Lysimeter trials to assess the impact of different flood–dry-cycles on the dynamics of pore water concentrations of As, Cr, Mo and V in a contaminated floodplain soil

Abstract

We hypothesize that the dynamics of water soluble arsenic (As), chromium (Cr), molybdenum (Mo), and vanadium (V) in soils might be controlled by the period of flooding due to changes of redox potential (EH), pH, and carriers of metals such as dissolved organic carbon (DOC), iron (Fe), manganese (Mn) and sulfate (SO42−). Therefore, we aimed to assess the impact of different flood–dry-cycles on the temporal dynamics of pore water concentrations of As, Cr, Mo and V as affected by changes of soil EH/pH and dynamics of DOC, Fe, Mn and SO42− in a contaminated floodplain soil collected at the Elbe River (Germany). For this purpose a specific groundwater lysimeter technique with two separate lysimeters which served as replicates was used. The groundwater level inside the lysimeters was controlled to simulate different flood–dry-cycles sequentially as follows: the long term (LT) includes 94days of flooding followed by similar drying term. The short term (ST) comprises 21days flooding followed by a similar drying term and was repeated six times. The entire experimental period (LT_ST) was about 450days. The presented data are mean values of both lysimeters.Flooding of the soil caused a significant decrease of EH and pH. Concentrations of soluble As, Cr, Fe, Mn, Mo, and DOC were higher under reducing conditions than under oxidizing conditions in LT. However, As and Cr tended to be mobilized under oxidizing conditions during ST, which might be due to slow kinetics of the redox reaction of As and Cr. Dynamics of Mo were more affected by changes of EH/pH as compared to As, Cr, and V and governed mainly by Fe–Mn chemistry. Concentrations of V in ST were higher than in LT and were controlled particularly by pH and chemistry of Fe. The interactions between the elements and carriers studied were stronger during long flood–dry-cycles than during short cycles, which confirmed our hypothesis. We conclude that the dynamics of As, Cr, Mo, and V are determined by the length of time soils are exposed to flooding, because drivers of element mobility need a certain time to provoke reactions in soils under changing conditions.