Impervious Surface
Spatial distribution of weighted average of impervious surface of HUC-14s in the Raritan River watershed as of 2012
Impervious surface is also known as impervious cover and has been recognized as a critical driver to degrade water quality in the watershed. Impervious surface is defined as the area that prohibits penetration of water into underlying ground layers, as a result, rain and snow are unable to infiltrate into the ground and runs off. Examples of impervious surface include residential rooftops, public buildings, parking lots, commercial structures, and bedrock close to the soil surface. In general, concrete, asphalt, rooftops, and highly compacted soils that do not absorb water are known as impervious surfaces.
The expansion of impervious surface cover poses a threat to water quality in the watershed. For example, if forest or agricultural land is converted into residential or commercial uses, the impervious surface area increases resulting in a reduction of the amount of infiltration and ultimately leads to lower groundwater recharge. Increases in impervious surface cover also increases the runoff that is likely to carry non-point source pollutants into nearby waterbodies. This leads to deposition of excessive amounts of nutrients such as phosphorus and nitrogen resulting in algal blooms as well as the potential to carry oil, gas and other contaminants from roadways and parking lots into local waterbodies. Increases in impervious surface areas can also add to the volume of runoff and increase the velocity of flows in streams during a precipitation event. The increased volume and velocity of water can erode stream banks increasing sediment loads in streams that further degrades water quality.
Significant research has been conducted on impervious surface cover as it relates to water quality and researchers have found a high correlation between amounts of impervious surface cover and the degree of water quality impairment. When the impervious surface cover is greater than 10 percent of total watershed area, the water quality degrades in the watershed; if it is greater than 25 percent the water quality degrades severely (NJWSA, n.d.). When the impervious surface cover is less than 10 percent of total watershed area, the water quality is considered protected (Arnold and Gibbons, 1996).
Watershed with more than 10% impervious cover are considered to have impaired water quality; those with > 25% impervious cover are considered degraded. The Raritan had 12.1% impervious cover across the basin in 2012, with the Lower Raritan watershed management area exceeding 22.4%.