Millions of tons of bottom sediments are dredged annually
all over the world. Ports and bays need to extract the sediments to
guarantee the navigation levels or remediate the aquatic ecosystem. The
removed material is commonly disposed of in open oceans or landfills. These
disposal methods are not in line with circular-economy goals and
additionally are unsuitable due to their legal and environmental
compatibility. Recovery of valuables represents a way to eliminate dumping
and contributes towards the sustainable extraction of secondary raw
materials. Nevertheless, the recovery varies on a case-by-case basis and
depends on the sediment components. Therefore, the first step is to analyse
and identify the sediment composition and properties. Malmfjärden is a
shallow semi-enclosed bay located in Kalmar, Sweden. Dredging of sediments
is required to recuperate the water level. This study focuses on
characterizing the sediments, pore water and surface water from the bay to
uncover possible sediment recovery paths and define the baseline of
contamination in the water body. The results showed that the bay had
high amounts of nitrogen (170–450
Ports, lakes and semi-enclosed water bodies require continuous dredging of
bottom sediments to guarantee proper navigation levels and preserving the
aquatic ecosystem (DelValls et al., 2004). Therefore, millions of tons of
dredged sediments are generated around the world (Mymrin et al., 2017).
Europe alone produces
The recycling of dredged sediments in beneficial uses could be a promising route compared to the traditional disposal methods (Baxter et al., 2004). The recovery of valuables, like sediments as secondary raw materials, may contribute towards achieving sustainable circular economies. However, minimum attention has been given to developing and implementing methods for recycling or recovering dredged sediments. Recycling options may include the use in construction industries, habitat creation, plant-growing media, or the creation of dykes and covering landfills (Jani et al., 2019). In all uses, sediments can be employed as a replacement of natural raw material addressing the depletion of the Earth resources.
Recycling of dredged sediments varies on a case-by-case basis and depends on the sediment composition and physicochemical properties. Defining the composition as well as characterizing the physicochemical properties of the dredged sediments is therefore regarded as an essential step towards identifying the future route of sediments (Couvidat et al., 2018). Analysing the trace elements and organic-matter contents, nutrients and organic compounds is essential to identify potential end users and whether any prior treatment is needed (Mattei et al., 2016). High organic-matter content, nitrogen and phosphorus are essential for using the sediments in agriculture (Dang et al., 2013). However, high organic-matter content also limits the use of sediments in the construction industry due to the reduction in the durability of final products. Identifying the particle size distribution gives an essential understanding of the type of sediments and the amount of sand, silt and clay. Sandy sediments are easy to recover by separating the sand and using it in construction projects (Siham et al., 2008). Fine-grain sediments are more associated with contamination since clay and silt is bound with pollutants, and commonly chemical or biological treatment is required prior to using dredged sediments (Yoo et al., 2013).
Analysing the pore water extracted from sediments can also define the occurrence of pollutants and other components in sediments (Hammond, 2019). Additionally, characterizing sediments and surface water creates a baseline of current contamination level in a water body. Some water quality parameters are the concentration of nutrients, chlorophyll, turbidity, electrical conductivity, pH and dissolved oxygen (WHO, 1996). The status quo is vital to defining quality objectives and creating monitoring plans (Hartwell et al., 2019; Zhang et al., 2019).
This study focuses on characterizing the sediments, pore water and surface water from Malmfjärden Bay located in Kalmar, southeastern Sweden, to determine possible sediment recovery paths and define the baseline of contamination in the water body.
Malmfjärden is a semi-enclosed bay located in the city centre of Kalmar, southeastern Sweden, with minor connections to the Baltic Sea and the Western Gotland basin (Fig. 1). The bay has a special value for the municipality due to its central location which provides significant landscape and ecosystem services such as a habitat for wild birds and recreational space for inhabitants and visitors of the city. Currently, the water body is shallow, and there is a need to dredge bottom sediments to recover the normal water level to allow activities such as swimming, canoeing and wildlife development. Moreover, around the bay, there are residential and commercial areas rather than industrial sites. The bay lacks domestic and industrial sewage discharges and only receives runoff inputs. There are no published studies concerning the nutrient content and physicochemical characterization of the sediments.
