Cotton Bayou and West Fork Double Bayou Special Study

In 2006, H-GAC teamed up with the USGS, the Trinity River Authority (TRA) and the TCEQ to conduct a systematic monitoring study to assess Cotton Bayou and West Fork Double Bayou. TCEQ had listed both coastal bayous as being impaired for dissolved oxygen. The study showed that while both bayous tested as impaired for dissolved oxygen, the impairments were not as severe as reported when data was collected at different times and from different areas.

The City of Mont Belvieu was just beginning the renewal process for their wastewater permit in 2005 and questioned the validity of TCEQ's listing of Cotton Bayou for impaired DO. Knowing any impairment would affect the reporting limits applied to their new permit, the City's objections were two-fold. First, the assessment was based on only one set of samples and, second, those measurements were collected from the tidal portion of the bayou only. The impaired status, however, was applied to all of Cotton Bayou even though the freshwater portion where the City discharged had not been evaluated. The City requested H-GAC conduct an assessment of the entire bayou to determine the current environmental status of the water body and provide a baseline to aid in recognition of emerging problems and water-quality trends. Anticipating the need, a similar project was performed concurrently for West Fork Double Bayou. USGS gathered data describing the physical, chemical and biological characteristics to determine if a dissolved oxygen problem exists and how often it occurs, and to collect enough data to determine the aquatic life use in the freshwater portion of both water bodies.

The Cotton Bayou watershed is located east of Houston, with a drainage area of 16.5 square miles. With headwaters located north of Mont Belvieu, Cotton Bayou extends 6.6 miles to the southeast and drains into Cotton Lake, which empties into Trinity Bay (Segment 2422). The watershed is largely non-urbanized. Inputs to Cotton Bayou include runoff from unused land, agricultural fields, small municipalities, and municipal wastewater treatment plants. The principal tributary to Cotton Bayou is Hackberry Gully and has similar land-cover and inputs.

The West Fork Double Bayou watershed is located east of Houston near the City of Anahuac and has a drainage area of 35.1 square miles. With headwaters that extend to the Anahuac oil field east of the City of Anahuac, West Fork Double Bayou flows southwest approximately 14 miles to Trinity Bay. The watershed is largely non-urbanized, and like the Cotton Bayou watershed, contains unused land, agricultural fields, small municipalities, and municipal wastewater treatment plants.

H-GAC collected 24-hour DO information over two years to calculate the mean concentration and record the minimum DO measurement. Cotton Bayou above tidal fully supported the default high aquatic life use designation because all five measurements for both the 24-hour minimum and mean DO measurements were above the standards. However, Hackberry Gully, the primary tributary, did not appear to support the default high aquatic life use. During the critical period, both the 24-hour mean and minimum DO measurements were below 4.0 and 3.0 mg/L respectively. Since this is the only data ever collected on Hackberry Gully, data does not show whether the 7Q2 flow requirement was met during the critical period. Additionally, the minimum DO during one of the index period measurements was significantly below the 3.0 mg/L, but all 24-hr mean DO results from the index period events were above the 24-hr mean.

Double Bayou West Fork, an unclassified water body (segment 2422B), is non supporting of both the 24-hr average and the 24-hr minimum.

The habitat quality indices for representative reaches at each of the four monitoring locations were mostly intermediate with a few scores barely breaking into the high category. Aquatic life use scores, as calculated by the TCEQ, were generally 'high' at three of the four study reaches but fell to 'intermediate' during the summer months when the lower DO measurements were documented. The index of biotic integrity calculated for the catches from each reach indicated either 'high' or 'intermediate' aquatic life uses for all the reaches.

See the full study here.  (PDF 2.65MB)

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