Impervious surface as an indicator of pH and specific conductance in the urbanizing coastal zone of New Jersey, USA

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Abstract

Impervious surface is often used as an indicator of non-point source pollution in urban areas due to the strong relationship between percent impervious surface cover and water-quality impacts. In many cases, a threshold effect exists where water quality rapidly degrades above a given percent cover, but the exact threshold level appears to vary across regions. This study explores the relationship between pH, specific conductance, and percent impervious surface cover in the urbanizing coastal zone of New Jersey (USA) to determine the nature of the relationship in the region. The results of the analysis suggest the system is very sensitive to impervious surface, with a threshold potentially existing between 2.4% and 5.1% impervious surface cover. An examination of future conditions suggests that by 2020 water quality in more than 50% of the catchments in the study area will be negatively impacted by non-point source pollution associated with impervious surface.

Introduction

Urban development along the US coasts is occurring three times faster than in non-coastal regions of the country (Clark, 1996). The rapid rate of change is primarily a result of coastal metropolitan expansion and exurban development fueled by people attracted to the natural amenities of the coastal zone (Bartlett et al., 2000). In many places, recent growth has contributed to declines in water quality (Kennish, 1992; Beatley et al., 2002), modification of shorelines (Nordstrom, 2000), and destruction of terrestrial and aquatic habitat (Beatley et al., 2002). Of particular concern in many urbanizing coastal regions are increases in non-point source pollution, leading to altered water-quality impacts in headwaters and downstream estuaries. Urban non-point source impacts begin during construction, with the removal of existing land covers often leading to increases in sedimentation, runoff velocity, and stream volume (Charbonneau and Kondolf, 1993). Potential post-construction threats include high nutrient loading, oxygen depletion, erosion and alteration of the stream bed, toxic contamination, changes in salinity, and bacterial contamination (Robinson, 1976; Laxen and Harrison, 1977; Omernik, 1977; Klein, 1979; Charbonneau and Kondolf, 1993).

The extent of specific land uses within a catchment can serve as an indicator of many water-quality impacts, due to the strong relationship between land-based activities and non-point source pollution (Tong and Chen, 2002). Percent impervious surface cover has been proposed as a more refined measure of non-point source pollution associated with urban activities (Schueler, 1994). Arnold and Gibbons (1996) identified a step relationship between impervious surface cover and water quality with two observable thresholds: (1) 10% impervious surface cover, above which water quality begins to be impacted and (2) 30% impervious surface cover, above which water quality is degraded to such an extent that most waterways are no longer able to support aquatic life. The lower threshold is similar to the 15% threshold identified by Klein (1979) and the 10–20% threshold identified by Schueler (1994). More recently, in a review of the literature examining the relationship between percent impervious surface and invertebrate communities, Morse et al. (2003) identified impervious surface thresholds ranging from 5% to 25%, above which there was a decrease in diversity, population density, and/or sensitive species. The range of impervious surface thresholds reported in the literature suggests the potential for regional variations in the relationship between percent impervious surface cover and water-quality impacts.

In this paper, the relationship between water quality and different aspects of land use and land cover in New Jersey's coastal zone is analyzed to determine the applicability of impervious surface as an indicator of non-point source pollution in the region. The following questions are addressed through an examination of water-quality samples and their corresponding drainages: (1) what is the specific relationship between percent impervious surface cover, pH, and specific conductance? and (2) is impervious surface alone a sufficient indicator of non-point source pollution's impacts on water quality or do other aspects of land use or land cover need to be considered? The indicators that emerged from the analysis associated with the above questions were then used to evaluate the impact of future development on water quality within the study area.

Section snippets

Study area

The study area includes three watersheds (Barnegat Bay, Mullica River, and Great Egg Harbor) located on the Atlantic Coastal Plain in New Jersey, USA (Fig. 1). The watersheds fall within the Pinelands ecosystem, which is characterized by Pine-Oak upland forests and sandy acidic soils (Douglass and Trela, 1998). There are linear wetlands throughout the region and extensive salt marshes along the edges of the estuaries. Several confined and unconfined aquifers exist in the region, with the

Data

The relationship between pH, specific conductance, impervious surface, other land cover, and land uses was analyzed for 18 sites from the Barnegat Bay, Mullica River, and Great Egg Harbor watersheds (Fig. 1). Water-quality data were collected by the USGS from 1995 to 1997 and obtained from the NWISWeb site. The sample sites included in this analysis did not have nested drainage areas, to avoid problems associated with spatial autocorrelation. None of the drainages include any known point source

Results

The water-quality samples for the 18 sites are summarized in Table 1. The pH values range from 4.0 to 7.5 and specific conductance ranges from 26 to 263 μS cm−1. The highest specific conductance value represents a site with only one sample, so results associated with this value should be viewed with caution. The amount of LULC varies greatly between drainage areas (Table 2). Total altered land ranges from <1% to 65%, forest land ranges from 21% to 88%, while wetlands range from 4% to 40% of each

Discussion

Impervious surface appears to be a suitable indicator of water quality for the study area based on the sample sites included in this analysis. Surprisingly, these results suggest that differentiating urban uses by percent impervious surface cover may not be needed to understand urban non-point source pollution impacts. The slightly stronger correlation associated with percent urban may be because the urban class represents use, incorporating impervious, grass and other land covers. Future work

Conclusions

This paper describes an analysis of the relationship between two water-quality measures, impervious surface, and other aspects of land use and land cover for three watersheds in the coastal zone of New Jersey, USA. Percent impervious surface cover appears to be an appropriate indicator of urban non-point source pollution's impact on water quality. This system may be particularly sensitive to impervious surface cover, with water-quality changes starting at impervious surface levels as low as 2%.

Acknowledgments

Robert Zampella and Robert Nicholson provided useful insights, while Richard Lathrop, Mathew Baker, and one anonymous reviewer made helpful comments on earlier versions of the manuscript. John Bognar provided assistance with delineating the drainage areas, with John Bognar and Richard Lathrop supplying the 2000 land-use data. This project was funded through NOAA's NERR Fellowship program.

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