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Article: Mad about mud? Monitoring and aftercare gives clarity. A new policy is needed for depots for dredging spoil

Authors:

Ing. Aldert van der Kooij, Ir. Tony Kok, Ir. Mark de Groot, ∆

Ir. Jenco de Groot, Π

26 April 2007

∆: DHV, P O Box 1132, 3800 BC Amersfoort, the Netherlands

Π: Province of Zuid-Holland, the Netherlands

Summary

After the closure certificate for the dredging deposit in the Braassemer Lake was issued in 2001, the

province of Zuid-Holland has been in charge of the aftercare and monitoring in line with the drawn-up

aftercare plan. The monitoring results were evaluated and the aftercare plan was adjusted in 2006. It

appears that to this day the groundwater has been contaminated as a result of small emissions from the

deposit.

The concentrations are below the background values that are, to a large extent, affected by the quality of

the infiltrating surface water.

Testing against the requirements of the policy statement on the removal of dredge spoil has shown that

the deposit does not comply in this case (particularly with regard to naphthalene). What should be the

solution: geohydrologic control or a sheetpile wall around the pit? In this instance we are dealing with only

minor contamination from naphthalene detected below the deposit, which will dilute when spreading out

further. For that reason the position appears to be less suitable for such testing.

How should testing be performed? For that purpose a new policy is needed, also taking the European

Framework Directive on Water and the Groundwater Subsidiary Directive into consideration.

Introduction

East of Roelofarendsveen is the Braassemer Lake, stretching out over approximately 450 hectares with an

average depth of 3 metres. During 1954-1958 and 1962-1963 around 4 million cubic metres of sand was

extracted from the lake, leaving an 18-metre deep pit. The original top layer of peat-containing clay was

then returned into the pit.

From the 1960’s through to the end of the 1980’s the pit was filled with around 1 million cubic metres of

dredge spoil. These materials dumped into the pit are called the “old deposit”. Under the regime of the

Environmental Management Act 1980, dumping resumed in 1992 and ended in 1997. This spoil is called

the “new deposit”.

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The final inspection of the deposit in 1998 showed that the cover layer brought in over approx. 25 hectares

did not at all places comply with the one-metre thickness as prescribed in the permit issued under the

Environmental Management Act and the Pollution of Surface Waters Act, and was sometimes even

missing altogether. However,

since part of the side wall of

the deposit had collapsed, the

maximum height of the silt in

the deposit had already been

reached. That is the reason

why it was not permitted to

add more to the cover layer.

Based on the quality

measurements of the surface

water, the closure certificate

for the deposit was issued in

2001. In addition to the

existing monitoring process of

groundwater, it was agreed to

monitor the surface water

above the deposit for any

effects from the deposit. At

this time, the responsibility for

aftercare and monitoring went

from the licensee, the Baggerdepot Zuid-Holland BV (BZH), to the province of Zuid-Holland. Aftercare and

monitoring is performed under the authority of the province since then.

In line with the aftercare plan, an evaluation was conducted in 2006. The most important findings are

explained in this article.

Geohydrology

The water level of the Braassemer Lake is more than 4 metres above the polder water level. Infiltration of

the surface water into the lake bed and seepage to the surrounding eastern polders play an important role.

The water level of the Braassemer Lake is also above the height of rise in the first aquifer. This will cause

infiltration from the surface water to the first aquifer. The height of rise in the second aquifer is above the

one in the first aquifer, causing the water to seep through from the second to the first aquifer.

Figure 2 shows that extraction of sand created a situation where a “leak” was made in the resistive layer

between the surface water and the first aquifer.

Figure 1. Regional situation with contour lines first aquifer

(m + Normal Amsterdam Water Level)

-4,4 -4,6-4,2 -4,8

-4,0

-4,0

-4,2

depot

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Figure 2. Schematic cross section of the dredging deposit in the Braassemer Lake

This creates a higher risk of surface water

infiltrating the soil. The leak has later

partially been fixed thanks to the returned

top layer and the dredge spoil.

