Abstract
The clear and reliable detection of effluent plumes using satellite data is especially challenging. The surface signature of such events is of a small scale; it shows a complex interaction with the local environment and depends greatly on the effluent and marine water constitution. In the context of remote sensing techniques for detecting treated wastewater discharges, we study the surface signature of small river plumes, as they share specific characteristics, such as higher turbidity levels and increased nutrient concentration, and are fresh compared to the salty marine water. The Bulgarian Black Sea zone proves to be a challenging study area, with its optically complex waters and positive freshwater balance. Additionally, the Bulgarian Black Sea coast is a known tourist destination with an increased seasonal load; thus, the problem of the identification of wastewater discharges is a topical issue. In this study, we analyze the absorption components of the Inherent Optical Properties (IOPs) for 84 study points that are located at outfall discharging areas, river estuaries and at different distances from the shoreline, reaching the open sea area at a bottom depth of more than 2000 m. The calculations of IOPs take into account all available Sentinel 2 cloudless acquisitions for three years from 2017 until 2019 and are performed using the Case-2 Regional CoastColour (C2RCC) processor, implemented in the Sentinel Application Platform (SNAP). The predominant absorber for each study area and its temporal variation is determined, deriving the specific characteristics of the different areas and tracking their seasonal and annual course. Optical data from the Galata AERONET-OC site are used for validating the absorption coefficient of phytoplankton pigment. A conclusion regarding the possibility of distinguishing riverine, marine and coastal water is derived. The study provides a sound basis for estimating the advantages and drawbacks of optical satellite data for tracking the extent of effluent and fluvial plumes with unknown concentrations of optically significant seawater constituents.
Highlights
The analysis requires substantial computational resources if the raw data are processed. These challenges are approached by studying the riverine plume surface signatures, which has a similar manifestation in satellite data, and comparing them with areas located in the discharge zone of wastewater treatment plants in order to identify common traits in the detection techniques
To be see how they change with increasing distance from the shoreline and sea depth, we study their change with increasing distance from the shoreline and sea depth, we study their IOPsIOPs and and compare to inwater points
We investigated the surface signature of riverine plumes, assuming that, in a similar way, we could apply the results to the treated wastewater as the two types of water share similar optical characteristics
Summary
Holt et al [15] performed a study on stormwater runoff plumes by comparing SAR and MODIS Aqua ocean color images, together with in situ measurements They suggested a concept to complement water quality monitoring based on SAR and optical sensor data to guide the in situ collection and assessment of beach closures due to contamination. Recent research by Lebedev et al [26] used the concentrations of suspended matter acquired from a MediumResolution Imaging Spectrometer (MERIS) on board Envisat to establish a relationship between river runoff and the river plume area for two rivers on the Russian coast in the northeastern Black Sea. Osadchiev and Sedakov [27] applied an optical flow algorithm on near-simultaneous ocean color satellite imagery from Landsat 8 and Sentinel 2 to study the spread of small river plumes on the northeastern shore of the Black Sea and successfully reconstructed surface currents along the shore. An anomaly value that deviates from the typical IOPs on the satellite image could be a sign of the presence of an unusual water source in the area
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