Steinmetz, H., Organic micropollutants discharged by combined sewer overflows – Characterisation of pollutant sources and stormwater-related processes, Water Research (2016), doi: 10.1016/j.watres.2016.07.068. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Organic micropollutants discharged by combined sewer overflows

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– Characterisation of pollutant sources and stormwater-related

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processes

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Marie A. Launay a, Ulrich Dittmer a, Heidrun Steinmetz b

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a

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Management (ISWA), Bandtäle 2, 70569 Stuttgart, Germany

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b

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Wastewater Technology, Paul-Ehrlich-Str. 14, 67663 Kaiserslautern, Germany

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Corresponding author: Marie Launay, Phone: +49 711 685 65445, E-mail address:

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University of Stuttgart, Institute for Sanitary Engineering, Water Quality and Solid Waste

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University of Kaiserslautern, Department of Civil Engineering, Resource Efficient

[email protected]

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Abstract

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To characterise emissions from combined sewer overflows (CSOs) regarding organic

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micropollutants, a monitoring study was undertaken in an urban catchment in southwest

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Stuttgart, Germany. The occurrence of 69 organic micropollutants was assessed at one CSO

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outfall during seven rain events as well as in the sewage network at the influent of the

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wastewater treatment plant (WWTP) and in the receiving water. Several pollutant groups like

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pharmaceuticals and personal care products (PPCPs), urban biocides and pesticides,

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industrial chemicals, organophosphorus flame retardants, plasticisers and polycyclic

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aromatic hydrocarbons (PAHs) were chosen for analysis. Out of the 69 monitored

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substances, 60 were detected in CSO discharges. The results of this study show that CSOs

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represent an important pathway for a wide range of organic micropollutants from wastewater

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systems to urban receiving waters. For most compounds detected in CSO samples, event

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mean concentrations varied between the different events in about one order of magnitude

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range. When comparing CSO concentrations with median wastewater concentrations during

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dry weather, two main patterns could be observed depending on the source of the pollutant:

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ACCEPTED MANUSCRIPT (i) wastewater is diluted by stormwater; (ii) stormwater is the most important source of a

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pollutant. Both wastewater and stormwater only play an important role in pollutant

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concentration for a few compounds. The proportion of stormwater calculated with the

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conductivity is a suitable indicator for the evaluation of emitted loads of dissolved wastewater

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pollutants, but not for all compounds. In fact, this study demonstrates that remobilisation of

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in-sewer deposits contributed from 10 % to 65 % to emissions of carbamazepine in CSO

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events. The contribution of stormwater to CSO emitted loads was higher than 90 % for all

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herbicides as well as for PAHs. Regarding the priority substance di(2-ethylhexyl)phthalate

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(DEHP), this contribution varied between 39 % and 85 %. The PAH concentrations found

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along the river indicate environmental risk, especially during rainfall events.

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Keywords: Combined sewer overflows; PPCPs; priority pollutants; urban biocides; urban

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catchment; surface water quality

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1. Introduction

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The European Water Framework Directive (WFD) came into force in 2000 to achieve and

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maintain good chemical and ecological status of surface waters and groundwater. Decision

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2455/2001/EC of November 2001 defined 33 priority substances, including metals, industrial

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chemicals, biocides and pesticides, which represent a significant risk for the aquatic

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environment. Directive 2008/105/EC adopted environmental quality standards (EQS) for

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these 33 substances, indicated as annual average (AA-EQS) and maximum allowable

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concentration (MAC-EQS). The European Commission has to re-examine and update this list

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at least every four years. In 2013, a new Directive (Directive 2013/39/EC) amended the

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previous ones regarding priority substances. A new list of 45 substances was defined. Newly

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identified substances were added and EQS of some existing compounds were revised.

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In order to apply measures for reducing the pollutant emissions into the aquatic environment,

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it is necessary to have a precise knowledge about the pollutant origin and behaviour in the

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urban catchment and in wastewater systems as well as about the main pathways to the

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ACCEPTED MANUSCRIPT environment. Several studies have shown that aside from wastewater treatment plant

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(WWTP) discharges, principal sources of micropollutants in urban surface waters are

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stormwater runoff and combined sewer overflows (CSOs) (Ellis, 2006; Welker, 2007; Phillips

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et al., 2012; Luo et al., 2014). In particular CSO discharges contain pollutants originating

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from domestic sewage, industrial wastewater and stormwater runoff. The need to minimise

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the environmental risk of CSO events represents a major challenge for public utilities

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because of the high number of CSOs in urban catchments. In fact in Germany, more than

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71,700 CSO structures are currently listed (German Federal Statistical Office, 2015).

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Regarding environmental standards, inputs from CSOs with very high pollutant

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concentrations can affect the surface water quality (Phillips and Chalmers, 2009; Weyrauch

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et al., 2010; Launay et al., 2013). However, previous studies focused only on a few selected

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micropollutants or groups of pollutants, due to the high human and analytical efforts and the

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costly pollutant analysis. To our best knowledge, only Gasperi et al. (2012) dealt with a

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comparable number of substances as this study. However, the monitoring was focused on

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priority substances. In contrast to previous monitoring programs, this study assesses

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concentration levels of a vast variety of micropollutants including pharmaceutical and

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personal care products (PPCPs), X-ray contrast agents, biocides, herbicides, industrial

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chemicals, flame retardants, plasticisers and polycyclic aromatic hydrocarbons (PAHs) in

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sewage, in CSO discharges as well as in the receiving water.

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This monitoring study was undertaken in an urban catchment in southwest Stuttgart,

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Germany to characterise CSO emissions regarding organic pollutants. It aims at (1)

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assessing the occurrence of a wide range of organic micropollutants with different

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physicochemical properties and different origins in dry and wet weather flow, (2) evaluating

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the importance of the pollutant transport and in-sewer processes for micropollutant

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concentrations and fluxes discharged by CSOs, (3) improving the understanding of the

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impact of CSO discharges on water quality.

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2. Materials and methods

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2.1. Study site and sampling strategy

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This study was conducted in an urban catchment (35 km²) in the southwest of the city of

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Stuttgart, Germany (see Figure 1).