In spite of this “stop” around 450 cubic

metres of surface water a day infiltrates via

the edges and the bottom of the deposit to

the first aquifer. As table 2 shows, this is

30% of the entire water balance of the first

aquifer below the lake bed.

Using this balance, a geohydrologic model has been made. Figure 3 shows the groundwater spreading

after infiltration via the deposit.

The question is therefore whether contamination escapes from the dredge spoil and how such

contamination will spread. To that effect the monitoring results have been evaluated through testing

against the background value and against the criteria of the 1993 policy statement on the removal of

dredge spoil.

Aftercare and monitoring results

The installed monitoring system (figures 3 and 4) consists of a series of probes surrounding the deposit

below the water surface (first-

line network) and at a further

distance away on the

embankment (second-line

network).

Samples are taken from this

system once every two years

since 1994. The results of the

second-line monitoring network

are used to establish the local

background values of the first

aquifer. These probes were

placed on the embankment at

such distance from the deposit

that they could not yet lie within

Table 2. Water balance 1st aquifer below the Braassemer Lake

M3/day %

Inflow west end (aquifer 1) 680 47 %

Infiltration lake bed 195 14 %

Infiltration wall deposit 350 24 %

Infiltration soil deposit 100 7 %

Seepage from the second aquifer 110 8 %

Outflow east end (aquifer 1) 1435 100 %

Figure 3. Water spreading from the deposit after 50 years.

100% 1% 10% 50%

1e scheidende laag

2e watervoerend pakket

Braassemermeer

Deklaag (klei)

Veenlaag

1e watervoerend pakket

Polderpeil = NAP -5,1

10 m

0,5 m

28 m

10 m

Boezempeil = NAP -0,6 m

NAP -0,1 mNAP -4,0 m

6 m

Polderpeil =

NAP -4,7

depot

slib

aangroeilaag

klei (gestort)

C=3000 [d]

kD=1500 [m2/d]

C=5900 [d]C=4700 [d]

C=1000-1500 [d]

1e scheidende laag

2e watervoerend pakket

Braassemermeer

Deklaag (klei)

Veenlaag

1e watervoerend pakket

Polderpeil = NAP -5,1

10 m

0,5 m

28 m

10 m

Boezempeil = NAP -0,6 m

NAP -0,1 mNAP -4,0 m

6 m

Polderpeil =

NAP -4,7

depot

slib

aangroeilaag

klei (gestort)

1e scheidende laag

2e watervoerend pakket

Braassemermeer

Deklaag (klei)

Veenlaag

1e watervoerend pakket

Polderpeil = NAP -5,1

10 m

0,5 m

28 m

10 m

Boezempeil = NAP -0,6 m

NAP -0,1 mNAP -4,0 m

6 m

Polderpeil =

NAP -4,7

depot

slib

aangroeilaag

klei (gestort)

C=3000 [d]

kD=1500 [m2/d]

C=5900 [d]C=4700 [d]

C=1000-1500 [d]

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the current effects of any contamination from the deposit. From these figures the 97.5 percentile score has

been calculated (chance of exceeding is therefore 2.5%) and assumed as the upper limit of the local

background. This is in line with the system as described in the Implementation Scheme of the Decree on

Waste Disposal and Landfills. Exceeding these background values means that further investigation must

be made into any measures to be taken and if so, what kind of measures.

Figure 4 Groundwater sampling around the

deposit

Until now, in line with the existing

aftercare plan, testing has been

performed against the reference values

based on the local background values

around the deposit. At the evaluation,

testing was also done against the policy

statement on the removal of dredge spoil

on the basis of the checklist aftercare

dredge spoil deposits, drafted at IPO

level in 2002. On the other hand, the

background values on the embankment

were calculated in order to exclude any influence from the deposit.

Testing against the Policy statement on the removal of dredge spoil

For the evaluation in accordance with the policy statement on the removal of dredge spoil, testing is

performed against the following three criteria:

1. Concentration criterion: Does the concentration development in the interstitial water of the pondage

exceed the target value for deep groundwater (Ministry of Housing, Spatial Planning and the

Environment (VROM), 2000)?

