Quantifying Contaminant Transport with On-Line Monitoring

Anita Nybakk (NGI, Trondheim, Norway)Gijs Breedveld (NGI, Oslo, Norway)Marianne Kvennås (NGI, Trondheim, Norway)Randi Skirstad Grini (NGI, Trondheim, Norway)

Monitoring sediment remediation

And real-time analysis:• On-line measurements of current

velocity and direction• On-line measurements of turbidity

The last 5-10 years NGI has been monitoring several sediment remediation projects along the Norwegian coast.

The monitoring has consisted of laboratory analysis:

• Sediment samples• Sediment traps• Water samples• Passive sampler• Diffusion chambers

TurbidityCloudiness of a fluid

Quantified by measuring the scattering of light

The light is spread mainly by particles in the water

Different types of sediments were expected to give different turbidity to suspended material rate

How much will the ratio of turbidity to suspended material vary between different types of sediments?

Monitoring with real-time analysisEstimate the transport of pollutants out of the remediation area

Continuous data set

The laboratory analysis can be used to calibrate the continuous data set from the real-time analysis.

Monitoring with real-time analysisMake sure that the

remediation is done within the permit

Limits for turbidity and current velocities are determined on the basis of • concentrations of pollutants in

the sediments• measurements of the current

in the area• experience from other similar

projects.

Aim of this projectBetter basis for deciding turbidity limit

Improve the quality of data used to estimate transport of polluted sediments in remediation projects

Why is this necessary?

• If the limits are too low, the contractors might not be able to work as fast as they would otherwise.

• Simplify the decision procedure for turbidity limits

• If the limits are too high, there will be a higher transport of polluted sediment than desired.

Why?

ExperimentsThree 2 m high columns filled

with sea water

Wet sediments were added and homogenized with the water

Water samples were drawn from the columns

Turbidity was measured and the water samples were filtered for suspended material

Turbidity versus time

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Turb

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Ramstadlandet

Roald

Fosnavåg

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Turb

idity

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Time (min)

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The three stages of sedimentation

ResultsThe assumption that

coarser sediment types sediment faster than finer sediment types does not count.

Turbidity from shell sand stays higher for a long time

The sedimentation can be divided into 3 stages• Fast stage• Slow stage• Very slow stage

Turbidity versus suspended material

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Turb

idity

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Suspended material (mg)

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Fosnavåg

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Turbidity (NTU

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Suspended material (mg)

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Sediment fractions and concentrations

SummarySedimentation rate differ from sediment to sediment,

and it is hard to predict the sediment rate by measuring the grain size.

Turbidity – suspended material ratio do not vary much from sediment to sediment in the area of interest.

The fraction with highest concentration of pollutions vary from sediment to sediment, and from pollutant to pollutant.

To decide limits of turbidity one need to run sedimentation tests and fraction analysis in the lab.