|
SENT BY:DPR.4 St.Paul, MN 4-12-33 ; 3:31AM ; 612
<br />612 407 4191 4 A
<br />impervious coverage increases, the velocity and vol-
<br />ume of surface runoff increase, and there is a corre-
<br />sponding decrease in infiltration. The larger volume of
<br />runoff and the increased efficiency of water convey-
<br />ance through pipes, gutters, and artificially straight-
<br />ened channels result in increased severity of flooding,
<br />with storm flows chat are greater in volume and peak
<br />more rapidly than i4 the case in rural areas (Career
<br />1961; Anderson 1968; Leopold 1968; Tourbier and
<br />Westmacoct 1981). The shift away from infiltration rr.
<br />duces groundwater recharge, lowering water cables.
<br />This. bath. threatens water supplies and reduces the
<br />groundwater contribution co scream flow, which can
<br />result in intermittent or dry stream beds during low
<br />How periods (Dunne and Leopold 1979; Harbor 1994).
<br />Hydrologic disruption gives rise to physical and
<br />ecological impacts. Enhanced runoff causes increased
<br />erosion from construction sites, downstream areas
<br />and scream banks. The increased volume of water and
<br />sediment, combined with the "flashiness" of these
<br />peak discharges, result in wider and straighter scream
<br />channels (Arnold, Boison, and Pacton 1982), Loss of
<br />tree cover leads to greater water temperature fluctua-
<br />tions, making the water warmer in the summer and
<br />colder in the winter (Galli 1991). There is substantial
<br />loss of bock screamside (riparian) habitat through ero-
<br />sion, and in -scream habitat as the varied natural
<br />stream bed of pebbles, rock ledges, and deep pools is
<br />covered by a uniform blanket of eroded sand and silt
<br />(Schueler 1992). Engineered responses to flooding like
<br />stream' diversion, channelization, damming, and pip-
<br />ing further destroy scream beds and related habitats
<br />like ponds and wetlands. Finally, with more intensive
<br />land uses comes a corresponding increase in the gener-
<br />arion of pollutants. Increased runoff serves to crans-
<br />porr these pollutants directly into waterways, creating
<br />nonpoint source pollution, or polluted runoff.
<br />Major categories ofnonpoint source pollutants in -
<br />dude pathogens (disease -causing microorganisms),
<br />nutrients, roxic contaminants, and debris. Pathogen
<br />eontaininarion indicates possible health hazards, re-
<br />u'.
<br />d
<br />r 'salting inosed beaches and shellfish beds. Over-
<br />abundance of nutrients such as nitrogen and
<br />phosphorous can threaten well water supplies, and in
<br />surface waters can lead co algal "blooms" chat, upon
<br />..decaying, rob the waters of life -sustaining oxygen
<br />d Toxic contaminants like heavy metals and pesticides
<br />pose threats to the health of aquatic organisms and
<br />their human consumers, and are often persistent in
<br />the environment. Debris, particularly plastic, can be
<br />hazardous co..animal and human alike, and is an aes-
<br />thetic concern. Sediment is also a major nonpoint
<br />source pollutant, both for its effects on aquatic ecol-
<br />ogy and because of the fact that many of the other
<br />pollutants tend to adhere to eroded soil particles (En-
<br />vironmental Protection Agency 1992, 1993a).
<br />The results of polluted runoff are evident in every
<br />corner of the United States. According to the Environ-
<br />mental Protection Agency (1994), nonpoinr source
<br />pollution is now the number one cause ofwater qual-
<br />ity impairment in the United States, accounting for
<br />the pollution of about 4096 of all waters sur-
<br />vcyed across the nation. The effects of nonpoint
<br />source pollution on coastal waters and their living re-
<br />sources have been of particular concern (U.S. House of
<br />Representatives 1988; Environmental Protection
<br />Agency 1993a). Urban runoff alone ranks as the see-
<br />ona-most common source of water pollution for lakes
<br />and estuaries nationwide, and the third most common
<br />source for rivers (Environmental Protection Agency
<br />1994).
<br />As point source pollution is increasingly brought
<br />under control, the true impact of urban nonpoint
<br />source pollution is being recognized. For instance,
<br />even in an urbanized estuary like Long island Sound,
<br />where the major environmental problems have been
<br />strongly linked to point source discharges from sewage
<br />treatment plants, an estimated 47% of the pathogen
<br />contamination is from urban runoff (Long Island
<br />Sound Study 1994)•
<br />Imperviousness as an
<br />Environmental Indicator
<br />Planners wishing to protect their community's wa-
<br />ter resources against these threats may not know
<br />where to begin. The site -specific and diffuse nature of
<br />polluted runoff seems co demand extensive technical
<br />information on pollutant loadings, hydrologic model-
<br />ing, and the effectiveness of various management
<br />practices. This information is difficult co acquire, nor
<br />only because of the cost of such studies, but because
<br />nonpoint-source-related research and engineering are
<br />new and evolving fields.
<br />Enter impervious surfaces. When doing
<br />eorununity-level planning, or where derailed sire in-
<br />formation is unavailable, impervious coverage may of-
<br />ten be the most feasible and cost-effective vehicle for
<br />addressing water pollurion. TWo major factors argue
<br />for its potential utility to the local planner.
<br />First, imperviousness is integrative. As such, it can
<br />estimate or predict cumulative water resource impacts
<br />without regard to specific factors, helping to cur
<br />through much of the intimidating complexity sur-
<br />rounding nonpoint source pollution. Although imper-
<br />vious surfaces do not generate pollution, they: (1) are
<br />a critical contributor to the hydrologic changes chat
<br />degrade waterways; (2) are a major component of the
<br />APAIOURNAL•S"'Na 1996 245
<br />
|