Municipal Discharges Along the Slovenian Littoral(The Northern Adriatic Sea) - Community Planning

and the Environmental Load

Vlado Malacic1, Boris Petelin1, Aleksander Vukovic1, Branko Potocni~

1Marine Station Piran, National Institute of Biology, 6330 Piran, Fornace 41, Slovenia

2Public Municipal Enterprise of Koper, 6000 Koper, 15. maja 4, Slovenia

Abstract

In three towns along the Siovenian littoral municipial sewage is treated mechanically.Municipial wastewater from the town of Koper empties into the estuary of the small RiianaRiver (annual mean flow is less than 5 m3/s), the wastewater from the town of Piran emptiesinto the sea from two diffusers at the end of adjacent submarine pipes (length 3.5 km), whilethe wastewater from the town of Izola is discharged into the sea from a primitive diffuser at adistance of about 300m from the shore. In all three towns the sewerage system is in a renewalphase and modem sewage treatment plants (biochemical treatment) are to be built.

Since even the best treatment plants do not operate continuously, there should be a safe,complementary back-up system that would prevent sewage contamination of the littoralstrip. The model results of initial and secondary dilution of sewage that emerges from thesubmarine diffuser in front of the Bay of Koper demonstrate that a submarine outfall is sucha system. It is shown that the total dilution (primary and secondary) of a sewage thatemerges from a diffuser in the Gulf of Trieste, that is at distance of around 4 km from theshore, is of a factor around 104before the sewage reaches the coastline.List of abbreviationsPE Population equivalentCTD Conductivity, temperature, depth

Introduction

The treatment and spreading of sewage into shal-low bays is a problem in populated coastal areas.Along the southern coast of the Gulf of Trieste threetowns (Koper, Izola and Piran) are forced to meet highstandards of sewage handling. Therefore, operativeplans for the renewal, reconstruction and redesign oftheir sewage treatment are in preparation. High qualitysewage manipulation requires bio-chemical treatment,which usually follows mechanical treatment. Nowa-days, there is only mechanical treatment in use in thecommunities around the Gulf of Trieste.

In the seventies and eighties plankton, nutrientsand bentic communities were monitored around the

first diffuser of Piran that has been operating since 1976[1-3]. That study did not show any severe environmen-tal deterioration due to sewage disposal. A more recentstudy examined the distribution of the fluorometricsignal in the wastewater near-field [4] and the benthic

Period bioi, Vol 102, Supplement 1, 2000

community around the diffusers. The findings ascer-tained more than 20 years ago were confirmed: there isabundant benthic biocenosis around the diffusers,therefore no significant deterioration of environmentalconditions in the proximity of the diffusers. Recently,different nutrients were analysed in near-field sam-ples. Among them ammonia was confirmed as a sub-stance that follows the presence of bacteria in dilutedwastewater [5].

This paper describes the study that was a part ofthe activities related to the potential construction of thesewage diffuser in the community of Koper. First, wewill present the initial dilution of sewage that emergesfrom the orifice of the diffuser, located in the Bay ofKoper, and then rises toward a buoyantly neutral layer.Second, we will examine two potential locations for thediffuser, from the point of view of secondary dilutionof wastewater that is caused by wind-driven and tidal

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currents.Wewillpresent the spread of a group of parti-cles that were released at the location of diffuser. Onelocation is more suitable for the diffuser than the other

due to higher dilution of sewage that may reach thecoastline. We will discuss the results of model simula-tions and summarise them in the conclusion.

Study Area

The Gulf ofTrieste (Figure 1) is a shallow (depth 20m) semi-enclosed gulf with a flat bottom at the south-ern side. The water mass in the gulf exchanges with the

Figure1- Distributionof the loadof theGulfofTriestewithsewagewa-ter (in percentage),the location of existingsubmarinediffusers in Piranand the two potential locationsof the diffuser in Koper (dots), and thestation in the Bayof Koper,where the sea-densitywas measured(empty

circle).

mass of the rest of the northern Adriatic Sea throughthe gulf's entrance. About 23 % of the sewage loadaround the gulf of Trieste arrives in the gulf from thesouthern (Slovenian) coast [6]. The benthic respirationrates, the consumption and production of oxygenabove the sea floor showed that during June-Octoberthe central part of the gulf may exceed its capacity toassimilate and deposit the load of organic matter [7].

