Dissolved Constituent Concentrations at 21 Stream-Water Monitoring Sites in the City of Atlanta from 2003 to 2006

Abstract

During the summer of 2003, the U.S. Geological Survey, in cooperation with the City of Atlanta, began routine manual stream-water sampling of 21 sites with drainage areas ranging from 3.7 to 232 square kilometers (km2). During approximately 12 manual sampling visits per year, concurrent equal width increment (EWI) and grab or point samples are collected to evaluate the homogeneity of the stream-water chemistry in the cross section. In addition, real-time water-quality and discharge monitoring at 11 sites is augmented by automatic samplers for collection of samples during storms. For the routine samples, 2,441 have been collected through June 2006; the samples were analyzed for a broad suite of dissolved and sediment-associated constituents. This paper summarizes an evaluation of inorganic properties including specific conductance (KSC, a general measure of the amount of dissolved solutes in the stream water) dissolved oxygen, pH and turbidity, and concentrations of major dissolved ions, nutrients (nitrogen and phosphorus), and bacteria among sites and with respect to watershed characteristics. The concentrations of all major dissolved constituents and nutrients were the same in EWI and grab samples for each site indicating that the streams are well-mixed. However, the concentrations are statistically different among sites for several constituents, despite high variability both within and among sites. Mean KSC varied with respect to the percentage of commercial and industrial land use. The highest mean KSC were in two streams with drainages having the highest percentage of industrial and commercial land use; the lowest mean KSC were in streams draining high percentages of residential-plus-forested areas. Although the maximum nitrate-nitrogen concentration (3 milligrams per liter—mg l–1) was much less than the public health standard for potable water (10 mg l–1), the average concentration at two sites was greater than 1 mg l–1, which was significantly higher than any other sites. The drainage area for one site contains the highest percentage of high-density residential area and golf course area. Most of the sample concentrations were below reporting limits for dissolved total phosphorus (51 percent) and phosphate (83 percent). Fecal coliform bacteria concentrations of several individual samples at each site exceeded Georgia’s waterquality standard for any water-use class including public water supply, recreation, or fishing. The bacteria concentrations at most sites were statistically indistinguishable due to the large within-site concentration variability. Statistically significant differences for some properties and constituents were identified among sites.