INFLUENCES OF WASTEWATER TREATMENT HISTORY ON ORGANIC MATTER DYNAMICS WITHIN SALEM SOUND (MA) AS INTERPRETED FROM DATED SEDIMENT CORES

Salem Sound, MA, is a mesotidal, well-mixed and tidally-dominated estuary (mean z=13m). The South Essex Sewerage District Wastewater Facility (SESD) is the major wastewater treatment facility in the region and its outflow is located in the outer sound near Haste Island. We hypothesize that primary productivity in Salem Sound, and the subsequent flux of organic matter to the sediment, has been influenced by SESD activity. Sediment cores were taken next to the Haste Outfall (HO), at the mouth of Salem Harbor (SHM), and from inner Salem Harbor (SH). Organic matter was quantified down each core via loss on ignition (LOI), and age constraints were calculated using ¹³⁷Cs and ²¹⁰Pb.

The HO core, proximal to the SESD outfall, contains mean background values of 2.7% LOI below 31cm (mid-nineteenth century), and values increase above this depth. A rapid increase in organic matter concentration from 6.6% to 11.8% is observed at 21cm. This depth corresponds to ~1905 which is contemporaneous with construction of the outfall pipe discharging raw wastewater. At a depth of 7cm (mid 1970s), LOI values decrease from 11.7% to 9.3%. This shift is likely attributed to SESD beginning primary treatment in 1977. At 2cm (late 1990s), LOI values drop from 7.8% to 6.3% and remain at 6.1% to the modern surface, likely a result of SESD upgrading to secondary treatment in 1998. The Salem Harbor cores (SHM and SH) do not display the same shifts in LOI as are evident at HO, likely a result of their distal locations and the well-mixed nature of the estuary. LOI values from SH and SHM have background values of 7.0% and 5.4%, respectively, and exhibit increasing trends with maximum values of 11.0% and 10.8%, respectively. Between 2-4cm, LOI values in both cores drop. The decrease in organic matter concentration at the top of all three cores suggests a system-wide decrease in organic matter deposition, perhaps linked to the 1998 upgrade to secondary treatment. The results of this study reveal eutrophication of Salem Sound due to anthropogenic inputs of nutrients. “Natural” organic matter accumulation is greater in shoreward locations, which is expected due to terrigenous nutrient sources. The record of SESD activity only being evident in the HO core can be explained by Salem Sound’s short residence time and the proximity of the core to the point source.