The Importance of Precedent Hydro-climatological Conditions for the Mass Transfer of Pollutants in...

The Importance of Precedent Hydro-climatologicalConditions for the Mass Transfer of Pollutantsin Separated Sewer Systems and CorrespondingTributaries During Storm Events

Andreas Krein & Merce Salvia-Castellvi & Jean Francois Iffly &

Laurent Pfister & Lucien Hoffmann

Received: 22 June 2006 /Accepted: 14 January 2007 / Published online: 30 January 2007# Springer Science + Business Media B.V. 2007

Abstract Runoff events were analysed in separatedsewer systems in the town of Luxembourg. The rela-tionships between Event Mean Concentrations of dif-ferent pollutants and runoff patterns were evaluated. Inaddition, the inter-storm and intra-storm variability ofthe material transport were determined. Primarily, thevariations in pollutant concentrations and loads aredetermined by the antecedent weather conditions. Thepresence of illicit sanitary inputs in one of the sewersproduced a significant first flush effect as well ashigher Event Mean Concentrations for pollutants.Furthermore, near the town of Trier 40 storms wereanalyzed in a small natural basin mainly influenced byrunoff from a separated sewer system. Natural andartificial storm events were investigated in order toestimate the relationship between the pollutant sourcesin the channel and from the separated sewer system.Just like in the canalization of Luxembourg City thepollutant dynamics during natural storms are stronglyinfluenced by pre-event hydrological conditions. Theartificial storms behave differently. Despite little pre-rain, the maximum concentrations of toxic substances

are comparatively low. A resuspension of sedimentonly occurs in the natural channel system, without theintroduction of fines from the sewer system.

Keywords first flush . Luxembourg . resuspension .

separated sewer overflows . urban stormwater

1 Introduction and Objectives

Separated sewer overflows are well known for theirdischarge of untreated water during storm events (Hatt,Fletcher, Walch, & Taylor, 2004). The main processescontributing to the contamination of urban storm run-off are the wet and dry deposition of contaminantsfrom the atmosphere, the exhaustions from road trafficand the abrasion processes of tires, breaks and roadmaterial (Greenstein, Tiefenthaler, & Bay, 2004). Thebuild-up of contaminants on impervious surfaces,wash off from surfaces into formed channels or pipes,and the transport along these pathways are observedin detail (Lee, Bang, Ketchum, Choe, & Yu, 2002). Insome cases, urban runoff contains nutrients at similarconcentrations to treated sewage, as well as signifi-cant levels of suspended solids, heavy metals, organictoxicants and pathogens (Charles et al., 2003; Rocheret al., 2004).

Mobilized fines from these sources can be stored atintermediate locations within a basin, such as on floodplains and in the channel. The consequences of finesediment transfers and storage in aquatic systems are

Water Air Soil Pollut (2007) 182:357–368DOI 10.1007/s11270-007-9347-7

A. Krein (*) :M. Salvia-Castellvi : J. F. Iffly :L. Pfister : L. HoffmannCentre de Recherche Public – Gabriel Lippmann,Département Environnement et Agro-Biotechnologies,41, rue du Brill, L-4422 Belvaux,Grand Duchy of Luxembourge-mail: [email protected]

environmentally significant as fine sediments act asboth a vector for the transport of contaminants anddirectly as a pollutant, especially in the context of habitatquality (Newcombe & MacDonald, 1991). Fine-grainedsediments are known to be a major potential sink forhydrophobic pollutants in the aquatic environment. It hasbeen demonstrated that high levels of fine sediment havea deleterious effect on the survival of the incubatingembryos of trout and salmon or macroinvertebrates(Soulsby, Youngson, Moir, & Malcom, 2001).

The objectives of this study are to investigate thecharacteristics and the dynamics of pollutant overflowafter rain events in the two separated sewer systemsSt. Quirin and Route d’Esch of the town of Luxembourg.An additional aim is the investigation of the amount ofpollutants transported during storm events in a smallnatural brook, mainly influenced by stormflow from aseparated sewer system. Factors should be isolatedwhich, along with the increased input of pollutants, affectthe overall concentrations of relevant harmful substances.The natural catchment under investigation was addition-ally chosen because of the possibility to use controlledreservoir releases to generate storm events. The artificial-ly generated flood simulated bank-full flow conditionssimilar to those of a storm event and provided an oppor-tunity to measure sediment mobilization without super-imposition by material from the sewage network. Theseexperiments enable the resuspension of sediment in largeparts of the natural channel system, without the introduc-tion of sediment transported from the sewer system.