2. Emission criterion: Does the emission over the edge of the pondage (g/ha/year) exceed the maximum

emission from the policy statement on the removal of dredge spoil (adjusted to the target value for

deep groundwater)?

3. Spreading criterion: Will the affected soil volume downstream from the pondage (contour of the target

value deep groundwater with zero background concentration) during 10,000 years exceed the

pondage volume?

If the answer to the above three questions is “yes”, the deposit fails to comply with the criteria of the policy

statement on the removal of dredge spoil. In that case the emission must be restricted to an absolute

minimum according to the ALARA principle, e.g. applying an insulating layer or by geohydrologic control.

Tests have shown that most substances meet the criteria, apart from the most mobile metal (Zinc) and the

most mobile PAH (Naphthalene). Zinc and Naphthalene meet neither the concentration nor the emission

criterion.

Naphthalene is the only substance that does not meet the spreading criterion either. Zinc spreads harder.

After 10,000 years the naphthalene-affected soil volume will be 64 times the volume of the deposit itself.

Since these are very small concentrations remaining below the background value to date (see also the

evaluation of spreading risks) it would not be sensible to take action for this reason at this point in time.

The testing framework, as formulated in the policy statement on the removal of dredge spoil, therefore

appears to be unsuitable for existing deposits. The question should be asked which testing framework

should be used instead. A policy should have to be drawn up accordingly.

Testing for urgency in line with the new Soil Protection 2006 circular is deemed less suitable. The new

deposit is no historic event. The duty of care and standstill principle would be abandoned. In addition, such

testing is not consistent with the existing aftercare plan.

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On the basis of the evaluation, testing should be performed against the newly set background values. This

fits in with the monitoring system as described in the Decree on Waste Disposal and Landfills, the

regulations for dumping dredge spoil on land, as well as in the existing aftercare plan.

Testing to background values

The results of groundwater and surface water monitoring are shown in table 3. The concentrations of just

about all monitored components in the first aquifer measured by the first-line network are significantly

lower than those measured by the second-line network. This situation seems contradictory: one would

rather expect to find higher concentrations near the deposit. These higher concentrations are only found

for sulphate and naphthalene/PAH. The cause appears to be the infiltration of the surface water with lower

or equal substance concentrations along the edges of the deposit.

Only sulphate was found in higher concentrations in the surface water, leaving higher concentrations in the

groundwater around the deposit. The higher naphthalene/PAH content is attributed to emissions from the

deposit. So there seems to be more effect from the surface water to the quality of the groundwater than

from water of the deposit.

Assessment of the risks of spreading

The geohydrologic model has created a picture of the most mobile substances found in the deposit: zinc

and naphthalene. The details in table 5 have been used for the calculations. The adsorption coefficients in

the deposit and the concentrations entered are bases on field surveys conducted in the deposit in 2006.

The decomposition of substances and the reduced mobility of zinc under anaerobic field conditions have

not been taken into consideration. These assumptions are therefore the worst case scenario.

Table 3. Comparing the monitoring results of the first aquifer with the background values and surface water

First-line

network

Near deposit

Average

Second-line

network

On embankment

Average

Background value

On embankment

97.5 per cent.

N Reference

value

Near deposit

Surface water

Average

Arsenic (µg/l) 7.5 11.2 28.5 1 89 5.1

Cadmium (µg/l) 0.4 0.4 0.5 0 1.5 0.3

Chromium (µg/l) 1.4 1.9 3.9 1 1.5 2.5

Copper (µg/l) 4.8 4.5 15 (1) 0 15 5.0

Lead (µg/l) 5.5 6.1 15 (1) 0 15 4.5

Nickel (µg/l) 9.8 7.2 30.4 2 15 5.2

Zinc (µg/l) 21.9 79.3 209 0 150 24.4

Naphthalene (µg/l) 0.06 0.04 0.11 7 0.42 0.02

Ammonium(mgN/l) 3.0 16 31.4 0 15 0.06

Sulphate (mg S/l) 59.1 28.6 88.1 14 150 87.3

Calcium (mg/l) 90.6 97.6 157 0 219 92.8

Potassium (mg/l) 12.4 23.6 43.1 0 18 12.3

Magnesium(mg/l) 23.3 45.2 83.7 3 42 20.0

Sodium (mg/l) 94.6 120 219 1 100 98.5

Chloride (mg/l) 149 162 231 0 183 157

PAH total (µg/l) 0.12 0.07 0.26 3 0.64

N: no. of exceeding 1) value equal to target value

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Table 5. Entered contamination and adsorption coefficients

