Abstract

To assess the environmental impact of the Massachusetts Bay outfall for Boston's treated sewage effluent, grain size and characteristics of aggregates in bottom sediments have been measured during a comprehensive monitoring program. Both a sandy and a muddy bottom location within 2.5 km of the outfall have been sampled three times per year since 1990. These samples have provided about 11 years of pre-discharge baseline data used to evaluate natural variability.

The sandy location (station #2) showed a very consistent textural trend over the monitoring period. Mean textural percentages were slightly over 97% sand, 1.5% silt, and 0.9% clay. The within-station variability of the sand was less than 2%. No seasonal differences in grain size were observed. There was very little change in the grain size distribution at this site following unusually large storms in October 1991 and December 1992. However, the concentration of aggregates, measured separately from the texture, decreased from about 1.5% to 0.5% after the December 1992 storm perhaps because of its longer duration. The aggregates at this station were composed primarily of quartz, with small concentrations of fecal pellets, worm tubes, and amorphous organic material all bound in an elastic mucus-like matrix. The majority of the fecal pellets were larger than 0.25 mm, consistent with the size produced from an assemblage of benthic polychaetes, isopods, and amphipods.

The muddy location ( station #3) contained 50-90% silt plus clay. No seasonal textural trends were evident, but clay concentrations increased 35% following the severe storm in December 1992. Increases in clay concentration were also detected during February and May of 2001. These dates mark the 4th and 7th months after the September 2000 start of the new outfall. The ongoing monitoring study will compare these most recent and any future textural changes with chemical analyses of the sediments. The comparison will help to determine if a clear linkage can be made between the operation of the outfall and any changes observed in the sediments.

The organic mineral aggregates at station #3 peaked (54%) in concentration during the early fall of 1993. Except for this peak, values were typically between 5% and 15% during the 11 year monitoring period. There were no seasonal trends. Speculation suggests that the peak in aggregate concentration may have been caused by larger populations of benthic organisms, which developed following the influx of fine-grained organic-matter-rich sediment after the unusual storm of December 1992. High concentrations of aggregates are restricted to the upper 2 cm based on profiles in sediment cores up to 50 cm long, and suggests that this form of sediment encapsulation is fragile in the sedimentary record.