Retention av tungmetaller i en anlagd våtmark
studier av Vattenparken i Enköpings kommun
The aim of this study was to examine the amounts of heavy metals in the sediment of a wetland constructed for stormwater treatment. The wetland is called Vattenparken and is located in the city of Enköping. The heavy metals which were analysed were Pb, Cd, Ni, Zn, Cu, Cr and Hg. The total metal concentrations were determined using HNO3 chemical extraction. From 8 sampling sites 5 sediment cores were taken. Each core was sectioned into 3 layers (0-1.5, 1.5-4 and 4-7 cm). The metal concentrations were compared with previous studies of stormwatersediments, arable land and the quality criteria published by the Swedish Environmental Protection Agency.
Previous studies of stormwater- and lake sediments from Uppsala County had metal concentrations of the same magnitude as the sediments in Vattenparken except for Pb, which was found in higher concentrations in Vattenparken. According to the Swedish Environmental Protection Agency classification system for effects on the local environment, the concentrations of Pb are of moderate magnitude and are not regarded to have any big impact on the local environment. The other metals are also found in this range of magnitude or lower.
However most of the heavy metal concentrations in the sediments of Vattenparken are higher than the concentrations of heavy metals that the Environmental Protection Agency regarded as normal concentrations for south of Sweden, even though the concentration of Hg is very low in the sediments of Vattenparken. Then concentration of Hg is even lower than the mean original concentrations for Swedish conditions according to the Environmental Protection Agency. Previous studies of the arable land in the municipality of Enköping indicate that the low concentration of Hg is not a coincidence. Also the arable land in the municipality of Enköping has very low concentrations of Hg.
Measurements on the suspended material indicate an extensive sedimentation in Vattenparken. Several heavy metals have their highest values just after the inlet of the wetland. There is no trend in metal concentrations with depth.
There was also an attempt made to use data on the heavy metal transport in Vattenparken to adjust a one-dimensional computer model. A proper adjustment of a computer model to a wetland requires large amounts of measured data. The parameter that should be considered with special interest is the fall velocity of particles because it was the factor, which was found to have biggest impact on the metal retardation in the model.