Why are rocks, sand, and dirt considered pollutants?

It can seem surprising that rocks, sand, and dirt are considered pollutants, since these materials are part of the natural environment.  However, as with many other things, it is the amount that counts.   Natural systems are not adapted to receiving these materials, and the nutrients and toxics they carry, in the amounts associated with some human activities.   "Rocks, sand and dirt" have been included in the federal Clean Water Act's definition of pollutant since its initial enactment in 1972, and are part of Maine's law.  Discharge of these materials can have serious adverse impacts on surface water quality and on the biological communities that inhabit our surface waters.

These pollutants carry nutrients to our waters and feed the algal growth that chokes some of our lakes.  Maine's 1994 water quality assessment report describes over 225,000 acres of lakes as not in attainment of water quality standards due to organic and nutrient enrichment.  The enrichment comes, wholly or in part,  from the eroded soil washing into the lakes.  An additional 42,000 acres of lakes not in attainment were directly affected by siltation.

Also, siltation in small streams reduces habitat required for fish spawning, and available for other aquatic life.  This happens in two ways.  Although fish and other forms of aquatic life encounter these materials every day, they are not adapted to encountering large or continuous amounts.  Sand and dirt act as abrasives, damaging soft tissues such as gills.  Sand and dirt also cover and clog the porous substrate in streams.  This destroys habitat important for the support of small aquatic life forms, and for the protection of eggs.  It also inhibits the flow of water necessary to keep the habitat aerated.  Adverse effects are long term, impairing the habitat long after the visual presence of the silt in the water is gone.