SES analyses suggested that a significant fraction of the metal loading in Poole sediments may not be stable and thus has potential for remobilisation (Langston, 1982; Cundy and Croudace, 1995), and only remains stable because of the sheltered, low-energy conditions in Poole Harbour (Langston et al., 2003).
Dredging, to keep navigational channels open, amounts to an average displacement of 70,000 m3 sediment per year (Langston et al., 2003). In 2005, a large dredging operation commenced to deepen the main channel in Poole Harbour (Figure 21).
In addition to the main dredging operation in the main channel, another dredging operation in Holes Bay will be undertaken for the construction of a new bridge: the “Twin Sails Ahead” project. The engineering consultants Gifford and Partners Ltd (GPL) have extracted core samples in Holes Bay and concluded that dredging (in this case for the construction of a new bridge) would not pose an imminent danger to the environment (GPL, 2004). By contrast, other studies (Langston et al., 2003; Wardlaw, 2005; Underhill-Day, 2008) suggest that metals and metalloids could be remobilised when the
sediment is disturbed. Since these reports, no further data has been published to support either viewpoint.
Figure 21: The dredging operation in Poole Harbour (adapted from PHC, 2004b)
This large-scale dredging in a small embayment with very poor flushing characteristics will, however, presumably cause a strong impact; therefore scientific sampling must be concluded before this operation commences.
Originally, the start was scheduled for 2007, but, fortunately for this project, postponed (pers. comm. Rice 11 , 2007). Early September 2009 PHC announced the start of the operations (Murphy, 2009). Monitoring of the site confirmed that works have commenced immediately thereafter by the "C. H.
Horn" (140 gross tons), equipped with a grab system. Compared to a suction based dredging system, grab dredging may lead to a substantially higher remobilisation as the sediment is constantly washed out (Figure 22).
11 John Rice, engineering manager for the Full Sail Ahead Project
Figure 22: The "C. H. Horn" during dredging operations in Holes Bay, 05.09.2009
Analyses in the approaches to Poole Harbour, the Swash Channel, showed hardly any contamination at all in the top-layers, only tin and mercury showed elevated levels (Hübner, 2009). All other analysed metal concentrations in the upper part of the sediment (interaction layer) were on average only 10.4 % of their respective TEL-values (cf. Chapter 10.4.6). It is therefore reasonable to conclude that the majority of contamination remains within the harbour, primarily owing to its minimal flushing characteristics.
In the contaminant study in Poole Harbour, the null hypotheses are that
a) the metal/metalloid concentrations are below the threshold effect levels, which would indicate that adverse effects are to be expected only rarely (cf. Chapter 2.2.6);
b) there are no significant differences between the concentrations in the different sampling zones, which would indicate that the different impacts (for example the considerable industrial use in Holes Bay, cf.
Chapter 5.1.2) did not cause noteworthy differences in the distribution of different contaminants.
5.2 Material and methods
Detailed cm-by-cm analyses can provide much information on trends over long time frames (Smol, 2002); however, owing to the large size of the estuary, a high number of cores would need to be extracted to avoid the common problem associated with other studies (cf. Chapter 5.1), and a cm-by-cm analysis would produce a prohibitively large number of samples. In order to obtain a reasonable balance between spatial coverage and temporal resolution 53 cores have been extracted in the present study. This allowed a reasonable coverage and it is very unlikely that additional cores would have provided significantly more information (cf. Figure 23-Figure 30). Samples have been extracted with a Beeker corer type 04.29.SA (cf. Chapters 2.1.2 and 10.2.1) between 2007 and 2009.
Figure 23: The sampling locations in Poole Harbour
The literature recommends using sites with low turbation to get reference samples with a clear profile (Smol, 2002). Unfortunately, using only such sites would exclude the majority of Poole Harbour from analyses; however, there is a site which offers such conditions: a sea wall was constructed on the eastern side of Brownsea Island in order to drain St Andrew's Bay and create a meadow as cattle pasture (pers. comm. Thain12, 2006). According to Mr Thain and the Dorset Wildlife Trust Webpage the wall was constructed in the 1850’s; the map-material shows that at least a provisional construction was already existing in 1811 (Makenzie and Hurd, 1829). The area sheltered by this wall resulted in undisturbed reference samples, while the Southern Bights (i.e. Brand’s Bay, Newton Bay and Ower Bay) provided nearly contamination-free samples for the estimation of the natural background values.
Cores were processed with a resolution of 5 cm (for core lengths, see Chapter 5.3 and Table 7 respectively) and the previously described methodology has been applied. Samples have been dried with a temperature not exceeding 40 °C and prepared with a 0.3 mm sieve (cf. Chapter 2.1.3).
Typically, for each sample 3 replicates were analysed, but higher variations between the replicates triggered the inclusion of further replicates (cf.
Chapter 2.2.5). The (pseudo-)total metal content was analysed using aqua regia digests (cf. Chapter 2.2.2); the mobile fraction using the standardised procedure DIN 19730 (cf. Chapters 2.2.4 and 4.4).
