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Nitrogen, Septic Systems, and Water Quality

Historically, Cape Cod’s coastal waters were renowned and appreciated for their abundant resources. Vast meadows of eelgrass, a light-loving flowering plant carpeted the bottom of many of our shallow bays.

Eelgrass meadows provided an ideal habitat for commercially and recreationally important fish and shellfish, like winter flounder, scallops, and blue crab.

As human development increased on the land, the amount of nutrients, like nitrogen, entering our coastal waters also increased. Nitrogen stimulates the growth of algae (seaweeds and microscopic phytoplankton). Too much algae decreases the amount of light reaching the bottom, causing eelgrass to die. Too much algae also can contribute to low oxygen conditions in the water, stressing or killing fish and shellfish.

On Cape Cod, the major source of nitrogen to our coastal waters is on-site septic systems. Septic systems use the soil to provide basic treatment of wastewater from toilets, sinks, showers, washing machines, garbage disposals and dishwashers. But, septic systems, even Title V systems, were designed to remove viruses and bacteria, not nitrogen.

In order to resolve the problem of excessive amounts of nitrogen in our coastal waters, we must treat our wastewater more effectively. Doing so will be very expensive and will depend on communities working together.

To read more about our coastal waters, nitrogen and on site septic systems, below is a list of downloadable CapeKeeper fact sheets:

Nitrogen and coastal water quality

What your septic system does and doesn't do

The Color of Water: Bring Back the Blue

The Color of Water: Bring Back the Blue (Short version)