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The urbanised areas are mainly drained by a combined sewer system. The coefficient of

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imperviousness of the catchment is 50 %. The WWTP has a capacity of 160,000 population

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equivalents and its average daily inflow is 22,985 m³/d. Under dry weather conditions, the

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ratio of daily mean river flow to daily mean effluent flow is between 1:2 and 1:4. However,

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during the summer months, when the river flow is at its lowest, this ratio can reach 1:9. This

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critical situation is very often the case for small urban catchments. Upstream of the WWTP

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discharge, the river Körsch receives discharges from 37 CSO structures. Among them 16 are

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combined with storage tanks. The discharged volumes from each CSO during storm events

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were calculated for the entire year 2014 based on dynamic rainfall-runoff simulations using

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EPA SWMM5 (U.S. Environmental Protection Agency’s Storm Water Management Model).

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According to this simulation, the volume discharged from the last CSO outfall upstream of the

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WWTP represented 44 % of the total overflow discharge. Therefore, this outfall was selected

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for sampling. Seven events from July to October 2014 were sampled volume-proportionally

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to get representative composite samples of CSO events. For this purpose, a flowmeter (PCM

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Pro, NIVUS) was installed in the discharge pipe. For the level measurement, both hydrostatic

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pressure and ultrasonic sensing techniques were used. The flowmeter used an ultrasonic

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cross correlation method and digital pattern detection to determine velocity. Flow rate data

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was calculated and registered at an interval of 1 minute by a data logger. Pulse signals were

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sent to an automatic refrigerated sampler so that 150 mL sub-samples were taken per

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300 m³ volume increment. 10 sub-samples were taken to build up one representative

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sample. In total, 25 CSO samples were analysed. The main characteristics of the sampled

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events are given in Table 1. Precipitation data were obtained from three rain gauges and an

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ACCEPTED MANUSCRIPT arithmetic average was calculated for each sampled event. The location of the rain gauge

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stations is shown in Figure 1.

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In order to determine the relative contributions of wastewater and stormwater to pollutant

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loads discharged by CSOs, data for wastewater were also collected during dry weather. For

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this purpose, 24h flow-weighted composite samples were collected at the WWTP influent in

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February and July 2014 during dry weather conditions (n = 9). Both winter and summer

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sampling was carried out to assess the influence of different levels of infiltration water in

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sewers and of possible seasonal effects on the pollutant occurrence. To determine the

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effects of the WWTP effluent and CSO discharges on the water quality, grab samples were

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collected manually at four different locations along the river (R2 to R5 in Figure 1) for nine

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dry days in February and July 2014 and at five locations (R1 to R5 in Figure 1) during the

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following four CSO events: D1, D3, D4 and D7.

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2.2. Selection of organic micropollutants and analytical methods

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All samples were analysed by measuring pH and electrical conductivity. The following criteria

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were selected for establishing the micropollutant list for this study: the substances must be

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representative for various sources and pathways (municipal and industrial wastewater, urban

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runoff) and should have different chemical and physical properties. Another criterion was the

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classification as priority substance by the European Commission in the WFD. A total of 69

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organic micropollutants were chosen for chemical analysis: 15 PPCPs, 2 artificial

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sweeteners, 5 X-ray contrast media, 18 industrial chemicals, 7 pesticides and biocides, 6

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PCBs and 16 EPA PAHs. Priority substances analysed in this study are: anthracene,

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fluoranthene, naphthalene, benzo(a)pyrene, diuron, isoproturon, terbutryn, di(2-

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ethylhexyl)phthalate (DEHP), 4-nonylphenol (4NP) and 4-tertiary octylphenol (4tOP). DEHP

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is one of the most common phthalates used as a plasticiser to improve the plasticity and

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flexibility of materials and is present ubiquitously in the aquatic and terrestrial environment

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(Magdouli et al., 2013). Both alkylphenols 4NP and 4tOP are defined as priority substances

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in the WFD due to their estrogenic effects and ability to accumulate in aquatic organisms. 5

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ACCEPTED MANUSCRIPT The complete list of 69 micropollutants with abbreviations can be found in Table S1

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(Supplementary material).

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Depending on the characteristics of the compound, two different analytical methods were

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used: the analysis was performed by gas chromatography coupled to mass spectrometry

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(GC-MS) for PAHs, PCBs, flame retardants, synthetic musk fragrances and phenolic

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xenoestrogens. For pharmaceuticals, urban pesticides, X-ray contrast media, artificial

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sweeteners and corrosion inhibitors, analysis was performed by liquid chromatography

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coupled to a tandem mass spectrometry (LC-MS/MS). The analysis was performed with

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homogenised original and membrane-filtered samples, so that both dissolved and particulate

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fractions of each compound could be determined. More detailed information about pollutant

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analysis as well as detection and quantification limits can be found in the supplementary

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material. Blank samples were analysed for each measurement campaign. Due to high blank

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values for naphthalene (NAP), results in sewage and in CSO samples could not be

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discussed for this compound. The analysis was performed by the laboratory of Stuttgart’s

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wastewater utility service. Results shown in this article are the concentrations of

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homogenised samples including dissolved and particulate matter.

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2.3. Calculation of event mean concentration (EMC) and mixing ratios

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In order to determine the Event Mean Concentration (EMC) for each sampled CSO event,

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the equation below was used:

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EMC =

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where EMC is the event mean concentration of pollutant, Vi is the overflow volume

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corresponding to the volume in sample i, Ci is the pollutant concentration in the sample i, n is

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the number of samples taken during one event and V is the total overflow volume.

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Proportions of surface runoff and wastewater were calculated for each CSO event based on

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the electrical conductivity as proposed by Passerat et al. (2011), using the following formula:

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x ∗ EC + x ∗ EC = EC (2)

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x + x = 1 where ECDW is the electrical conductivity of wastewater, ECSW is the electrical conductivity of

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surface runoff , ECCSO is the mean electrical conductivity of each sampled CSO, xDW and xSW

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are the proportions of wastewater and surface runoff in CSO events respectively.

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Conductivity values of wastewater were taken on a dry day before CSO events and ranged

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from 1020 µS/cm to 1250 µS/cm. Measurements in the catchment during a rainfall event

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showed that the runoff conductivity was 75 µS/cm.