2 Material and Methodology

2.1 Study Areas

One part of the study focused on the monitoring oftwo of the nine separated sewer overflows of the townof Luxembourg, a town with 85,000 inhabitants(Fig. 1). The St. Quirin drainage area occupies 12.6 ha,mostly in the pedestrian zone of the town. The streetsurface amounts to 1.6 ha and the roofage is 7.5 ha. Thedrainage area of Route d’Esch accounts for 27.3 ha,mostly covered with residential areas (7.1 ha) but in-cluding some major roadways of the town (3.9 ha).Both systems are pluvial sewers consisting of concretepipes of 1 m of diameter for Route d’Esch and 0.8 mfor St. Quirin. Along a 30 m section the slope at bothmeasurement points is 2%. These separated sewer sys-

tems only drain stormwater runoff. The receiving waterbody is the Pétrusse stream, which has a catchmentarea of 43.8 km2. The hydrological study of the streamshows that the urbanized part of the basin, whichamounts to 7% of the total catchment, contributes from40 to 60% of the total runoff during storm events. Asan answer to precipitation events the sewer systemsproduce sharply shaped hydrographs with short andsteep falling limbs showing limited water retention.The water levels can surge from zero to 60 cm.

The work in the natural environment was under-taken in the northern part of the Olewiger Bach basin.This is located in the Hunsrück Mountains near theGerman city of Trier having a catchment area of about35 km2 (Fig. 1). Devonian shales with quartz and dia-base veins dominate the underlying bedrock. The landuse is a mixture of arable land on the plateau, forest onthe north and east facing slopes of the valley, and vine-yards on the south facing slopes. The valley bottom isoccupied by pasture. De Sutter, Krein, and Verhoeven(2001) provide detailed information about streambedmorphology and sediment characteristics. In summer,multi-peaked storms, which can be traced to consecu-tive contributions of tributaries, are characteristic in thecatchment. The long lasting, low intensity winter preci-pitation causes a singular broad discharge maximum,which is primarily composed of laterally flowing soilwater and groundwater. Runoff from several separatedsewer systems influence the water quality of the river.For the Olewiger Bach basin the separate sewer systemonly delivers stormwater runoff to the river, with allsewage effluent being delivered to a treatment plant.

The Trier municipal water works can regulate thedischarge of the Olewiger Bach through releases fromtheir drinking water reservoir that is located in anadjacent basin. A pipeline from the drinking water re-servoir can be opened at a waterworks inlet to releasewater into the Olewiger Bach. The reservoir water canbe released at a known rate producing an artificial floodin the downstream reaches. It is characterized by lowconductivity values and low suspended sedimentconcentrations. The main advantage of artificial fieldexperiments is that it is possible to exclude certaingoverning factors (for instance runoff) and to controlothers (for instance hydrograph form). Furthermore,the transport of suspended particles is supply con-trolled: natural events activate sources within the entirebasin, whereas artificial storms exclusively use sourceswithin the channel.

358 Water Air Soil Pollut (2007) 182:357–368

2.2 Sampling Strategy

In the sewer systems water-levels were recorded bymeans of ISCO 4120 flow loggers between August2003 and March 2004. The time step for water-levelmeasurements was 1 min, as discharges in the sewerswere extremely variable in very short time periods.Discharge was obtained with level-to-flow conver-sions applying the Manning equation. Autosampler

were connected to the flow loggers in order to triggerthe sampling after a fixed water-level was reached.Sampling was performed at a two or five-minute inter-val throughout the whole duration of the 22 investi-gated events. A representative selection of samples waschosen for analysis, due to watercolor or smell.

