PH: S0273-1223(98)OO211-X

8) Pergamon Wat. Sci. T~ch. Vol. 37. No. 6-7. pp. 299-305.1998.@ 1998 IAWQ. Published by Elsevier Science Ltd

Printed in Great Britain.0273-1223/98 S19'00 +0'00

REMEDIATION OF CONTAMINATEDSEDIMENTS IN OSLO HARBOUR,NORWAY

Audun Hauge*, Roger M. Konieczny**,Per 0. Halvorsen*** and Arild Eikumt• Norwegian Geotechnical Institute, P.O. Box 3930, Ullevaal Hageby, N-0806 Oslo,Norway•• Norwegian Water Research Institute, P.O. Box 173, Kjelsaas, N-0411 Oslo,Norway••• Oslo Port Authority, P.O. Box 230, Sentrum, N-Ol03 Oslo, Norwayt Aquateam AS. P.O. Box 6326, Etterstad, N-0604 Oslo, Norway

ABSTRACf

During the past 60-70 years. contaminated sediments from local rivers have been settling in the harbour basinof Oslo. This contamination. combined with that from other harbour activities, has developed into a seriouspollution problem with both organic and inorganic contaminants. Since 1992 dredging activities have beenrestricted until the effect of dredging was determined and a safe disposal was found for the sediments.Because of this restriction. a conflict situation arose in terms of decreasing sailing depths. and dispersal oflarge quantities of contaminated sediments into the water column each day from ship traffic. A complete planof action to remediate Oslo Harbour was started in 1994 and delivered to the State Pollution ControlAuthority (SFT) in August 1996. An evaluation of environmental risks connected to the remediation workhas been carried out along with the determination of the requirements for the monitoring programme duringand after the work. Preliminary evaluation of dredging techniques indicates that mechanicJhydraulicequipment will cause the least dispersion of sediment. Several options for treatment of the sediments havebeen evaluated. Study results point out that conditioning prior to disposal of the sediments in a confineddisposal site in the harbour area. is the most cost effective solution. Seven disposal sites with a capacity of0.5 mill. m3 have been identified. Two of the sites are integrated with a subsea road tunnel crossing theharbour basin. The other disposal sites will serve as new dock area after closure. @ 1998 IAWQ. Publishedby Elsevier Science Ltd

KEYWORDS

Disposal; dredging; remediation; treatment

INTRODUCfION

Oslo. the capital of Norway, is located at the head of the Oslo Fjord, approximately 40 km from the opensea. There is therefore relatively little exchange of sea water in the harbour by the coastal currents.

A number of rivers and streams have their outlets in the harbour basin, necessitating dredging on severaloccasions to maintain sufficient sailing depths. In the past, dredged sediments were disposed uncapped onthe sea bottom close to the harbour.JW5T 37,6/7-1( 299

300 A. HAUGE et al.

During the past 60-70 years, the depositing of the contaminated sediments from the rivers in the harbourbasin has, combined with the other harbour activities, developed into a serious pollution problem.

A preliminary investigation of the sediments in 1992, proved the seriousness of the pollution situation. As aresult, all dredging activities were restricted until the effects of dredging were determined and a safe disposalwas found for the sediments. In the ensuing period, a conflict arose in terms of decreasing sailing depths,and the daily dispersion of large quantities of contaminated sediments into the water column from seagoingvessels.

In 1992, the Oslo Port Authority initiated a project to elucidate the problem in the central part of the harbour.However, the plans were turned down due to plans for regulation and cultural restoration of this part of Oslo.

A complete plan of action to remediate Oslo Harbour was started in 1994 and submitted to the StatePollution Control Authority (SFT) August 1996.

GOALS FOR PLANNING AND ENGINEERING

The main goals for the work carried out concerning the contamination situation in Oslo Harbour have been:

To find an environmentally friendly technique for dredging the contaminated sediment.To find the optimal method for treatment/disposal of the dredged sediment.

To achieve these main goals the following work tasks have been carried out:

Characterisation of the sediments with respect to contaminants, estimated volume of contaminatedsediments, grain size, water content, content of organic material etc.

Evaluate the impact from ships traffic in terms of release of contaminants from the sediments to thewater column.

Evaluate options for treatment of the sediment combined with different techniques for dredging.

Investigate different options for disposal of dredged material both under water combined withcapping or near shore in a confined disposal facility (CDF).

LEVEL OF CONTAMINAnON

Contaminated sedjments

The concentration levels of most contaminants in the sediments of Oslo Harbour are generally high com•pared to national standards (Table I), and may be identified as one source of contaminants to the biota whichhas resulted in a local advisory against the consumption of fish livers.