Sediments samples were collected from four areas in the bay to cover all
the shallow region of the water body properly. The sampling area was also
defined according to plans of Kalmar municipality to dredge the bay. The
sampling areas covered the area that will be subject to intervention in the future. Each
area contained several sampling stations. Figure 1 shows the location of the
study and the sampling areas in the bay. In all stations, triplicates were
collected using a manual core sampler. Sediment cores (total height of
around 60 cm) were divided into top (0–20 cm) and bottom (20–60 cm) layers
to identify a possible differences in their composition. The layers were
separated using a plastic spatula, and composite samples of each of the four
groups were created by manually mixing the sediments using pre-cleaned
polyethene bags. Samples were stored in pre-cleaned polyethene bags at 4
Pore water was extracted following the procedure used by Charbonnier et al. (2019). The sediments were centrifuged in a Beckman Avanti J-25 (USA)
centrifuge at a speed of 4000 rpm for 15 min. One general sample was formed
mixing the pore water extracted from each core. Only one sample, instead of
several samples per group, was delivered to analysis due to the high volume
requirement from the private laboratory. The extracted pore water was
collected in a pre-clean acid-washed (with 15 % HCl) container and
preserved at 4
Surface water samples were taken from the same sediment stations. The
surface water was sampled at a depth of around 10 cm. The samples were
stored in pre-cleaned acid-washed containers (rinse with 15 % HCl) at 4
Location of the study area and sampling stations.
The analyses performed on sediments, pore water and surface water are shown in Fig. 2. The analytical methods are further explained in Sect. 2.3.1 and 2.3.2.
Analyses performed on sediments, pore water and surface water.
Composite samples of top and bottom sediments from each of the four groups were
employed to determine the composition of the sediments. The solid content
was measured by drying 50 g of each sample in an oven at
The chemical characteristics of the sediments were analysed at the commercial laboratory SYNLAB- Sweden. The samples were digested and processed according to EN-ISO-11885, and the elements were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Organic compounds were measured by gas-chromatography–mass-spectrometry (GC-MS). The total nitrogen was analysed following the Kjeldahl procedure (SS-EN 16169:2012). Samples for trace elements and nutrients were analysed in duplicates, while organic compounds were analysed without replicating it due to previous indications of low organic-pollutant levels at this site.
The sediment particle size distribution was found by the wet-sieve methodology followed by the pipette method given by Poppe et al. (2000). According to previous knowledge, the particle size distribution in the bay has a constant uniformity; therefore, only one general composite sample from the top layers and one from the bottom layers were generated to carry out the test. The accumulated particle size graph was plotted using the software DPlot version 2.3.5.7.
Contents of trace elements and organic compounds in both pore and surface water were analysed at the commercial laboratory SYNLAB – Sweden. The trace element contents were measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES) while total nitrogen was analysed following the Kjeldahl procedure (SS-EN ISO 11905-1:1997).
The pH, electrical conductivity and dissolved oxygen were analysed using the
portable instrument WTW Multi 340i (Germany). The turbidity was measured
using the equipment WTW Turb 550 IR (Germany). The chemical oxygen demand
(COD) was analysed by the spectrophotometer HACH DR5000 with the cuvette
LCI500 (low range: 0–150 mg L
The correlation between trace elements and organic matter was calculated to identify a possible relationship among the variables. The Pearson correlation analysis was carried out using the statistical software IBM SPSS (version 24).
The contamination factor (
Figure 3 shows the accumulated particle size
distribution of the samples from top and bottom layers of sediments on a semi-log scale. The
predominant fraction in both samples was fine particles
Dredged sediments are typically related to high water content. For handling, recycling or transporting sediments, the water content requires a reduction to useful levels (Ali et al., 2014). Table 1 illustrates the average water content in the top and bottom layers. The water content from top and bottom layers was between 71.9 % and 80.1 % and between 67.8 % and 71.7 %, respectively. Top layers contain a higher amount of water since they are closer to the water column. When a machine dredges the sediments, it is expected that the water content will increase up to 99 % and dewatering processes will be necessary to improve the transportation of sediments (Ali et al., 2014).
Composition of nutrients and the percentage of organic-matter and
water content in sediments (mean
Accumulated particle size distribution in a composite sample from top and a composite sample from bottom layers of all core sediments (semi-log scale).
Organic-matter content in sediments interacts with clay minerals to absorb or desorb anthropogenic or natural trace elements and nutrients (Ali et al., 2014). Table 1 shows the results of the organic matter of sediments represented by loss on ignition. In the study site, organic content in the top and bottom layers of sediments ranged from 12.5 % to 14.5 % and 11.4 % to 12.8 %, respectively. Both average values highlighted moderate organic content (SS-EN ISO 14688-2:2018). Results of the organic matter showed low variances between the top and bottom layers. Moreover, there is no significant difference between the two layers. The results suggested that organic matter is well distributed throughout the sediments of the sampled area of the bay.