Between 1996 and 2005, 40 natural storm eventsand three artificial events were sampled by hand at thegauging station of the Olewiger Bach. Artificial events

Fig. 1 Areas of investiga-tion – Trier region and thelocation of the St. Quirinand Route d’Esch sewers inthe town of Luxembourghaving the Pétrusse streamas tributary

Water Air Soil Pollut (2007) 182:357–368 359

in the Olewiger Bach basin were generated in order tostudy in-channel substance transport and erosion pro-cesses. To analyze the influence of hydraulic boundaryconditions on sediment transfer variable dischargeamounts and flood durations as well as flood serieshave been investigated. Samples of 60 l were takenmidstream at the gauging station when changes ofwater-level, turbidity, smell or colour were observed.The suspended particles were separated by continuousthrough flow centrifugation within 24 h after sampling.Moreover, with the intention to understand the charac-teristics of one of the main sources of pollutants, streetdust was gathered daily with a special low vacuumpump in the Olewiger Bach catchment. The carriagewayof the Gustav-Heinemann-Straße has been sampled,which has a daily traffic volume of 35,000 vehicles.

2.3 Measurements

Nineteen water quality parameters have been analysedin the laboratory of the CRP – Gabriel Lippmann forthe collected samples from the sewer system ofLuxembourg City. The electrical conductivity wasmeasured with a WTW 197i conductivity meter. Theamount of suspended sediment (SS) was analysed byweight difference of the filters (WHATMAN GF/Fglass fiber filters, 0.7 μm pore size) drying the materialfor 2 h at 105°C. Turbidity was measured with a MerckTurbiquant 1500 IR. The biological oxygen demand(BOD5) was quantified with the dilution method viaoxygen measurements with a WTW Oxi 539. Thechemical oxygen demand (COD) was determined withthe dichromate method (Hach DR/2000). Extractablelipophilic hydrocarbons like oil and greases wereanalysed gravimetrically from the filters after n-hexaneextractions of the filter cake. In addition, the ammonium-nitrogen (NH4-N, filtered GF/C samples, blue indo-phenol’s method), soluble reactive phosphorus (SRP,filtered GF/C samples, ascorbic-molybdate method),total Kjeldahl-nitrogen and total phosphorus (TKNand TP, measured as NH4-N and SRP after unfilteredsamples were digested by acid potassium persulfate inautoclave), chloride (Cl−), nitrite-nitrogen (NO2-N)and nitrate-nitrogen (NO3-N) by ionic chromatogra-phy (Dionex DX-500), sodium and potassium byionic chromatography (Dionex DX-600), and lead(Pb), iron (Fe), copper (Cu) and zinc (Zn) by ICP-EOS after acid digestion of unfiltered samples havebeen determined.

In the Olewiger Brook water samples were col-lected midstream with two-liter polyethylene bottles.During the sampling procedure, electric conductivityand temperature were measured in situ using a mobileWTW conductivity meter (Cond 340i). In the labora-tory of the University of Trier the water samples werefiltered through WHATMAN GF/F glass fiber filters,acidified with concentrated nitric acid for metal ana-lyses and stored prior to analysis at 4°C. The dissolvedanions (nitrate, sulphate, chloride) were analysed byion chromatography (METROHM 690 IC). Dissolvedorthophosphate was determined with a photometerusing the standardized method of molybdenum bluerecommended by the Deutsche Einheitsverfahren. Thedissolved metals calcium (Ca), magnesium (Mg),potassium (K), sodium (Na), iron (Fe), manganese(Mn) and zinc (Zn) were analysed by atomic absorp-tion spectroscopy (AAS VARIAN-SpectrAA-640GTA100). Furthermore, the analytical program of thesuspended particles and the street dust coveredparticle-bound heavy metals (digestion with nitricacid), the amount of suspended matter and organiccarbon (Corg). Corg was determined with an elementanalyzer (RC 412, LECO). Reference materials wereanalysed and several measurements were repeated tomeet analytical quality assurance standards. Relativestandard deviations of methods and determinationswere lower than 5%. In addition, each sample wasinvestigated for the 16 polycyclic aromatic hydro-carbons (PAH) of the United States EnvironmentalProtection Agency (1984). For the analysis of PAHsthe samples were spiked with internal standards andsolvent extracted with acetone/hexane in a Soxhletsystem for eight h. The two deuterated PAH anthra-cene-d10 and benzo(a)anthracene-d12 were used asinternal standards as well as benzo(b)chrysene. PAHswere analysed using a Hewlett Packard 5890 GCcoupled to a HP 5970B mass selective detector.

