Detection of illicit sand mining and the associated environmental effects in China's fourth largest freshwater lake using daytime and nighttime satellite images.

Illegal sand mining activities are rampant in coastal and inland water around the world and result in increased water turbidity, reduced water transparency, damage to fish spawning sites and adverse effects on the health of aquatic ecosystems. However, many sand dredging vessels hide during the day and work at night, rendering conventional monitoring measures ineffective. In this study, illegal sand dredging activities and the associated aquatic environmental effects were investigated in Lake Hongze (the fourth largest freshwater lake in China) using both conventional daytime satellite data, including MODIS/Aqua and Landsat TM/ETM data as well as VIIRS Day/Night Band (DNB) nighttime light (NTL) data, the following results were obtained. (1) The Landsat data revealed that sand dredging vessels first appeared in February 2012 and their number (monthly average: 658) peaked in 2016, and sand dredging stopped after March 2017. (2) The VIIRS NTL data were satisfactory for monitoring nighttime illegal dredging activities, and they more accurately reflected the temporal and spatial distribution characteristics of dredging vessels due to their high frequency. (3) Observations from the MODIS data acquired since 2002 showed three distinct stages of changes in the suspended particulate matter (SPM) concentrations of Lake Hongze that were consistent with the temporal distributions of sand dredging vessels. (4) The contribution of dredging vessels to the increases in SPM concentration was quantitatively determined, and nighttime sand dredging activities were found to have disturbed the waters more significantly. (5) The effectiveness of government measures implemented at various stages to control illegal sand dredging activities were scientifically evaluated. This study provides technological support for government monitoring and the control of illegal sand dredging activities and can serve as a valuable reference for water bodies similar to Lake Hongze worldwide.