Groundwater quality: Global threats, opportunities and realising the potential of groundwater.

Groundwater is a critical resource enabling adaptation due to land use change, population growth, environmental degradation, and climate change. It can be a driver of change and adaptation, as well as effectively mitigate impacts brought about by a range of human activities. Groundwater quality is key to assessing groundwater resources and we need to improve our understanding and coverage of groundwater quality threats if we are to use groundwater sustainably to not further burden future generations by limiting resources and/or increasing treatment or abstraction costs. Good groundwater quality is key to progress on a range of Sustainable Development Goals, but achievement of those goals most affected by groundwater contamination is often hindered by of a lack of resources to enable adaptation. A range of threats to groundwater quality exist, both natural and anthropogenic, which may constrain groundwater use. However, groundwater often provides good quality water for a range of purposes and is the most important water resource in many settings. This special issue explores some of the key groundwater quality challenges we face today as well as the opportunities good groundwater quality and treatment solutions bring to enhance safe groundwater use. Legacy anthropogenic contaminants and geogenic contaminants may be well documented in certain places, such as N America, Europe and parts of Asia. However, there is a real issue of data accessibility in some regions, even for more common contaminants. This paucity of information can restrict our understanding and ability to manage and protect groundwater sources. Compared to surface water quality, large scale assessments for groundwater quality are still scarce and often rely on inadequate data sets. Better access to existing data sets and more research is needed on many groundwater quality threats. Identification and quantification of these threats will support the wise use and protection of this subsurface resource, allow society to adequately address future challenges, and help communities realise the full potential of groundwater.

Application of stable isotopes to identify problems in large-scale water transfer in Grand Canyon National Park.

Waters on, and below, the South Rim of the Grand Canyon were sampled for stable isotopic analysis to determine the hydrologic effects of the transcanyon pipeline. The transcanyon pipeline transports North Rim water discharging at Roaring Spring across the Grand Canyon to South Rim. Ultimately this water is discharged through the sewage treatment plant at the Clearwell Overflow wash on the surface expression of the Bright Angel Fault. The North Rim water is some 8 per mil more depleted in deltaD than most of the water issuing from springs on the South Rim except for that from Indian Garden Spring which lies below the Clearwell Overflow wash. Such a composition of Indian Garden Spring must come from discharged wastewater onthe rim, percolating downward approximately 1,000 m vertically through the Bright Angel Fault. The difference in stable isotopic composition of the North Rim water renders it not only traceable in Indian Garden Spring water, but the proportions may be determined as well which result in projecting an admixture of up to half the total discharge. Curiously however, Indian Garden Spring contains no appreciable amounts of the anions associated with wastewater. More recently, a leak in the transcanyon pipeline was discovered above Indian Garden Spring, suggesting that a portion of that spring's discharge may have its origin in water directly from the pipeline. Nevertheless, these data provide information relevant to the National Park Service policy of precluding anthropomorphic forces impacting national parks. In addition, the stable isotopic ratios of park water provide a mechanism to assess the potential for future degradation, as well as the origin of any future degradation, of the water quality of Indian Garden Spring.