7 EXISTING CONDITIONS IN THE PROJECT AREA
7.3 Sediment quality
7.3.3.8 Chemical warfare agents
Chemical munitions were dumped in areas of the Baltic Sea, including the Bornholm Basin, after the end of WWII. Since then, shell cases of many chemical munitions have corroded and CWA have been released into the surrounding marine environment, where they have been accumulating in seabed sediments.
CWA break down at varying rates into less toxic, water-soluble substances. Some CWA, however, have extremely low solubility and degrade slowly (e.g. mustard gas, Clark I and II and Adamsite).
Given their low solubility, these compounds cannot occur in higher concentrations in water, and wide-scale threats to the marine environment from dissolved CWA can be ruled out. However, direct contact with CWA in sediments is dangerous for many forms of life, including humans, other mam-mals, birds and fish. Knowledge of the interactivity of CWA with microorganisms is still fragmentary /277/.
The most frequently occurring CWA in the chemical munitions dumped east of Bornholm and the consequences should humans be exposed to them are shown in Table 7-10.
Table 7-10 Examples of CWA contained within chemical munitions dumped in the Bornholm Basin /277/.
Name Composition CAS no. Dumped (t) Consequences
As discussed in section 7.1.3, a survey to determine CWA concentrations in seabed sediments along the proposed NSP2 route in Danish waters was conducted in November-December 2017. A number of CWA and CWA degradation products were measured, as summarised in Table 7-11.
Table 7-11 CWA analysed in seabed sediments.
Chemical Description CAS number
Sulphur Mustard (SM) Dumped CWA 506-60-2
1,4-Dithiane Degradation product of SM 505-29-3
1,4-Oxathianine Degradation product of SM 15980-15-1
1,4,5-Oxadithiepane Degradation product of SM 3886-40-6
1,2,5-Trithiepane Degradation product of SM 6576-93-8
Adamsite, Dumped CWA 578-94-9
5,10-Dihydrophenarsazin-10-oxide Degradation product of Adamsite 4733-19-1
Clark I (C1) Dumped CWA 712-48-1
Clark II (C2) Dumped CWA 23525-22-6
Diphenylarsinic Acid Degradation product of C1/C1 4656-80-8
Diphenylpropylthioarsine Degradation product of C1/C2 17544-92-2
Triphenylarsine (TPA) Dumped CWA 603-32-7
Triphenylarsine Oxide Degradation product of TPA 1153-05-5
Phenyldichloroarsine (PDCA) Dumped CWA 696-28-6
Phenylarsonic Acid Degradation product of PDCA 98-05-5
Dipropyl Phenylarsonodithionite Degradation product of PDCA 1776-69-8
α-Chloroacetophenone (CN) Dumped CWA 532-27-4
Lewisite I (L1) Dumped CWA 541-25-3
Dipropyl(2-Chlorovinyl) Arsonodithi-onite
Degradation product of L1 677354-97-1
Lewisite II (L2) Dumped CWA 40334-69-8
Bis(2-chlorovinyl)Arsinic Acid Degradation product of L2 157184-21-9 Bis(2-chlorovinyl) Propylthioarsine Degradation product of L2 677355-04-3
Tabun Dumped CWA 77-81-6
Trichloroarsine (TCA) Component in dumped arsine oil 8011-67-4
Tripropyl arsenotrithionite Degradation product of TCA 5582-57-0
Tripropyl arsenite Degradation product of TCA 15606-91-4
A total of 40 samples were analysed for their content of CWA and CWA degradation products. As summarised in Table 7-12, only two samples contained measurable amounts of any of the CWA or CWA degradation products. One of the two samples from station EEZ_08 contained 0.8 µg/kg DW of 1,2,5 trithiepane, a degradation product of sulphur mustard, and one of the two samples from station EEZ_19 contained 8.3 µg/kg DW of bis(2-chlorovinyl)arsenic acid, which is the oxidised form of Lewisite II.
Table 7-12 Results of CWA measurements performed during the 2017 survey. Concentrations are shown in μg/kg DW.
