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Assessment of Water Quality, Benthic Invertebrates, and Periphyton in the Threemile Creek Basin, Mobile, Alabama, 1999–2003

U.S. Geological Survey, Scientific Investigations Report 2004–5302
by Ann K. McPherson, Amy C. Gill, and Richard S. Moreland

This report is available as a pdf below


The U.S. Geological Survey conducted a 4-year investigation of water quality and aquatic-community structure in Threemile Creek, an urban stream that drains residential areas in Mobile, Alabama. Water-quality samples were collected between March 2000 and September 2003 at four sites on Threemile Creek, and between March 2000 and October 2001 at two tributary sites that drain heavily urbanized areas in the watershed. Stream samples were analyzed for major ions, nutrients, fecal-indicator bacteria, and selected organic wastewater compounds. Continuous measurements of dissolved-oxygen concentrations, water temperature, specific conductance, and turbidity were recorded at three sites on Threemile Creek during 1999–2003. Aquatic-community structure was evaluated by conducting one survey of the benthic invertebrate community and multiple surveys of the algal community (periphyton). Benthic invertebrate samples were collected in July 2000 at four sites on Threemile Creek; periphyton samples were collected at four sites on Threemile Creek and the two tributary sites during 2000 –2003. The occurrence and distribution of chemical constituents in the water column provided an initial assessment of water quality in the streams; the structure of the benthic invertebrate and algal communities provided an indication of the cumulative effects of water quality on the aquatic biota. Information contained in this report can be used by planners and resource managers in the evaluation of proposed total maximum daily loads and other restoration efforts that may be implemented on Threemile Creek.

The three most upstream sites on Threemile Creek had similar water chemistry, characterized by a strong calcium-bicarbonate component; the most downstream site on Threemile Creek was affected by tidal fluctuations and mixing from Mobile Bay and had a strong sodium-chloride component. The water chemistry at the tributary site on Center Street was characterized by a strong sodium-chloride component; the water chemistry at the second tributary site, Toulmins Spring Branch, was characterized by a strong calcium component without a dominant anionic species. The ratios of sodium to chloride at the tributary at Center Street were higher than typical values for seawater, indicating that sources other than seawater (such as leaking or overflowing sewer systems or industrial discharge) likely are contributors to the increased levels of sodium and chloride. Concentrations of fluoride and boron also were elevated at this site, indicating possible anthropogenic sources.

Dissolved-oxygen concentrations were not always within levels established by the Alabama Department of Environmental Management; continuous monitors recorded dissolved-oxygen concentrations that were repeatedly less than the minimum criterion (3.0 milligrams per liter) at the most downstream site on Threemile Creek. Water temperature exceeded the recommended criterion (32.2 degrees Celsius) at five of six sites in the Threemile Creek basin. The pH values were within established criteria (6.0 – 8.5) at sites on Threemile Creek; however, pH values ranged from 7.2 to 10.0 at the tributary at Center Street and from 6.6 to 9.9 at Toulmins Spring Branch.

Nutrient concentrations in the Threemile Creek basin reflect the influences of both land use and the complex hydrologic systems in the lower part of the basin. Nitrite-plus-nitrate concentrations exceeded U.S. Environmental Protection Agency ecoregion nutrient criteria in 88 percent of the samples. In 45 percent of the samples, total phosphorus concentrations exceeded the U.S. Environmental Protection Agency goal of 0.1 milligram per liter for preventing nuisance aquatic growth. Ratios of nitrogen to phosphorus indicate that both nutrients have limiting effects.

