Acoustic reflector of opportunity distribution as a surrogate for inferring effluent distribution in a survey of Massachusetts Bay

In 1998 the Massachusetts Water Resources Authority (MWRA) expects to complete the construction of a new oceanic wastewater outfall system which will have the capacity to release as much as 1.2×10⁹ (gal) of treated wastewater per day. The design of the system is such that during summer months the effluent plume should be trapped beneath the seasonal thermocline. It is proposed that the spatio-temporal distribution of acoustic reflectors (probably biological) present in the seasonal thermocline is a reasonable surrogate for inferring characteristics of the spatio-temporal distribution of acoustic reflectors that will result from the release of wastewater effluent. In September 1995, the authors conducted a joint U.S. EPA, MWRA, NOAA survey of Massachusetts Bay. The primary data sets gathered were acoustic backscatter profiles at 20 kHz and 200 kHz made every 0.5 (sec) with 0.5 (m) vertical resolution. Also, CTD casts and water bottle samples were taken at selected locations. Oceanographic phenomena such as internal waves and tidal surges in conjunction with topographic features cause the surrogate scattering layers to be vertically modulated with possible mixing of materials normally trapped within layer boundaries. Analysis of acoustic backscatter measurements provide visualizations of such events and allow for the estimation of the surrogate scattering layer distributions in the vertical and horizontal planes. Quantitative comparison of acoustic backscatter strength with CTD derived density and transmissometry measurements give an indication of the relationship of the surrogate scattering layers to the water density structure at selected sites in the study (and an indication of the reliability of the surrogate). In addition to its role as a surrogate for wastewater, the relationship of the surrogate scattering material layers with respect to the isopycnal surfaces is also considered