Watershed Scale Variation in Water Quality in Lake Pontchartrain Basin

Abstract

The Lake Pontchartrain Basin is located in Southeast Louisiana and it covers a drainage area of 12473.72 km 2 . The basin is the most rapidly developing area in Louisiana and it has experienced significant changes in land use, hydrological and fluvial processes, and water quality. In this study thirty years of water quality data from 1975 – 2005 were collected and analyzed using a statistical approach. The water quality data include nutrients, solids and dissolved oxygen. To understand spatial variation in water quality, the data were split into upstream and downstream sets for each river. To understand temporal variation in water quality, the data were split into two groups corresponding to the means of the two periods of 1975 – 1990 and 1990 – 2005 for each river. Results of the statistical analysis show that the dissolved oxygen is normally distributed. Concentrations of nutrients and suspended solids vary in a wide range and follow a 3-parameter Log-normal distribution, which can be attributed to parental Log-normal distribution of precipitation. At the downstream site of Amite River the dissolved oxygen level dropped from 6.76 mg/L in 1980 to 5.73 mg/L in 2000. Comparatively higher mean dissolved oxygen was 7.53 mg/L at the downstream site of Tangipahoa River. Overall, the dissolved oxygen showed a decreasing trend in all watersheds in the Basin. The concentrations of total suspended solids were higher at downstream but no significant temporal change was observed. The Amite River carried higher sediment concentration (450 mg/L) than the Tickfaw River as the Amite watershed is subjected to sand and gravel mining. The highest nitrite-nitrogen concentrations were observed in the Tangipahoa and Tickfaw watersheds (higher than 0.75 mg/L). The total phosphorus concentrations were as high as 0.7 mg/L at the upstream station and 1 mg/L at the downstream station. The highest total organic carbon values (< 20 mg/L) were observed in Tickfaw, Tangipahoa and Tchefuncte watersheds. The watershed-scale analysis of water quality variation is important to the restoration of the Lake Pontchartrain Basin.