Substance Entered

concentration

[µg/l]

Adsorption

coefficient

in the

deposit

[l/kg]

Retardation

factor

in the

deposit

[-]

Adsorption

coefficient

outside the

deposit

[l/kg]

Retardation

factor

outside the

deposit

[-]

zinc 360 152 964 20 128

naphthalene 2.75 109 691 1.1 8

The water balance, quality details and the geohydrology were then used to calculate as to how zinc and

naphthalene would spread over a 50-year period. The results are shown in figures 5.1 and 5.2. We may

draw the conclusion that the concentrations measured to date fall considerably short of the calculations.

That would confirm the worst case scenario of the model.

Fig. 5.1 Zinc spreading after 50 years

Fig 5.2 Naphthalene spreading after 50 years

For naphthalene the slightly higher concentrations may be attributed to the deposit. In the past, the

established background value for naphthalene near the deposit has been exceeded occasionally.

However, the concentrations that could consequently build up on the embankment over the years, are far

below the background values for naphthalene and even further below the intervention value of 70 µg/l.

This is confirmed by monitoring, where the results were noticeable in the early years only.

As for the spread of zinc outside of the deposit, this will be hardly observable. The spread is very slow,

while the dilution with infiltrating surface water and groundwater will quickly dissipate the concentrations.

Just like other metals, zinc will also have a reduced mobility under anaerobic field conditions than the

model assumes.

Should the deposit be closed off completely, the result would be that zinc and naphthalene concentrations

would be about twice as high, while slowing down the spread. That is because the “leak” is further closed

off, reducing the dilution.

Conclusions

Infiltration

Measurements taken since 1994 have shown that the deposit resulted in a small emission of

contamination to the groundwater. It has been demonstrated that surface water along the edges of the

dredging deposit infiltrates to the groundwater in the first aquifer.

The impact of infiltrated surface water on the quality of the groundwater is considerably greater than by

emission from the deposit. The infiltration is most noticeable along the edges of the former sand extraction

pit. The question is how such an impact relates to the European Water Framework Directive and

particularly to the Groundwater Subsidiary Directive.

Background value

The emissions are so small that, in combination with the dilution of the infiltrated surface water along the

walls of the deposit, the concentrations in the first aquifer around the deposit remain below the local

3 µg/l

0,01 µg/l

1 µg/l

360 µg/l

65 µg/l

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background and target values. The concentrations will remain equally low, reducing the chance and the

need for intervention, and may not show up until many years from now. If there is a need for intervention,

any measures will be technically hard to accomplish and therefore quite costly.

Policy statement on the removal of dredge spoil

Although Naphthalene will not meet the spreading criterion of the policy statement on the removal of

dredge spoil over time, in a technical and social sense it would not be meaningful to take any controlling or

insulating measures at this point in time. The risks are too small. The impact immediately below the

deposit is merely a slight naphthalene contamination (no more than 3 ug/l) and will probably remain below

the background and target value through dilution of infiltrated surface water over time.

Continuous monitoring

The evaluation offers arguments ranging from active intervention to meet the spreading criterion of the

policy statement on the removal of dredge spoil in future years, to stopping the monitoring process

because, under the Soil Protection Act, an urgent clean-up situation will never arise. Since the current

policy offers insufficient clarity, it has been proposed to proceed with the monitoring process under a lower

frequency in which testing is performed against the background value on the embankment. Monitoring the

surface water above the deposit will be continued. In the meantime we must define new policy.

