Watershed-estuary dynamics in the mobile river watershed-mobile bay estuary (mr-mb) continuum examined by combined geochemical and satellite approaches

Abstract

The Mobile River System acts as a vital economic, social, and cultural center for Alabama and the northern Gulf of Mexico region. With increasing levels of urbanization and land-use and development, the ecological integrity of this system becomes more vulnerable to imbalance due to pollution, anthropogenic alteration of stream course, and other land-use activities. This study focuses on tracking suspended sediment material transported by the Mobile River System, from upstream origin of erosion through the Mobile Estuary, and into Mobile Bay, by combining established geochemical and remote sensing methods and approaches. To accomplish this, I posed three research questions, (Q1) where in the upper reaches of the system are the suspended particulates originating? (Q2) what is the magnitude of flux for suspended sediment (SS) and associated major and trace metals from the river system? and (Q3) can the distribution of the suspended sediment material be effectively tracked within the Mobile Bay basin? The origin of sediment, i.e. identifying sources from the two major tributaries of the Mobile River, the Alabama and the Tombigbee Rivers (Q1) was determined using geochemical fingerprinting of radioisotopes and trace metals. The concatenation of fingerprinting properties of each tributary and of downstream suspended sediments in a geochemical mixing model resulted in 61% of suspended sediment material originating from the Tombigbee Basin, and 39% from the Alabama Basin. The flux of material out of the river system into Mobile Bay (Q2) was determined through compositional analysis of suspended sediment material collected by passive suspended sediment capture within the Mobile-Tensaw Delta. Using this sampling approach, SS flux entering the Delta varied between 981 g/s during low flow regime and 23,509 g/s during high flow. Associated trace metal fluxes were below EPA regulated limits. (Q3) Calibration of existing remote sensing algorithms with in-situ data from Mobile Bay proved successful in generating remote sensing algorithms which can track sediment movement in Mobile Bay across seasonal and hydrologic conditions.