The minimum, maximum and average concentrations of nutrients in the top and
bottom layers of sediments are illustrated in Table 1. On the one hand,
total phosphorus ranged from 1000 to 1300 mg kg
On the other hand, total nitrogen in top and bottom layers ranged from
7700 to 11 000 and 7400 to 9500 mg kg
Differences in variance in concentrations of nitrogen in top and bottom layers were insignificant (top layer standard deviation of 14 % and bottom layer standard deviation of 12 % compared to the mean). Likewise, the average concentration of the top and bottom layers was similar (difference of 10 %), suggesting that nitrogen is well distributed through the sampled area. Additionally, the sediments were always exposed to a similar nitrogen rate, probably related to the atmospheric component of the element source that provides a similar input of nitrogen over time.
Concentration of organic compounds in sediments (mean
Table 2 illustrates the concentration of target organic components in the top
and bottom layers of the sampled sediments as well as their maximum
permissible concentration for sensitive and less sensitive land uses
(Swedish EPA, 2009). The concentration of aromatic compounds (8–10, 10–16
and 16–35), benzene, toluene, ethylbenzene, xylene (BTEX) and aliphatic
compounds 5–16 were all below detection limits. The concentration of
aliphatic compounds 16–35 was between 84 and 120 mg kg
The concentrations of organic components in the top and bottom layers showed insignificant variations, expressing that all the sampled area is exposed to a similar rate of pollution. However, the top presented a significantly higher concentration of aliphatic compounds 16–35. The concentrations of aliphatic compounds 16–35, PCB7, PAH-L, PAH-M and PAH-H exhibited insignificant differences between the top and bottom layers. The results suggested that organic compounds have been discharged in a similar rate except for the aliphatic compounds 16–35 that have been more abundant in recent times.
Sediments from Malmfjärden exhibited a low concentration of organic compounds. The low concentrations of these compounds can be explained by the fact that the bay has no industrial and domestic discharges and the contamination source was the runoff inputs, which is usually less polluted than sewage (Metcalf and Eddy, 2003). Regarding the Swedish guideline to determine the extent of pollution in sediments (Swedish EPA, 2009), the mean concentration of organic compounds in bottom layers met the maximum permissible concentration for sensitive lands. However, average concentrations of organic compounds in the top layer only met the less sensitive land thresholds since aliphatic compounds 16–35, PCB7 and PAH-H slightly exceeded the maximum permissible values for sensitive lands. Aliphatic compounds 16–35 and PAH-H have a higher molecular weight compared to other organic components. Therefore, they are less biodegradable and persist more in the environment. PCB7 is also a well-known persistent compound that, over time, easily prevails in the environment (Speight, 2017).
Concentration of trace elements in sediments (mean
Minimum, maximum and average concentrations of trace elements from the top
and bottom layers are shown in Table 3. Low variation was observed for the
trace element concentration in each layer. Arsenic varied between
8.1 and 13.0 mg kg
Correlation coefficient matrix of trace elements and organic matter
in sediments from the study area (
On the one hand, the similar distribution of trace elements in each
layer displayed that all the sampled areas were exposed to a similar rate of
trace element which also suggested a common source of trace elements. The same
results were confirmed by the positive correlation between average
concentrations of trace elements from all sampling stations. Pearson's
correlations (two-tailed) coefficients are shown in Table 4 and ranged from
0.76 to 0.99. Due to the lack of industrial and domestic sewage discharges
to the study site, it is suggested that the source of trace elements could
be the runoff inputs to the bay. The highest correlations were among Cu–As,
Cr–As, Cr–Cu, Zn–Pb (
On the other hand, Pb, Zn, Cd and Cu displayed a major difference in average concentration between the top and bottom layers. As, Cr and Ni also revealed higher concentrations in top than in bottom layers; however, the difference was not significant. The higher concentration in top layers can be explained by the fact that, in earlier times, Kalmar had fewer vehicles, which are potentially one of the primary sources of pollution in the runoff through contaminated road dust (Hwang et al., 2016).
Sediments from Malmfjärden showed a low to medium concentration of trace elements. In top and bottom layers, Cd was the major contaminant, exceeding by 25 % the maximum permissible concentration established by the Swedish guideline for sensitive land use (Swedish EPA, 2009). As and Pb exceeded the limits for less sensitive land use by 10 % and 25 %, respectively.
