A spatio-temporal sand mining pressure index to support the management of estuary health.

Sand mining is ubiquitous given the universal demand for aggregate materials for construction and other manufacturing industries. This demand is mostly met by mining fluvially derived sediment from rivers and estuaries, resulting in extensive, enduring changes to the morphological, physico-chemical and biological characteristics of aquatic ecosystems. The extent of these activities in South Africa is largely unknown due to the dynamic, ephemeral, and often clandestine operations, which are mainly located in remote or inaccessible areas. This study used freely available remote sensing imagery to conduct an accurate and objective appraisal of sand mining risks to small subtropical estuaries on the east coast of South Africa. It aimed to produce the first quantitative data on sand mining in the region, and to develop a Mining Pressure Index (MPI) as an indicator of the relative sand mining pressure between estuaries. Sand mining has resulted in the cumulative transformation of 4.7% of the Estuarine Functional Zone (EFZ) habitat of temporarily closed estuaries (TCEs) in the region, and up to 8.7% of the EFZ of individual TCEs, revealing that these activities are more extensive than previous qualitative surveys suggested. However, when extensive riverine sand mining immediately upstream of the EFZ was included, almost half of the subtropical temporarily closed estuaries in KwaZulu-Natal TCEs were subject to direct or indirect sand mining pressure between 2009 and 2018. The MPI used information on changes in the cumulative, annual mining footprint over time, as well as potential passive habitat recovery over time. The highest MPIs were in systems classified as Moderately to Largely Modified estuaries, and increasing mining pressure was associated with declines in indicators of estuary health, including poor physical habitat health, and poor biological community health scores. The MPI can be used to support future sand mining regulation, guide compliance monitoring and enforcement, and to prioritize restoration and management interventions to avoid further declines in estuary health in the region. The continued expansion and intensification of sand mining warrants continuous close surveillance, and the methods used here can be easily and inexpensively replicated and are transferable to other estuary types and other bioregions.