Simulated upland placement of estuarine dredged materials

Abstract

Placement of estuarine dredged materials on the land surface can result in severe environmental issues if acid sulfate conditions develop. In this study, upland placement of marine dredged material was simulated using a mesocosm experiment. Dredged materials were sampled from eight low energy and high energy subaqueous soil-landscape units of four estuaries in Rhode Island. Soils were mapped in the low energy landscape units (i.e. coves and bottoms) as Sulfiwassents and in the high energy units (i.e. deltas and fans) as Psammowassents. Dredged materials were characterized for various physical and chemical properties, placed into mesocosms, and exposed to natural climatic conditions outside. In addition, high energy and low energy materials were mixed at ratios of 20:1, 10:1, 5:1, and 2.5:1 and placed in mesocosms. Mesocosm leachate was collected and analyzed for pH, conductivity, and sulfate content for over 2years (one year for the mixed mesocosms). Dredged materials ranged from sand to silt loam textures with the Sulfiwassents having finer textures and higher levels of carbon and calcium carbonates. Inorganic sulfide concentrations ranged from 56 to 3410μgg−1, with an average of 228μgg−1 for the Psammowassents and 2319μgg−1 for Sulfiwassents. Concentrations of Zn in the Sulfiwassent materials decreased by more than half after 2-years suggesting leaching of Zn during oxidation of the dredged materials. Leachate from low energy Sulfiwassents showed a large drop in pH (pH≤4.0) associated with sulfide oxidation and creation of acid sulfate conditions, while leachate from coarser textured Psammowassents increased in pH to >8.0, presumably from the formation of salts. These conditions generally persisted for the duration of the experiment. Salts washed out of the dredged material fairly quickly such that leachate conductivities reached <5dSm−1 in 10months. Sulfate content of the leachate varied for the first 18months with the Sulfiwassent materials having higher levels and taking longer to wash out. Leachate from mixing as little as 5% Sulfiwassent with Psammowassent materials resulted in acid sulfate leachate (pH<4) in less than a year. Our results suggest that developing soil interpretations for upland placement of dredged materials of estuarine subaqueous soils should be focused on limitations as the result of creation of acid sulfate conditions and leaching of metals for Sulfiwassents and potentially high pH values for Psammowassents.