Spatial and Temporal Distribution of Dredged Sediments Disposed at the Pensacola, Florida, Ocean Dredged Material Disposal Site Using Naturally Occurring Gamma Radiation

Abstract

The fate and effects of large volumes of dredged sediments disposed at offshore disposal sites are of particular interest to the federal and state agencies responsible for its management. During the period from Jan. to April 1990, 3. 7 million m3 of dredged sediments were disposed of at the Pensacola, FL, Ocean Dredged Material Disposal Site (ODMDS). From May 1990 to Oct. 1993, the dispersal of dredged sediments in the area surrounding the disposal site was studied using bathymetry, sediment particle size distribution, and the differential abundance of naturally occurring gamma radiation emitters in dredged sediments and native seafloor sediments. Gamma radiation measurements in the 0.4- to 3.0-MeV energy range, made in May 1990, showed that disposal activities at the Pensacola site resulted in the accumulation of fine (silts and clays) dredged sediments throughout the 91-km2 survey area. The average proportion of fine-grained sediment particles (silts and clays) in surface sediments outside the boundaries of the designated disposal site more than doubled after disposal. By the end of the 3-yr study period in 1993, the bulk of the fine-grained dredged sediment had been transported away from the area surrounding the ODMDS. Gamma radiation measurements were able to detect the presence of dredged sediments in the disposal zone and in several areas outside the disposal site boundaries.