3 Results

3.1 Importance of Antecedent Conditionsfor the Material Transport in the Sewer System

A continuous base flow was observed in both sewers,attaining about 3 l s−1 in the Route d’Esch sewer and1.5 l s−1 in the St. Quirin sewer. This discharge mainlyconsists of parasite water infiltrations from the subsoil


as well as direct runoff from commercial activities orrunoff from illicit sanitary connections.

In order to survey intra-daily variations, a contin-uous recording of electrical conductivity was per-formed for 4 days during a dry period with a time stepof 10 min. In the Route d’Esch sewer the conductivityremained stable around 1000 μS cm−1. On the con-trary, the baseflow presented daily acute peaks up to3500 μS cm−1 in St. Quirin, suggesting the presenceof illicit inputs. Illegal discharger were confirmed bythe chemical composition of samples as well as by thestructural analysis of the extractable lipophilic hydro-carbons collected in both sewers. Especially the St.Quirin sewer showed important quantities of organicacids, more precisely palmitic, stearic and oleic acid.

Due to short thunderstorms of high precipitationintensity the first hydrograph peak occurs 10–15 minafter the precipitation started. The discharge increaseis brought about by the influx from the paved areas.The sewers drain the surface water of heavily fre-quented main streets as well as neighboring residentialareas. Particles are mobilised from all over the catch-ment. In addition, material deposited in the sewers isresuspended and mixes with the illicit sullage. Sharppeaks of pollution runoff are introduced in the Pétrussestream with a duration of only a few hours. Figure 2gives an example of a runoff event in the Route d’Eschsewer on August 29th 2003. The difference in concen-trations between the beginning and the end of singlestorms was very clear. Normally, the chemograph peakoccurs in front of the hydrograph peak, leading to amarked first flush effect. The extractable lipophilicsubstances like oil and greases react 5 min prior to the

COD peak. Precipitation events of very small intensityand amount make themselves visible often registerableonly through a peak in the conductivity, but convey ahighly contaminated cloud of suspended particles tothe water. The storm events are generally highly conta-minated by harmful substances.

In Table 1, for both separate sewer systems the min-imum and maximum concentrations of different pol-lutants are presented for a storm that was sampled onFebruary the 7th 2004. The results show high withinstorm variations for all of our analysed parameters. Inthe St. Quirin sewer system the extractable lipophiliccompounds amount to 229 mg l−1. Furthermore, thechemical oxygen demand exhibits high intra-stormvariations. Especially in the St. Quirin sewer the ratiobetween maximum and minimum substance concen-trations within storms often exceeded one order ofmagnitude. This ratio ranges between two and fourfor most events in the Route d’Esch sewer. The varia-bility applies prior to the parameters related to sus-pended solids like COD, BOD5, TP, or heavy metals.To a lesser extend these results are valid for dissolvedions like Cl, Na or K. The variability of dissolvedcompounds like the nutrients NH4-N, NO2-N or NO3-Nis comparatively low. The composition of St. Quirinrunoff waters appears to be closer to untreated domesticwastewater than to storm water runoff. Other studiescorroborate our results (Lee & Bang, 2000).

However, the minimum and the maximum concen-trations were extremely variable from one storm to thenext. According to these inter-storm variations, Table 2shows the variability of the COD concentrations ob-served for different storms in both sewers. The COD

Fig. 2 Hydrographs and se-lected chemographs (chemi-cal oxygen demand,extractable lipophilichydrocarbons) of a runoffevent in the separated sewerof Route d’Esch on August29th 2003


concentrations exceeded 10,000 mg l−1 O2 in three ofthe 11 measured storms for St. Quirin. Accumulatedsediments mainly originating from the illicit inputsare washed off during the rain events, even if theseare of a weak intensity. Moreover, accumulated sedi-ments, organic layers and biofilms are flushed out bya small volume of water, leading to brief but hugepeaks of pollution. This was the case, for example, forthe small storm sampled on September 22nd 2003. Asummer rainstorm of weak intensity occurs after a longantecedent dry period. If discharged into the streamwithout any treatment, especially this kind of runoff

event represents a high ecological stress for the down-stream receiving waters. These high COD values werenever observed in the Route d’Esch sewer.

In order to compare among storms and to charac-terize the runoff constituents, the index Event MeanConcentration (EMC) was calculated for each eventand single parameters.

