Water-Resources Investigations Report 014225
U.S. Geological Survey Water-Resources Investigations Report 014225, 131 pages (Published 2001)
Prepared in cooperation with the Mobile Area Water and Sewer System
By C.A. Journey and A.C. Gill
This report is available online in pdf format: USGS WRI 014225 (6 MB)
J.B. Converse (Converse) Lake is a 3,600- acre, tributary-storage reservoir in Mobile County, southwestern Alabama. The lake serves as the primary drinking-water supply for the city of Mobile. The Converse Lake watershed lies within the Coastal Plain Physiographic Province. Semiconsolidated to unconsolidated sediments of sand, silt, gravel, and clay underlie the watershed, and are covered by acidic soils. Land use in the watershed is mainly forest (64 percent) and agriculture (31 percent). Residential and commercial development account for only 1 percent of the total land use in the watershed.
Converse Lake receives inflow from seven major tributaries. The greatest inflows are from Big Creek, Crooked Creek, and Hamilton Creek that had mean annual streamflows of 72.2, 19.4, and 25.0 cubic feet per second, respectively, for the period 1990 to 1998, which represents about 72 percent of the total annual streamflow to the lake. The total mean annual inflow to the lake is estimated to be about 163 cubic feet per second.
In general, water quality in Converse Lake and its tributaries meets the criteria established by the Alabama Department of Environmental Management (ADEM) for drinking-water supplies, whole-body contact, and aquatic life. The exceptions include acidic pH levels, iron and manganese levels above secondary or aesthetic criteria, and fecal bacterial levels in some tributaries above whole-body contact (swimmable) criteria. The pH levels throughout the watershed were commonly below the criteria level of 6.0, but this appears to have been a naturally occurring phenomenon caused by poorly buffered soil types, resistant sediments, and forested land use. Median iron and manganese levels were above aesthetic criteria levels of 300 and 50 micrograms per liter, respectively, in some tributaries. All tributary sites in the Converse Lake watershed had median and minimum dissolved-oxygen concentrations above the ADEM criteria level of 5 milligrams per liter except for Boggy Branch, which had a minimum dissolved-oxygen concentration of 3.7 milligrams per liter.
The degree to which nutrient contributions from tributaries were causing nutrient enrichment and eutrophication in Converse Lake was assessed. Trend analysis detected little or no change in nutrient concentrations at the tributary and lake sites in the Converse Lake watershed from the 1991 to 1998 water years. Nutrient concentrations at most tributary sites exhibited a significant, positive relation with streamflow that indicated the dominant source of nutrient input to the watershed is from nonpoint contributions. From 1990 to 1998, computed mean annual loads of 75,400 kilograms of total nitrogen, 36,950 kilograms of total Kjeldahl nitrogen, 28,870 kilograms of total inorganic nitrogen, and 3,480 kilograms of total phosphorus were contributed to the lake by Big Creek, Hamilton Creek, and Crooked Creek combined. These mean annual loads of nutrients corresponded to borderline eutrophic/mesotrophic conditions in the lake. Of the combined loads, 62 percent of the total nitrogen, 70 percent of the total Kjeldahl nitrogen, 54 percent of the total inorganic nitrogen, and 47 percent of the total phosphorus originated from the forested subbasin of Big Creek. The more residential and agricultural subbasins of Crooked Creek and Hamilton Creek, however, yielded over twice the total phosphorus load per hectare of land use. Crooked and Hamilton Creek subbasins also had higher yields of the more bioavailable total inorganic nitrogen. A simplistic empirical model could not explain the relation between year-to-year nutrient contributions to Converse Lake from the tributaries and the lakes ability to assimilate those contributions.
The potential presence of pathogens in the lake and its tributaries was assessed based on fecal bacterial concentrations. Fecal bacterial concentrations at some tributary sites were above existing criteria for swimmable uses. Contributions of fecal bacteria from tributaries to the lake, however, did not appear to affect the lake because fecal bacterial concentrations at lake sites were one to two orders of magnitude lower than tributary sites and well below criteria levels. Juniper Creek had the highest fecal bacterial concentrations during the study. Trend analysis showed that flow-adjusted fecal streptococcus concentrations increased at Juniper Creek during the 8 years of data collection. Giardia cysts and Cryptosporidium oocysts were detected infrequently at sites in the lake and the raw-water intake during the monitoring effort. Most of the detections occurred during the summer months, but no clear relation between season and cyst density or cyst concentrations in the water supply can be established because of the infrequent detections.
Naturally occurring organic carbon compounds derived from the decay of plant material on land were the major source of organic carbon in the Converse Lake watershed. Reactive organic carbon had a greater potential to form trihalomethanes as a result of chlorination during water treatment. In the lake, algae was considered an important source of organic carbon as indicated by strong, positive, statistically significant correlations among chlorophyll a, total organic carbon, and dissolved organic carbon. The algalderived organic carbon, however, was not considered the major source of the reactive organic carbon as indicated by the absence of significant statistical correlations between chlorophyll a, total organic carbon, dissolved organic carbon, ultraviolet absorbance at 254 nanometers, and trihalomethane-formation potential.
Purpose and scope
Major hydrogeologic units
Major soil types
Basic water chemistry
Water-quality analysis methods and approach
Results of water-quality analysis
pH and alkalinity
Nutrient loading and trophic response
Nutrient analysis methods and approach
Results of nutrient analysis
Eight-year trend analysis
Spatial and temporal distribution
Comparison of nutrient loads with trophic state of lake
Microbiological analysis methods and approach
Results of microbiological analysis
Organic carbon characterization
Organic carbon analysis methods and approach
Results of organic carbon analysis
Estimation of reactivity of the organic carbon
Spatial and temporal distribution
Sources of reactive organic carbon
Summary and conclusions
Appendix 1. Summary of descriptive statistics for selected surface-water sites in the J.B. Converse Lake watershed, Mobile County, Alabama, October 1990June 1998
Appendix 2. Monthly mean load tables for selected surface-water sites in the J.B. Converse Lake watershed, Mobile County, Alabama, 199098
Appendix 3. Methods and results for Giardia and Cryptosporidium analyses for October 1996May 1998
This report is available online in pdf format: USGS WRIR 014225 (6 MB)
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Journey, Celeste A.; Gill, Amy C., 2001, Assessment of Water-Quality Conditions in the J.B. Converse Lake Watershed, Mobile County, Alabama, 199098: U.S. Geological Survey Water-Resources Investigations Report 014225, 131 p.
For more information, contact the Alabama Water Science Center.