The largest municipal sewage system along thesouthern coast of the Gulf of Trieste is that of the town

of Koper. The system is loaded with 45.000 PE (popula-tion equivalent), while its central sewage plant has acapacity of 50.000 PE and is loaded with 38.000 PE. Inthe central sewage treatment plant the sludge is treatedanaerobicaly and rough particles are removed throughsedimentation and flotation.

In the community of Piran the sewage water ar-rives in a central sewage treatment plant with a capac-ity of 30.000 PE, that is loaded with 13.000 PE. The me-chanical treatment is composed of the screening ofgrease and floating particles, while the sludge is oxy-genated. After this treatment the sewage water arrives

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Figure2 - Top: settlement of the polychaet Spirographis spallanzani on

the sewage diffuser next to the orifice (photo T. Makovec).Bottom: in-creased abundance of the polychaet at the diffuser is interesting also for

the vagile organisms (photo T. Makovec).

in the sea through two submarine pipes (3.450 m and3.600 m long) that end with diffusers. They were de-signed for a maximum flow rate of 0.15 m3/ s, which isadequate for an excess load of 50,000 PE (during thetourist season). Discharge of sewage water by a systemof diffusers at depth of21 min the Bay of Piran does notshow a negative effect on benthic biocenosis (Figure 2),as was the case more that three decades ago when thesewage was discharged at the shoreline. Moreover, theconstruction elements of the diffuser are a substratum

suitable for the colonization of organisms that cannotsettle on a silty or detrital sea floor.

The sewage treatment plant of the community ofIzola is the least developed. It is composed only of apumping system with rough and fine rakes in the treat-ment basin. The sewage water outflows from the plantinto the sea through a pipe of length 350 m. The town ofIzola with its industry and small surrounding villages(about 20.000 PE) is undergoing a renewal of its sewagesystem.

The towns Koper and Izola are without submarinesewage pipes and face the Bay of Koper. Its hydro-graphy is similar to the hydrography of the southernpart of the gulf [8], the latter was described in [9]. We

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V. MALACIC et al.:

will focus on this bay for the calculations of initial andsecondary dilution of sewage. While stratification (ver-tical gradient of the sea-density) is most important forthe initial dilution and spreading of sewage above thediffuser, wind is the most important agent for second-ary dilution. Although there are no measurements ofthe distribution of the wind field around the Gulf of

Trieste, we reasonably suppose that in the central partof the gulf the winds are similar to those that were mea-sured at the Meteorological station Portoroz (Beli Kriz)by the Hydrometeorological Institute of Slovenia in theperiod 1976-1990: the »Bora« wind (NE) is the mostpowerful (monthly mean speed 5.0 m/s) and the sec-ond most frequent one (35.5 %), while »Jugo" windsfrom the south (SE and S), are the most frequent (39.4%) with monthly mean speeds between 3.0 m/s (SE)and 4.4 m/ s (S) [10).The most significant wind weatherpattern is the passage of a cyclon over the northernAdriatic. This transient wind forcing will be applied asa forcing mechanism in the estimate of the secondarydilution of sewage that would spread from the diffuserlocated in front of the Bay of Koper.

Materials and Methods

Initial dilution

Water samples were taken in the central part of theBay of Koper (depth 16 m) from 13/03/1978 until

y(kg/m') y(kg/m')

Figure 3 - Vertical profiles of seasonal distribution of density anomaly

in the Bay of Koper. Dots: mean stratification, rectangles: sharpened

stratification (see text), z is the distance from the sea-floor. Symbols are

at depths at which the water samples were taken. They are connected

with lines of cubic splines with zero second derivative at the sea-floor

and the sea-surface.

