Arbejdsrapport fra Miljøstyrelsen, 28/2005 – Undersøgelse af piletræers evne til at vokse i saltholdigt havneslam, samt optagelse og nedsivning af udvalgte stoffer

Undersøgelse af piletræers evne til at vokse i saltholdigt havneslam, samt optagelse og nedsivning af udvalgte stoffer

Summary and conclusions

Samples of bottom sediment were collected from 9 different sampling sites and samples of sea water from 5 different sampling sites inside and outside Kalvehave Harbour. The sea water contained 9-10‰ salt and had a conductivity of about 17 mS/cm. The water content and the content of organic matter (estimated as loss on ignition after heating to 550°C for 20 hours) were higher in the harbour (about 30-70% and 10-20%, respectively) than outside (25-35% and 0-5%, respectively), which was attributed to the higher degree of mixing and thereby removal of fine grained material at the open sea.

An approximately linear correlation was found between water content and the content of organic matter for organic matter contents up to 10%. With a few exceptions, the clay and silt fraction amounted to less than 10% of the inorganic part. There was a positive correlation between clay- and silt fraction, and loss on ignition which also includes water bound in clay minerals. The total carbon content followed the trend for organic matter with values ranging from 2 to 15% in the harbour. Accordingly, the total sulfur content varied between 0.13 and 3.22%, which is a strong indication of sulfate reduction and thereby anaerobic conditions and likely also a lack of iron- and manganese(hydr)oxides in the bottom sediment. There was a weak positive correlation between total carbon and total sulfur as well as between total carbon and loss on ignition. In general, the sediment with a high organic matter content appears more microbially active and more reduced.

In general, the content of zinc in the bottom sediment was between 500 and 1000 mg/kg dry matter (DM), while the content of copper typically varied between 50 and 150 mg/kg DM. According to guideline values brought about by the Danish Environmental Protection Agency in 1992, these values make the bottom sediment potentially harmful for the sea environment. Outside the harbour, the concentrations of both heavy metals were much lower and do according to the guideline not represent an environmental problem. There was a clear tendency towards increasing concentrations of Zn and Cu with increasing total carbon content, indicating that the heavy metals are primarily bound to the organic part of the bottom sediment. This could be expected because of the low clay and silt content. The strong binding to the solid phase is manifested by much lower concentrations of Zn and Cu in the water phase, which were far below the drinking water guideline.

Larger amounts of bottom sediment were collected from 2 sampling sites in the old part of the harbour. The sediment was mixed in storage tanks and subsequently used in the phytoextraction experiments at the Technical University of Denmark. Precipitation experiments were conducted with “thin“ sediment from the top and “thick” sediment from the bottom of the storage tank. As expected, the thin sediment precipitated faster with 90% relative sinking after 24 hours. During the same period, the thick sediment sank 76%. Total precipitation was attained after 3 days. Precipitation under pressure (1-3 bar) did not proceed much faster. Accordingly, in practice it is hardly necessary to precipitate harbour sediment for more than 24 hours.

The sediment used in the experiments contained 140-157 mg Cu/kg DM, 358-526 mg Zn/kg DM, 40.5-43.2 mg Ni/kg DM, 2.7-2.9 mg Cd/kg DM, and approximately 25 mg Li/kg DM, 18.4% dry matter, 6.2% total-carbon, 2.2% carbonate-carbon, 9400 mg total-nitrogen/kg DM, 1142 mg total-phosphorus/kg DM, 654.5 µg TBT-Sn/kg DM, 206.5 µg DBT-Sn/kg DM, and 129.3 µg MBT-Sn/kg DM.

In the phytoextraction experiments the 3 willow clones ”Orm” (× Salix viminalis), ”Aage” (× Salix schwerinii), and ”Bjørn” (Salix schwerinii × Salix viminalis) were used. The willow trees were planted in c. 30 cm sand/gravel in wooden boxes 1.25×2.4 m a year prior to the experiments. During this period no fertilizers were applied. At the beginning of the experiments in June 2001, the willow trees in box 1 (”Bjørn”) had reached a height of c. 105 cm. ”Aage” in box 2 was c. 165 cm, while ”Orm” in box 3 was c. 95 cm. Only the clone ”Aage” appeared to be in good condition at the first application of harbour sediment on June 20th. During the following 3 months, 2355-2420 L sediment with an average dry matter content of 5% was applied in each box. Box 1 was thoroughly watered several times during the summer, while box 2 received c. 2900 L water (rain water included), and box 3 received rain water only (approximately 770 L) during the 3 months. Accordingly the three boxes had different experimental conditions during the period. In addition, the willows were three different species in different conditions at the beginning of the experiments. This complicates a direct comparison of experimental results.

