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Effects of urban development on water quality in the Piedmont of North Carolina--The NAWQA urban land-use gradient study

Poster
By Douglas A. Harned, Thomas F. Cuffney, Elise M.P. Giddings, and Gerard McMahon

Abstract

A study of urban basins located in the Piedmont of North Carolina is underway as part of the U.S. Geological Survey National Water-Quality Assessment (NAWQA) to determine the relation between level of urban development and water quality. Data were collected from 30 basins on water chemistry (nutrient, pesticide, and ion concentrations), geomorphic and habitat characteristics, hydrologic stage, discharge, water temperature, pH, dissolved-oxygen concentration, specific conductance, benthic algae, invertebrate communities, and fish communities. Collection frequency for water chemistry ranged from 2 samples (at 20 sites) to 6 samples (at 10 sites). Biological data were collected in each basin twice.

Investigation of the effects of urbanization on water quality must control for the effects of natural factors, while varying the degree of urbanization between study basins. A regional framework was used to control variability in natural factors that influence water quality. The urban intensity in each basin was measured by using an index to integrate information on human influences. The Urban Index includes information about land cover, infrastructure, population, and socioeconomic characteristics. Sites were selected to represent the full gradient of undeveloped to fully urbanized basins.

A prelminary review of the stream water-chemistry data indicates distinct relations between ionic composition and the Urban Index. Mean specific conductance was positively correlated with the Urban Index (Spearman correlation coefficient (r) = 0.77; 95-percent confidence limits (95CL) 0.61-0.93; probability (pr) <0.0001; N = 30). Specific conductance ranged from 56 microsiemens (uS) at the least developed site to 607 uS at the most developed site. Dissolved sulfate (r = 0.74; 95CL 0.57-0.91; pr <0.0001) and chloride (r = 0.71; 95CL 0.52-0.90; pr <0.0001) were also positively correlated with the Urban Index. Sulfate ranged from 2.3 to 66 milligrams per liter (mg/L) and chloride ranged from 3.5 to 96 mg/L. Urban sources of sulfate include domestic sewage and emissions from the combustion of automotive and diesel fuels. Sources of chloride include sewage and road salting.

pH was positively correlated with the Urban Index (R = 0.60; 95CL 0.38-0.84; pr = 0.0005) with a range from 6.5 at the least urban site to 7.5 at the most urban site. The increase in pH may be due in part to conversion of organic forest soils to less acidic soils of urban lawns.

The overall trend of increasing total dissolved nitrogen (r = 0.46; 95CL 0.12-0.80; pr = 0.0103) and nitrite plus nitrate (r = 0.46; 95CL 0.09-0.83; pr = 0.0109) concentrations, with increasing Urban Index may reflect sources such as sewage and lawn fertilizer use in the more urban basins. However, some of the least urban basins also had elevated nitrogen concentrations reflecting possible agricultural influences such as fertlizer use and animal waste. Total nitrogen concentration ranged from 0.31 to 14 mg/L.

Unit-area stream discharge during low-flow periods was negatively correlated with the Urban Index (r = -0.56; 95CL -0.74 - -0.37; pr = 0.0014). Reduced discharge with greater urban development may be a result of reduced infiltration caused by imperious surfaces. Unit discharge ranged from 0.47 to 2.27 cubic feet per second per square mile of drainage area.

Harned, D.A., Cuffney, T.F., Giddings, E.M.P., and McMahon, Gerard, 2004, Effects of urban development on water quality in the Piedmont of North Carolina--The NAWQA urban land-use gradient study, AGU 2004 Fall Meeting, December 13-17, 2004, San Francisco, California: Supplement to Eos, Transactions, American Geophysical Union, v. 85, no. 47, Abstract B11B-0150 [poster].