3.2 Event Mean Concentrations in the Sewer System

The integration of the pollutant load of each stormwas calculated with a time step of 1 min. This is cor-

Table 2 Variability of chemical oxygen demand observed for different runoff events in the St. Quirin and Route d’Esch separatedsewer systems

Chemical oxygen demand St. Quirin (mg l−1 O2) Chemical oxygen demand Rte d’Esch (mg l−1 O2)

Date of storm min. conc. max. conc. ratio Date of storm min. conc. max. conc. ratio

August 18th 2003 7 280 40 August 29th 2003 41 610 15August 28th 2003 21 12,010 572 September 6th 2003 144 924 6August 29th 2003 32 768 24 September 7th 2003 68 95 1September 6th 2003 350 11,992 34 September 9th 2003 149 1,488 10September 7th 2003 184 670 4 September 28th 2003 87 120 1September 22nd 2003 516 15,520 30 October 3rd 2003 67 154 2September 23rd 2003 128 1,376 11 December 20th 2003 a 72 358 5December 20th 2003 a 682 9,890 15 December 20th 2003 b 86 266 3December 20th 2003 b 148 281 2 December 20th 2003 c 62 111 2February 7th 2004 37 2,478 67 February 7th 2004 27 351 13February 8th 2004 27 552 20 February 8th 2004 29 228 8

Table 1 Variability of pollutant concentrations observed in St. Quirin and Route d’Esch separated sewer overflows for a runoff eventon February 7th 2004

Parameter scale unit St. Quirin concentrations Rte d’Esch concentrations

minimum maximum minimum maximum

BOD5 mg l−1 29 936 16 80COD mg l−1 37 2,478 27 351Cond μS cm−1 576 1,931 1,159 1,999Cu mg l−1 0.014 0.408 0.033 0.077Fe mg l−1 0.08 10.6 0.22 1.42NH4-N mg l−1 0.32 6.1 0.05 1.5NO2-N mg l−1 0.3 2.5 2.2 3.1NO3-N mg l−1 0.1 3.3 0.8 4.2Extr. lipo. comp. mg l−1 1.1 229 1.4 6.9Pb mg l−1 0.009 0.44 0.021 0.059SRP mg l−1 0.04 1.50 0.07 0.37SS mg l−1 10 1,441 12 185TKN mg l−1 3.2 43.4 0.4 2.8TP mg l−1 0.3 5.1 0.2 1.6Zn mg l−1 0.08 10.4 0.89 2.06


responding to the time step of our water-level measure-ments. To solve the high temporal resolution, the pol-lutant concentrations were linearly interpolated fromnearby sample points. Pollutant fluxes were calculatedas the product of water discharge and pollutant concen-tration. EMC values were computed by dividing theload by the runoff volume.

According to both sewers and different parametersTable 3 shows the average of the calculated EMCvalues as well as extreme values. For the Route d’Eschsewer the values are in agreement with those describedby the United States Environmental Protection Agency(1999) for urban runoff in separated sewers. On thecontrary, for the St. Quirin sewer, the EMC of selectedwater quality parameters are comparable to that foundin untreated domestic water, highlighting the greatinfluence of illicit connections. The inter-storm vari-ability attains two orders of magnitude for St. Quirinand one order of magnitude for Route d’Esch. Particu-larly the length of the antecedent dry weather perioddetermines the pollutant concentration. The longer thisperiod continues, the higher the accumulation extent ofpollutants into the sewers. Applying a multipleregression with the chemical oxygen demand-EMCas the dependent variable, and the storm’s total volume(V) and the number of antecedent dry weather days astwo independent variables, 85% of the EMC variancefor COD is explained for St. Quirin (significance F=0.008) and 81% for Route d’Esch (significance F=0.03). Concentrations were directly correlated with the

length of antecedent dry weather and inverselycorrelated with the aggregated storm volume. A similarpicture is given by Fig. 3, illustrating the number ofdays without rain versus the maximum storm runoffconcentrations of dissolved zinc and ammonium. Thisrelationship shows a high linear correlation. Accord-

Table 3 Variability of the event mean concentrations of different measured parameters in St. Quirin and Route d’Esch

