Raium isotopes in Port Phillip Bay : estimation of the rate of bioirrigation of sediments, and water residence time

Recent work has shown that estuarine sediments are a source of Ra to coastal waters. Ra is soluble in saline water and is rapidly desorbed into pore water from deposited fluvial sediments where it is continuously generated by insoluble Th parents. The rate at which Ra effuses into surface water has been used to determine the rate of surface-water porewater exchange. Once in the water column, the behaviour of Ra is essentially conservative, enabling the determination of water residence time in a semi-enclosed estuary. Here we use measurements of Ra in an estuary to estimate two water mixing processes. Port Phillip Bay is a semi-enclosed estuary adjacent to the city of Melbourne, one of the highest density population centres in Australia. The Bay is approximately 50 km in diameter, and has an average depth fo 14 m. A recent study found that the potential for eutrophication and algal blooms in the Bay was intricately linked to the fate of nutrients, particularly nitrogen discharged into the Bay from rivers, drains, and sewage treatment plants. Two of the most important processes controlling the levels of inorganic N in the water column were identified as bioirrigation of bottom sediments, and the rate of exchange of Bay water with ocean water via Bass Strait. In this paper we describe how Ra isotopes can be used to estimate the rates of these processes, and we compare these rates with estimates made using conventional techniques. (author). 10 refs