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near surface Urban runoff

Open pits

Pesticides

Detergents

Public transport

Chemicals spills

Chemicals spills

Cleaners

Storage facilities

Storage facilities

Livestock waste

Motor oil

Below surface

Landfills Sewage systems

Pipes

Underground storage

Mines

Wells

Underground storage

Wells Septic systems

Below surface

Landfills Sewage systems

Pipes

Underground storage

Mines

Wells

Underground storage

Wells Septic systems

Large gaps in the reliability of predictions about the availability freshwater attach more importance to the comparatively stable and huge groundwater reservoir in the subsurface of our continents. Groundwater plays an important role because it is often the principal supply for drinking water, agriculture, and industry. Recognition of demand and quality deteriorations is particularly important for groundwater because it has only limited capacities for re-naturation due to slow flow rates and long memory effects. This renders aquifers fundamental, but also sensible and often uncertain resources of freshwater. In addition, regions of intensive groundwater use are also often the ones that severely compromise groundwater quality through urban or agricultural influences, causing double pressure on water resources in regions where water is most needed. Such double pressures are particularly critical in areas with excessive groundwater abstractions and poor or missing groundwater management.

We also know that by mass groundwater remains the most mined resource compared to oil, gravel, and other mineral and metal resources. Despite this unique status, the numbers on available useable groundwater quantities, particularly on large scales, remain uncertain. This does not mean that surface waters and moisture stored in the atmosphere have higher priorities; they are just easier to access and thus availability of data about their quantities is generally better. Thus, more investigations are needed of the dynamics, quality management, and treatment of groundwater on local and larger scales. The most accurate results of groundwater stocks and recharge can be generated at local scales and thus should be further combined to produce global estimates and used for calibrating large-scale models. Groundwater quantity and quality evaluations remain challenging due to costs of high-resolution monitoring via piezometers and wells or other geophysical and geo-electric methods or satellite monitoring and GIS with systematic and international use of data. Even with currently available and advanced future investigation methods, we have to accept that it will hardly be possible to establish an exact picture of the entire subsurface situation. This includes the risk that poor definition of groundwater resources may lead to their overexploitation. If better quantification of groundwater is a necessity, detailed research is needed to understand the interactions between the ground- and surface water, particularly with the ocean.

Of course, groundwater is not the only source of freshwater. Other promising methods to reduce pressures on water stocks include further development of desalination, artifi cial groundwater recharge, new economic use of water (e.g., drip irrigation practices), and storage of water masses in aquifers rather than surface water structures. Although the latter storage form does not enable the generation of hydropower, it would prevent destruction of ecosystems through damming, reduce evaporative losses, and prevent salinization of soils if the groundwater levels are deep enough below the land surface.

Acknowledgments

This work was supported by a grant from the Ministry of Science, Research and the Arts of Baden-Wuertemberg (AZ33-7533.18-15-02/80) to Johannes Barth and Peter Grathwohl. This work was also partially undertaken within the European Integrated Project AquaTerra (GOCE 505428). The project has received research funding from the Community's Sixth Framework Programme.

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