In 1992, the local authorities introduced restrictions on both dredging and disposal in the Port of Oslo toprevent further impact by uncontrolled resuspension of contaminated sediments into the water column. Thissituation called for a restorative action plan including further mapping of sediment qualities, point sourcesand remediation alternatives.

Resuspensjon of sediments

On an annual basis, there are about 8000 arrivals/departures of ships in the Oslo Harbour which means aconsiderable erosion and resuspension of sediments every two hours. Monitoring of a single arrival showedthat between 50-200 kg of sediment was suspended into the water column, containing 1-5 g mercury (Hg)and 200-700 g lead (Pb). The settling rate is, however, high and showed between 20-30% decrease of the

Remediation of contaminated sediments 301

particle load after a period of 15 minutes and 40% within 20 minutes. After 2 hours only 5% of the totalparticulate matter, probably present as a colloidal fraction, remained suspended.

Table 1. Concentrations of selected heavy metals and organic compounds in the upper 50 cm sediment levelin various harbour localities, Oslo Harbour 1992-1995

Contaminant

ppmHgppm CdppmPbppmPAHppmTHCppbPCBppbDDTppbHCB

Bj~rvika

area 415.824.88801500

195006501

32207

Gr~nlibukta

area 55.27.6355291

123263180

4111

Kongshavnarea 6

3.57.1315114

740065083

Otherlocalities

32.314.02400103

58103385161648

These results indicate that 95% of the contaminated sediments are predominantly relocated within the har•bour area. Due to water movements and the maritime traffic, the colloidal fraction is continuously trans•ported out of the harbour area causing negative effects on several organisms living within the fjord system.

Effects of remediation operation

The main problems associated with remediation work of contaminated sediments are the resuspension ofsediments during dredging and, in the case of disposal, leakage of contaminants during stabilisation. Thiscontamination, compounded with that from other harbour activities, has developed into a serious pollutionproblem containing both organic and inorganic contaminants

The possibility of leakage during stabilisation in a disposal facility is discussed in the next chapter. Afterclosure of the disposal site, it is possible that the colloidal material can seep through the barriers. However, itis anticipated that this transport will be limited.

EVALUATION OF DREDGING METHODS

Extensive work was carried out to evaluate various dredging methods for possible use in Oslo Harbour.Usually a distinction is made between mechanical and hydraulic methods, although these two methods canbe combined. Emphasis was made to choose the dredging method that causes the least environmental impacton the water quality in the harbour during dredging operations. This would normally favour a hydraulicmethod. On the other hand one tries to achieve as high solids concentration in the dredged sediments aspossible, which would normally favour a mechanical dredging method. The final selection of dredgingmethod will be based on the environmental impact the selected method will have on the fjord duringoperation, as well as the conditionability and dewaterability of the sediments and overall costs.

CHARACTERISATION AND TREATMENT OF SEDIMENTS

The sediments in the Oslo Harbour contain both organic and inorganic pollutants. This will normally make itdifficult to find cost effective treatment methods. During the treatability studies, emphasis was made todescribe the sediment properties relevant to pre-treatment. This is shown in Fig. 1.

Based on the various tests shown in Fig. I, the optimal flux (kg DS/m2h where DS is dissolved sediment),was determined for hydraulic dredging of sediments.

302 A. HAUGE el al.

Sediments II

Physical PropertiesWater content %

Particle size distributionpH

Thickening properties Conditionability DewaterabilitySedimentation lest Standard conditioning test (CST) Frozen image centrifuge

Thickening test

Figure I. Characterisation of sediments with respect to pre-treatment.

In addition, the best type and dosage of conditioning chemicals was determined. Some of the finalconclusions made from the treatability studies were as follows;

I. With the use of hydraulic dredging without conditioning the maximum flux that can be applied to thesedimentation basin behind the embankment wall is 52 kg DS/m2h.

2. Without conditioning and with solids concentration in the dredged material >31 % it was difficult toachieve water/solid interface during separation.

3. Good solid separation can be achieved without conditioning with dry solids concentration in thedredged material between 7,5 and 22 percent.

4. Conditioning using a combination of iron and polymer significantly improved the water/solidseparation.

Based on the treatability it was suggested to use hydraulic dredging, including conditioning, as shown in Fig.2.

i

Iron c:hJclrideoSialic rn&.

Figure 2. Proposed dredging operation for Oslo Harbour.

DISPOSAL OF DREDGED MATERIAL

As previously mentioned, remediation work in harbours is characterised by large quantities of material,complex contamination, very soft material and problems with hydrogen sulphide (H2S) odours if thesediment is brought above sea level. To treat or clean the 0.5 mill. m3 of sediments from Oslo Harbourwould result in very high cost based upon techniques that were available at the beginning of this project. The

Remediation of contaminated sediments 303

project therefore looked for a combined solution consisting of conditioning/stabilisation and safe disposal ofthe sediment.