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Municipal Discharges Along the Siovenian Littoral

18/09/1997, approx. once monthly at depths 0,3 m, 5m, 10 m and 16 m. We grouped the density data in fourseasonal intervals, each of a length of three months.Then we calculated seasonal means and standard devi-

ations. Since we also wanted to capture with the modelof initial dilution the variability of the solutions withineach season, sharp stratification conditions were con-sidered in addition to the average ones: From the statis-tical data of density anomaly we calculated the sharpstratification in such a way that the standard devia-tions were subtracted from the mean values in the toppart of the water column (height above the sea-floor z~11 m; Figure 3), and they were added to the mean val-ues in the bottom part (z < 10 m). Numerous of valuesranges between 26 (z = 0 m, spring) and 52 (z = 16 m,autumn). Stratification weaker than the average cannotbe obtained in a similar way - the only reasonable

weaker stratification is that of the homogeneous watercolumn, which is very similar to winter stratification.Interpolation with cubic splines was thereafter used toestimate the densities at arbitrary heights above thesea-floor, that were called for by the numerical model.Cubic splines lead to statically unstable bottom layer ofthe water column, which would homogenize underreal situations in nature. This deficiency of the method,

however, barely influences the solution of the model.

The numerical model of primary sewage dilutionfollows the conservation of volume and buoyancy flux,and of horizontal and vertical components of momen-tum flux [11, 12]. The model supposes the Gaussian

profiles of the buoyant jet velocity and of sewage den-sity. Using the Runge-Kutta method with the adaptive

step-size along the plume's trajectory the model solvesa system of coupled nonlinear equations for the corevelocity and the core density of a buoyant jet, the jet ra-dius at which the velocity falls to 1/ e of its core value,and the angle of inclination of a-buoyant jet. The pro-gram stops when the angle of the plume's inclinationbecomes negative, or when the sea-surface is reached.

The model was calibrated with known expressions for

linear variations of density with height [13].

Secondary dilution

After the rise of buoyant plumes that emerge fromthe orificesof adiffuser, the plumes merge and form aninitial cloud of diluted sewage in a layer of neutralbuoyancy. This cloud is stretched by currents. For thesake of simplicity a constant flux of sewage was sup-posed in simulations. Secondary dilution was moni-tored through the spread of artificial particles that orig-inate in the location of the potential diffuser and arereleased with a lapse time of one hour. Along the pathof each of the labelled particle we supposed that thebacteria decay exponentially with the decay time T90=3.5 hour. This value of decay time is the upper limit of

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the time interval suggested by [14], and we chose it tobe on the safe side of the estimate of secondary dilu-tion.

The motion of particles (their dispersion) is sup-posed to be governed by two types of dynamics:advection and turbulent diffusion. As far as the first is

concerned, we considered two kinds of separate dy-namics: surface wind-driven dynamics and tidal dy-namics. We supposed that particles in the wind-drivencurrents are trapped in the surface layer, while whenparticles are released in tidal currents they originate inthe interior of the water column - the tidal currents cal-

culated numericaly are the depth averaged.

For the wind field above the Gulf of Trieste a par-ticular yet typical synoptic situation has been chosenfrom early October 1999 [15]: the cyclon, which wasformed above the Gulf of Genova during the progres-sion of a cold front, moved over northern Italy (3rd Oc-tober 1999). Then, the southern winds Ougo) over theAdriatic Sea started to blow (Figure 4). During the pas-sage of the cyclon over the Adriatic Sea on the after-noon of 4th October 1999 the wind turned within a four

hour period into an eastern wind (Bora). Winds overthe Gulf of Trieste for this particular meteorological sit-uation were obtained from a numerical model

ALADIN/SI that simulated the weather for the periodof 2-5 October 1999. That model was run with bound-

ary conditions that were updated every 12 hours and

10~ 5

i 0

i" -5-10

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72 84 96o 12 24

Figure 4 - Hourly wind vectors of prognostic model AladinjSI at the po-

tentiallocation of the diffuser in front of the Bay of Koper (Figure 1).

starting with 2nd October 1999 01 :00 UTe. WN is the northern compo-

nent of the wind.

were extracted from the synoptic model ALA DIN-/LACE. The ALADIN/SI ran with a resolution of 2.5km when the dynamic adaptation of topography wasapplied. The record of wind data at 10 m height for a lo-cation in the Gulf of Trieste (Figure 4), where the dif-fuser is likely to be placed, was thereafter extractedfrom the model with a linear space interpolation ofdata from four surrounding numerical cells.