To link the contamination with the source, and determine its development with time the age of the sediment layers must be known. Radiometric dating is an effective means, and 14C is commonly used as evidence of recent climate change and environmental effects of human activities (Chernicoff and Whitney, 2007). 241Am also has considerable potential as dating tool, its main advantage is its lower mobility (Appleby et al., 1991; ATSDR, 2006). If the sedimentation rate is known, the depth can be related to the age. Studies by Warneke (2002) and Cundy and Croudace (1996) considered this issue and
12 Chris Thain, Reserve Manager Dorset Wildlife Trust
provided sedimentation rates for Poole Harbour with the assistance of radiometric dating. For this approach, a corer with a very low compression is essential (cf. Chapters 4.2.1 and 10.2). In addition, contamination horizons can be used to calibrate local differences. Events like the catastrophic BDH fire in 1988 (cf. Chapter 5.1.2) usually cause horizons in the sediment, furthermore large-scale catastrophes resulting in worldwide fallouts (e.g.
atmospheric nuclear bomb testing and volcanic eruptions) can provide evidence for establishing a chronology (Smol, 2002).
Bryan and Langston (1992) provided a comprehensive list of average concentrations for 19 different sites, primarily UK estuaries, including Poole Harbour. These values have been used a reference values in this study.
Bauer et al. (1996) described an equation to estimate the potential risk of adverse effects of contamination (“Gefährdungspotential”), here translated as risk potential index (RPI). The original formula (RPI0; Equation 3) is based on the German sewage sludge ordinance (AbfKlärV; BMU, 1992) and takes account of both mobile and (pseudo-)total concentration together with one guideline values for each: the sewage sludge threshold value (“Grenzwert der Klärschlammverordnung”; KSVG) and the threshold value for mobile fractions ("Prüfwert für Mobilanteile"; PWM). Although this approach was originally not intended for sediments, the basic concept is sound and has been adapted for the use in the present study. For this purpose, effect levels for marine and estuarine sediments (cf. Table 6), as discussed in Chapter 2.2.6, have been implemented in the calculations. For the first formula (RPI1; Equation 4) only one essential modification was implemented: the sewage sludge threshold level was replaced by the SQuiRTs threshold effect level for marine sediments.
In the second formula (RPI2; Equation 5) the same basic concept was used, but this time the equation was reorganised: the absolute concentrations were set in relation to the probable effect levels and the mobile fractions to the threshold effect levels, thus basing the equation solely on up-to-date values. If the total available concentration equals the PEL and the mobile fraction equals the TEL, the RPI2 would take the value of 1.
Equation 3:
RPI0 = risk potential index [non-dimensional ratio value], original sewage sludge formula RPI1 = risk potential index [non-dimensional ratio value], threshold level exchanged RPI2 = risk potential index [non-dimensional ratio value], new calculation method TCA = aqua regia extractable content [mg·kg-1]
PES = ammonium nitrate extractable content [mg·kg-1]
KSVG = sewage sludge threshold value (“Grenzwert der Klärschlammverordnung”) [mg·kg-1] PWM = monitoring value for mobile fraction ("Prüfwert für Mobilanteile") [mg·kg-1]
TEL = threshold effect level [mg·kg-1] PEL = probable effect level [mg·kg-1]
Table 6: Threshold effect levels (TEL); probable effect levels (TEL) and monitoring values for mobile trace metal contents (PW), all values in mg·kg-1
TEL(1) PEL(1) PW(2)
5.3 Results
Depending on locations and sediment parameters (most notably grain size) cores were between 30 and 100 cm in length (Table 7), with 100 cm constituting the maximal length possible with the equipment used (Table 1);
which is, considering the temporal range this length allows, adequate for the objectives of this study (cf. Chapter 1.4.1) and also lies well within the range suggested by Ahlf et al. (2002). Shorter cores were obtained in Holes Bay at the PPPS, where the solid chalk layer prohibited deeper sampling, in the middle of the main basin and at the Eastern Sandbanks, where the sandy bottom with only small clay fractions made penetration of the ground difficult.
In all other areas, cores typically were between 70 and 90 cm in length.
Table 7: Corelengths in Poole Harbour
sampling zone min [cm] max [cm] mean [cm]
Holes Bay 30 100 76.4
Lytchett Bay 100 100 100.0
Wareham Channel 65 100 84.0
Eastern Sandbanks 40 50 46.6
Southern Bights 45 100 88.2
Brownsea Island 50 85 63.3
Σ 76.0
Given that the sampling area was much larger in size and the contamination levels differed substantially, it has been difficult to display the whole sampling area with 3D-plots as was done for the Christchurch Harbour case study (Figure 15). Therefore, first the major elements have been compared for the whole estuary (Figure 24-Figure 30), and then different zones (Holes Bay, Wareham Channel, Southern Bights, Brownsea Island lagoon, Eastern Sandbanks and Lytchett Bay) were assessed in more detail. In addition, three
known major point sources (PSTW, PPPS and RNCF; cf. Chapter 5.1.2) were analysed in detail.