Water polution studies on Lake Illawarra. I. Salinity variations and estimation of residence time

Abstract

Surface and bottom water salinities were determined at 10 monitoring stations over a 2-year period. Mean salinity is controlled primarily by rainfall and varied from 12.8 to 31.3%, during the monitoring period. The rate of flushing of Lake Illawarra by tidal action and by freshwater entry was estimated from salinity variations and by mathematical modelling; for dry periods a flushing time (for reduction by a factor of e) of about 60 days was calculated for a conservative substance introduced into the lake. Mean tidal volume was 1 % of the lake volume. Vertical mixing by wave action is complete within 2-4 weeks. Although the major creeks are on the western side of the lake, no significant east-west salinity gradient was found. Lake Illawarra is characterized by the small ratio of catchment to lake area and the short distance from the very elevated watershed to the lake shores, resulting in rapid response to rainfall. The volume of direct precipitation was estimated to be comparable with creek input.