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2.4. Correction of sewage concentrations taking into account infiltration water

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To eliminate the effect of seasonally varying dilution due to infiltration water, micropollutant

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concentrations in sewage were calculated by dividing daily pollutant loads by the wastewater

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flow. The proportion of infiltration water at the WWTP influent was calculated based on the

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hypothesis of an average water consumption of 125 L per person per day. Figure 2a shows

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the corrected values in sewage at the WWTP influent during dry weather conditions (n = 9).

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Substances are classified into groups based on their application. Due to the wide range of

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values, results are presented in a logarithmic scale. Corrected pollutant concentrations in

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sewage are also reported in Table S3 (Supplementary material).

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3. Results and discussion

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3.1. Concentrations in wastewater during dry weather

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Among the 69 analysed organic pollutants, eight compounds were below the limit of

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detection for all samples: all 6 PCBs, methyltriclosan (MTCS), a transformation product of the

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during dry weather are reported in Table S2 (Supplementary material).

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3.1.1. Wastewater micropollutants

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In this study, the group of wastewater micropollutants includes 15 PPCPs, the artificial

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sweeteners acesulfame (ACE) and sucralose (SUC) and 5 iodinated X-ray contrast agents

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(iomeprol, iopamidol, iohexol, iopromide and diatriozate). As Table S2 in the supplementary

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material indicates, the highest levels in wastewater were detected for caffeine (60–110 µg/L)

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and acesulfame (19–38 µg/L). On the other hand, the lowest detected levels in the WWTP

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influent were found for the beta blocker propranolol PNL (13–30 ng/L). Comparable values

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(median of 40 ng PNL/L) were also detected by Wick et al. (2009) in the influent of another

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German WWTP. Triclosan (TCS) is an antibacterial agent found in personal care products,

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detergents, toys, socks and shoes. Its concentrations in WWTP influent ranged from

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0.56 µg/L to 1 µg/L, which is comparable with values reported in previous studies in Europe

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(Bester, 2003; Bendz et al., 2005). Regarding the insect repellent N,N-diethyl-m-toluamide

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(DEET), concentrations in sewage ranged from 70 ng/L to 530 ng/L in February (median

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concentration of 155 ng/L) and from 360 ng/L to 770 ng/L in July (median concentration of

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470 ng/L, three times higher than in winter), demonstrating clear seasonal fluctuations. The

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seasonal variability of DEET concentration in WWTP influent was already established in

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previous studies (Knepper, 2004; Merel et al., 2015). In this group, the widest range of

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concentrations during dry weather conditions was observed for the iodinated X-ray contrast

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media (34–6,900 ng/L), due to the irregularity of their application in hospitals and radiological

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practices.

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3.1.2. Biocides and herbicides

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The group of biocides and herbicides includes terbutryn (TBY), carbendazim (CZIM), diuron

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(DIU), isoproturon (ISO), 2-methyl-4-chlorophenoxyacetic acid (MCPA), mecoprop (MCP)

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and dichlorprop (DCP). Values for DCP were below the limit of detection (LOD) of 5 ng/L in

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ACCEPTED MANUSCRIPT all samples. TBY had the highest concentration with 120 ng/L. MCPA was detected in

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samples taken in summer whereas concentrations in samples taken in winter were below the

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LOD value of 5 ng/L, demonstrating the seasonal occurrence of this compound in

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wastewater. No clear seasonal variations were identified for the other substances at the

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WWTP influent. One possible explanation for the presence of the fungicide CZIM in urban

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areas may be the important role of indoor use of household products containing this

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compound (Wittmer et al., 2011). So biocides can also be detected in sewage during dry

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weather conditions, probably due to household activities. Similar conclusions were drawn for

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several urban catchments in Denmark (Bollmann et al., 2014).

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3.1.3. Industrial chemicals

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The group of industrial chemicals includes the three corrosion inhibitors 1H-benzotriazole

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(BTR), 5-methyl-1H-benzotriazole (5-MBT) and 4-methyl-1H-benzotriazole (4-MBT), the

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phenolic compounds 4-t-octylphenol (4tOP), 4-nonylphenol (4NP) and bisphenol A (BPA),

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the phthalate plasticiser DEHP, the vulcanisation accelerators in tire and rubber

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manufacturing processes benzothiazole (BT) and 2-methylthiobenzothiazole (MTBT). The

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highest concentrations in wastewater were detected for BTR (7.3 µg/L to 19 µg/L) and DEHP

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(4.1 µg/L to 16 µg/L). The lowest concentrations were detected for 4tOP (100 ng/L to

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140 ng/L), BT (170 ng/L to 270 ng/L) and MTBT (230 ng/L to 420 ng/L).

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3.1.4. Organophosphorus compounds

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A total of nine organophosphorus flame retardants and plasticisers were selected in this

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study: triethylphosphate (TEP), triisobutyl phosphate (TIBP), tributyl phosphate (TBP), tris(2-

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chloroethyl)phosphate (TCEP), tris(1-chloro-2-propyl)phosphate (TCPP), tris(1,3-

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dichloroisopropyl)phosphate (TDCPP), tris(2-butoxyethyl)phosphate (TBEP), triphenyl

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phosphate (TPP) and triphenylphosphine oxide (TPPO). The highest concentrations in

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wastewater were found for TBEP (3.5–20 µg/L) and TCPP (0.46–4.5 µg/L). Similar high

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levels of TBEP have been reported in several WWTP influents in Sweden (Marklund et al.,

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ACCEPTED MANUSCRIPT 2005). The lowest concentrations were found for TBP (30–130 ng/L) and TPPO (50–

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140 ng/L). TIBP and TDCPP showed relatively constant levels in wastewater (from 180 ng/L

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to 350 ng/L and from 170 ng/L to 310 ng/L respectively).

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3.1.5. Polycyclic aromatic hydrocarbons (PAHs)

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The PAH concentrations in wastewater ranged from 3 ng/L for acenaphthene (ACN),

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acenaphthylene (ACY) and fluorene (FL) to 330 ng/L for fluoranthene (Fluo). The sum of all

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15 PAHs considered ranged from 365 ng/L to 1,730 ng/L. Fluoranthene and pyrene had the

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highest concentrations and contributed to 14 % and 13 % of the total PAH concentration

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respectively.

