Ammonium interaction with coastal marine sediments: influence of redox conditions on K*

Ammonium adsorption on sediments is an important process in aquatic environments where often about twice as much exchangeable NH4+ is found adsorbed on sediments as that measured in associated porewaters. Adsorption of NH4+ on fine-grained terrigenous sediments from the Louisiana Shelf west of the Mississippi River delta and from Lower Laguna Madre (LLM), Texas was studied under reducing and oxic conditions. Sediment from both sites yielded K* values of 2.3 and 4.0 ml g dw−1 under oxidizing and reducing conditions, respectively, even though the NH4+ concentration was about 10 times greater in Laguna Madre sediments than sediments from the Louisiana Shelf. K* determined under reducing conditions was a better predictor of exchangeable NH4+ in sediments from the two sites, based on NH4+ porewater concentrations, than K* determined under oxidizing conditions (the mean square error of reducing K* was ∼1/4 that of oxidizing K*). However, when considering potential NH4+ release from sediments if they are resuspended into overlying oxic waters, the oxic adsorption coefficient should be used.