Abstract
Industrial effluent from the Norco Manufacturing Complex (NMC), a major refinery located 22 mi west of New Orleans, was diverted from Bayou Trepagnier to the Mississippi River in February 1995. The possibility that effluent diversion might liberate contaminants from bottom sediments and generally degrade water quality generated concern among environmentalists and regulatory agencies. Hydrologic studies completed to address these concerns indicate that the effluent was generally a minor component of the water budget, comprising about 10 percent of tidal flow into the drainage basin. However, during intermittent, low-water conditions when the bayou was confined to its banks, effluent volume on average probably exceeded tidal flow. Although the effluent generally constituted only a small proportion of total discharge, it often had a profound effect on water quality because low flow velocities and shallow depths limited mixing of water masses. Comparison of water quality data before and after diversion shows that median temperature, salinity, and pH have decreased at stations in the headwaters near the NMC outfall, whereas the dissolved oxygen content of the water column has increased or remained the same. Monthly monitoring data also indicate that, in general, Pb remains bound in bottom sediments despite significant changes in water chemistry after diversion. Hardness, total dissolved solids, and chlorinity have increased along the bayou, reflecting a greater influence of Lake Pontchartrain on water chemistry after diversion. Overall, water quality has improved in Bayou Trepagnier, which should benefit the biota, particularly in the headwaters where harsh conditions existed prior to diversion.