Previous analysis of wind and current-meter mea-surements that were performed for the Italian side ofthe Gulf of Trieste [16]were the basis for our estimate ofthe wind-driven surface currents in the central part ofthe Gulf of Trieste. We chose the current speed to be 3 %of the wind speed, while we supposed the current di-

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rection (azimuth) to be 250clockwise from the directionof the wind. Although this relationship has some diffi-culties that will be discussed later, it is quite efficient.The relation between the wind and current speed fol-lows from the conservation of the stress of momentum

at the sea-surface [17].

We simulated tidal dynamics with the 2D numeri-cal model [18], adapted for the northern Adriatic Seawith a space resolution of 556 m [18]. We calibrated thetidal model with the constants of tidal constituents in

five ports north of the open-boundary line that con-nects Pesaro (Italy) and Pula (Croatia) [19].As far as thetracking of artifical particles is concerned we appliedthe full Lagrangian approach in which the depth-aver-aged velocities of surrounding numerical cells werespacially interpolated to the positions of released parti-cles at each time step. In this way the advective motionof the particles was resolved.

During the integration of the particles' movement,we added turbulent diffusion to the advective drift of

particles at each time step. While for the latter the ve-locity field that was calculated previously was applied,for the former we inserted a random-walk method dur-

ing the integration. We ignored vertical diffusion sinceit is smaller than the horizontal diffusion for at least an

order of magnitude - we wanted to be on the safe sideof the estimate of the secondary dilution. Moreover,under stratified conditions the sewage patch is sand-wiched within a buoyantly neutral layer and the verti-cal migration of particles is even more reduced.

Horizontal diffusion was introduced with two in-

dependent random number generators R, and R,I"onefor the random step M, and the other for its direction qJ:

6r = R,.J12KH6t; t/J= 27tR;

where it has been supposed that the coefficient of thehorizontal diffusion KH=10 m2/s is a value that was

chosen for the spread of oil slicks in a shallow bay [20]in the Arabian Gulf, for the modelling of the oil spreadover the whole Arabian Gulf [21], as well as for the

spread of Po river plume in northern Adriatic [22] inBora wind conditions. The coefficient 12 under the

square root of the expression for 6r follows from theprinciple of four degrees of freedom of a horizontal dis-placement of an artificial particle, with a variance 4KH6t [20].

ResuIts

Initial dilution and spreading

We ran a numerical model of initial dilution for the

sewage that emerges horizontally from an orifice of adiffuser in the Bay of Koper with seasonal stratifica-

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V. MALACIC et al.:

tion. The initial velocity of sewage at the orifice of di-ameter 0.1 m that we took was 1.5 mls - this corre-

sponds to a flow rate of 0.176 m3/s, when a diffuser iscomposed of 15 orifices. This value for the flow rate ismuch larger than the present value (between 0.08 and0.13 m3/s), but is within the range of the planned flowrate if the town of Izola is connected to the central sew-

age plant of Koper along with its surrounding villages,and when the collector system is in good repair (notleaking). Lower outflow velocities would slightly de-crease the rise height, but would increase the dilution.We also supposed that the orifices are far enough apartso that the plumes do not merge before the sewagereaches a layer with neutral buoyancy. Since the simu-lations show that the radius of a plume is alwayssmaller than 3 m (Figure 5), a distance between the (al-

oo 2 4 6 8 10 0 2 4 6 8 10

x(m) x(m)

Figure 5 - Simulations of spreading of a buoyant jet of sewage that

emerges horizontaly from an orifice of diameter of 0.1 m near thesea-floorin the Bayof Koper with an outflow velocity of 1.5m/s. Dotsmarkthe height of the plumethat risesin sharpenedstratified conditions.

