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Scientific Investigations Report 2005–5171

Water Quality and Simulated Effects of Urban Land-Use Change in J.B. Converse Lake Watershed, Mobile County, Alabama, 1990–2003

U.S. Geological Survey Scientific Investigations Report 2005–5171, 124 pages (Published 2005)

By Amy C. Gill, Ann K. McPherson, and Richard S. Moreland

This report is available online in pdf format: USGS SIR 2005–5171 (8 MB)


Cover of SIR 2005–5171.

J.B. Converse Lake, a 3,600-acre manmade reservoir in western Mobile County, Alabama, is used as a source of drinking water for the city of Mobile, and for recreational fishing. The watershed of the lake is predominantly rural. Residential and commercial development is expanding westward from the city of Mobile, however, and the potential for water-quality changes as a result of land-use change is of concern in the J.B. Converse Lake watershed.

Tributary and lake water-quality data were collected during 1998–2003 and used with previously collected (October 1990–June 1998) data to assess water-quality conditions, identify temporal trends, and calibrate the BATHTUB reservoir water-quality model. Selected stream and lake samples were analyzed for concentrations of fecal-indicator bacteria and a group of wastewater-indicator compounds to aid in identifying sites receiving wastewater contamination. The calibrated BATHTUB model was used to predict lake response to changing land use in the watershed.

Median nutrient concentrations and trophic state indicators measured in the tributaries and lake in the study area during 1998–2003 were compared to U.S. Environmental Protection Agency criteria for streams and lakes, respectively, in the Southeastern Plains ecoregion. Tributary median concentrations exceeded the total Kjeldahl nitrogen criterion (300 micrograms per liter) at Jackson Branch, Crooked Creek at Wulff Road, and Hamilton Creek below Semmes; the nitrite plus nitrate criterion (95 micrograms per liter) at all sites except Jackson Branch; the total nitrogen criterion (395 micrograms per liter) at all sites except Collins Creek, Boggy Branch, and Hamilton Creek below Semmes; and the total phosphorus criterion (22.5 micrograms per liter) at Crooked Creek at Wulff Road and Juniper Creek at Coleman Dairy Road. Median nutrient concentrations exceeded the total nitrogen criterion (329 micrograms per liter), the total Kjeldahl criterion (320 micrograms per liter), and the nitrite plus nitrate method detection limit (20 micrograms per liter) at some lake sampling sites. Median total phosphorus concentrations, in contrast, were below the ecoregion criterion of 10 micrograms per liter at all lake sites. Median Secchi depths exceeded the criterion of 2.041 meters at all but one lake site, indicating good water clarity. Median chlorophyll a concentrations exceeded the fluorometric chlorophyll a criterion (5.125 micrograms per liter) at the intake to the water treatment system and at one other site in the lake.

The range of tributary total organic carbon concentrations, 0.4 to 13 milligrams per liter, was similar to the range of concentrations observed at the lake sites, 0.4 to 9.2 milligrams per liter. Tributary concentrations appeared to be greater in the predominantly forested watersheds of Big Creek and Jackson Branch, but no statistically significant correlation was found between forested land-use area and 1998–2003 organic carbon concentrations. Lake organic carbon concentrations and chlorophyll a concentrations were not significantly correlated, even though the occurrence of greater organic carbon concentrations during late summer indicated a potential algal source.

Trend analyses of nutrient concentrations in Big, Crooked, and Hamilton Creeks during 1990–2003 indicated only a few statistically significant changes in nutrient concentrations. Flow adjusted and unadjusted total and dissolved inorganic nitrogen concentrations exhibited significant decreasing long-term trends of about 0.01 milligram per liter per year at Crooked Creek near Fairview.

Annual nutrient loads and yields were calculated for Big, Crooked, and Hamilton Creeks. Mean annual instream nutrient loads were greater in Big Creek than in Crooked or Hamilton Creeks. Nutrient loads for all three creeks generally were influenced by nonpoint sources. Seasonal influences on nutrient loads were less apparent. In contrast, mean annual yields generally were greater for Crooked and Hamilton Creek watersheds than the Big Creek watershed, reflecting the greater prevalence of agricultural and urban land uses in the Crooked and Hamilton Creek watersheds.

