Assessing the role of pit lakes in riverine landscape rehabilitation: a comprehensive study in the Po River basin via multi-source satellite and in situ data

Abstract

Since the 1950s, an intense sand and gravel quarrying from the river floodplains led to the formation of pit lakes (PLs) which now dot floodplains and have changed the morphology and the drainage system of several Italian river basins. In the last decade, a renewed interest in PLs has grown as tools for rehabilitating rivers and their floodplains, in order to provide a similar ecological service once offered by pristine riverine wetlands, which were lost due to anthropogenic pressures. Despite the potential importance of these environments, systematic data on their location, morphology, and water quality remains limited. The primary aim of this study was to quantify the number, distribution, and major morphometric characteristics of PLs in the Po River basin, in order to assess their relevance and suitability for ecological restoration purposes. To achieve these aims, a synergistic approach was used including remote sensing imagery (Landsat, Sentinel-2, and PRISMA), satellite archives, and regional databases. In addition, by exploiting the synoptic and multispectral view of these satellites, we evaluated the physical and optical properties of a large sample of PLs from 1990 to 2021. Specifically, we focused on two key parameters of water quality: the dominant wavelength (λdom, i.e., the water colour) and the concentration of Suspended Particulate Matter (SPM), splitting it into its inorganic (Suspended Particulate Inorganic Matter, SPIM) and organic (Suspended Particulate Organic Matter, SPOM) fractions. The results show that more than 12,650 small lentic water bodies are present in the Po River basin, with agricultural ponds the most represented category, followed by PLs. Specifically, 1580 PLs were identified, divided into three subcategories: active (338), ceased (1188), and doubtful (54). These artificial aquatic systems, when compared with pristine riverine wetlands, are more numerous, widely distributed, and on average more extensive, although more regularly shaped. The temporal analysis of satellite imagery showed that the number and the total area of PLs have increased from 1990 to 2021. PLs currently account for 63.5 km2 surface area, an average water volume of 378x106 m3, and for a removal of 26.6 mg N m-2 d-1 via denitrification. The results also show a very strong correlation between SPM concentrations obtained in situ and those obtained from satellite images, both for data derived from Landsat (R2 = 0.85) and Sentinel-2 images (R2 = 0.82). In general, it appeared that PLs with the highest mean SPM concentrations and the highest mean λdom (i.e., poor water quality) are located along the main Po River, and more generally near rivers. Active PLs exhibit a poor water quality status, especially those of small size (<5 ha) and directly connected to a river. Seasonal comparison shows the same trend for both SPM concentration and λdom: higher values in winter gradually decreasing until spring-summer, then increasing again. In addition, it emerged that the end of quarrying activity led to a reduction in SPM concentration from a minimum of 43% to a maximum of 72%. Finally, we demonstrated that the PRISMA hyperspectral satellite is a reliable tool for estimating the inorganic and organic fraction of SPM concentration. In this context, the use of multi-source satellite imagery enabled the quantification and analysis of PLs and other water bodies in a vast area such as the Po River basin, as well as the evaluation of the temporal evolution of the physical and optical properties of the PLs. In particular, the Sentinel-2 images consistently proved to be a reliable resource for capturing episodic and recurring quarrying events and portraying the ever-changing dynamics of these aquatic systems.