INITIAL EXAMINATION OF CHEMICAL VARIABILITY WITHIN ATCHAFALAYA BASIN DURING SPRING-SUMMER FLOOD OF 2011

The water chemistry in the Atchafalaya Basin (located in south-central Louisiana) is a dynamic variable system. Water quality varies throughout the basin depending on location, distance south from the Morganza spillway, position relative to the basin’s axis, depth from water surface, and time after the start of the flood event starting May 24^th, 2011 when gates were opened on the Morganza spillway.

Between May 19^th and July 21^st several agencies (Louisiana Department of Wildlife and Fisheries, US Geological Survey, and US Fish and Wildlife Service) collected field chemistry measurements (pH, turbidity, dissolved oxygen (DO), Cl^-, temperature, and electrical conductivity (EC)) at approximately 45 sites throughout the basin (which is south of 32^o 23’ and is 20 km by 80 km, long axis trending at approximately 145^o) and collected water samples for later laboratory analysis. Field measurements and sample collection for later laboratory analysis occurred weekly between May 19^th and June 23^rd and biweekly on July 7^th and July 21^st. Later laboratory analysis was completed by several agencies/individuals. The staff of the Louisiana Geological Survey determined concentrations of Br^-, Fl^-, Cl^-, NO₃^-2, PO₄^-3, and SO^4-2 by ion chromatographic analysis; phosphate by colorimeter; and total suspended solids (TSS) by gravimetric analysis.

Generally DO, Fl^-, TSS and Turbidity decrease over time, and Cl^-, EC, SO₄^-2, and temperature increases over time. For nutrients of -, NO₃^-2 and PO₄^-3 the average concentration appears to be fairly steady during the flood, but the variability depending on location generally increases over time. Some of the variables measured vary depending on location as well as time. Depth of field measurement appears not to influence concentrations of dissolved species such as DO, Cl^-, EC, pH, but influences turbidity which increases significantly with depth. Lateral position also influences concentration of some parameters, DO increases from east to west across the basin; turbidity and TSS appear to have higher concentrations near basin’s central axis and decrease towards the edges of the basin; and nutrient concentrations decrease southward and from east to west across the basin.