There are several reasons that imperviousness is a key environmental indicator: there is a definite link between impervious surfaces and the hydrologic changes that degrade water quality; impervious surfaces are characteristic of urbanization; impervious surfaces prevent natural pollutant processing in the soil by preventing percolation; and impervious surfaces carry pollutants into waterways (Weng, 2001).
Climate, geology, soils, land use and vegetation shape a waterway and how it is affected by impervious surfaces. Human action can only affect the last two factors. Impervious area reduces the amount of water available to recharge groundwater aquifers and increases the volume of surface runoff. In early stages of urbanization, removal of trees and vegetation may increase evapotranspiration and interception and increase stream sedimentation. Later, impacts include decreased infiltration, lowered groundwater tables, increased storm flows and decreased base flows during dry periods. With more development, increased imperviousness will reduce time of runoff and concentrate it so that peak discharges are higher and occur earlier soon after rainfall starts. The volume of runoff and the potential for flooding is greatly increased. Sewers and storm drains also accelerate runoff (Brabec, et al, 2002).
Studies on the effect of increased impervious areas in a watershed have documented these impacts (Beach, 2002):
Studies suggest that some water quality parameters can be modified by altering local riparian conditions but the dominant water quality trends in streams are related more to catchment-wide land use and geology. Other studies suggest that there is a higher correlation between the proportion of basin in forest and water quality than the proportion of the stream channel with riparian forest (Clausen, et al, 2003).
Forested riparian areas can help provide better physical stream habitat by reducing direct stormwater flows that reshape channels and provide beneficial detritus and woody debris that enhances stream habitat. Wetland adjacent to streams also can mitigate the introduction of sediment, nutrients, and temperature changes.
Urbanization has also been determined to be a significant anthropogenic source of nitrogen in receiving waters of the northeastern United States (Clausen, et al, 2003). It also has a large role in increasing levels of sodium chloride in freshwaters. A recent study using data from the Baltimore Long Term Ecological Research project as well as data from New York and New Hampshire has shown a relationship between increased concentrations of sodium chloride in freshwaters and impervious surface area (Kaushal et. al, 2005) primarily from the use of salt as an ice melting agent on roadways. This same study also found that chloride concentrations in many suburban and urban streams now already exceed the maximum limit (250 mg/l) recommended for the protection of freshwater life. Increased levels of chloride also threaten the both surface and groundwater drinking supplies.
Much of the discussion regarding the level of impervious surface area centers around whether there is a percent coverage of a watershed which is a “tipping point” for seeing negative ecological impacts. A Wisconsin study showed that when the level of connected impervious surface area reaches 8-12% impervious surface area, small changes in urbanization can cause significant changes in the following stream conditions: fish densities, species richness, diversity and index of biotic integrity (IBI), bank erosion, and base flow (Wang, et al, 2001). Some watersheds may be even more sensitive to impervious surface area - a USGS study in Alaska found that populations of some aquatic invertebrates start to decline when a watershed reaches 5% ISA, (Ourso, 2003).
Recent studies indicate that, because each watershed possesses unique characteristics, using a single threshold (percent imperviousness) is not an accurate way to assess watershed impacts. The studies suggest that impact thresholds have ranged from 4% to 12% for fish populations, 8 to 15% for macroinvertebrates, and 4 to 50% for abiotic measurements like water quality and habitat (Clausen, et al, 2003).
Recent GIS analysis of data from aerial photographs indicates that about 14% of the land area in the Narragansett Bay watershed (RIDEM, 2007) is covered by impervious surfaces. It is important to note that impervious surface area is distributed unevenly within watersheds; here in New England, those areas tend to be on or near watercourses, in or adjacent to established community centers. The effect these areas on waterways is dependent on how directly connected these surfaces are to water. Studies in Australia found that stream health is strongly affected by the proportion of a catchment area that consists of impervious areas that are directly connected to waterways; that is, there is a direct physical connection (drain, pipe, etc.) that allows water to be channeled into a watercourse (Ladson, et. al. 2004). Until recent times, prevailing management practices were to move water off of hard surfaces into local waterways as fast as possible.
Based on analysis of satellite data, as of 2004, 10% of the Rhode Island is covered by impervious surfaces. The level of coverage in major population centers is over 30%. Heavily settled suburban communities have percentages between 10% and 30%. Only 17 of 39 municipalities have less than 10% impervious surface area and these areas are more extensive along coastal areas than interior areas. In Rhode Island, impervious surface areas increased 43% between 1972 and 1999 (Zhou, 2004).
In Massachusetts, it has been estimated that, if land it built out to the extent allowed by current zoning, an additional 15,760 acres of impervious surfaces will be created along with a loss of nearly 320,000 acres of natural, vegetated lands and open space (Blackstone River Coalition, 2008).
% Impervious Surface in the Narragansett Bay Watershed Map
% Impervious Surface by Sub-watershed Map
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