Full linesmark the trajectories of sewagealong the axisof eachplume,dashedlinesmark the boundariesof plumesalong which the risingveloc-

ity falls to 1/e of the corevelocityJigure 5.Simulationsof spreadingof abuoyantjet of sewagethat emergeshorizontalyfrom anorifice of diame-ter of 0.1 m nearthe sea-floorin the Bayof Koper with an outflow veloc-

ity of 1.5m/s. Dotsmarkthe height of the plumethat risesin sharpenedstratified conditions.Full linesmark the trajectoriesof sewagealong theaxis of each plume, dashed linesmark the boundariesof plumesalong

which the risingvelocity falls to 1/e of the corevelocity.

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ternatively drilled) orifices of 5 m would be more thanenough to prevent merging during the uplift. Thelength of the whole diffuser would then be of 75 m.

During ~hewinter period the sewage rises towardsthe surface (Figure 5), while during other seasons itforms a patch at the plumes' tops that is well below thesea-surface. In the plots the dashed lines that markplumes' widths are drawn just up to the last two stepsof integration at which point the plumes' radii expandradically. The simulation shows that under mean sum-mer conditions the sewage may popup to the surface.

A word is appropriate about the factor of initial di-lution in the core of the plume of sewage. This rangesbetween 52 (sharpened summer and autumn stratifica-tion) and 129 (average winter stratification). In all runsthe rise time of sewage towards the layer of neutralbuoyancy is shorter than one minute. It takes only 28 sfor the plume to rise in summer with sharpened strati-fication. All factors of dilution increase with a decrease

of initial velocity of sewage at the orifice: when this ve-locity is of 0.5 mls (expected during the night in dryseason), then the factor of dilution varies between 82and 209.

Secondary dilution

Wind-driven spread of sewage - In the simula-tion, the time series of a surface wind-driven current,which we calculated out from the wind data at thesea-surface (ALADIN/SI model), was applied. We re-leased artificial particles of pollutant at location 1 (Fig-ure 1) of the potential diffuser in front of the Bay ofKoper with a time delay of one hour, starting at 01:00UTM on 2 October 1999. We followed the trajectories of72 particles for 12 hours each over an interval of threedays (Figure 6, top). Along the path of each particle theamount of bacteria exponentially decreases, and allparticles have the amount of bacteria normalized to 1 atthe start of wandering. When all trajectories are plottedtogether, regardless of their starting time, the isolinesof the envelope of bacteria (Figure 6, bottom) along alltrajectories show the extension of bacteria around thediffuser. This plot is not, however, the plot of instanta-neous distribution of bacteria. That would be muchnarrower, and would be confined to a few trajectoriesof particles that were released most recently. Theisolines of the envelope of bacteria form two elonga-tions from the point of release that correspond to thesouthern and eastern wind. The point of release is al-most 4 km westward from the nearest land point. Theisoline 0.01 extends northward (in italian waters) forless than 2 km. Since it is reasonable to suppose that theinitial dilution of the diffuser would be 100 at thesea-surface, the isoline 0.01 corresponds to a total dilu-tion of 104- this figure is in accordance with the UNEPrecommendations [14].

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Figure6-Top: trajectoriesof artificial particlesthat were releasedin a surfacewind-driven layer,starting with the first one at 2nd October 199901:00UTe.72 trajectories that were releasedwith a delay of one hour present 12hoursof movementof particlesduring the passageof a cyclonover thenorthern Adriatic sea.Bottom: isolinesof the envelopeof the amount of bacteriathat were releasedin the seawith artificial particlesat different

times. Theamount of bacteria is normalizedto 1 in the starting position.The straight line that dividesthe Gulf of Triesteisthe border line betweenSloveniaand Italy.

