Epsilon Archive for Student Projects

Abstract

Human activities have increased nutrient losses to agricultural ditches and streams, including phosphorus. Phosphorus leakage leads to a worsening of water quality in downstream ecosystems, including lakes and seas. In addition, higher phosphorus concentrations lead to eutrophication, which negatively affects aquatic systems. Many land and water management solutions to reduce nutrient losses have been proposed. One of them is a two-stage ditch (SD) built from a traditional trapezoidal ditch (TD) by its widening and introducing floodplains. During high flow conditions, the floodplains will be flooded, which will, in turn, reduce the water velocity and reduce losses of phosphate downstream. The slower water velocity makes it possible for the sediment to settle and adsorb phosphate, potentially creating a sink for phosphate dissolved in stream water and improving water quality downstream.
In this study, SD’s capacity for retention of phosphate was tested. The study was conducted in four SDs as part of a project Two-stage ditches in Sweden. Sediment samples were collected upstream/downstream floodplains and channels. The studied SDs are located in Sörmland, Östergötland and Skåne. The sediment samples were collected between February to April 2022. The collected sediment samples were analysed in a laboratory to determine the equilibrium phosphate concentration at net zero sorption (EPC0) to measure the sediments buffering capacity to phosphate. The EPC0 was correlated with the phosphate concentrations in the stream water to determine if the sediment adsorb ore desorb phosphate. The EPC0 was later analysed to find conjunction with the stream water pH and DO (oxygen levels) and clay content referenced in the literature (Smolders et al. 2017; Bergström et al. 2015; Trentman 2020; Palmer-Felgate et al. 2011). Statistical analysis was made to find differences in EPC0, retention capacity, and SRP concentration within the SD. The analysis was made between sites, stream location, and between the terraces and channel. The result pointed the EPC0 value varies between the sites but with similar patterns, and the study show fluxes of phosphate in both ways from all SDs among the samples. The highest source of phosphate came from SD7 upstream with a release of 144 PO4-P ug/L/gDW by the channel in March 2022 and the highest retention by 82 PO4-P ug/L / gDW was observed in SD3 upstream in February 2022 on the terrace. SD2 showed the best result with more retention among the sample in both the channel and terraces and SD7 showed more phosphate release among the samples on channels and terraces. The statistical analysis showed no differences within the sites nor in SRP concentration within the SD. This could indicate drainage water attached to the SD leaching SRP. There was no conjunction that point pH, DO, and clay content influenced EPC0 during this investigation.