Fecal coliform and Escherichia coli test results indicated smaller concentrations of fecal-indicator bacteria in the lake than in the tributaries. Maximum concentrations in the tributaries were well above the single-sample maximum criteria established by the Alabama Department of Environmental Management and U.S. Environmental Protection Agency for public water supplies and full-body contact. Median concentrations of fecal indicator bacteria were below these criteria at all tributary sites. Two lake sites each had elevated concentrations of fecal indicator bacteria in a single sample. Subsequent sampling at the two lake sites did not indicate recurring concentrations above criteria.

Eighty-six samples from throughout the J.B. Converse Lake watershed were analyzed for a group of organic wastewater compounds commonly found in wastewater and urban runoff. Twenty-nine of 87 compounds were detected in at least one sample in the J.B. Converse Lake watershed. Ten of the detected compounds have been identified as potential endocrine disruptors. Organic wastewater compounds were detected in 64 percent of the samples collected in the J.B. Converse Lake watershed; potential endocrine disruptors were detected in 29 percent of the samples. Organic wastewater compound detection frequencies from the J.B. Converse Lake watershed were lower than detection frequencies in the nearby Threemile Creek watershed and in a nationwide reconnaissance study.

The BATHTUB model was calibrated using tributary and lake water-quality data from the 2001–03 growing seasons (April–September). The calibrated model was used to predict changes in lake nutrient concentrations, trophic state, and frequency of algal blooms resulting from increases in urban land use. Based on estimated nutrient loading rates, total phosphorus and total nitrogen loads to the lake would be expected to increase by 156 and 39.1 percent, respectively, if 100 percent of the developable land in the watershed were urbanized. BATHTUB simulations indicated changes in trophic state and increases in algal bloom frequency in response to loading increases of this magnitude. Simulated trophic state indices calculated from phosphorus and chlorophyll a concentrations and from Secchi depths indicated increases in trophic state in shallower lake segments. Simulations indicated that moderate to severe algal blooms (defined by chlorophyll a concentrations greater than or equal to 30 micrograms per liter) can occur in some areas of the lake on as many as 8 days during the growing season.




Purpose and Scope

Description of the Study Area

Previous Investigations


Approach and Methods

Water-Quality Sampling

Data Analysis and Review

Load Estimation and Analysis of Trends

Modeling Approach

Quality-Control Methods and Results

Tributary Water Quality

Nutrient Concentrations

Nutrient Loads and Yields

Organic Carbon Concentrations

Lake Water Quality

Field Properties

Nutrient Concentrations

Organic Carbon Concentrations

Trophic Status

Fecal Indicator Bacteria and Organic Wastewater Compounds

Occurrence and Distribution of Fecal Indicator Bacteria

Occurrence and Distribution of Organic Wastewater Compounds

Organic Wastewater Compounds as Endocrine Disruptors

Comparison of Wastewater-Indicator Groups in the Threemile Creek and Converse Lake Watersheds

Comparison of Organic Wastewater Compounds in the Converse Lake and Threemile Creek Watersheds with Nationwide Results

Relations Between Bacterial and Chemical Wastewater Indicators

Trends in Nutrient and Bacteria Concentrations

Estimation of Increased Nutrient Loading as a Result of Land-Use Change

Simulation of Lake Water Quality and Trophic Status

Changes in Nutrient Concentrations

Changes in Chlorophyll a Concentrations

Changes in Secchi Depths

Changes in Trophic Status

Changes in Algal Bloom Frequency

Summary and Conclusions

Literature Cited



This report is available online in pdf format: USGS SIR 2005–5171 (8 MB)
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Suggested citation:

Gill, Amy C.; McPherson, Ann K.; Moreland, Richard S., Water Quality and Simulated Effects of Urban Land-Use Change in J.B. Converse Lake Watershed, Mobile County, Alabama, 1990–2003, 2005: U.S. Geological Survey Scientific Investigations Report 2005–5171, 124 p.

For more information, contact the Alabama Water Science Center.

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