Heavy metal-bearing sediment particles enter river systems by discharge of mine or processing waste, tailings dam failures, remobilization of mining-contaminated alluvium and mine drainage. The mineralogy and geochemistry of these particles is dependent upon the original ore mineralogy, and on processes that have occurred in the source areas, during transport and deposition and during postdepositional early diagenesis. This paper reviews the research carried out to date on the sources, mineralogy, chemistry and fate of heavy metal-bearing particles in mining-affected river systems, and identifies six important avenues for further investigation.

Fifty years after the hyporheic zone was first defined (Orghidan, 1959), there are still gaps in the knowledge regarding the role of biodiversity in hyporheic processes. First, some methodological questions remained unanswered regarding the interactions between biodiversity and physical processes, both for the study of habitat characteristics and interactions at different scales. Furthermore, many questions remain to be addressed to help inform our understanding of invertebrate community dynamics, especially regarding the trophic niches of organisms, the functional groups present within sediment, and their temporal changes. Understanding microbial community dynamics would require investigations about their relationship with the physical characteristics of the sediment, their diversity, their relationship with metabolic pathways, their interactions with invertebrates, and their response to environmental stress. Another fundamental research question is that of the importance of the hyporheic zone in the global metabolism of the river, which must be explored in relation to organic matter recycling, the effects of disturbances, and the degradation of contaminants. Finally, the application of this knowledge requires the development of methods for the estimation of hydrological exchanges, especially for the management of sediment clogging, the optimization of self-purification, and the integration of climate change in environmental policies. The development of descriptors of hyporheic zone health and of new metrology is also crucial to include specific targets in water policies for the long-term management of the system and a clear evaluation of restoration strategies.

Headwater woodland streams are primarily heterotrophic: they receive substantial inputs of organic matter from the riparian vegetation, while autochthonous primary production is generally low. A substantial part of leaf litter entering running waters may be buried in the streambed because of flooding and sediment movement. Although the general significance of the hyporheic zone for stream metabolism has been reported early, organic matter storage within the sediment of streams has received less attention, with most studies only quantifying accumulations at the streambed surface and ignoring other stream compartments. In the present study, the amounts of three fractions of coarse particulate organic matter (CPOM; >16, 4–16 and 1–4 mm) were determined in late autumn and early spring in the interstitial and benthic zones of three headwater streams of the Montagne Noire (South-Western France) differing in their substratum grain size. Our findings demonstrated that the total CPOM content in the interstitial zone can be much (up to one order of magnitude) higher than at the sediment surface. The sandy bottomed stream exhibited a higher amount of CPOM (whatever the size fraction) than the two other streams, suggesting that the sediment particle size may be a major determinant of CPOM storage. Given the large amount of organic matter stored in the interstitial zone, this compartment may play an important role for the carbon turnover and associated trophic dynamics in the stream ecosystem.