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3.1.6. Concentration variations due to infiltration water

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Figure 2a shows the corrected values in sewage at the WWTP influent during dry weather

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conditions (n = 9). In July, the proportion of infiltration water in sewers was 30 % and reached

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50 % in February. Seasonal fluctuations in infiltration water levels are a crucial issue and

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should not be neglected when planning wastewater sampling campaigns. Although the

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results shown in Figure 2a do not include the effect of dilution by infiltration water, most of

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the compounds have significant variations in dry weather concentrations. To investigate if

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these variations are due to seasonal effects, ratios between corrected values and mean

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concentrations were calculated. Figure 3 shows the results for triclosan (TCS), the X-ray

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contrast agent iohexol (IOH) and the insect repellent DEET. Clear seasonal variations could

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be found for DEET and for MCPA (see Section 3.1.2). The IOH concentrations like other X-

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ray contrast media fluctuated randomly. On the contrary, triclosan (TCS) showed quite

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constant levels in sewage, as Figure 3 indicates. Similar conclusions could be drawn for

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caffeine (CAF), acesulfame (ACE), sucralose (SUC), metoprolol (MPL), carbamazepine

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(CBZ), atenolol (ANL), 5-methyl-1H-benzotriazole (5-MBT), 2-methylthiobenzothiazole

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(MTBT), 4-t-octylphenol (4tOP), triisobutyl phosphate (TIBP) and tris(1,3-

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dichloroisopropyl)phosphate (TDCPP).

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3.2. Concentrations in CSO discharges

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Figure 2b shows the total concentrations of 62 organic micropollutants measured in 7 CSO

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events, plotted in logarithmic scale. Measured values in all CSO samples are reported in

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Table S4 (Supplementary material). CSO events showed a wide range of micropollutant

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concentrations. The highest total concentrations in CSO discharges were observed for CAF

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(3.5–18.5 µg/L) and ACE (0.81–5.3 µg/L), like in dry weather flow conditions. The third

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highest concentrations in CSOs were found for the industrial chemical DEHP (0.70–

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5.4 µg/L). In general, wastewater micropollutants as well as industrial chemicals showed

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much wider concentration ranges in CSOs than in wastewater.

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3.2.1. Comparison with dry weather concentrations

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Figure 4 indicates the ratios between CSO concentrations and median wastewater

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concentrations at the WWTP influent during dry weather. Three different patterns could be

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observed. In the case of PPCPs, artificial sweeteners, X-ray contrast agents and most of the

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industrial chemicals, ratios between CSO and wastewater concentrations were all below one,

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meaning that wastewater is diluted with stormwater runoff. Moreover, concentrations of the

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plasticiser tributyl phosphate (TBP) dropped below the LOD in CSO samples, due to dilution

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with stormwater. On the other hand, for other compounds like urban biocides, PAHs and the

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two industrial chemicals BT and 4tOP, concentration ratios between CSO and wastewater

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samples were greater than one, meaning that stormwater concentrations are higher than

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wastewater concentrations. This indicates an input through surface runoff. BT is used as

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vulcanisation accelerator in tyre and rubber manufacturing processes and can be deposited

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on the road surface through traffic activities (Zeng et al., 2004). The tyre road debris, like

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street dust, are washed off during rain events, which explains the high BT concentrations in

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CSO samples. Regarding 4tOP, ratios between CSO and median wastewater concentrations

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were about 2.5. 4tOP is used in the production of varnishes, lacquers and as an adhesive

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agent in rubber for tyres. High concentrations of 4tOP can also be found in surface runoff

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and therefore in CSO samples.

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ACCEPTED MANUSCRIPT For DIU, ISO, DEHP, 4tOP, Fluo and Pyr, similar ranges of concentration ratios between

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CSOs and wastewater were found in both areas. For other PAHs, the same trend could be

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observed, but as it can be seen from Figure 4, ratios obtained by Gasperi et al. (2012) are

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about ten times higher than ratios obtained in our study. One possible explanation may be

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the higher ambient air pollution from local PAH sources like traffic in the French capital.

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For some compounds like CZIM and the organophosphorus flame retardants and plasticisers

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TCPP, TEP, TPP, TIBP and TPPO, median concentrations in wastewater and total

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concentrations in CSO were in the same range, showing that both stormwater runoff and

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wastewater play an important role as pollutant sources. In contrast to urban herbicides,

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indoor applications of products containing CZIM appear to be a significant source of this

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compound in sewers.

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3.2.2. Relative contribution of wastewater and stormwater runoff

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The relative proportions of wastewater and stormwater runoff were calculated based on the

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electrical conductivity (see Formula 2). Results for each CSO event are presented in Table 2.

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For all seven CSO events studied, the proportion of runoff ranged from 73 % for the

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discharge on 12th September 2014 to 95 % for the discharge on 17th October 2014. As Table

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2 indicates, runoff proportions are comparable with results in Paris based on the same

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approach (Passerat et al., 2011; Gasperi et al., 2012).

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In order to assess if the mixing ratios calculated using the electrical conductivity are

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appropriate tracers for wastewater micropollutant fluxes in CSOs, relative wastewater and

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runoff proportions were calculated using wastewater micropollutants with the help of the

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same formula used for conductivity. Results presented in Figure 5 were calculated for all

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sampled CSOs based on the assumption that the concentration in stormwater was zero. In

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fact, due to their use, the detection of these substances is not expected in surface runoff

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(Madoux-Humery et al., 2015).

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ACCEPTED MANUSCRIPT As it can be seen from Figure 5, results are satisfying for sucralose (SUC), naproxen (NPX),

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ibuprofen (IBF) and diclofenac (DCF), for which the difference in wastewater proportion with

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conductivity calculation is mostly below 25 %. For these substances, nearly no in-sewer

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processes (biological degradation, transformation, sorption, remobilisation of sewer deposits)

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occurred. Therefore, it can be concluded that the electrical conductivity is an appropriate

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tracer to determine the relative proportions of stormwater and wastewater in combined

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sewers. It is important to note that the conductivity is not an appropriate wastewater tracer in

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winter, more specifically under snowmelt conditions, due to the use of de-icing agents and

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salt with high conductivity values. For sampling studies including snowmelt, other tracers

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must be applied (Madoux-Humery et al., 2013). An ideal wastewater tracer: should be

318

abundant in sewage; should not be present in stormwater; should have a conservative

319

behaviour; and its input into the sewer system should be quite constant. From the results of

320

this study, artificial sweeteners (ACE and SUC), NPX, IBF and DCF appear to be potential

321

suitable wastewater tracers in combined sewers during rain events. However, further

322

research is needed to evaluate the input dynamics of these compounds into the sewer

323

system during dry weather flow.

