Analysis of spatial and temporal heterogeneities of methane emissions of reservoirs by correlating hydro-acoustic with sediment parameters

The quantification of methane emissions from reservoirs is still imprecise. This study aims on the improvement of methods to understand the relevant processes causing heterogeneities. By conducting hydro-acoustic surveys, morphometric information was obtained. Seabed classification was conducted, including extensive ground truthing, which allowed spatial interpolation. It was possible to link the sediment distribution to quality parameters and to characteristics determining methane production

Acoustic Mapping of Gas Stored in Sediments of Shallow Aquatic Systems Linked to Methane Production and Ebullition Patterns

Bubble-mediated transport is the predominant pathway of methane emissions from inland waters, which are a globally significant sources of the potent greenhouse gas to the atmosphere. High uncertainties exist in emission estimates due to high spatial and temporal variability. Acoustic methods have been applied for the spatial mapping of ebullition rates by quantification of rising gas bubbles in the water column. However, the high temporal variability of ebullition fluxes can influence estimates of mean emission rates if they are based on reduced surveys. On the other hand, echo sounding has been successfully applied to detect free gas stored in the sediment, which provide insights into the spatial variability of methane production and release. In this study, a subtropical, midsize, mesotrophic drinking water reservoir in Brazil was investigated to address the spatial and temporal variability of free gas stored in the sediment matrix. High spatial resolution maps of gas content in the sediment were estimated from echo-sounding surveys. The gas content was analyzed in relation to water depth, sediment deposition, and organic matter content (OMC) available from previous studies, to investigate its spatial variability. The analysis was further supported by measurements of potential methane production rates, porewater methane concentration, and ebullition flux. The largest gas content (above average) was found at locations with high sediment deposition, and its magnitude depended on the water depth. At shallow water depth (12 m), the gas stored in the sediment is released episodically during short events. An artificial neural network model was successfully trained to predict the gas content in the sediment as a function of water depth, OMC, and sediment thickness (R$^2$ = 0.89). Largest discrepancies were observed in the regions with steep slopes and for low areal gas content (<4 L m$^{−2}$). Although further improvements are proposed, we demonstrate the potential of echo-sounding for gas detection in the sediment, which combined with sediment and water body characteristics provides insights into the processes that regulate methane emissions from inland waters

Assessment of Phosphorus Input from Urban Areas in the Passaúna River and Reservoir

Elevated phosphorus loads play an important role in the deterioration of water quality and can subsequently pose a threat to the aquatic organisms in a river or a standing water body. The accurate assessment of total phosphorus (TP) fluxes from a catchment is of high importance to the well-being of the entire river ecosystem. In this study, we assessed the yearly input of TP from the urban areas of the Passaúna catchment in southern Brazil. The catchment drains into the eponymous reservoir, which provides drinking water for more than 800,000 inhabitants of the Curitiba Metropolitan region. The protection of the water quality in the river as well as in the reservoir is of paramount importance, yet high phosphorous inputs have been detected. For adequate protection, the catchment emissions need to be accurately assessed. Initially, the TP concentration in the river sediment was determined in order to assess the relationship between the TP export of the urban areas and the TP stock of the river. It was found that in areas with a higher share of urban land cover and especially in areas with a lack of sewage treatment, the TP concentration in the sediment reached up to 6700 mg/kg. The assessment of the overall TP input from urban areas was based on a regionalized emission-modeling approach, combined with data from long-term water quality monitoring of the river. The monitoring station established upstream of the Passaúna Reservoir inflow provided an initial assessment and the necessary output for the validation and calibration of the model. From the drainage basin of the monitoring station, an overall TP input of 2501 kg/a (0.31 kg/(ha a)) was measured between 1 May 2018 and 1 May 2019 (3508 kg TP/a or 0.23 kg/(ha a) when extrapolating the overall catchment of the Passaúna Reservoir). The monitoring data indicated that the TP input increases during the wet months of the year. The sediment stock of the river also plays an important role in the interannual budget of TP. During the timespan of one year, many deposition–resuspension events happen. The resuspended material is included in the baseflow and hinders the differentiation between urban and nonurban input. After calibration, the model was able to predict the yearly input of TP from the urban areas of the Passaúna catchment. In addition, the share of inhabitants who are not connected to the sewer system was assessed. Overall, the combination of monitoring and modeling in this study offers a valuable overview of the TP dynamics of the system, while the model ensures reproducibility with high accuracy at the same time

To what extent can a sediment yield model be trusted? A case study from the passaúna catchment, brazil