New policy

The policy statement on the removal of dredge spoil seems less suitable as a testing instrument for

groundwater monitoring in existing deposits. A new policy is needed, preferably in line with the policy for

monitoring dumping sites on land. In addition, the policy must fit in with the Groundwater Framework

Directive.

The location on hand was chosen to proceed with testing against the background values (where the

background values on the embankment have now been set). This fits in with the Implementation Scheme

of the Decree on Waste Disposal and Landfills and with the Dumping sites for dredge spoil on land

Scheme.

Such situations frequently occur in the Netherlands. The Netherlands displays an ever growing trend of

dumping contaminated dredge spoil into such pits (Government Statement on Aquatic Sediments,

29 August 2005). Slowly but surely, all those locations will be showing slightly increased concentrations of

the most mobile contamination from dredge spoil. The chance and the need for intervention will depend on

the chosen policy on these matters. In view of the risks of spreading as under the Soil Protection Act for

historic contamination, the need for intervention is hardly an issue – if at all. However, in view of the

‘standstill principle’ of the quality of groundwater, it is certainly something to reflect on and for which there

is a need to intervene. The realisation of the IPO checklist for the evaluation and aftercare of dredging

deposits in 2007 will have to take this into consideration.

From 2004 the new policy has been under intense scrutiny via the “Leaching and Spreading from

Deposits” project by the Ministry of Waterways and Public Works. From the survey conducted in 2006 by

the Aquatic Sediment Expert Centre (AKWA) it appears that contamination from dredging deposits is

spreading at a much lower speed than was first assumed. But the image of dredging will always be muddy,

as “De Volkskrant” daily newspaper wrote. Clarity is required for the aftercare of such deposits. The

Groundwater Subsidiary Directive should also be taken into consideration.

References

1. Braassemer Lake dredge spoil deposit, Aftercare plan version 5, DHV Environment and Infrastructure, registration

no. MM-PU931871, dated 3 February 1994

2. Final report Braassemer Lake ground and groundwater survey, GeoDelft CO-335620/66 dated 17 August 1993

3. Final inspection, Braassemer Lake dredge spoil deposit final report, Grontmij Advice and Technology, soil and

waste products department, doc.:98002416.doc./jan/mv, dated 9 December 1998

4. DHV memo “Optimisation groundwater monitoring Braassemer Lake, registration no. RB-SE20031271, dated 1

April 2003

5. Groundwater monitoring Braassemer Lake intake 2002, DHV, registration no. RB-SE20031275, dated 2 April 2003

6. Monitoring Braassemer Lake 2002-2004 (surface water), Rijnland District Water Control Board, November 2004

7. Groundwater monitoring Braassemer Lake 2004, Grontmij, memo no. 99055966_F, dated 4 March 2005

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8. Analysis summaries surface water monitoring 2005 and 2006.

9. Supplementary survey Braassemer Lake dredge spoil deposit for evaluation of aftercare and monitoring, DHV,

registration no. RB-SE20062660, dated 11 October 2006

10. Braassemer Lake dredge spoil deposit. Evaluation of aftercare and monitoring. DHV, registration no. RB-

SE20063481, November 2006

11. Government Statement on Aquatic Sediments, 29 August 2005. Letter DGW/BI 2005/1262 to the Lower Chamber

12. Braassemer Lake set up as IBC deposit. Article in Land + Water 1&2, 1996.

13. IPO checklist aftercare planning dredging deposits, 18 December 2002. Final report 4L0167.A2, IPO Project group

AF4a

14. Policy statement on the removal of dredge spoil. Parliamentary documents II, 1993-1994, 23450 no. 1

15. Dumping sites for dredge spoil on land scheme. Ministry for Housing, Regional Development and the Environment,

13 July 2001

16. Implementation Scheme of the Decree on Waste Disposal and Landfills. Ministry for Housing, Regional

Development and the Environment,13 July 2001

17. Extracting and spreading from deposits. Summary results 2004 and 2005, and looking at 2006. August 2006

AKWA report no. 06.002

18. The image of dredging remains muddy. De Volkskrant, 18 April 2007.