324

3.2.3. Relevance of in-sewer processes

325

Regarding caffeine (CAF), the wastewater proportion in CSOs was underestimated for all

326

events. Our results indicate that caffeine may be partly eliminated and/or transformed in

327

sewers. Conversely, it means that emitted CAF loads are overestimated by using the mixing

328

ratios calculated with the conductivity. Results from previous studies show that CAF is readily

329

biodegradable (Buerge et al., 2003; Thomas and Foster, 2005; Sui et al., 2010), so it can be

330

assumed that CAF is partly biologically degraded in sewers. Although CAF is one of the most

331

used indicators for wastewater contamination of aquatic systems (Chen et al., 2002; Buerge

332

et al., 2006; Hillebrand et al., 2012), little is known about its biodegradation in sewer

333

systems. More research is needed to understand better the fate of caffeine and its biological

334

degradation in combined sewers. Contrasting results were found for carbamazepine (CBZ).

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ACCEPTED MANUSCRIPT For five CSO events the wastewater proportion was highly overestimated (see Figure 5). As

336

CBZ should not be found in surface runoff (Madoux-Humery et al., 2015), CBZ

337

concentrations in CSOs can be explained partly by resuspension of sewer deposits. In order

338

to quantify the effect of in-sewer processes on CBZ fluxes, concentrations and loads due to

339

urban stormwater were calculated for CBZ using the mixing ratios obtained with conductivity

340

values. Relative wastewater and runoff proportions to total emitted CBZ loads in all sampled

341

CSO discharges can be seen in Figure 6b. Remobilisation of in-sewer deposits play a major

342

role for CBZ loads in CSO events, as it contributes from 10 % (D3) to 65 % (D6) to CBZ

343

emissions. These results were confirmed when analysing the phase distribution of this

344

compound in wastewater and in CSO samples. During dry weather conditions, particle-bound

345

CBZ represented 11 % (± 7 %) of the total concentration. In CSO samples, the mean ratio for

346

particle-bound CBZ was 17 % and reached up to 35 % in one sample during the first flush of

347

the CSO event on 17th October. Our findings confirm previous recent results from a sampling

348

campaign of CSO discharges in Montreal, Canada (Pongmala et al., 2015). In their case,

349

remobilisation during wet weather flow was responsible for 30 % of CBZ load discharged by

350

one CSO event on October 2009 (this proportion reached up to 65 % during the first flush of

351

the event).

352

3.2.4. Stormwater-related loads of priority substances

353

Based on the mixing ratios calculated using the conductivity, the relative wastewater and

354

stormwater-related loads discharged by CSOs were determined for priority substances.

355

Since the differentiation between urban runoff and remobilisation of sewer deposits as

356

pollutant sources during rain events is difficult, we defined stormwater-related loads as

357

pollutant loads resulting from both processes. Figure 6 shows the results for priority

358

substances for the discharge D7 and for all seven sampled CSOs (relative contribution

359

based on the sum of volumes and loads). The discharge D7 had the highest proportion of

360

stormwater (95 %) and the contributions of runoff and remobilisation of sewer deposits were

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ACCEPTED MANUSCRIPT also the highest of the seven discharges for ISO (99.2 %), 4tOP (98.3 %), 4NP (84.3 %) and

362

DEHP (84.6 %).

363

Regarding all sampled CSOs, the highest contribution of stormwater to discharged loads was

364

obtained for the group of urban herbicides: the median values for each substance ranged

365

from 93 % (MCPA) to 99 % (MCP). It can be explained with the fact that these substances

366

are mainly washed off from green areas and roofs during rain events and this phenomenon

367

represents the main source of urban herbicides and pesticides in combined sewers. The

368

group of PAHs had the second highest contribution of stormwater to the emitted loads. The

369

median values of the stormwater contribution ranged from 90 % (PHE) to 98 % (ACY). For

370

this group of compounds, both urban runoff and remobilisation of in-sewer deposits play an

371

important role. A study of Gasperi et al. (2010) showed similar results. They determined the

372

contributions of wastewater, runoff and sewer deposit erosion to PAH loads in several

373

combined sewer systems in Paris. For all urban catchments, in-sewer processes represented

374

an important source of PAHs in wet weather discharges (from 35 % up to 85 %).

375

Different trends were observed for the alkylphenols 4NP and 4tOP. The contribution of

376

stormwater to 4NP emissions by CSOs ranged from 25 % to 84 % (median of 50 %),

377

whereas the contribution for 4tOP was around 90 % for all sampled events. The ratios

378

between CSO and median wastewater concentrations (see Figure 4) confirm that urban

379

runoff is an important source of 4tOP. Both compounds have high octanol-water partition

380

coefficients (log KOW = 4.48 for 4NP, log KOW = 4.12 for 4tOP [Ahel and Giger, 1993]), which

381

indicate their hydrophobicity and their tendency to adsorb to particles. Nevertheless, these

382

values suggest that 4NP is more hydrophobic than 4tOP and therefore more 4NP is

383

accumulated in sediments than 4tOP. These findings are consistent with previous results

384

from monitoring of river water and riverine sediments in Japan (Isobe et al., 2001). So it can

385

be concluded that remobilisation of sewer deposits is the major process involved for 4NP,

386

explaining that the contribution of stormwater varies a lot, depending on the rain event

387

characteristics and antecedent dry weather period. By contrast, the quite constant

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ACCEPTED MANUSCRIPT contribution of stormwater for 4tOP could be explained by the fact that urban runoff is the

389

main source of this compound in wet weather flow and that remobilisation of sewer deposits

390

does not play such an important role as it is the case for 4NP.

391

DEHP levels in dry weather flow ranged between 4.1 µg/L and 16 µg/L and concentrations in

392

CSOs ranged between 0.7 µg/L and 5.4 µg/L (see Figure 2). Considering all sampled CSO

393

events, 60 % of DEHP emissions were due to stormwater (from 39 % for D5 to 85 % for D7).

394

DEHP has been already detected at high levels in stormwater in several catchments (Rule et

395

al., 2006; Gasperi et al., 2012; Kalmykova et al., 2013). Moreover, DEHP is a highly

396

hydrophobic compound with a log KOW of 7.6. Adsorption to sewer sediments during dry

397

weather conditions and remobilisation during rain events may also play a major role in DEHP

398

concentrations in CSO discharges.

