A method to incorporate ecology into residence time of chemicals in embayments: local effect time.

Residence times are defined classically by the physical and chemical aspects of water bodies rather than by their ecological implications. Therefore, a more clear and direct connection between the residence times and ecological effects is necessary to relate these timescales quantitatively to ecology. The concept of local effect time (LET) is proposed in this paper as a timescale with spatial resolution that relates to ecological components and their spatial distribution within an embayment. The LET can predict the susceptibility of real-world ecological components to change from one condition to another. It can provide an efficient way to allow managers and agencies to evaluate the degree of stress or relief from current or projected changes in the loading of contaminants or nutrients. The steps for calculating LETs and defining their correlation with the existing ecological components in an embayment are presented along with illustrative applications to loading from riverine inflow and a wastewater treatment plant. The LETsuccessfully identified the areas within the water body that could be prone to ecological changes due to perturbations in the loading rate of riverine water and its constituents. An example is given that shows how the LET method can be used to delineate the distribution and duration of high levels of coliform bacteria due to a pulsed effluent from the treatment plant.