Spread of sewage with tides -Model simulationsof tides showed that at location 1 of a potential diffuser(Figure 1) the mean velocity along the principal axiswithin a ten-day period of spring tides (starting with 20December 1999) is small - of the order of 10-3 mis,while the standard deviation along the principal axis(aligned with the gulf's axis) is of the order of 10-2m/ s.However, within a period of three days particles canstill be displaced by tides for a few kIn from the point ofrelease. Within a period of ten days an individual parti-

96

cle passes very close to the point of release about 10-20times. This wobbling of particles is causing a smallmean value of currents over a longer period of time.

Instantaneous distribution of faecal bacteria at the

time of 00:00 UTC on the 1st January 2000 was exam-ined in detail (Figure 7, top) around the diffuser at loca-tion 1. It is clear that out of the last 24 particles that werereleased hourly, only the last few particles influence thedistribution of bacteria around the point of release. The

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Figure 7 - Top: Instantaneous distribution of bacteria with the die-off time T90= 3.5 h due to tides at 00:00 1st December 2000 around location 1 of a

potential diffuser. Empty rectangles denote the positions of 24 artificial particles that were released previously during the 24 hours period.

Bottom: the envelope of all instantaneous distributions of bacteria within a period of 30 days, starting with 1st December 1999.

envelope of all distributions of bacteria at differenthours within a period of one month, starting with 1stDecember 1999, is elongated along the principal axis ofthe tidal current (Figure 7, bottom). The main axis ofthe envelope is about 600 m in length. As was pointedout earlier the isoline 10-2corresponds to a total dilu-tion of 104,and if the initial amount of faecal coliformsis 107/(100 ml), the isoline 10-2shows a spread of 103coliforms per 100 ml of sample. According to UNEP[14]this is within the acceptable limits for bathing wa-ters, and this amount of bacteria is several km awayfrom bathing areas. Several reasons, that we will dis-

Period bioi, Vol 102, Supplement 1, 2000

cuss later, lead to the conclusion that the amount ofbacteria would be even lower, for an order of magni-tude or more.

Spread of sewage in the worst case scenario -This case happens when the western wind (»Tramon-tana«) drifts the surface water directly towards thenearest land point. The wind must last for a few hoursto carry the sewage towards the land. According to therecommendations [14] the faecal coliforms should bebelow 103/(100 ml) at the edge of a coastal strip ofwidth 300 m. The distance from point 1 of the diffuser's

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location (Figure 1) towards the nearest point of theouter edge of a coastal strip Xl =3500 m, while the dis-tance from location 2 to the same point is X2=1700m. Itis supposed that the sewage patch would be elongatedfrom location1 or 2 towards the same point at the coast-line. The width of the patch increases with the distancefrom the diffuser and since the wind-driven advection

would be very strong only lateral turbulent diffusionwould be meaningful. We ignored the vertical diffu-sion. The statistics of the wind data, taken from the au-tomatic weather station in the Port of Koper (Hydro-meteorological Institute of Slovenia) for the periodbetween 3 December 1991 and 31 December 1997 [24]was applied for our purposes. For winds withhalf-hour average speeds between 3.3 m/s and 7.9m/ s, the western wind (WSW) occurs with a frequencyof only 0.6 %, and its half-hour average speed is smallerthan 5 m/ s. We shall take for the wind speed 5 m/ s thatcorresponds roughly to a surface current of: Uo=0.15m/s. The estimate of the dilution under described as-

sumptions was developed in [25]and described in [11].Travel time of sewage from the first location g2057 tl =6.48 h, while from the second location t2 =3.15h. Thecoefficient of lateral turbulent diffusion increases with

the width L=L(x) of the sewage patch according to thelaw of 4/3 [26]. Writing this relation in engineeringunits yields

We suppose for the initial width L =b =100 m (theprojection of the length of a diffuser that is orthogonalto the current). The concentration of a pollutant Cfollws the steady advection-diffusion law

u ac + kC= {cIC

)o ax ax2 '

where the bacteria die-off was accounted for with a sec-ond term at the left hand side with a factor k =2.3026/T90=0.65789/h.