399

3.3. Concentrations in the receiving water

400

The European Commission established environmental quality standards (EQS) for certain

401

priority substances in surface water in order to ensure the protection of both aquatic

402

environment and human health (Directive 2013/39/EU). Two different types of standards

403

were set: EQS expressed as annual average concentrations (AA-EQS) when considering

404

long-term exposure and chronic effects and maximum allowable concentrations (MAC-EQS)

405

when considering short-term exposure and acute effects.

406

In Germany, a federal surface water ordinance was adopted in 2011

407

(Oberflächengewässerverordnung, 2011). This ordinance adopted the list of priority

408

substances including environmental quality standards set in Directive 2008/105/EC for

409

classification of chemical status. For ecological status, the ordinance provides a list of river

410

basin specific substances. The EQS for these substances need only to be determined by

411

annual average. The herbicides mecoprop, dichlorprop and MCPA are not considered as

412

priority substances in the European Water Framework Directive but belong to the list of river

413

basin specific substances of the German surface water ordinance. Therefore concentrations

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16

ACCEPTED MANUSCRIPT of these herbicides were compared with AA-EQS and results are shown in Table 3. In the

415

first revision of the list of priority substances of the WFD, the European Commission

416

introduced an AA-EQS for diclofenac. This value was set at 100 ng/L for inland surface

417

waters.

418

3.3.1. Comparison with AA-EQS

419

The comparison of pollutant concentrations in the receiving water (grab samples) with AA-

420

EQS allows assessing the relevance of main pollutant pathways in the urban catchment. Out

421

of the 12 substances with AA-EQS monitored in this study, only 3 had concentrations always

422

below these standards at all sampling points: dichlorprop, isoproturon and DEHP (see Table

423

3).

424

During dry weather conditions, very high values exceeding EQS for diclofenac (DCF) as well

425

as high values for terbutryn (TBY) and 4NP were found downstream of the WWTP discharge

426

only. Concentrations of both PAHs fluoranthene (Fluo) and benzo(a)pyrene (BaP) exceeded

427

AA-EQS in all sampling locations. The highest PAH concentrations were found upstream of

428

the WWTP discharge point and for these substances, there is a dilution effect in the river by

429

the WWTP final effluent, as PAHs are effectively eliminated via sorption on the sewage

430

sludge during wastewater treatment.

431

During rainfall events, all samples downstream of the WWTP discharge (R4) had higher DCF

432

concentrations than the proposed EQS of 100 ng/L, but the values were much lower than

433

during dry weather conditions due to dilution with rainwater. Upstream of the WWTP

434

discharge (R3), exceeding of EQS due to a CSO discharge was observed for one sample.

435

For herbicides TBY, MCPA and mecoprop (MCP), the EQS values were exceeded during

436

rainfall events due to CSO discharges. Downstream of the WWTP discharge point,

437

exceeding values of EQS for TBY, DIU, 4tOP, 4NP and MCP were more frequent or higher

438

than during dry weather conditions, despite the higher river flow during rain events. So it can

439

be concluded that for the observed events, the impact of higher WWTP effluent loads was

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17

ACCEPTED MANUSCRIPT stronger than the effect of higher river dilution. BaP is considered in the WFD as a marker for

441

priority substances of PAHs and the comparison of its concentration with AA-EQS gives the

442

reference for this group. All river samples showed higher concentrations than the AA-EQS

443

value of 0.17 ng/L. The ratios between measured concentrations and AA-EQS ranged

444

between 5 and 494. The high Fluo and BaP concentrations found at the site R1 (upstream of

445

CSO and WWTP discharges) during rainfall events highlight the importance of the

446

contribution of nonpoint source pollution of surface waters resulting from runoff. In urban

447

areas, street runoff appears to be the main source of PAHs in total surface runoff, but the

448

contribution of atmospheric deposition is not negligible (Motelay-Massei et al., 2006). Our

449

results indicate that PAHs may have a significant environmental impact on water quality in

450

this urban watershed.

451

3.3.2. Comparison with MAC-EQS

452

Table 4 indicates the frequency of exceeding MAC-EQS and the ratios of measured

453

concentrations and MAC-EQS. Since not all pollutants have both types of EQS, the

454

substances listed in Tables 3 and 4 are not identical. No risk of exceeding MAC-EQS was

455

observed for terbutryn, diuron, isoproturon, 4-nonylphenol and benzo(a)pyrene. CSOs

456

represent an important pathway to receiving water for benzo(b)fluoranthene,

457

benzo(k)fluoranthene and benzo(ghi)perylene but exceeding of MAC-EQS were also

458

detected upstream of CSO discharges at the sampling point R1. The concentrations found

459

for these compounds along the river indicate their environmental risk, especially during

460

rainfall events.

461

4. Conclusions

462

The results of this study show that CSOs represent an important pathway for a wide range of

463

organic micropollutants including PPCPs, urban biocides, industrial chemicals, flame

464

retardants, plasticisers and PAHs to urban receiving waters. Out of the 69 substances

465

chosen for chemical analysis, 61 were detected in wastewater during dry weather and 60

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18

ACCEPTED MANUSCRIPT were detected in CSO discharges. For most micropollutants detected in seven sampled

467

CSOs, event mean concentrations varied within about one order of magnitude. When

468

comparing CSO concentrations with median wastewater concentrations during dry weather,

469

two main patterns could be observed depending on the compound: (i) wastewater is diluted

470

by stormwater (PPCPs, X-ray contrast agents, BTR, 4-MBT, 5-MBT, BPA, TBEP); (ii)

471

stormwater is the most important source of pollutant. The contribution of stormwater to CSO

472

emitted loads was extremely high for all herbicides, 4tOP as well as for PAHs. Only for CZIM

473

and a few organophosphorus compounds (TCPP, TEP, TPP, TIBP and TPPO), both

474

wastewater and stormwater contribute to the loads discharged through CSOs.

