Boston Harbor and Massachusetts Bay
MWRA Environmental Quality Department

Boston Harbor Sediment Recovery

Some of MWRA’s most exciting findings in Boston Harbor have come from studies of the soft-bottom benthic community through sediment profile imaging (SPI) and benthic sampling that examine benthic species diversity and abundance. Large amounts of data on sediment quality were quickly gathered at more than 50 stations in Boston Harbor by photographing a cross-section of the top several inches of the sediment. Sediment profile imaging can measure oxygen penetration, an important measure of benthic health, as “redox potential discontinuity” (RPD) depth.

The sediment profile images below show that, at a heavily polluted site (A), little or no oxygen penetrates into the sediments and few benthic animals survive. At a recovering site (B), there is a thick surface layer of light-colored oxygenated sediment and abundant evidence of animal activity. The depth of oxygen penetration (RPD) is indicated by the thickness of the oxygenated sediments.

The depth of oxygen penetration (RPD) is shown by the thickness of light colored, oxygenated sediments on top of black sediments lacking oxygen.
A: This shows mostly dark sediments, oxygen-deprived, and there is no visible life on the surface. In this extreme case, hydrogen sulfide produced by anaerobic bacteria feeds mats of sulfur bacteria that build up on the sediment surface.
B: This shows a recovering site that has developed a deep RPD. Individual Ampelisca tubes protrude from the surface. Ampelisca pump water into their tubes, aerating the sediments below.