Stormwater Runoff

For many years, DEM has been actively monitoring bacteria levels from 11 stations throughout Green Hill Pond



Source: Department of Environmental Management
Note: The three squiggly purple lines in the lower right are transatlantic coaxial cables that were installed between 1970 and 1996. They come ashore at the end of Green Hill Beach road.

Nine of the eleven stations meet the state’s MEAN water quality standard for shell fishing. Only one station (16) significantly exceeds that standard. So, on average, Green Hill Pond meets one of the state’s water quality standards.


Source: Department of Environmental Management

However, all Green Hill Pond stations fail the state’s PEAK water quality standard for shellfishing (less than 30 CFU/100ml). This situation is why there is a shellfishing ban and why it is hazardous to swim.


Source: Department of Environmental Management

Rain is the major component in producing peak levels of bacteria. As can be seen in the chart below, the first five days after a rain event result in bacteria levels that are up to 10 times the readings for the next five days. That said, the water is not guaranteed to be safe six or more days after rain. Other less common events, such as dead animals in the streams, can also cause a peak bacteria surges at any time.


Source: Department of Environmental Management; FGHP analysis
Note: The data used came from 30 measurements for each station over five years 2017-2022.

According to DEM, the two freshwater tributaries—Factory Brook and Teal Brook (noted in blue on the map) –are the major sources of bacteria loading in Green Hill Pond. As DEM concluded in 2006: “If the tributaries meet water quality standards, it is DEM’s judgement that the Pond will support designated uses.” In other words, fix the tributaries and we fix Green Hill Pond—at least as far as bacteria is concerned.