475

Due to the inter-event variability of CSOs, it is necessary to monitor several rainfall events

476

with different characteristics to obtain representative results. Regarding monitoring strategy,

477

we have demonstrated that the use of mixing ratios as surrogate parameter to estimate

478

discharged loads of pollutants from sewage and stormwater runoff is suitable. However it is

479

important to choose substances with stable concentrations in dry weather flow in order to

480

interpret correctly results about pollutant variability in CSO events. The electrical conductivity

481

appeared to be a suitable parameter to calculate the proportions of wastewater and surface

482

runoff in combined sewer systems. Not only the pollutant source (sewage or stormwater)

483

play a major role in CSO micropollutant concentrations, but also the occurring in-sewer

484

processes. More research is needed for a better understanding of these in-sewer processes

485

involving micropollutants, especially regarding biological degradation as well as adsorption to

486

sewer sediments during dry weather and remobilisation during rain events. In addition, our

487

results demonstrate that surface water quality monitoring should include sampling during wet

488

weather conditions and that information on CSO events in the catchment should be

489

considered for a meaningful analysis and a better interpretation of the results.

490

The comparison of concentrations in the receiving water with EQS showed that depending

491

on the compound, WWTP or CSO discharges are the main sources of priority substances in

492

the receiving water body. In order to reduce the total emitted loads effectively, it is necessary

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19

ACCEPTED MANUSCRIPT to combine different measures regarding both CSO and WWTP management strategies.

494

CSO discharges have a significant impact on the surface water quality and further research

495

is needed to evaluate the effect of CSO discharges on the chemical status of rivers.

496

Regarding PAHs, measures at wastewater systems are not sufficient to achieve good water

497

quality; concrete measures to reduce atmospheric emissions of PAHs should be

498

implemented.

499

ACKNOWLEDGEMENT

500

This work was supported by the Ministry of the Environment, Climate Protection and the

501

Energy Sector Baden-Württemberg, Germany. The authors acknowledge the technical staff

502

of the WWTP for the cooperation during the study. The authors would also like to thank the

503

technical and laboratory staff of the Stuttgart Municipality for their valuable contributions to

504

this work. Our special thanks go to Bertram Kuch for his helpful input with regards to the

505

micropollutant analysis and Asya Drenkova-Tuhtan for proofreading this article.

506

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ACCEPTED MANUSCRIPT Figure 1. Map of the catchment and surface water sampling locations (background map source: LGL BW, 2016).

Figure 2. Micropollutant concentrations in sewage (corrected values) at (a) the WWTP

RI PT

influent during dry weather (triangles) and (b) in CSO samples (diamonds). n.d. means not detected. *: Substance with EQS in WFD, **: Substance with EQS in German surface water

SC

ordinance.

Figure 3. Ratio between concentration in sewage and mean concentration in 24h composite

M AN U

samples at the WWTP influent during dry weather.

Figure 4. Ratios between CSO and median wastewater concentrations. n.d. means not

TE D

detected. Crosses represent the values published by Gasperi et al. (2012).

Figure 5. Wastewater proportion in CSO discharges calculated with electrical conductivity

EP

versus calculation with wastewater micropollutants. Dashed lines represent a variation of

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± 25 % with conductivity results.

Figure 6. Relative contribution of stormwater and wastewater to total overflow volumes and pollutant loads for (a) one CSO event on 17th October 2014 (D7) and for (b) all seven sampled CSO events. *: Substance with EQS in WFD, **: Substance with EQS in German surface water ordinance, ***: Substance with EQS proposal.

ACCEPTED MANUSCRIPT Table 1. Characteristics of the studied rain events and CSO discharges. Overflow

Date

Rainfall Depth (mm)

Volume (m³)

Duration (min)

D1

2014/07/28

37

61,406

D2

2014/08/02

8.4

D3

2014/08/26

D4

Maximum Flowrate (L/s) 4,250

5,571

180

507

1,560

31.4

47,903

677

1,178

2014/09/12

26.3

29,701

1,056

467

D5

2014/09/21

5.3

917

70

221

D6

2014/09/21

10.5

8,771

306

476

D7

2014/10/17

20.7

30,889

470

1,093

AC C

EP

TE D

M AN U

2

1

Antecedent dry period (d) 6 3

RI PT

865

Average Flowrate (L/s) 1,182

3,420

1

985

12

516

1

1,130

0,5

SC

1

2,730

1

ACCEPTED MANUSCRIPT 1

Table 2. Runoff proportions in sampled CSO events based on electrical conductivity

2

measurements and comparison with values from the literature.

D1

2014/07/28

146

Runoff proportion in CSOs (%) 94

D2

2014/08/02

254

82

D3

2014/08/26

220

85

D4

2014/09/12

326

73

D5

2014/09/21

289

82

D6

2014/09/21

224

85

D7

2014/10/17

127

95

89

(Passerat et al., 2011)

69–95

SC

(Gasperi et al., 2012)

AC C

EP

TE D

M AN U

3

Runoff proportion in CSOs in Paris (%)

RI PT

Date

Electrical conductivity (µS/cm)

1

ACCEPTED MANUSCRIPT 1

Table 3. Frequency of exceeding AA-EQS (%) and range of measured

2

concentrations/AA-EQS ratios. DW: dry weather, WW: wet weather. Empty fields mean

3

that measured concentrations were below EQS in all samples.

R1

of main CSO / (upstream of upstream of WWTP main CSO) discharge)

WW

WW

DW

R4 (downstream R5

of WWTP discharge)

WW

DW

WW

DW

WW

100

25% (2.8)

100% (8.422)

100% (1.6-9)

100% (6.513)

100% (1.314)

TBY*

65

25% (2.3)

22% (1.41.5)

100% (1.4-2)

11% (1.2)

75% (1.11.4)

DIU*

200

ISO*

300

DEHP*

1300

4tOP*

100

4NP*

300

MCP**

100

MCPA**

100

DCP**

100

Fluo*

6,3

BaP*

0,17

SC

DCF***

M AN U

25% (2.3)

25% (1.8)

Concentration below AA-EQS Concentration below AA-EQS

TE D

11% (1.5)

33% (4.4)

25% (1.8) 25% (1.3)

11% (1.2)

25% (1.2)

75% (1-3)

75% (1.919)

75% (1.715)

25% (2.9)

25% (2)

25% (1.3)

Concentration below AA-EQS

100% (4-27)

22% (1.43.8)

100% (1.312)

33% (1.11.3)

100% (1-14)

33% (1.62.2)

100% (3.516)

100% (22-382)

100% (82494)

100% (9324)

100% (23270)

100% (5-26)

100% (19312

100% (12177)

100% (65447)

9

4

EP

75% (2-10)

AC C

4

R3 (downstream

R2

RI PT

AA-EQS Substance (ng/L)

n 4 3 9 4 9 4 *: EQS in WFD, **: EQS in German surface water ordinance, ***: EQS proposal

1

ACCEPTED MANUSCRIPT 1

Table 4. Frequency of exceeding MAC-EQS (%) and range of measured

2

concentrations/MAC-EQS ratios. DW: dry weather, WW: wet weather. Empty fields

3

mean that measured concentrations were below EQS in all samples.