Station Sample ID Detected CWA/CWA
degrada-tion product North-east and north of Bornholm area
EEZ_01 EEZ_01_CWA_01_1 -
EEZ_08 EEZ_08_CWA_01_1 0.8 μg/kg DW of 1,2,5 trithiepane
EEZ_09 EEZ_09_CWA_01_1 -
EEZ_19 EEZ_19_CWA_01_1 8.3 μg/kg DW of
bis(2-chlorovi-nyl)arsinic acid
The Baltic Sea constitutes a complex mix of environments, where water characteristics vary from freshwater to marine and from oxygenated (aerobic) to hypoxic/anoxic (anaerobic). These charac-teristics and their spatial and temporal variations are controlled by the hydrography of the Baltic Sea, as discussed in this section. The hydrography in the Baltic Sea is therefore considered an important receptor.
Hydrography of the Baltic Sea
The semi-enclosed Baltic Sea forms a large estuary. The area is permanently stratified because it receives freshwater from rivers and saltwater from the North Sea, which flows into the Baltic Sea via the Danish straits. The inflow of saltwater from the Kattegat to the Baltic Sea causes a horizontal salinity gradient from almost oceanic conditions in the northern Kattegat to almost freshwater con-ditions in the innermost Gulf of Finland /110/.
The temperature in the bottom water in the Bornholm Basin is typically within the range of 5-7 °C throughout the year, and is sensitive to inflows from the Kattegat and the North Sea. In winter, the temperature of the bottom water is warmer than the overlying water due to the inflow of warm but dense saline water through the Danish straits. The average temperature of the surface water in the Bornholm Basin is 15 °C during the summer and 4 °C during the winter.
In general, the currents in the Baltic Sea are weak, except for in the transition area, i.e. the Belt Sea. On average, the surface current may be described as cyclonal horizontal, with a speed of a few cm/s. Wind-driven currents of higher velocities appear in the upper layers. At deeper levels, small-scale vortices may appear due to the influence of bathymetric variations /111//112/.
The deep-water renewal processes in the Baltic Sea depend on specific meteorological circum-stances that force substantial amounts of salt- and oxygen-enriched seawater from the Kattegat through the Danish straits into the western Baltic Sea. From there, it slowly moves as a thin bottom layer into the central Baltic Sea basins, replacing aged water masses. The saltwater inflows from the Kattegat are sporadic but ecologically important. The principle of a major inflow is shown in Figure 7-19. Before 1980, such events were relatively frequent and could be observed on average once a year. In the last two decades, however, the frequency has decreased /113/.
Figure 7-19 The heavy, saline water flows along the bottom, and the less saline surface water flows out of the Baltic Sea. The water becomes stratified, and a halocline separates the layers of varying salinity /113/.
The Arkona Basin is the first basin that new deep water flowing into the Baltic Proper encounters after crossing the entrance sills in the Sound (Drogden Sill) and Fehmarn Belt (Dars Sill). The deep water flows along the bottom as a gravity-forced dense bottom current that mixes with resident Baltic Sea surface water /113/. The salinity of the inflowing deep water therefore decreases as the flow proceeds into the basin, and at the same time the volume flow increases due to mixing with the ambient water.
Dense bottom currents build up a deep-water pool in the Arkona Basin that loses water via a dense bottom current carrying water through the Bornholm Strait and into the Bornholm Basin. This builds up the deep-water pool in the Bornholm Basin, which is drained through the Stolpe Channel. This water sustains the deep water in the large basins in the interior of the Baltic Proper.
Average wave heights in the Arkona Basin are in the range of 0.5-1 m during the summer and 1-1.5 m during the winter. Higher waves up to >4 m occur during storm events /132/. The
fre-7-20 /133/. Such storm events occur mainly during the winter months (November to February) and are very rare in the months of May to August.
Figure 7-20 Annual number of storm events with significant wave heights of 4 m or more in the Baltic Sea /133/.
The mean and extreme significant wave heights at the end of the twenty-first century are antici-pated to increase compared with present conditions. The changes are expected to be greatest in the Bothnian Bay and Bothnian Sea because of reduced ice coverage causing unstable marine at-mospheric boundary layers with increased surface speed /138/.