In Oslo Harbour there are several docks and piers where a Confined Disposal Facility (COP) can beconstructed resulting in addition new valuable dock area after closure. One location for a COF isGl'litnlibukta (see Fig. 3).

Rock FillDam

Figure 3. Oslo Harbour, East. Confined Disposal Facility (CDF>, GllInlibukta.

The construction of the CDF is based on a semipermeable rockfill dam between the two piers. Contaminatedsediment is pumped in behind the dam. The rockfill is from a tunnel bored through Ordovicianshalellimestone. i.e. relatively fine grained rockfill. The seabed soil is soft clay to more than 40-50 m depthand the seabed slopes from approximately 10m to IS m water depth over a distance of about 100m.

To obtain satisfactory stability of the dam. the strength of the soil need to be improved. Vacuum consoli•dation will be used for this purpose. the technique is based on installing a number of vertical drains in thesoft clay. covering the footing area of the dam by a membrane and establishing suction under the membrane.The effect of draining the pore water out of the soil is increased effective stresses and increased soil strength.

The intention of the semipermeable barrier is to contain the contaminants contained in the disposal facility.The contaminants are heavy metals. chlorinated organics and hydrocarbons. Except for the oil components.most of the contaminants are very tightly attached to particles. The intention of the rockfill is therefore tohold back all dissolved material (particles >0.45 Jlm in diameter).

Figure 4 shows in principle the construction of the barrier which is a combination of rockfill and filter ofsandlgeotextile on the inside to prevent any transport of particles.

The groundwater flow into the area is limited, due to bypassing of a river through a drainage tunnel to theeast of Gl'litnlibukta. A new drained road tunnel is also constructed east of GI'litnlibukta. The infiltration ofrainwater will be minimised by establishing a surface water system, combined with impermeable surfaces.

304

0.5

-5

A. HAUGE et al.

N Rockfill

~ Sandi gravel 0-30 mm

~ Geotextile 300 glm', O'lO :5 0.1 mm

~ Sedimenll rockfill

® Geotextile

EB Rockfilll gravel

Figure 4. Construction of a Confined Disposal Facility (CDF).

The tidal influence on the deposits will also be limited by the CDF design.

Taking all these items into account, the transport of contaminants with water out of the disposal site will bevery limited.

The disposal site is supposed to serve two options:

To confine as much contaminated sediment as possibleThe area must be taken into use as dock area as soon as possible

The additional loading from the deposits is expected to give 1.5-2.0 m of settlement in the soft undergroundover the next 30-50 years. To speed up the settlement, a grid of vertical drains will be installed resulting in75-80% of the settlement in less than two years.

To obtain a reasonable quality of the confined material, rockfill will be mixed with contaminated sediment.In a normal rockfill the void ratio is approximately 40%. This means that of the total volume of 100 000 m3,approximately 40 000 m3 of sediment will be filled in. The combination sediment/rock will be filled inlayers of 1-2 m.

MONITORING

Dredging and clean-up operations cause release of contaminants to the water column within the remediationarea. The national environmental authorities have set acceptable water quality standards and has approvedthe suggested monitoring program. The program given in Table 2 includes combined measurements ofphysical, chemical and biological parameters for operational guidance and short time monitoring. Thismonitoring is to be performed at different times and various distances (50-100m, 200-500 m and 1-2 km)from all activity areas.

Remediation of contaminated sediments

Table 2. Suggested monitoring frequency on site (50-100 m), in adjacent areas (2DO-SOO m) and in theinfluence area of reference site (1-2 km)

305

Period of operation

Start up levels prior tooperation andconstructionDuring the dredgingoperation and fill in

During the constructionof the disposal facility

Physicalturbidity measurements

Background turbidity 3-5replicates in all areas andreference siteContinuous measurementsevery 2-4 hours on site,occasionally in the adja•cent areaOccasionally or daily mea•surements on site

Chemicalcontaminants in water

Background levels in 3-5water depths in all areasand reference siteDaily on site and in theadjacent areas 1-2 waterdepths. Weekly on thereference localityWeekly measurements inthe adjacent area

Biologicalaccumulation in mussels

Background levels in tran•splanted mussel popu•lationsAfter dredging alternative•ly one additional measure•ment during dredging

After completion of theconstruction period

Monitorin& of the disposal site

To monitor the disposal site a number of wells will be installed locally, in the piers and in the rockfill dam.The system will be monitored frequently during operation of the disposal site and in the first period afterclosure.