Let us write also the definition for the square of thewidth of the patch [25]

'"

Jy2CdyL2=12~

JCdy

where the integrations are performed across the patchat a distance X from the diffuser, with the Gaussian pro-file of the concentration of pollutant. The width L thenincreases with the distance from the diffuser as

L =b(l + ~2X)3/2

3b '

98

where ~= 12 ~ /(uob) = 0.168,and ~ = d,L=b).The solu-tion for the concentration Cmaxalong the axis of thepatch yields as

and this decreases with the distance x because of expo-nential mortality (t =x/uo), and because of the lateralspread of sewage.

For sewage that arrives at the coast from the loca-tion 1, time tl and the distance Xl is entered in the solu-tion above. The value of the error function is then

0.1266, while the exponential decrease is 0.0141. If thediffusor is efficient and the initial dilution is 100, thenthe total dilution at the edge of a coastal strip equals100/(0.0141*0.1266) =5.6 Hr. Similar calculation of di-lution of sewage that arrives at the coast from point 2gives for the error function a value of 0.2792, and forthe exponential factor a value of 0.1260. The total dilu-tion in this case is equal to 2.8103.

Discussion

Under sharpened stratification (Figure 5) the ris-ing plume of sewage reaches almost the same height insummer (8.2 m) as in autumn (8.3m). It reaches a lowerheight (10.4 m) under the mean autumn stratificationthan under the mean summer stratification (11.6 m).The reason for this lies in the structure of the stratifica-

tion: while the mean stratification just below the sea--surface is higher in summer than in autumn, it is lowerin the bottom part of the water column, at mid-depths,where stratification is crucial for the height of theplume.

We have to emphasize that the simulations of ini-tial dilution were performed for the stratification con-ditions in the central part of the Bay of Koper, wherethe depth is 16 m. The future diffuser, however, wouldnot be placed there - the anchoring area. Instead, itwould be placed at point 1 or 2 (Figure 1), where thedepth is 21 m. Since close to the sea-floor there is nearlyalways a bottom boundary layer that is well mixed (un-published data of CTD profiles in the central part of theGulf of Trieste), the sewage plume would spreadhigher, and the dilution would then be larger than thatpresented in this work (between 52 and 129 for thedepth of 16 m and for the initial velocity of 1.5 m/s).With the careful design of a diffuser the factor of initialdilution would nearly always be around 100. More-over, we supposed for the estimate of initial dilutionthat the currents are not important - to be on the safeside of the estimate. Currents increase the factor of ini-

tial dilution [27].

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After sewage rises in plumes it forms an initialcloud either at the sea-surface, or in a layer of neutralbuoyancy within the water column. Its thickness at thesurface is of the order of 1110 of the height of the watercolumn (23), while in the interior of the stratified watercolumn its thickness is even less. This conclusion was

confirmed with fine-scale CTD profiling and small--scale vertical profiling of bacteria above the diffusersof the outfall of Piran (5).

Within 12 hours of the introduction sewage intothe sea, the number of faecal bacteria has decayed atleast for a factor of 10-1213,5< 0.04 % of the initialamount. This means that the dilution due to die-off af-

ter 12 hours of particles' wandering is larger than 2680.In 12 hours particles could move 10 km away from itsinitial position. Since at these distances over the smallgulf the inhomogeneity of the wind-field may be im-portant, it is pointless to observe particles for a longerperiod. A caution is needed for the interpretation of themovement of artificial particles, if the sewage remainsbelow the sea-surface. The wind-driven current at the

surface may be oriented in a completely different direc-tion from the current at depth due to the recirculationeffect in the gulf: the surface water mass that is drivenout from the gulf by the Bora wind is replenished withthe mass inflow at depth. The analysis of wind-drivencirculation deserves full attention in the future, but isbeyond the scope of this work.

