Rivers are important ecosystems under continuous anthropogenic stresses. The hyporheic zone is a ubiquitous, reactive interface between the main channel and its surrounding sediments along the river network. We elaborate on the main physical, biological, and biogeochemical drivers and processes within the hyporheic zone that have been studied by multiple scientific disciplines for almost half a century. These previous efforts have shown that the hyporheic zone is a modulator for most metabolic stream processes and serves as a refuge and habitat for a diverse range of aquatic organisms. It also exerts a major control on river water quality by increasing the contact time with reactive environments, which in turn results in retention and transformation of nutrients, trace organic compounds, fine suspended particles, and microplastics, among others. The paper showcases the critical importance of hyporheic zones, both from a scientific and an applied perspective, and their role in ecosystem services to answer the question of the manuscript title. It identifies major research gaps in our understanding of hyporheic processes. In conclusion, we highlight the potential of hyporheic restoration to efficiently manage and reactivate ecosystem functions and services in river corridors.

The use of fiber‐optic sensors has flourished in many fields over the past 30 years. One particular branch of fiber‐optic sensing, distributed temperature sensing, has become a well‐explored and widely‐accepted tool for a diverse range of environmental applications over the past decade. Peer‐reviewed work on fiber‐optic distributed temperature sensing advanced significantly, moving from innovations in instrumentation, deployment techniques, calibration, and analysis methods to applications. However, exciting advancements in other branches of fiber optics, such as fiber Bragg gratings, optical frequency domain reflectometry, and distributed acoustic sensing, have thus far been underutilized in environmental studies and await exploitation by environmental scientists. These additional techniques offer immense possibilities for novel applications in hydrology, hydrogeology, geophysics, and other environment fields where high‐accuracy, high‐frequency, and/or high spatial resolution measurements are needed.

Groundwater provides a vital source of drinking water for rural communities in many parts of Africa, particularly in the dry season when there are few safe alternative sources. This paper summarises results from a study (n = 428) assessing dry season water quality, both microbiological and inorganic chemistry, in handpump equipped boreholes (HPBs) across the Ethiopia Highlands (n = 142), Malawi (n = 162) and Uganda (n = 124) using a stratified, randomised sampling design. This study seeks to examine general water quality by randomly sampling rural groundwater supplies across larger areas with different geology and climate. The majority, 72%, of HPBs surveyed provide good quality dry season drinking water as defined by WHO drinking water quality criteria. Within this overall picture, the most notable constraints were from thermotolerant coliforms (TTCs), which exceeded the WHO drinking water guideline of zero colony forming units (cfu/100 ml) in 21% of sites (range 0–626 cfu/100 ml). TTC contamination was found to have a significant and positive correlation with annual average rainfall (ρ = 0.2, p = 0.00003). Across all three countries, WHO health based chemical drinking water quality values were exceeded at 9% of sites and were found for manganese (4%), fluoride (2.6%) and nitrate (2.5%); arsenic concentrations were below the guideline value of 10 μg l⁻¹ (range < 0.5–7 μg l⁻¹). The high percentage of Mn exceedances (14% ± 5.2% >400 μg l⁻¹) found in drinking water sources in Uganda challenges the decision by WHO not to formalise a health-based guideline for Mn. While the overall level of microbiological contamination from HPBs is low, results from this study strongly suggest that at a national and regional level, microbiological contamination rather than chemical contamination will provide a greater barrier to achieving targets set for improved drinking water quality under the UN-SDG 6. Efforts should be made to ensure that boreholes are properly sited and constructed effectively to reduce pathogen contamination.

Numerous basin aquifers in arid and semi-arid regions of the world derive a significant portion of their recharge from adjacent mountains. Such recharge can effectively occur through either stream infiltration in the mountain-front zone (mountain-front recharge, MFR) or subsurface flow from the mountain (mountain-block recharge, MBR). While a thorough understanding of recharge mechanisms is critical for conceptualizing and managing groundwater systems, distinguishing between MFR and MBR is difficult. We present an approach that uses hydraulic head, chloride and electrical conductivity (EC) data to distinguish between MFR and MBR. These variables are inexpensive to measure, and may be readily available from hydrogeological databases in many cases. Hydraulic heads can provide information on groundwater flow directions and stream–aquifer interactions, while chloride concentrations and EC values can be used to distinguish between different water sources if these have a distinct signature. Such information can provide evidence for the occurrence or absence of MFR and MBR. This approach is tested through application to the Adelaide Plains basin, South Australia. The recharge mechanisms of this basin have long been debated, in part due to difficulties in understanding the hydraulic role of faults. Both hydraulic head and chloride (equivalently, EC) data consistently suggest that streams are gaining in the adjacent Mount Lofty Ranges and losing when entering the basin. Moreover, the data indicate that not only the Quaternary aquifers but also the deeper Tertiary aquifers are recharged through MFR and not MBR. It is expected that this finding will have a significant impact on the management of water resources in the region. This study demonstrates the relevance of using hydraulic head, chloride and EC data to distinguish between MFR and MBR.