All values (mg l−1) St Quirin Rte d’Esch

EMC average EMC minimum EMC maximum EMC average EMC minimum EMC maximum

BOD5 335 8 1,300 30 5 90COD 1,152 30 4,800 138 25 400Cu 0.17 0.04 0.5 0.07 0.03 0.2Fe 2.88 0.5 10 1.72 0.4 5NH4-N 3.4 0.2 17 1.1 0.1 5.3NO2-N 0.54 0.02 1.9 0.96 0.03 2.8NO3-N 1 0.2 2.5 1.1 0.3 2.2Extr. lipo. comp. 135 1.4 570 1.8 1.4 2.7Pb 0.08 0.02 0.13 0.05 0.02 0.09SRP 0.68 0.1 2.1 0.15 0.06 0.44SS 592 30 2500 131 30 300TKN 7.4 1 24 2.3 0.6 7.8TP 3 0.3 12 0.7 0.2 2Zn 3.33 0.8 11.7 1.17 0.5 4.1

Fig. 3 St Quirin sewer system – Maximum concentrations oftotal dissolved zinc and ammonium of single storm eventsdependent on the precedent number of days without rain


ing to St. Quirin there is a strong relationship betweenthe COD-EMC values and the volume of storm waterrunoff (R2=0.90) indicating the importance of thedilution conditions on the pollutant response.

3.3 First Flush Effects in the Sewer System

The first flush phenomenon described by Lee et al.(2002) can be evaluated in our sewer systems underinvestigation. This is done by drawing the curve thatgives the variation of the cumulative pollutant massdivided by the total pollutant mass (dimensionlesscumulative pollutant mass) in relation to the cumula-tive volume divided by the total volume (dimension-less cumulative runoff volume) (Bertrand-Krajewski,Chebbo, & Saget, 1998). If the concentration remainsconstant during the storm event the pollutant mass isproportional to the volume and the double frequencycumulating curve follows the line of origin with agradient of one. If the data for a particular storm fallsabove this, a first flush is suggested. Figure 4 showsan example of the suspended sediment cumulativeload curves for the observed storms in both sewers.All the curves are clearly above the 45° line for St.Quirin whereas they are closer to the bisector forRoute d’Esch. The maximum divergence of the cumu-lative load curve from the diagonal was used as a

measure of the magnitude of the first flush. A signif-icant first flush was considered to have occurred whenthe percentage of the deviation exceeded 20%. Re-garding the chemical oxygen demand, 87% of themeasured storms presented a significant first flusheffect in St. Quirin, whereas only 29% of these stormsshowed a first flush in Route d’Esch. The presence ofaccumulated materials in the pipes of St. Quirin seemsto be responsible for the first flush phenomenon. Tostudy the pollutant load distribution a large amount ofmonitored events are needed to have a representativeset of mass-runoff curves. The first flush phenomenonwas observed for all the parameters related to the sus-pended sediment, but it was inexistent for dissolvedsubstances. Particle-bound pollutants from sealed sur-faces, e.g. streets or roofs, are quickly exhausted byprecipitation events. Renewed contamination beginsimmediately afterwards. Other studies support our re-sults (Deletic, 1998; Gupta & Saul, 1996).

Retaining solids with an adapted treatment can con-tribute to protect and improve the quality of the re-ceiving surface water. In order to capture 80% of theCOD load, 30–65% of the runoff should be interceptedin the St. Quirin sewer, respectively 60–80% in theRoute d’Esch sewer. A clever sewer management,green roofs or impounding basins to retain the pol-lutants, can possibly guarantee this retention.

Fig. 4 Dimensionless cumulative runoff volume and runoff mass curves for suspended sediment measured in the St. Quirin (on theright) and Route d’Esch (on the left) separated sewer system


3.4 Importance of Antecedent Conditionsin the Natural Environment

Due to short thunderstorms of high precipitation inten-sity the first hydrograph peak occurs in the OlewigerBach 15–30 min after the precipitation started. Thedischarge increase is brought about by the influx ofthe separated sewer system in the northern part of thebasin. Here, this drains the surface water of a heavilyfrequented main street as well as neighboring residen-tial areas and mobilizes particles originating from thestreet surface. These are contaminated with PAHs andheavy metals. On the contrary, the long lasting winterprecipitations of low intensity cause a singular broaddischarge maximum, which is primarily composed oflaterally flowing soil water and groundwater. Thetransport of eroding riverbanks and sediment materialsdominates.