Soil degradation and reservoir siltation are two of the major actual environmental, scientific, and engineering challenges. With the actual trend of world population increase, further pressure is expected on both water and soil systems around the world. Soil degradation and reservoir siltation are, however, strongly interlinked with the erosion processes that take place in the hydrological catchments, as both are consequences of these processes. Due to the spatial scale and duration of erosion events, the installation and operation of monitoring systems are rather cost- and time-consuming. Modeling is a feasible alternative for assessing the soil loss adequately. In this study, the possibility of adopting reservoir sediment stock as a validation measure for a monthly time-step sediment input model was investigated. For the assessment of sediment stock in the reservoir, the commercial free-fall penetrometer GraviProbe (GP) was used, while the calculation of sediment yield was calculated by combining a revised universal soil loss equation (RUSLE)-based model with a sediment delivery ratio model based on the connectivity approach. For the RUSLE factors, a combination of remote sensing, literature review, and conventional sampling was used. For calculation of the C Factor, satellite imagery from the Sentinel-2 platform was used. The C Factor was derived from an empirical approach by combining the normalized difference vegetation index (NDVI), the degree of soil sealing, and land-use/land-cover data. The key research objective of this study was to examine to what extent a reservoir can be used to validate a long-term erosion model, and to find out the limiting factors in this regard. Another focus was to assess the potential improvements in erosion modeling from the use of Sentinel-2 data. The use of such data showed good potential to improve the overall spatial and temporal performance of the model and also dictated further opportunities for using such types of model as reliable decision support systems for sustainable catchment management and reservoir protection measures

Methodical Optimization of Model Approaches on Pollutant Balancing in River Basin Districts to Promote the Implementation Strategy for the Water Framework Directive. Final Report on the Project FZK: 370 822 202/01

Within a feasibility study the existing quantification approaches of the model system MONERIS have been methodically developed and their adequacy for modeling in the scale of large river basins has been tested. Additionally, input data sets of spatially and temporally higher resolution have been implemented. The following approaches and methods have been developed based on their relevance for pollutant emissions: water balance and runoff components, sewer systems, erosion and sediment delivery into surface waters, in-stream retention of particulately transported pollutants and the metrological recording of river loads in case of floods. Furthermore, a consistent and transparent instrument for quantifying and visualizing emissions into surface waters has been developed which will facilitate the reporting commitments. The architecture of MoRE allows users without any knowledge of programing to be able to add other input data and quantification approaches as well as embed other substance groups. Thus this instrument complies with the follow-up and development capabilities concerning the consideration of scenarios

Methodische Optimierung von Modellansätzen zur Schadstoffbilanzierung in Flussgebietseinheiten zur Förderung der Umsetzungsstrategie zur Wasserrahmenrichtlinie. Endbericht für das Umweltbundesamt-Vorhaben FZK: 370 822 202/01

Im Rahmen einer Machbarkeitsstudie wurden bestehende Quantifizierungsansätze des Modellsystems MONERIS methodisch weiterentwickelt und hinsichtlich ihrer Eignung für die Modellierung auf der Skala großer Flussgebiete getestet. Zudem wurden räumlich und zeitlich höher aufgelöste Eingangsdatensätze implementiert. Auf Grund ihrer Bedeutung für Schadstoffeinträge wurden die Ansätze und Methoden für die Wasserbilanz, Kanalisationssysteme, Erosion und Sedimenteintragsverhältnis, gewässerinterne Retention von partikulär transportierten Schadstoffen und messtechnische Erfassung von Gewässerfrachten im Hochwasserfall weiterentwickelt. Zudem wurden die Eingangsdaten für die Eintragspfade Kanalisationssysteme und die Erosion für die deutschen Einzugsgebiete räumlich und zeitlich höher aufgelöst. Darüber hinaus wurde ein konsistentes und transparentes Instrumentarium zur Quantifizierung und Visualisierung der Stoffeinträge in die Oberflächengewässer entwickelt, mit dessen Hilfe die Berichtspflichten erfüllt werden können. Die Architektur von MoRE erlaubt dem Nutzer, ohne Programmierkenntnisse die Basisvariante durch andere Eingangsdaten und Quantifizierungsansätze zu ergänzen sowie weitere Stoffgruppen einzubinden. Somit entspricht das Werkzeug allen Anforderungen an die Fortschreibung und Weiterentwicklungsfähigkeit im Hinblick auf Szenario-Berechnungen

Linking Sediment Gas Storage to the Methane Dynamics in a Shallow Freshwater Reservoir

Freshwater reservoirs are globally relevant sources of the greenhouse gas methane. Organic matter rich sediments are hot spots of methane production and can store large amounts of methane dissolved in porewater and as free gas. Yet, in situ data on the gas storage as free gas (bubbles) in freshwater sediments are scarce. Here, an acoustic approach was tested and used to map the gas content in the sediment of a shallow temperate reservoir. The sediment gas storage was linked to the methane budget obtained from almost 2 years of in situ monitoring. The emission fluxes were dominated by ebullition and degassing at the reservoir outlet, which combined accounted for 93% of the total methane emissions. 66% of the ebullition variability was explained by a combination of environmental parameters. Mappings of sediment gas content using echo sounder surveys revealed the accumulation of free gas in regions of elevated sediment deposition. Temporally, the gas storage in the sediment was related to methane emissions, in which a period of intensified emissions resulted in a reduction of sediment gas storage. The sediment could store an equivalent of 4 to 13 days of accumulated potential methane production, which could supply the mean ebullition flux for more than 2 months. We suggest that sediment gas storage plays an important role in buffering and modulating methane emissions in aquatic systems and need to be accounted for in process-based models