Thibodaux, LA

Monitoring 2000-2001

     Until 1992, the City of Thibodaux, Louisiana operated a sewage treatment plant with a capacity to treat 4 million gallons per day average daily flow and 6 million gallons per day peak flow. The treatment system consisted of an aerated lagoon and a high rate trickling filter that produced an effluent with average characteristics of 15-20 mg/l BOD, 10-18 mg/l TSS, and fecal coliform less than one MPN through the use of chlorine disinfection.

     The plant discharged the secondarily treated effluent into the Terrebonne Lafourche drainage canal, a water quality-limited stream segment. As part of a systematic program to upgrade the treatment system, The City constructed a distribution system to disperse the effluent over an area of the Pointe au Chene Swamp west of the city. As part of this effort, an ecological study of the forested wetland was initiated in 1989 to determine the efficiency of the wetland in improving water quality and the impact of the effluent on the area. The two-year study consisted of analyses of hydrology, water quality, soils, vegetation, and aquatic fauna of the area.

     Treated effluent has been discharged into the wetland since 1992. The first four years of monitoring was completed in 1996 and a report was prepared by LSU in 1998. A LPDES permit for the second phase of monitoring was granted April 15, 1999 by LDEQ. Comite Resources, Inc. contracted with the City to carry out monitoring in May 2000. The first year of monitoring occurred during a period of severe drought in South Louisiana and this is reflected in the results. The control site was often dry and the treatment site had lower water levels. Results of the first year of study after initiation of effluent discharge indicate that the effluent is being assimilated in the wetland system.

Reductions of N0₃, N0₄, and PO₄

     Phosphorus concentrations were somewhat elevated in the treatment site. Hydrological and climatic showed that 2000-2001 was generally a very dry period, and the control area remained dry from much of the year. The treatment area remained flooded due its lower elevation and the input of the effluent but water levels were lower.

     The plant discharged the secondarily treated effluent into the Terrebonne Lafourche drainage canal, a water quality-limited stream segment. As part of a systematic program to upgrade the treatment system, The City constructed a distribution system to disperse the effluent over an area of the Pointe au Chene Swamp west of the city. As part of this effort, an ecological study of the forested wetland was initiated in 1989 to determine the efficiency of the wetland in improving water quality and the impact of the effluent on the area. The two-year study consisted of analyses of hydrology, water quality, soils, vegetation, and aquatic fauna of the area.

     Treated effluent has been discharged into the wetland since 1992. The first four years of monitoring was completed in 1996 and a report was prepared by LSU in 1998. A LPDES permit for the second phase of monitoring was granted April 15, 1999 by LDEQ. Comite Resources, Inc. contracted with the City to carry out monitoring in May 2000. The first year of monitoring occurred during a period of severe drought in South Louisiana and this is reflected in the results. The control site was often dry and the treatment site had lower water levels. Results of the first year of study after initiation of effluent discharge indicate that the effluent is being assimilated in the wetland system. Precipitation, evapotranspiration, and regional water levels exert a much greater influence on water levels at the site than the effluent.

Litterfall during 2000-2001 was lower in both the control and treatment site when compared to the baseline study.

     This is a continuing result of Hurricane Andrew, which passed through the area in 1992 and blew down a number of trees. In addition, nutria killed a number of trees in both areas. The forest structure is similar to that measured during 1994 and 1995. The wetland system continues to assimilate nutrients without any detrimental impacts due to the wastewater input.

Treated Effluent Discharged from 40 Pipes

     Treated effluent is discharged from 40 pipes located on a 610 m spoil bank that serves as the northern boundary of the site. The effluent flows down a concrete apron 10 m in length, through an open area 40 m wide (Pond) into the forested swamp. It then flows southward, between the ridge on the east and an oil access road on the west and exits at a point where these two features nearly meet. Water exits the confined wetland into a larger swamp before it finally empties into the Terrebonne-Lafourche Drainage Canal. The combination of ridge, spoil bank and access road, hydrologically isolates the treatment swamp from the rest of the 1425 ha basin.

     Litterfall is collected monthly in each plot using five 0.25 m2 boxes with 1 mm mesh bottoms. The boxes were placed randomly in each plot as part of earlier studies. The boxes are elevated to prevent inundation during high water periods. The litter is separated into leaves and woody material, dried to a constant weight, and weighed.