SPATIAL AND TEMPORAL TRACE METAL GEOCHEMICAL SIGNATURES IN URBAN PONDS: RECORDERS OF PAST LAND USE - INDICATORS OF FUTURE DEVELOPMENT

The Neponset River watershed is one of three main watersheds draining into Boston Harbor and has a unique history of industrialization and persistent contamination. The study site encompasses six urban ponds on the East Branch, each characterized by current and/or past industrial activity. In order to assess both long-term heavy metal loading in this watershed and to determine if current sedimentalogical conditions limit water quality or pose ecological risk, both sediment cores and surface sediment samples were collected. Sediment core samples taken with a Russian corer were sequentially sectioned in 5 cm depth increments to a maximum depth of 50 cm and analyzed by X-ray fluorescence. Depth versus concentration profiles for several key toxic metals illustrate both stratigraphically controlled maximum concentrations and varying trace element correlations. A timeline of probable industrial effluents contaminating the East Brach was created dating from 1800 to the present. The observed depth versus concentration profiles from each pond exemplify a unique historical geochemical signature that can be interpreted to reflect both industrial point sources and regional sources of heavy metals for elements such as lead, copper, zinc, and chromium.

In addition to tracing geochemical signals in the ponds, a preliminary ecological risk assessment was conducted through analysis of surface samples and GIS based mapping. In all ponds, surface sediment concentrations of lead, copper, zinc and chromium are close to or exceed the effects range median (ERM) value, the median metal concentration in sediment associated with toxic effects to the macroinveterate community. In Factory Pond the ERM exceedance for lead is by a factor of 4.2 and for copper is 4.0, while in Upper Forge Pond, which has been less developed, the ERM exceedance for lead is by a factor of 1.8 and copper values remain below the ERM. The range in ERM exceedance factors correlate with known variations in past land use patterns at each pond. Future land use changes affecting the East Branch study site include a 300 unit residential development which will lower infiltration/run-off ratios, change sediment deposition patterns, and increase the mobilization potential of sediments that contain the industrial legacy of the watershed and may affect downstream ecology.