Reactive transport and long-term redox evolution at the catchment scale

Abstract

Diffusive groundwater pollution caused by agricultural and atmospheric inputs is a pressing issue in environmental management worldwide. Various researchers have studied nitrate contamination since the substantial increase of nitrogen fertilization in agriculture starting in the second half of the 20th century. This study addresses large scale reactive solute transport in typical landscapes and aquifers exemplified by geological analogues of southwestern Germany. Firstly, fate of nitrate and other solutes was studied in a typical hilly landscape in a transect crossing two river valleys (Ammer and Neckar rivers) in Southern Germany. The numerical model compromises a 2-D cross-section accounting for geology, water-rock interaction, intra-aqueous reactions, and groundwater hydraulics. Results show that the groundwater divide significantly deviates from the surface water divide promoting inter-valley groundwater flow. Reactive transport modelling of redox-sensitive solutes (e.g. agricultural nitrate, natural sulfate and dissolved organic carbon) with MIN3P indicates that in the floodplains of both rivers, organic rich Holocene sediments allow reduction of agricultural nitrate. However, nitrate applied in the hillslopes of Ammer valley underlain by a fractured oxidized Triassic mudrock is transported towards the high yield sand and gravel aquifer in the neighboring Neckar valley. Therefore, nitrate in the Neckar valley groundwater may come to a large extent from the hillslopes of the neighboring valley and not from the agriculture sources in the valley itself. The study demonstrates cross catchment transport of groundwater pollutants, which occurs if water levels between adjacent valleys differ significantly. The more detailed reactive transport model of Ammer river floodplain shows that agriculture nitrate is reduced rapidly in the Ammer floodplain sediments. However, there is a potential for geogenic production of ammonium in sediment layers high in organic carbon and peat, which might be a major source of nitrate in the drains. Part of the nitrate in drains and creeks in the Ammer valley thus could be of geogenic origin. Such findings are relevant for regional land and water quality management. Secondly, a reactive transport model was developed for a fractured pyrite bearing limestone aquifer. The flow was assumed to pass through a connected system of fractures and karstified features providing the continuous exposure of water parcels to the limestone surface with subsequent reactive diffusive transport of solutes in the rock matrix. A series of scenarios was developed to understand the pathways of pollutant turnover (especially nitrate) if the activity of biota is suppressed in the limestone matrix due to small pore sizes. A sequence of abiotic and biotic steps has to be considered in pyrite oxidation in the matrix to provide a realistic source of Fe2+. Results showed that pyrite in the matrix alone cannot significantly affect the nitrate transport. Only the simultaneous presence of siderite in the limestone (and/or pyrite exposed directly on the fracture wall) may explain observed patterns of denitrification. Overall, this dissertation demonstrates the importance of individual and comprehensive investigation of complex hydrogeological systems such as adjacent valleys during the process of decision making in land use management. This dissertation also demonstrates the relevance of reactive transport modelling to identify potential reaction pathways of groundwater pollutants in large scale flow in fractured aquifers. This is a prerequisite to understand and predict long-term fate and transport of pollutants in groundwater.