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DMR Home > Health & Safety > Shellfish > Growing Area Program > Rainfall and Pollution

rain cloudRainfall and Pollution:

April showers bring May flowers… and the possibility of closures for shellfish harvest areas. Rain causes fecal coliform counts to spike in water surrounding shellfish flats across the state.

There are a number of ways that rainfall interacts with the environment to increase fecal coliform pollution on shellfish flats:

  1. Overland Runoff
    When the ground can’t hold any more water because it is saturated, frozen, or is an impervious surface, such as pavement, rainfall will flow along the land’s surface as overland runoff. As runoff moves, it can pick up the pollution that is deposited on the land’s surface and carry it to the nearest river, stream, lake, or coastal shoreline.
  2. Septic System Failure
    When rainfall saturates soils, this can cause septic systems that are normally functional to fail. Because the soil is already full of water, it cannot accommodate any additional water entering the leaching field of an in-ground septic system, so water and sewage from the system pool on the surface. Malfunctioning septic systems that are already failing before rainfall fail more severely and expose larger amounts of sewage at the surface. Overland runoff can transport the sewage from failing septic systems to a nearby lake, stream, or coastal shore.
  3. Increased Flow in Streams
    Streams and rivers drain water from large areas of land, so there are many sources of land-based pollution in their drainage areas. As a result, streams and rivers are typically more polluted than the coastal water that they flow into. The polluted water spreads out until the cleaner coastal water can dilute it. On a day with a significant amount of rain, there is more water flowing out of streams and rivers. The polluted water spreads out over a larger area, and it takes more ocean water to dilute the pollution.
  4. Overflow from Municipal Sewage Treatment Systems
    Some sewage treatment plants combine water from storm drains with sewage water for treatment. On days with heavy rains when there is a large amount of storm water, the volume of water can exceed the capacity of the sewage treatment plant. The treatment plants will release some of the water as a combined sewage overflow. This releases raw, untreated sewage directly into the river or coastal area where the plant is located.

How to make rainfall have a smaller impact on shellfish flats:

  • Maintain your septic system and pump it out regularly.
  • Use Best Management Practices on farms. Confine manure sources so that they are not available for overland runoff to carry.
  • Plant vegetative buffers along shorelines to catch pollution and slow runoff before it reaches the nearest water source. This is particularly important along livestock pastures and farmland where manure is spread as a fertilizer because both of these are areas where runoff has the potential to pick up large amounts of pollution.