One of the main culprits of water pollution in the watershed is stormwater - the water that runs over the landscape during and after a rainfall or when snowfall melts, picking up pollutants as it flows.


Because so much of our landscape now consists of impervious surfaces such as sidewalks, driveways, roads, parking lots, playgrounds, and roof tops – much of the rainwater and snow melt now flows over the ground for longer than it used to, picking up contaminants along the way.  The term that’s used for this dirty water is stormwater runoff.

Rainwater can pick up bacteria and parasites from pet waste – and chemicals from fertilizer, pesticides, herbicides, ice melt, motor oil, antifreeze, and windshield washer fluid.  There’s also plenty of litter floating around…

This dirty rainwater (now referred to a stormwater runoff) eventually washes into catch basins or storm drains along the street, which are connected via underground pipes to nearby water bodies.  All of the contaminated water flows directly into local streams, rivers, ponds, untreated.


Contaminated stormwater runs into local waterbodies.

Polluted stormwater causes problems for local drinking water sources; recreational activities like swimming, boating and fishing; and aquatic life.

Click here to learn how you can prevent polluted stormwater runoff from your property.

Protecting receiving waters

Before the landscape was developed, far more rainwater would seep into the ground and be filtered by soil, plant roots and microorganisms, before joining the groundwater or seeping from the soil into a waterway.

Today, realizing just how much of the ground has been covered in impervious material, communities are incorporating stormwater-cleaning structures, called BMPs (Best Management Practices) into the built landscape.  Examples of BMPs include tree-filter-boxes, bioretention cells, and rain gardens.

Typical catchbasin at left, and water-filtering tree-filter box at right, Milton.

Typical catchbasin at left, and water-filtering tree-filter box at right, Milton.


Tree-filter-boxes can be installed along sidewalks to receive and filter the water that runs off the street.

Tree-filter-boxes consist of a concrete box, an outlet pipe, a special soil mixture, and a small tree or shrub. The tree roots, the soil, and microorganisms filter out the contaminants from the street runoff. The filtered water then flows out of the box, via the outlet pipe, enters an underground pipe system, and is released into a nearby waterbody. By filtering stormwater, tree-filter-boxes reduce the amount of pollutants that enter the receiving waters.

Bioretention cells & rain gardens

Bioretention cells (“biocells”) and rain gardens are both excavated depressions in the ground that have been filled with a special soil mixture and a combination of plants, which help to treat stormwater.

Rain gardens can look “wild” and natural, or more manicured.

Both structures may contain an outlet that leads from the stormwater treatment structure to a nearby waterway. The plants, soil and soil microorganisms of the biocell or raingarden filter contaminants from the stormwater, and the cleaned water then either joins the groundwater or flows into an adjacent waterway.

Click here for more information on building a rain garden.

Neponset Watershed Association sites stormwater treatment structures

A variety of other stormwater treatment structures exist. In fact, Neponset River Watershed Association Environmental Scientist, Chris Hirsch, has been hard at work, siting stormwater treatment structures around Neponset River Watershed communities. To learn more about this effort, contact Chris Hirsch at 781-575-0354 x302 or

Applying a "no dumping" decal to a catchbasin, Milton.

Applying a “no dumping” decal to a catchbasin, Milton.

Beyond stormwater treatment structures

In addition to installing stormwater treatment structures, communities are implementing non-structural Best Management Practices, such as changing policies, and initiating public education campaigns to reduce the quantity of chemicals people apply to their yards and to people to properly dispose of pet waste.

Signage, mutt-mitt bins, and catchbasin decals are just a few of the tools used in education campaigns.

Click here to learn how you can prevent polluted stormwater runoff from your property.

Over the years, the Watershed Association has organized several work groups staffed by volunteers to install catchbasin decals in neighborhoods around Milton, especially in the Pine Tree Brook watershed. The goal has been to reduce the entry of stormwater-carried pollutants into the Brook.