Intra-annual variability of urban effects on streamwater quality: An examination of watersheds in the Atlanta, Georgia USA region

Abstract

There has been considerable research on urban impacts on streamwater quality, but studies have mostly ignored intra-annual variations in water-quality variables and thus seasonally-varying impacts. Therefore, this study uses daily water-quality data over eight years for 38 streams in the Atlanta, Georgia USA region and watershed-level, urbanization variables to assess month-specific impacts of urbanization on water quality. The water-quality variables were mean specific conductance, mean temperature, mean dissolved oxygen, median pH, and median turbidity. The watershed-level variables were population density, housing density, proportion of watershed that is high-intensity developed land, imperviousness, proportion of watershed that is urban land use, an urban-intensity index, and change in developed land. Both differences in water quality between urban, suburban, and exurban streams and correlations between water quality and the watershed-level variables were examined. During all months, urban streams had significantly higher specific conductance than suburban and exurban streams, and the urban intensity of a watershed was significantly positively correlated with specific conductance. Differences and correlations were intensified during winter and spring. For temperature and dissolved oxygen, only during non-winter months did urban streams have significantly higher temperatures and significantly lower dissolved oxygen as well as significant positive correlations between urban intensity and temperature and significant negative correlations between urban intensity and dissolved oxygen. Urban streams had the highest pH during all months, but none of the differences were significant. Significant correlations between pH and urban intensity were restricted generally to winter and spring months. Turbidity was highest for exurban watersheds and thus was not significantly positively correlated with urban intensity. Seasonal and stream-group differences in the weathering of concrete, use of road salts, solar heating of streams, urban surface temperatures, baseflow, and land-disturbance activities explain the water-quality findings. Most of the findings should be applicable to urban areas in humid subtropical climates within the Piedmont physiographic region.