Pit lakes from gravel and sand quarrying in the Po River basin: An opportunity for riverscape rehabilitation and ecosystem services improvement

Since the 1950s, an intense sand and gravel quarrying from the river floodplains led to the formation of pit lakes (PLs) that 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. The primary aim of this study was to quantify the number, distribution, and major morphometric characteristics of these ecosystems in the Po River basin, in order to assess their relevance and suitability for ecological restoration purposes. To achieve this aim, a synergistic approach was used including remote sensing techniques, satellite archives, and regional databases. 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 ecosystems, 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 378 × 106 m3 and for a removal of 26.6 mg N m−2 d−1 via denitrification. Therefore, PLs can contribute ecosystem services, e.g., water provisioning and regulation of the nitrogen cycle, and can be exploited for rehabilitating the riverscape. In this context, remote sensing is a very reliable tool for quantifying and analysing PLs and others water bodies in a vast area such as the Po River basin.