Median concentrations of enterococci and fecal coliform bacteria were highest at the two tributary sites and lowest at the most upstream site on Threemile Creek. In general, concentrations of bacteria increased in a downstream direction on Threemile Creek. Enterococci concentrations exceeded the U.S. Environmental Protection Agency criterion of 151 colonies per 100 milliliters of water in 52 percent of the samples; Escherichia coli concentrations exceeded the U.S. Environmental Protection Agency criterion (576 colonies per 100 milliliters) in 41 percent of the samples; and fecal coliform concentrations exceeded the Alabama Department of Environmental Management criterion (4,000 colonies per 100 milliliters) in 16 percent of the samples. Concentrations of bacteria at two sites on Threemile Creek were elevated during high flow rather than low flow, indicating nonpoint sources; concentrations of bacteria at the other four sites were elevated during low and high flow, indicating both point and nonpoint sources.

Stream samples were analyzed for 48 chemical compounds that commonly are found in wastewater and urban runoff, which may indicate contamination from a human source—37 of these compounds were detected in at least one sample. Twelve of the detected compounds are known or suspected to be hormonally active, with the potential to disrupt endocrine function. Organic wastewater compounds were detected in each of the 63 samples collected from the Threemile Creek basin. Of the 48 compounds analyzed, 9 wastewater-indicator compounds were detected frequently (50-percent or greater detection frequency). The most frequently detected compounds were atrazine (herbicide), caffeine (stimulant), 2-butoxyethanol phosphate (household cleaning agent), cholesterol (plant and animal steroid), diazinon (insecticide), bromacil (herbicide), triclosan (antimicrobial disinfectant), fluoranthene (polycyclic aromatic hydrocarbon), and N,Ndiethyl- m-toluamide (insect repellent commonly known as DEET). Steroids, detergent metabolites, plasticizer/flame retardants, and prescription/nonprescription drugs contributed almost 87 percent of the total measured concentration in all samples.

Benthic invertebrate communities at all but one site on Threemile Creek appeared to be impaired based on taxa richness, numbers of Ephemeroptera/Plecoptera/Trichoptera taxa, and measures of community diversity. When compared to the North Carolina biotic index values, however, average national pollution-tolerance values indicate very good water-quality condition at the most upstream site and good to fair conditions at all other sampling sites.

Algal data indicate that organic and nutrient enrichment contributed to occasional nuisance algal growth in the Threemile Creek basin. Benthic chlorophyll a occasionally was found at concentrations associated with nuisance algal growth. Ash-free dry mass, in contrast, did not indicate nuisance algal growth at any of the sites, but levels neared the nuisance level of 50 grams per square meter at Threemile Creek near Pine Grove during August 2003. Taxa richness indicated more diverse algal communities at the main-stem sites than at the tributary sites, perhaps because of the difference between concrete and tile substrates or the colonization period. Nitrogen-fixing algae accounted for less than 10 percent of the algal biovolume from artificial substrate samples at the mainstem sites, indicating that nitrogen was not a limiting nutrient. Nitrogen fixing Amphithrix janthina accounted for 40 to 45 percent of the algal community data at the tributary at Center Street, although water chemistry data indicate that both nitrogen and phosphorus were plentiful at the site.

Saprobic classifications of periphyton indicated that tributary algal communities are subjected to more organic enrichment than the main-stem sites. During September 2000, β-saprobic and α-mesosaprobic diatoms were more common than oligosaprobic diatoms at all sites, indicating the occurrence of organic enrichment. At the tributary sites, however, oligosaprobic diatoms were not present, indicating lower dissolved-oxygen concentrations and higher biochemical oxygen demand, in contrast to measured water chemistry at the two sites. The discrepancy between algal and water chemistry data may be due to the diel cycle of dissolved-oxygen concentrations induced by actively photosynthesizing organisms.

The results of this investigation provide a detailed survey of water-quality conditions in Threemile Creek for the 4-year period during October 1999 to September 2003. The water quality and aquatic-community structure in Threemile Creek are degraded, with degradation increasing in a downstream direction and most intense at the tributaries draining into Threemile Creek. The degree of degradation may be related to point and nonpoint sources of contamination originating within the basins. The results also have long-range watershed-management implications, demonstrating the association between urban development and stream degradation.

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