The substance concentrations of the source materialbehave according to the variable antecedent conditions.

With the intention to understand the characteristicsof one of the main sources of pollutants, street dustwas gathered with a special low vacuum pump in theOlewiger Bach catchment. The lanes of a heavily trav-eled street (35,000 cars per day) have been tested.Figure 5 illustrates the daily measured concentrationsof the polycyclic aromatic hydrocarbons, particle-bound zinc and particle-bound lead. A precipitationevent with 5 mm in a quarter of an hour caused a sharpdecrease in the pollutant concentrations on September12th. The following dry period allowed an increase insubstance concentrations (Table 4). The regressionanalysis of the rising curves results in a clear cor-relation. The concentrations of the dry mass increaselogarithmically. After 7 days without rain, the concen-trations of lead and PAH reach the starting concen-

Fig. 5 Concentrations ofparticle-bound lead, parti-cle-bound copper and theamount of total PAH instreet dust

Date of sampling Particulatelead (mg kg−1)

Particulatecopper (mg kg−1)

Total PAH(mg kg−1)

September 11th 1997 490 415 15.0September 12th 1997 491 420 15.0September 13th 1997 236 186 3.6September 14th 1997 321 202 7.8September 15th 1997 359 228 9.9September 16th 1997 385 254 11.2September 17th 1997 406 283 12.2September 18th 1997 420 309 12.6September 19th 1997 454 326 14.1September 20th 1997 473 346 14.7September 21st 1997 483 366 15.0September 22nd 1997 489 391 15.7September 23rd 1997 504 409 16.5

Table 4 Concentrations ofparticle-bound lead, parti-cle-bound copper and theamount of total PAH instreet dust


trations again. For particle-bound copper this increaseoverruns the seven days. Possibly, besides the pollu-tion by road traffic other sources are of relevance, e.g.material from nearby vineyards.

Increase PAH: c PAHð Þ mg kg�1ð Þ¼4:5 ln xð Þ þ 4:5,where x is the number of days, R2=0.98Increase lead: c Pbð Þ mg kg�1ð Þ ¼ 97 ln xð Þ þ 249,where x is the number of days, R2=0.98Increase copper: c Cuð Þ mg kg�1ð Þ ¼ 172 x0:314,where x is the number of days, R2=0.97

In the Olewiger Bach various pre-event hydrolog-ical conditions such as the number of prior storms, theprecedent amount of baseflow or the preliminary pre-cipitation quantity were related to maximum pollut-ants concentrations and particle characteristics ofsingle storm events. The investigations by Hunter,Sabatino, Gomperts, and MacKenzie (1979) as well asMance and Harman (1978) show that with floodingthere is very little dependency of the maximum sus-pended sediment substance concentration on the pre-rain amount. Hoffman, Mills, Latimer, and Quinn(1984) however attest a higher correlation. In orderto investigate the influence of pre-cedent hydrologicalconditions, the maximum concentrations of diversepollutants are connected to parameters sufficientlydescribed by the antecedent situation. There may be,for example, the maximum water-level 7 days prior tothe event, the ten-day pre-rain or the number ofstorms 7 days prior to the event to be tested. Certainlythese parameters are highly intercorrelated. A largenumber of events must be considered in order to docu-ment correlations and Fig. 6 presents the scattergraphsfrom 43 storm events.

Due to the tendency of some pollutants to accumu-late onto organic substances, the total organic carboncontent plays a major role in the explanation of theparticle-bound substance transport (Karickhoff,Brown, & Scott, 1979). Figure 6 a shows the relation-ship of the maximum organic carbon content of singlestorm events to the maximum water-level seven daysprior to the event. The light organic substance is rap-idly eroded with increasing pre-rain. The decrease inconcentration is clearly established. Moreover, of inter-est are those storm events, which concentrations de-viate from the general tendency to fall off with increasedpre rain and react independently. Concerning these out-liers further factors were identify that determine the

