Originally published in April 2019 in Vol. 2, Issue 1 of Earthworks newsletter.

Most cities have their own network of sewer conduit guiding stormwater to outfalls reaching a water of the U.S. This network of sewer conduit is known as an MS4 (Municipal Separate Stormwater Sewer System). You can find a lot of information regarding what MS4 your project may affect online with your local municipal website. The city of Phoenix, AZ has a great breakdown of their network and where their outfalls are located throughout the valley. When a builder decides to construct a subdivision, they have the ability to tie-in to an MS4 or construct their subdivision as a “self-retaining” development. This means that the new development will not directly affect any outside stormwater systems and will most likely have retention ponds with drywells. The stormwater that lands on a builder’s property flows into these retention areas and into their drywells. These drywells are separate from the MS4’s and disperse the stormwater through rock and sand before percolating underground. Most new developments are being built this way and will not involve the cities MS4 network.

The positive of building a “self-retaining” are minimal yet attractive. These retention basins usually act as a small park for the community and are landscaped with decorative rock and grass. Negatively  these areas use a lot of real estate in order to create these retentions and lose valuable land for additional homes to sell on their site. However, the builder is always subject to protect their site and surrounding area from pollutants leaving their property.