R1

WW

WW

R3 (downstream of main CSO / upstream of WWTP discharge)

DW

WW

R4 (downstream of WWTP discharge)

DW

WW

340

Concentration below MAC-EQS

DIU*

1800

Concentration below MAC-EQS

ISO*

1000

Concentration below MAC-EQS

4NP*

2000

Concentration below MAC-EQS

Fluo*

120

BaP*

270

BbF*

17

50% (1.6-6.5)

100% (1.1-7)

BkF*

17

25% (2.6)

67% (1.5-2.9)

BghiP*

8,2

75% (1.2-6.2)

100% (1.3-8.7)

4

3

33% (1.4)

DW

WW

M AN U

Concentration below MAC-EQS 50% (4-5.3)

50% (1.75.3)

11% (1.8)

75% (1.1-6.5)

11% (3)

50% (1.4-1.5)

25% (2.2)

11% (1.6)

50% (1.1-2.5)

11% (7)

75% (1.3-4.5)

50% (1.74.6)

11% (4)

100% (1.2-6.5)

9

4

4

9

4

TE D

11% (3)

AC C

EP

*: EQS in WFD

R5

SC

TBY*

n

4

R2 (upstream of main CSO)

RI PT

Substance

MACEQS (ng/L)

1

9

ACCEPTED MANUSCRIPT

Legend Watershed boundary

RI PT

River network

Rain gauge

SC

R1

Surface water sampling location

R2 R3

R4

R5

AC C

EP

TE D

M AN U

main CSO WWTP

Figure 1. Map of the catchment and surface water sampling locations (Map source: LGL BW, 2016).

a)

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

b)

Pharm.

Wastewater micropollutants

Herbicides

Contrast Biocides/ agents herbicides

Ind. chemicals

PAHs

Industrial Flame retardants/ chemicals plasticisers

PAHs

Figure 2. Micropollutant concentrations in sewage (corrected values) at (a) the WWTP influent during dry weather (triangles) and (b) in CSO samples (diamonds). n.d. means not detected. *: Substance with EQS in WFD, **: Substance with EQS in German surface water ordinance

ACCEPTED MANUSCRIPT TCS

DEET

RI PT

3

2

SC

1

0 0

1

2

3

February

M AN U

Cmean [-] Ci / concentration (-) Ci / Mean

IOH

4

5

6

7

8

9

10

July

24h composite sample at the WWTP influent at dry weather conditions

AC C

EP

TE D

Figure 3. Ratio between concentration in sewage and mean concentration in 24h composite samples at the WWTP influent during dry weather.

ACCEPTED MANUSCRIPT

1000,000 1000

RI PT M AN U

SC

10,000 10

1,000 1

TE D

0.1 0,100

EP AC C n.d.

CAF ACE IBF SUC HHCB DCF MPL NPX TCS BZF DEET CBZ AHTN SMX ANL PNL MTCS IOM IOH IOPA IOPM DIA TBY* CZIM DIU* ISO* MCPA** MCP** DCP BTR DEHP* 4-MBT 5-MBT 4NP* BPA MTBT BT 4tOP* TBEP TCPP TEP TPP TCEP TIBP TDCPP TPPO TBP Fluo* Pyr BaP* BbF* BaA Chr PHE BghiP* IP* BkF* ACN ANT* DahA ACY FL

0,001 0.001

n.d.

0.01 0,010

n.d.

Ratio CSO / Wastewater concentrations

100,000 100

Wastewater micropollutants

Contrast agents

Biocides/ herbicides

Industrial chemicals

Flame retardants/ plasticisers

PAHs

Figure 4. Ratios between CSO and median wastewater concentrations. n.d. means not detected. Crosses represent the values published by Gasperi et al. (2012).

CBZ

50%

SUC

ACCEPTED MANUSCRIPT

NPX

IBF

CAF

DCF

RI PT

40%

SC

35%

M AN U

30% 25%

TE D

20% 15%

EP

10%

AC C

Wastewater proportion in CSO discharges calculated with wastewater micropollutants

45%

5% 0% 0%

5%

10%

15%

20%

25%

30%

35%

Wastewater proportion in CSO discharges calculated with conductivity Figure 5. Wastewater proportion in CSO discharges calculated with electrical conductivity versus calculation with wastewater micropollutants. Dashed lines represent a variation of ± 25 % with conductivity results.

b)

Stormwater related load Wastewater load ACCEPTED MANUSCRIPT

90% 80% 70% 60%

RI PT

50% 40% 30% 20% 10%

SC

Relative contribution to total overflow volume/load

100%

0%

M AN U

a)

90% 80%

TE D

70% 60% 50%

30% 20%

EP

40%

AC C

Relative contribution to total overflow volume/load

100%

10%

0%

Pharm.

Herbicides

Ind. chemicals

PAHs

Figure 6. Relative contribution of stormwater and wastewater to total overflow volumes and pollutant loads for (a) one CSO event on 17th October 2014 (D7) and for (b) all seven sampled CSO events. *: Substance with EQS in WFD, **: Substance with EQS in German surface water ordinance, ***: Substance with EQS proposal.

ACCEPTED MANUSCRIPT

Highlights •

The occurrence of 69 organic pollutants was assessed at one CSO outfall during seven rain events. CSO discharges represent an important source of a wide range of organic

RI PT

•

micropollutants. •

The contribution of stormwater to CSO emitted loads was higher than 90 % for herbicides and PAHs.

Remobilisation of in-sewer deposits contributed from 10 % to 65 % to carbamazepine

SC

•

emissions in CSO events.

EP

TE D

pollutant loads effectively.

M AN U

Technical measures at both WWTP and CSOs are needed to reduce discharged

AC C

•