Dissolved Oxygen Impairment and BMP Effectiveness in Mermentau River Basin

The Mermentau River Basin is located in the southwestern part of Louisiana and encompasses a drainage area of approximately 12,396 km², consisting of urban area (2%), cropland (52%), forest (23%), wetland (10%), and water (13%). Water quality in the river basin has experienced significant impairments since 1980. Summer (June – August) average dissolved oxygen (DO) concentration has dropped below 5 mg/L generally in the river basin and even below 2 mg/L in some watersheds. As a result, 95% of the 20 waterbodies within the basin are not fully supporting their designated uses. Since 1991, a variety of Best Management Practices (BMPs) has been implemented in the Mermentau River Basin. However, the effectiveness of the BMPs in improving water quality and particularly in reducing DO impairment is largely unknown due to the spatio-temporal variability in hydrological and hydrometeorological parameters controlling DO impairment. This paper presents statistical analysis results on DO impairment and the effectiveness of BMPs in reducing DO impairment using water quality data monitored from 1980 – 2006 in the Mermentau River Basin. It is found that the dissolved oxygen concentration in the Mermentau River Basin varies both spatially and temporarily. In terms of temporal variation, the DO shows a clear seasonal variation characterized by a low DO concentration in summer (June – August) and a high DO concentration in winter (December – February). The summer average DO concentration displayed an overall decreasing trend over the 27 year period of 1980 – 2006 while water temperature exhibited a clear rising trend and increased by about 3°C over the same period. Some oxygen-consuming contaminants like the total organic carbon (TOC) have also shown a significant increasing trend since 1980. A strong correlation between decreasing DO level and increasing water temperature and TOC is found. In terms of spatial variation, DO concentration varies significantly from one watershed to another. It appears that DO concentration in the Bayou Queue de Tortue watershed is always the lowest and the DO concentration in the Bayou Nezpique watershed is always the highest in the river basin. The implementation of BMPs since 1990 has resulted in a noticeable improvement in water quality in terms of reducing DO impairment but it didn't change the overall decreasing trend in DO concentration in the river basin. BMPs need to be enhanced in the basin to meet water quality standards. The findings provide new insights into the overall effectiveness of BMPs and the restoration of coastal watersheds, especially coastal Louisiana watersheds.