The effect of hydrography on oxygen and hydrogen sulphide in the water
The surface waters of the Baltic Sea are aerated by wind mixing, and oxygen is further supplied by photosynthesis.The intermediate waters are also relatively well-oxygenated because most of the water from the Kattegat and the Great Belt is supplied to this depth range. The deep basins, how-ever, frequently experience oxygen depletion and a build-up of hydrogen sulphide (H2S) due to limited water renewal.
Inflows of oxygenated seawater from the North Sea to the deep basins occur irregularly, and lead to temporary increases in the salinity of the bottom water, as well as fluctuations in temperature (see Figure 7-21 shows an example of this effect measured during the field survey between No-vember 2017 and December 2018). These inflows of marine water are highly important for oxy-genating the deep-water areas of the Baltic Sea, and for supporting the physical environment for marine species. The inflows have been rare since the 1980s, but have had a slightly higher fre-quency in recent years /104/.
A relatively large saltwater inflow was detected in the western Baltic Sea during the winter 2011-2012. This event ventilated the Bornholm Basin and could be traced as far as the southern part of the eastern Gotland Basin. Oxygen conditions in the deep water have been improved by a series of inflow events since the end of 2013. First, a series of smaller inflow events occurred in November 2013, December 2013 and March 2014. These interacted positively and reached the deep water of the central Baltic Sea for the first time since 2003 /114/. In December 2014 and January 2015, a very strong inflow occurred, which transported 198 km³ of saline water into the Baltic Sea, and was followed by smaller events. An inflow of moderate intensity also occurred between 14 and 22 No-vember 2015. These events caused intensified oxygen dynamics in the Arkona Basin, Bornholm Basin and Eastern Gotland Basin, but the northern parts were not affected. As a result, the near-bottom oxygen concentrations in the Bornholm Deep ranged from 0.11 mg/l (in November 2015) to 7.2 mg/l (in February 2015), measured at 95 m water depth. In the Gotland deep, oxygen conditions ranged from -11.6 mg/l (in November 2013) to 3.9 mg/l (in April 2015 at 235 m depth;
/115/). The negative value measured in November 2013 represents the content of sulphide in the water given as oxygen equivalents, and is referred to as “oxygen debt”.
Salinity, temperature and oxygen were measured in the water column along the proposed NSP2 route at 27 stations. The measured depths of haloclines and thermoclines in the water column at each station as well as the bottom water oxygen content are listed in Table 7-13, and an example of the profiles is shown in Figure 7-21.
Table 7-13 A summary of water depth, the depth range with the major halocline and thermocline, and the bottom water oxygen content, temperature and salinity measured at the recording depth given in column 3.
South-west of Bornholm area, including Rønne Banke
EEZ_22 37 35 26 26 8.2 11 9.0
Stations with water depths roughly above 30 m exhibited signs of stratification both in regard to temperature and salinity. The profile shown in Figure 7-21 was recorded at station EEZ_09 in the Bornholm Basin.
Figure 7-21 Profiles of salinity (psu), temperature (°C) and oxygen (mg/l) in the water column at station EEZ_09.
The profiles in Figure 7-21 show several distinct water masses. Down to a depth of approximately 40 m, the water is well-mixed and with a salinityof 7.0 psu, a temperature of 7.5 °C and an oxygen concentration of 10.4 mg/l.Below this depth, the salinity increases gradually toward the bottom, and reaches 17 psu at 82 m. The temperature varies between about 6.5 and 8.9 °C and indicates the presence of at least three different layers at depths greater than 40 m. The upper part of the halocline (depths between 40 and 55 m) is relatively cold and with a salinity of 8 to 9 psu. Below this, a warmer layer with a salinity between roughly 10 and 14 psu extends down to about 65 m.
Finally, a cold bottom layer has a salinity above 15 psu. Due to the relatively high salinities of these water masses, they can be traced back to inflows of North Sea water through the Danish straits.
The varying temperatures show that these are separate inflows, which took place at different times.
Oxygen concentration decreases with depth through the three deep layers, from >10 mg/l at 40 m depth to 0.19 mg/l at 82 m depth.