Use of organochlorine contaminants to measure sedimentation rates in estuaries: a case study from the Manukau Harbour

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Abstract
Organochlorine contaminants are widely distributed in sediments of the Manukau Harbour. The first appearance of DDT in cores is a potential stratigraphic marker because it can have appeared only since extensive applications of DDT on New Zealand pastures for grass grub control began in c. 1950. The technique permits calculation of average annual sedimentation rates, but precision is limited by bioturbation. A nett sedimentation rate of about 5 mm yr-1 was determined from measurements of DDT for muddy tidal flats in Drury Creek. This compared well with a rate of 5 mm yr-1 based upon a pollen dating technique. The past widespread use of DDT may enable comparisons to be made of very recent sedimentation rates in other New Zealand estuaries. Other contaminants identified in the Manukau Harbour sediments include dieldrin, lindane, PCB's and chlordane.