Fig. 6 Maximum concentrations of organic carbon, particle-bound zinc and indeno(cd)pyrene in single storm events inrelation to parameters describing the antecedent hydrologicalconditions


concentrations of environmental pollutants in the en-vironment. The partial precipitation of a tributary brookis responsible for the increased carbon concentration inthe different behaving summer storm events (Fig. 6 aand i). In this tributary sewage ensures a wealth of nu-trients water, so that the thick organic growth on thewaterbed is eroded with the flood. The events labeledwith ii in Fig. 6a are preceded by multiple-weeklywinter storm events of a longer duration. An exhaustionof the material has already previously taken place.Additionally, in winter there is less autochthonousorganic material present. Particle-bound zinc shows asimilar behavior in relation to the pre-rain for 10 days(Fig. 6b). The concentrations of particle-bound lead,zinc and the total amount of PAH are to a large degreeconnected to the runoff from the heavily traveled road –described in Fig. 5 – with a daily volume of traffic of35,000 cars. The separate sewer system receives therunoff from the street surface and delivers it to theOlewiger Bach. These areas are directly connected viastorm drainage to the tributary and documented by rapidsource depletion. During the different storm events themaximum concentrations of solid carbon, and zinc(Fig. 6b) vary depending on the number, amount andstrength of antecedent storm events and the seasonaldependent material supply. Deviations from the generalbehavior patterns were induced by delocalized precip-itation events in tributary basins and minimal pre-rainamounts. The latter were able to exhaust potential con-taminant sources for instance from sealed anthropo-genic areas without producing storm water runoff at theinvestigated gauging station in the Olewiger Bach basin.The artificial storm events behave differently. They onlyhave sources in the channel bed area like sediment orriver bank material. Despite little pre-rain, the concen-trations of toxic substances are minimal. With indeno(cd)pyrene two further storm events depart from thegeneral behavior pattern (Fig. 6c and i). One of thestorm events is characterized through a dry-heat periodpreliminary phase in which the molecule was photo-chemically broken down. The other storm event had4 mm of pre-rain within 24 h, without however a run-off event was being created. Nevertheless, this flushmobilised the sources in the catchment area and partiallyremoved the toxic substance. Such minimal events inthe waters are often registerable only as suspended sedi-ment clouds with very high contents of pollutants.Sealed anthropogenic areas – in this case sources forindeno(cd)pyrene – are however cleaned.

4 Conclusion

The characterisation of the storm runoff pollutionin two separated sewer overflows of the town ofLuxembourg allowed us to identify an extremevariability of pollutant concentrations within-storms,among storms as well as a site-specific variability. Inconditions of low intensity rainfall events and a fewdays of antecedent dry weather, acute peaks of pollu-tion are discharged in the receiving waters. The presenceof illicit inputs in the St. Quirin sewer generates anessential difference between the two sewer systems.Whereas in the Route d’Esch the measured concen-trations correspond typically to those observed forseparated sewer overflows, they are roughly one orderof magnitude greater for St. Quirin. Here, the runoffstorm water composition appears to be similar to theuntreated domestic wastewater. The erosion of in-seweraccumulated sediments produces a significant first flusheffect. Furthermore, the study showed that dimension-less cumulative curves can be used to calculate theinterception range of pollutant load necessary to reachthe water quality objectives defined for the tributaries.The influence of housing areas, main roads and sewersystems are obvious. These are characterized by rapidsource depletion. The decrease in maximum substanceconcentrations due to the amount of pre-rain only occursif the source is spatially small and easily erodible. Inaddition, there must be a pool located well beyond thebackground concentrations of the catchment area underinvestigation. Antecedent conditions represent an im-portant factor in the description of maximum pollutantconcentrations both in separate sewer systems andreceiving watercourses. Precipitation events of verysmall intensity and amount make themselves visibleoften as single peak storm events which result predom-inantly from the sealed surface of this area, are oftenregisterable only through a negative peak in theconductivity, but convey a highly contaminated cloudof suspended particles to the water. The results of thenatural storms in the Olewiger Bach reveal theconsiderable role that pre-event hydrological conditionsplay in particle bound pollutant dynamics. Identificationand removal of illicit sanitary inputs can highlycontribute to the source control of urban storm waterpollution. More accurate assessment of pollutant loadsentering urban receiving water bodies is needed forimproving urban storm water management and meetingwater quality regulations.


Acknowledgement This project was conducted in collab-oration with the “Administration Communale de la ville deLuxemburg,” which partly supported this study. The GermanResearch Foundation granted the funding of another part.

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