Groundwater

Groundwater flow meddles with hydrological, environmental and geological processes. As water scarcity issues mount for people living above ground, the vast stores of freshwater in the subsurface require research attention.

Drought management is inefficient because feedbacks between drought and people are not fully understood. In this human-influenced era, we need to rethink the concept of drought to include the human role in mitigating and enhancing drought.

A global picture of the age structure and flow path of groundwater is lacking. Tritium concentrations and numerical modelling shed light on both the most recently replenished and the longest stored groundwater within Earth's continents.

Streamflow is a mixture of precipitation of various ages. Oxygen isotope data suggests that a third of global river discharge is sourced from rainfall within the past few months, which accounts for less than 0.1% of global groundwater.

Groundwater flow redistributes heat in the Earth’s crust. Numerical simulations of groundwater flow show net cooling of groundwater basins, as well as cooling of the underlying lithosphere in areas where groundwater flows over large distances.

Groundwater recharged less than 50 years ago is vulnerable to contamination and land-use changes. Data and simulations suggest that up to 6% of continental groundwater is modern—forming the largest component of the active hydrologic cycle.

Multiple factors determine how much water is and will be available in the river basins of Asia. To expose hotspots and help adaptation, these factors must be assessed together at the basin level.

Earthquake prediction is a long-sought goal. Changes in groundwater chemistry before earthquakes in Iceland highlight a potential hydrogeochemical precursor, but such signals must be evaluated in the context of long-term, multiparametric data sets.

The world's largest freshwater resource is groundwater. A review of our understanding of groundwater depletion suggests that although the problem is global, solutions must be adapted to specific regional requirements at the aquifer scale.

Microbe-mediated reactions remove nitrogen from river water as it flows through sediments. Simulations of the Mississippi River network suggest that denitrification due to flow through small-scale river bedforms exceeds that along channel banks.

Rivers and streams are key sources of CO₂. Estimated emissions and aquatic productivity from across the US show that small streams predominantly emit CO₂ produced in soils, but the contribution from aquatic metabolism increases with river size.

Droughts can cause dry-season productivity to decline in tropical forests. This decline occurs when precipitation is below 2,000 mm yr⁻¹, resulting in insufficient subsurface water storage to maintain constant production through the dry season.

Helium-4 is produced in the Earth’s crust and degassed to the atmosphere. Measurements of ⁴He and ⁸¹Kr dating in an aquifer in Brazil suggest that most crustal ⁴He reaches the atmosphere by the discharge of deep groundwater at the surface.

Precursor events to earthquakes are rarely reproduced. Measurement of groundwater chemistry in Iceland between 2008 and 2013 reveals distinct changes prior to two consecutive >M5 earthquakes.

Changes in continental water storage have been difficult to constrain from space-borne gravity data in regions experiencing both ice melting and glacial isostatic adjustment. Separation of the hydrologic and isostatic signals reveals increases in water storage in both North America and Scandinavia over the past decade.

Earthquake rupture is influenced by stress conditions in the crust before the quake. Analysis and modelling of surface deformation caused by the May 2011 earthquake in Lorca, Spain, indicate that groundwater extraction influenced the pattern of fault rupture.

Changes in terrestrial water storage are likely to affect sea level, but comprehensive and reliable data are scarce. Simulations of global terrestrial water stocks and flows, with an integrated model that specifically accounts for human activities, indicate that groundwater depletion and reservoir storage have together led to a sea-level rise of about 0.66 mm yr⁻¹ between 1961 and 2003, about 36% of the observed rise.

In the course of the transfer of precipitation into rivers, water is temporarily stored in reservoirs with different residence times. Analyses of precipitation and discharge records from Nepal suggest that in addition to snow and glacier melt and evapotranspiration, groundwater storage in a fractured basement aquifer also affects the annual discharge cycle of Himalayan rivers.

The subsurface of Mars could potentially have contained a vast microbial biosphere. An evaluation of the possibility of groundwater upwelling, which might provide clues to subsurface habitability, reveals evidence in the deep McLaughlin crater for clays and carbonates that probably formed in an alkaline, groundwater-fed lacustrine setting.