The estimation of source and flowpath dynamics of water and solutes in an Austrian headwater agricultural catchment

Abstract

diffuse pollution as compared to clearly defined point sources like waste water treatment plants (WWTPs), but also because the nutrients that are stored in the soil and transmitted to the surface waters are controlled by many complicated and heterogeneous natural processes which is much more difficult to control as compared to WWTPs. By understanding the natural mechanisms that control the storage and transmittance of nutrients and the water that carries them in typical agricultural catchments, better nutrient transport models can be made and subsequently better solutions can be found to help reduce the discharge of nutrients from agricultural lands to surface waters. The goals of the thesis are to develop the methodology to measure and analyze the data associated with solute transport, to identify the source reservoirs and the flowpaths of water and solutes, and ultimately to develop a qualitative and quantitative understanding of the interacting source and flowpath dynamics in an Austrian headwater agricultural catchment. With many flowpath inputs to the stream (e.g. tile drainages and springs) and some with little to no baseflow during periods of the year, the ability to continuously measure solute concentrations required a unique solution. In Chapter 2, a new device to house water monitoring devices is presented and successfully deployed in the Hydrologic Open Air Laboratory (HOAL) catchment. The device was called the Water Monitoring Enclosure (WME) and it ensures a minimum internal water level which ensures that the enclosed water monitoring devices remain submerged even when there is no flow into the WME. The limited diameter of the inflow pipe buffers the flow velocity within the WME as some devices are sensitive to dramatic changes in flow velocity. The WME also conveys sediment through the system to ensure that the aggregation of sediment would not interfere with the internal water monitoring devices. Clearly visible point inputs to the stream can be directly measured using water monitoring devices, but the truly diffuse water that enters the stream via groundwater must be estimated in a more indirect way. The continuous exchange of stream water and groundwater requires a more advanced methodology to estimate the fluxes as well as the groundwater solute concentrations. Chapter 3 presents a new methodology to estimate the stream to groundwater fluxes and associated groundwater solute concentrations, and a comparison to existing methodologies. The newly developed method assumes that the inflowing and outflowing fluxes occur simultaneously and uniformly along the entire stream reach. Through the use of artificial stream simulations, the new method had the highest performance compared to the other methods and that all methods produced significantly different results depending on the flux distribution assumptions. In the HOAL catchment, baseflow contributes the majority of the water and solute load throughout the year. Nitrate concentration was found to be seasonally variable throughout the year with higher concentrations in the winter and spring periods. Chapter 4 specifically addresses the question of the seasonal variability of the nitrate concentration by analyzing the seasonal source and flowpath dynamics in addition to other seasonal biochemical explanations. The cause of the seasonal nitrate concentration was due to the alternating source aquifer contributions throughout the year with the deep aquifer typically contributing 75% of the water during the summer and 50% in the winter. The shallow aquifer supplied the vast majority of the nitrate load with the perennial tile drainages acting as the dominant flowpath to the stream for the shallow aquifer water. Runoff events play an important role in the transport of many substances in addition to the water, but are significantly more difficult to estimate compared to baseflow due to additional processes that occur during runoff events. Using the developments from the work in the previous chapters, Chapter 5 determines the representative source and flowpath dynamics of water and solute load during runoff events in the HOAL. Two large runoff events contributed over 50% to the total event runoff flow and nitrate load. The shallow aquifer and the rain water contributed equally to the event flow and combined contributed over 80% of the total event runoff flow with the top soil contributing the remainder. Large runoff events had different source and hydrograph responses as compared to smaller events. Small events had little top soil water with most of the nitrate load originating from the shallow aquifer, while the large events had a significant contribution from the top soil water with the soil water and the shallow aquifer contributing equally to the nitrate load. The thesis has advanced the knowledge of the source and flowpath dynamics of water and solutes in a typical Austrian headwater agricultural catchment. This thesis has improved the collection of solutes in headwater catchments, improved source separation models and associated analysis methods, and provided additional knowledge for the development of large comprehensive transport models which will better identify and target significant pollutant sources and flowpaths that contribute to pollutant loads of surface waters.