Results (minimum, average and maximum) of trace element pollution indexes
(
Pollution index for trace elements in sediments of the study area
(
The Swedish Environmental Protection Agency estimated that sediments in Sweden have a background concentration of cadmium of approximately 0.2 ppm (Swedish EPA, 2000). The sediments from Malmfjärden presented levels of Cd around 1.3 ppm, which is 6 times higher than the background level, suggesting an anthropogenic source. Hence, it is likely that in this bay, where there are no industrial or domestic sewage discharges, the runoff may be the source of all metals including the moderate levels of Pb, Cu and Zn.
Characterization (nutrients, DO, electrical conductivity, pH,
turbidity, COD) of surface water from Malmfjärden Bay (mean
Characterizing the properties of the pore water contributes to identifying
the presence of pollutants and other components in sediments. Table 5
illustrates the concentration of trace elements, nutrients, organic
pollutants and other parameters in the pore water extracted from
Malmfjärden sediments. The pH was acidic (pH
The concentration of nutrients in pore water indicated 6.7 and 14 mg L
Lastly, chemical oxygen demand (COD) and electrical conductivity are water
quality indicators. COD measures the oxygen that can be consumed by
oxidizable compounds contained in water, and electrical conductivity is a
measure of the water strength to transmit electrical flow, which is directly
related to the amount of ions in the water (Metcalf and Eddy, 2003). Clean
surface water has a COD of 20 mg L
Water bodies with large amounts of nutrients are classified as eutrophic.
Globally, several surface water quality guidelines exist to assess
eutrophication levels. The common parameters are the concentration of
nutrients, dissolved oxygen, algal chlorophyll and transparency in the
water. Closely, core indicators are total nitrogen and phosphorus. Table 6
shows the minimum, maximum and mean concentration of nutrients, dissolved
oxygen (DO), electrical conductivity, turbidity, pH and COD. Total
phosphorus varied from 34.2 to 77.7
The electrical conductivity of the water body ranged from 11.38 to 11.57
Concentrations of trace elements and organic compounds in sediments from
Malmfjärden only met the maximum permissible concentration for less
sensitive land use. Hence, the sediments may be used directly in the
construction industry or as plant-growing substrate (for ornamental gardens
or vegetation beside roads) on industrial or commercial land. For
construction, the high organic content influences the durability of final
products, and therefore it is necessary to reduce it by thermal processes or
by mixing with fly ash or other inert components (Ali et al., 2014). For
plant-growing substrate, the lack of sand in the sediments suggests
creating a mix of substrates (with compost, coconut fibre or peat) to
improve the physical and nutrient characteristics (Mattei et al., 2018).
Additionally, sediments may also be used as a final covering layer for
landfills if future studies confirm the ability of sediments to create
waterproof covers (Yozzo et al., 2004). All the suggested uses of sediments
need to be investigated further to determine technical, environmental and
economic feasibility. However, in order to reuse these sediments in sensitive
lands (such as residential areas, schools and farmlands), the concentrations
of cadmium in the bottom and top layers must be reduced to less than 0.8 mg L
The characterization of sediments, pore water and surface water sampled from
the Malmfjärden Bay (located in southeastern Sweden) were assessed to
define pollution baselines of the water body and to determine potential
beneficial uses of dredged sediments. The sediment composition was dominated
by silt and clay with moderate levels of organic material and a medium–high
concentration of nitrogen. Organic compounds were found in low
concentrations in sediments. Additionally, trace elements showed low to
medium concentrations that were confirmed by the
The pore water lacked organic pollutants and had low pH values leading to low concentrations of metals or metalloids. Finally, the water body showed high concentrations of nitrogen related to eutrophication problems. However, the surface water showed levels of DO, pH, electrical conductivity, COD and turbidity in agreement with these introduce in the guidelines given by WHO (1996).
All data used were collected by the authors and are contained within this paper (see Table 1, 2, 3 and 6).
LF carried out planning, sampling, laboratory work, analysis of data and the writing of the article. YJ carried out planning, analysis of data and the writing of the article. LG carried out planning, sampling and laboratory work. WH carried out planning.
The authors declare that they have no conflict of interest.
This article is part of the special issue “European Geosciences Union General Assembly 2019, EGU Division Energy, Resources & Environment (ERE)”. It is a result of the EGU General Assembly 2019, Vienna, Austria, 7–12 April 2019.
The authors would like to thank Stefan Tobiasson and David Silfwersvärd for their contribution during the sampling campaign and Fabio Kaczala for his guidance during sampling and analysis of the results. Finally, the authors would like to state that part of the results of the study was shown at the EGU General Assembly 2019 (poster presentation).
This research has been supported by the Life Programme (grant no. LIFE15 ENV/SE/000279).
This paper was edited by Michael Kühn and reviewed by Michael Schneider and one anonymous referee.