In our analysis we have ignored the vertical diffu-sion, the die-off of bacteria during the transport of sew-age in pipes towards the sea (about 5 km in length), andthe enhanced dilution due to the treatment of sewagein a future central plant (bio-chemical treatment). Theywould all increase the dilution of sewage. We havestrong reason to believe that along the envelope 0.01 offaecal bacteria (Figure 6) the conservative pollutant (nodie-off), that would be released at location 1, would bediluted by a factor of 104 or more, while the bacteriawould be diluted by a factor of 106.Even if there wouldbe no bio-chemical treatment of sewage due to systemfailure, we expect that the envelope 0.01 marks theboundary of 10/(100 ml) faecal coliforms (in the treat-ment plant there are 107 1(100 ml) faecal coliforms).This would be the recommended upper limit accordingto UNEP (14) for aquaculture activities. If, however, thediffuser would be installed at the location 2 (Figure 1),the extension of sewage in Italian waters would belarger, and the coastal zone would be more affected bya discharge of sewage.

A final remark about secondary dilution under thetransient wind forcing mechanism concerns the valueof the coefficient of horizontal diffusion KH(10 m21s).Larger values would make the patch of sewage aroundthe diffuser a little more circular, not just directed along

Period bioi, Vol 102, Supplement 1, 2000

Municipal Discharges Along the Siovenian Littoral

the wind-driven current, while smaller values would

narrow the concentration of a pollutant close to a fewtrajectories. Qualitatively, the picture would notchange significantly.

When the worst case scenario is considered (asteady strong wind towards the nearest land point), itis worthwhile to emphasize once again that the analyti-cal estimates of dilution are grounded on the value ofthe wind with a speed of 5 ml s that blows from WSw.This happens less than 0.6 % of the time, which meansthat more than 99.4 % of the time of the diffuser's oper-ation the total dilution at the coast will be larger thanthat calculated. A WSW wind with a higher windspeed may, of course, occur, but the probability for thatwould be smaller than 0.6 %.

Conclusions

Since two towns of the Slovenian coast (Koper andIzola) may be connected to a central treatment plantfrom which the sewage would be transported into asubmarine diffuser, we concentrated on the factor of di-

lution and the spread of a sewage patch in front of theBay of Koper (the Gulf of Trieste). Underwater obser-vations of diffusers of the outfall of Piran, just 13 kmaway from the location of potential diffuser in the Bayof Koper, revealed that in the proximity of diffusersthere are no indications of severe deterioration of theenvironment.

While the initial dilution depends mainly on thestratification of the water column and its height, thesecondary dilution depends on the advection and tur-bulent diffusion of the sewage. We showed that an ini-tial dilution with a factor of 100 is reasonably expectedwhen the diffuser is carefuly constructed. Under windweather conditions the sewage patch that rises into thesurface layer spreads laterally quite a bit and may ex-tend for 10 km. When the wind turns from Jugo to-wards Bora, one may expect that the coastal zone ofwidth 300 m would not be severly affected with sew-age if the more distant location, almost four km awayfrom the land, is chosen. Then the factor of dilutionwould be larger than 104.The same conclusion followsalso from the analytical estimate for the worst case sce-nario, when the WSW wind blows.

Since even the best treatment plants do not operatecontinuously, there should be a safe, complementaryback-up system that would prevent sewage contami-nation of the littoral strip. Up to now a submarine out-fall that ends with a well designed diffuser is still themost reliable way to provide the mechanical dilution ofsewage in the sea at all times with a factor of 104andwithout considerable maintenance expense.

99

LITTORAL2000- ResponsibleCoastalZoneManagement

Acknowledgements

The authors are indebted to M. Vrhovec, and G.Gregoric from the Faculty of Mathematics and Physics,the University of Ljubljana, and especially to M. Zagarat the Hydrometeoro-logical Institute of Slovenia forthe provision of the data of the prognostic ALADIN ISImodel. We are also grateful to T. Makovec for the un-derwater photographs.

[

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Responsible Coastal Zone ManagementThe Challenge of the 21st Century

PeriodbioI,Vol 102, Supplement 1 P 91-100, Zagreb, September, 2000