Abstract
Geostatistics has been successfully used to analyse and characterize the spatial variability of environmental properties. Besides providing estimated values at unsampled locations, geostatistics measures the accuracy of the estimate, which is a significant advantage over traditional methods used to assess pollution. This work uses universal block kriging to model and map the spatial distribution of salinity measurements gathered by an Autonomous Underwater Vehicle in a sea outfall monitoring campaign. The aim is to distinguish the effluent plume from the receiving waters, characterizing its spatial variability in the vicinity of the discharge and estimating dilution. The results demonstrate that geostatistical methodology can provide good estimates of the dispersion of effluents, which are valuable in assessing the environmental impact and managing sea outfalls. Moreover, since accurate measurements of the plume's dilution are rare, these studies may be very helpful in the future to validate dispersion models.
Highlights
The physical and biological coastal processes that determine the values of environmental variables are complex and still poorly understood
Vertical oscillations of the Autonomous Underwater Vehicles (AUVs) were less than 1m in the 2m survey and less than 1m down and less than 1.5m up in the 4m survey
Patrícia Alexandra Gregório Ramos: Geostatistical Prediction of Ocean Outfall 9 Plume Characteristics Based on an Autonomous Underwater Vehicle estimated by the Block Kriging with Trend (BKT), while at a 4m depth the salinity was best estimated by the OBK
Summary
The physical and biological coastal processes that determine the values of environmental variables are complex and still poorly understood. As advanced by several investigations, the observed plume patchiness can occur due to one or a combination of factors which include: (1) variations in currents and stratification during time intervals (which can be hours in some cases), separating the beginning and the end of the field tests and resulting in different equilibrium depths or even distinct plume behaviours, (2) internal tides that can result in a given effluent concentration surface undergoing significant vertical excursions as it advects from the outfall and (3) limitations of sampling in terms of time resolution of time and space scales; in reality, field measurements are not truly synoptic ‐ a transverse of several kilometres can last a couple of hours Because of their extensive preparation and potentially negative impacts on the environment, detection methods using introduced tracer substances, either at the treatment plant or at the level of the diffusers in the waters, are not practical for routine monitoring of sewage effluents. Because of their easier field logistics, reduced cost per study, increased spatial resolution, reduced spatial aliasing effects and adaptive sampling capabilities, Autonomous Underwater Vehicles (AUVs) are especially valuable in detecting and mapping sewage plumes [9,10,11,12,13]
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