Five weeks after the first application of sediment on June 20th 2001, a willow from box 3 was analysed for TBT and degradation products, but the concentrations were below detection (0.5-5 µg -Sn/kg DM). Twigs (with leaves) from the 3 willow clones were analysed for the heavy metals Cu, Cd, Zn, and Ni at regular intervals. Cu concentrations never exceeded background levels (12-16 mg/kg DM). In contrast, Cd concentrations increased by a factor of 2 from a background level of about 1 mg/kg DM in 20 days. Similarly, Zn concentrations increased from 50-70 mg/kg DM to 300-350 mg Zn/kg DM during approximately 2 months. Ni concentrations also increased from 1-2 to 5-8 mg/kg DM during 30-40 days. Accordingly the uptake of cadmium proceeds most rapidly, while the remediation potential follows the sequence Zn > Cd > Ni > Cu.

On July 23^rd, samples were collected in box 3 from the upper part of the sediment (the crust), further downward in the sediment layer and from the underlying sand/gravel layer. The dry matter content had increased substantially, in particular in the crust, demonstrating an efficient drainage of the sediment layer. Overall, the sediment volume decreased by a factor of 10 after application. The concentration of TBT had decreased by a factor of 2 whereas the concentrations of DBT and in particular MBT were only slightly lower than the start values. All 3 compounds were below detection (<0.5 µg/kg DM) in the sand/gravel layer. Also in the drainage water, the organotin compounds were below detection. Accordingly, microbial or photolytic degradation of the TBT is the likely explanation for the decreasing concentration. Both the content of TBT and DBT continued to decrease until the last sampling on November 2^nd 2001, whereas the MBT concentration was constant. TBT appears to be degraded following first-order kinetics with a rate constant of 0.017 d^-1 and a half-life of 41 days. Accordingly, the TBT degradation is relatively rapid after application. It is possible that degradation will take place even without the willows, but it is likely that the plants enhances the degradation by improving conditions for the microorganisms in the top soil.

Both the Cu and Zn concentrations in the sediment had decreased after application, but the Cu/Li ratio was practically stable, indicating that the Cu was primarily washed out, whereas the Zn/Li ratio had decreased after application indicating a significant uptake of Zn by the willow trees. The drainage water from all three boxes was analysed for Cu, Cd, Zn, and Ni at regular intervals during the experiments, but only zinc slightly exceeded the drinking water guideline, indicating a very good quality of the drainage water with regards to the four heavy metals. In contrast, sea salt was present in the drainage water in concentrations only slightly lower than in the sea water.

Polycyclic aromatic hydrocarbons (PAH's) were analysed in the stock tank (sediment and water phase) and in the drainage water. All 29 PAH's analysed were detected in the sediment, which can be characterised as slightly contaminated with a total PAH concentration of 9.3 mg/kg DM. Also 2 phtalates were detected but in concentrations far below the isolation guideline value for waste water sludge. In the drainage water 5 PAHs were detected in concentrations of 0.01 µg/l, while the remaining 24 PAHs could either not be quantified or be detected. Accordingly, the PAH content exceeds the very low guideline value for surface water runoff (0.001 µg PAH/L).

All 3 willow clones were affected by the sea salt, and the growth stopped after application of the harbour sediment. Only the clone ”Aage” (× Salix Schwerinii) appeared in fairly good condition after the experiments. The average yield was approximately 3 tons DM/ha, which is 3-4 times less than the normal yield per year for willow trees. Lack of water and fertilizer may be part of the explanation. However, despite the salt all the willows have taken up Zn, Cd and to a lesser degree Ni. Copper was not accumulated in the willows. Under the experimental conditions, the removal of heavy metals by the willows amount to 15-30 g Ni/ha, 4-5 g Cd/ha, and 650-850 g Zn/ha. Extraction for at least 1-2 months is recommended, and the willows should be harvested before October. The clone ”Aage” seems the most suitable for phytoextraction of contaminated harbour sediment. The efficiency could probably be enhanced by washing the sediment with fresh water before application, but for a satisfying extraction of heavy metals, much smaller doses of sediment per surface area should be applied (i.e. a few tons dry matter per ha). The extraction potential could likely be substantially improved by growing the willows under ideal conditions. It is important to supply adequate amounts of water and fertilizer during the initial growth season, and to grow the willows in a sunny place.

The effect of a higher salinity was investigated by applying water from Gilleleje Harbour (c. 20‰ S) on smaller experimental tubs with willow trees. A week after the last dosage, the willows had lost all leaves and after a month the plants appeared dead. However, the willows started to shoot after the rainy September, but their growth seemed stunt for the rest of the growth season, which likely implies serious root damage. Accordingly, the willows can survive even a total saturation with 20‰ salt water for a short period, but at such high salinities dilution is recommended in order to avoid long-term effects.