Pollution
Imperviousness
Imperviousness is a measure
of the extent to which rain can filter into the ground. The
more hard surfaces in the form of rooftops, roads, and parking
lots, the more impervious the area becomes. As imperviousness
increases, and infiltration decreases, rain runs off more quickly,
filling streams more rapidly and carrying more sediment and
other pollutants into the Bay. Oil, grease, copper, and other
heavy metals all wash from the roads and, often, directly into
streams and the Bay. Other contaminants, such as nitrogen, can
come from lawns and fields through over-fertilization, from
the airborne emissions of cars and power plants, and from sewage.
In some cities, particularly older ones, sewage and storm
water are handled in combined sewers. Combined sewage systems
are a remnant from the days when household waste, privies,
and runoff were all routed directly to the rivers. In the
early 1900s cities began installing new lines that would direct
all wastewater to the treatment plant, but the old outfalls
were left in place as a sort of escape valve.
During heavy rains these combined systems get overloaded
and some of the mixed sewage and rainwater bypasses the treatment
plant through the old outfalls directly into the river. Many
of these combined sewer overflows (CSOs) are being taken care
of, but it is a very expensive, and therefore slow, undertaking.
Imperviousness is a two-headed coin; no matter how you flip
it, you lose. On one side, it increases surface runoff, flushing
pollutants and scouring streambeds, and on the other side,
infiltration and groundwater recharge decline, reducing flows
available to wells and streams during dry periods. Fluctuation
in stream flow makes it more difficult for marine species,
from Mayflies to trout, to survive. Long-term aquifers of
ancient water supplies can also be affected by the reduction
in infiltration. In conjunction with increasing demand from
private wells drawing down the aquifers, increased imperviousness
sometimes causes wells to dry up and land to sink.
Imperviousness leads to a host of negative impacts from higher
erosion and flooding risk to increasing nutrient pollution
to reduced recharge of underground water - in short, to lower
water quality. The Center for Watershed Protection (http://www.cwp.org)
has determined that watersheds with as little as 10-15% imperviousness
will experience degraded water quality. Consequently, low-density
development does not avoid the problem, but exacerbates it.
Fortunately, there are many ways to minimize hardened surfaces
and excess runoff, from reducing road widths and scattered
development to maintaining riparian buffers and natural landscaping.
Effect of Land Use Types
on Water Quality | Imperviousness
Sources of Air Pollution in the Watershed
| Water Pollution
Emission Calculator | Pollution
Resources
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