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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”.
2
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
3
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]
4
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.
5
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
6
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
7
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
8
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.