Abstract:For the period from 1958 to 1996, streamflow characteristics of a highly urbanized watershed were compared with less-urbanized and non-urbanized watersheds within a 20 000 km 2 region in the vicinity of Atlanta, Georgia: in the Piedmont and Blue Ridge physiographic provinces of the southeastern USA. Water levels in several wells completed in surficial and crystalline-rock aquifers were also evaluated. Data were analysed for seven US Geological Survey (USGS) stream gauges, 17 National Weather Service rain gauges, and five USGS monitoring wells. Annual runoff coefficients (RCs; runoff as a fractional percentage of precipitation) for the urban stream (Peachtree Creek) were not significantly greater than for the less-urbanized watersheds. The RCs for some streams were similar to others and the similar streams were grouped according to location. The RCs decreased from the higher elevation and higher relief watersheds to the lower elevation and lower relief watersheds: values were 0Ð54 for the two Blue Ridge streams, 0Ð37 for the four middle Piedmont streams (near Atlanta), and 0Ð28 for a southern Piedmont stream. For the 25 largest stormflows, the peak flows for Peachtree Creek were 30% to 100% greater than peak flows for the other streams. The storm recession period for the urban stream was 1-2 days less than that for the other streams and the recession was characterized by a 2-day storm recession constant that was, on average, 40 to 100% greater, i.e. streamflow decreased more rapidly than for the other streams. Baseflow recession constants ranged from 35 to 40% lower for Peachtree Creek than for the other streams; this is attributed to lower evapotranspiration losses, which result in a smaller change in groundwater storage than in the less-urbanized watersheds. Low flow of Peachtree Creek ranged from 25 to 35% less than the other streams, possibly the result of decreased infiltration caused by the more efficient routing of stormwater and the paving of groundwater recharge areas. The timing of daily or monthly groundwater-level fluctuations was similar annually in each well, reflecting the seasonal recharge. Although water-level monitoring only began in the 1980s for the two urban wells, water levels displayed a notable decline compared with non-urban wells since then; this is attributed to decreased groundwater recharge in the urban watersheds due to increased imperviousness and related rapid storm runoff.

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The major ion stream chemistry for an urbanized basin (Peachtree Creek) in the Atlanta (Georgia, USA) metropolitan region was analyzed and compared upon a time-series basis with the stream chemistry of a nearby, but far less developed basin (Sweetwater Creek). The major ion chemistry from both streams indicated that all parameters fell below safe drinking water standards as would be expected in watersheds underlain by low-solubility aluminosilicate bedrock. Base flow TDS concentrations for the urbanized basin (Peachtree Creek) were 30% greater than Sweetwater Creek and were characterized by elevated concentrations of base cations (Ca, Mg, Na), alkalinity, sulfate, and chloride. The less-urbanized Sweetwater Creek basin is underlain by a higher percentage of more soluble amphibolite and, therefore, the higher concentration of solutes within the urban base flow can not readily be explained by differential mineral weathering. The increased inorganic solute loads might have resulted from pollution input (leaky underground sewer lines?) and/or evaporative concentration; however, these explanations are presently speculative. Major ion concentrations within the urban Peachtree Creek basin were diluted by an average of 50% compared with only 25% in the Sweetwater Creek basin as a result of the increased contribution of direct surface runoff. The regression correlation coefficients for the ions that evolve through mineral weathering (Ca, Mg, Na, and HCO 3 ) are much higher in urban stream flow than in Sweetwater Creek waters. This might indicate that storm waters in the urban basin are more closely derived from a two end member mixture (groundwater and street runoff) than waters from Sweetwater Creek, which represent a more complex mixture.

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Dissolved oxygen (D.O.) concentration has a significant effect upon ground water quality by regulating the valence state of trace metals and by constraining the bacterial metabolism of dissolved organic species. For these reasons, the measurement of dissolved oxygen concentration should be considered essential in most water quality investigations. D.O. measurements have been frequently neglected in ground water monitoring. This is because O2 has often been assumed absent below the water table; measurement of O2, concentrations is not mandated by drinking water standards; and the redox potential has previously been considered an adequate and encompassing electrochemical measurement. Redox potentials, however, cannot adequately predict dissolved oxygen concentrations nor can D.O. concentrations be used to calculate redox potentials. D.O. concentrations can be measured precisely in the field by titration or electrode methods. The best methods of sample recovery are those that use positive pressure displacement devices. A fully adequate sampling procedure will isolate ground water from the atmosphere and will collect samples at restricted depth intervals at ambient temperature and pressure.