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Bacteria Water Quality in Dickinson Bayou

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In 1996, Dickinson Bayou was included on a list of impaired water bodies in the state of Texas, because fecal indicator bacteria levels were observed to exceed the established criteria to assure safe contact recreation. This impairment was extended in 2002 to include four major tributaries of Dickinson Bayou: Bensons Bayou, Giesler Bayou, Bordens Bayou, and Gum Bayou. These impairments resulted in the development and coordination of watershed protection planning as well as total maximum daily load (TMDL) development for fecal bacteria.

Dickinson Bayou is a small coastal prairie tidal stream that is primarily salt water at its mouth with freshwater inputs from the upper watershed. The lower tidal portion of the waterbody is used for commercial shrimping, recreational boating, fishing, canoeing and water skiing. The upper portion of the bayou is much more narrow, shallow, and is heavily forested along the banks. Dickinson Bayou watershed currently has a large amount of undeveloped land; however, it is undergoing rapid development like many coastal areas. The dominant land use currently is grassland, covering 46 percent of the area with high and low intensity developed land uses accounting for only 15 percent of the total watershed area. These developed land uses are increasing as the watershed undergoes transition from agricultural and rural to suburban and urban land uses.

Historical data show that fecal contamination of Dickinson Bayou is extensive, with many historical water quality monitoring stations having geometric means significantly above the contact recreation criterion. The Texas Commission on Environmental Quality (TCEQ) has two different bacteria standards which apply specifically to fresh and salt water. Standards for E. coli, which is an indicator used for fresh water, are 126 MPN/dL for the geometric mean and 394 cfu/dL for a single sample. Standards for enterococci, which is an indicator used for salt and brackish waters is 35 MPN/dL for the geometric mean and 89 MPN/dL for the single sample. Exceedance of the single sample standard for E. coli has ranged from 12 percent to over 69 percent across the bayou while exceedances of the enterococci single sample standard has ranged from 10 percent to over 92percent. Geometric means for E. coli range from 45 MPN/dL to 711 MPN/dL (standard is 394 MPN/dL), while enterococci oncentrations ranged from 12 MPN/dL to more than 321 MPN/dL.

This paper will present a review of historical data collected in the bayou and an analysis of the potential relationships between bacteria and other water quality parameters. While the causes for these high bacteria levels are currently not well understood, results of the analysis of historical water quality data does provide some insight. Relationships between bacteria levels and total suspended solids, salinity, and rainfall were observed as were relationships with temperature. There also appears to be a spatial trend in bacteria concentrations, with the highest levels being found in the upper watershed and decreasing concentrations towards the watershed terminus.

These results were used to guide the preparation of a sampling plan to further examine sources of bacteria in the watershed. This sampling was conducted during the Summer and Fall of 2008 and examined contributions from wastewater treatment plant, tributary, and other sources in the watershed as well as the impact of runoff on bayou bacteria levels.
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Keywords: TMDL; bacteria; fecal indicator bacteria; tidal stream

Document Type: Research Article

Publication date: 2009-01-01

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