The Øresund Link: Recognition of fine-grained sediments from excavation activities. Pilot study

The purpose of the investigation is to test a method of determining the amount of influx of sediment (spill) derived from construction activities to the sediments in Øresund. The method is suggested for use during environmental monitoring of the effect of spill from the construction activities associated with the Øresund Link, with emphasis on the fine grained sediments.

The sediment composition of 40 samples from 10 positions in the Øresund has been investigated together with 20 mixtures of natural sediment and spill material: In 10 samples natural sediment were mixed with spill from till and 10 with spill from limestone. Finally shells of 5 marine mollusc species from Øresund were analysed. A total of 66 samples were analysed.

The construction activities is mainly conducted in limestone and till, both characterised by high contents of calcium carbonate and consequently Chemical analyses of the spill show high contents of elements as calcium (Ca), strontium (Sr) and carbonate and low contents of eg. aluminum (Al), potassium (K) and yttrium (Y). This indicate that it will be possible to distinguish between natural sediments and sediments with an influx derived from the construction activities associated with the Øresund Link using the content of eg. Ca, Sr and carbon.

There is a varying natural content of eg. Ca, Sr and total carbon (TC) in the sediments, and because the content of these elements are correlated to the content of clay (increasing with increasing clay content) in the natural sediments, tree algorithms could be developed to calculate the influx of excess Ca, Sr and TC from spill.

The calcium carbonate contribution to the natural sediments is mainly from shell debris. The contribution from shell debris is found to be of nunor importance in the sediments analysed. It is found that carbonate derived from recent shell debris can be discriminated from eg. Danian limestone on the basis of the Ca/Sr ratio which is 488 in København Limestone (Danian limestone) and 237 in shell debris. A fourth algorithm using the different Ca/Sr in the spill relative to natural sediment is developed.

Spill was found at one position (FBC-C) using an average of the four algorithms mentioned above. The spill content is approx. 13 weight % in the uppermost 3 cm indicating that the spill-deposition on the location is equivalent to 4 mm if the density of the spill is equivalent to the natural sediment. The detection limit of the method is approx. 7 % spill.