Identifying the areas where paleowater or “fossil water” – water that recharged before the Holocene started 12,000 years ago – is pumped for drinking water supply helps managers decide whether groundwater can sustainably meet future demands.
This type of groundwater recharged during rainy periods under cooler and wetter conditions that ended about 10,000 years ago. Since the hydrologic conditions were quite different from those that are currently replenishing groundwater, pre-Holocene groundwater is considered a non-renewable groundwater resource.
In a recent study, researchers from Lawrence Livermore National Laboratory (LLNL), CSU East Bay and UC Santa Barbara identified paleowater using three key isotopic indicators of groundwater residence time: tritium, radiocarbon and radiogenic helium-4.
By comparing results of these three tracers from a dataset from more than 2,000 drinking water production wells in California (collected by LLNL and the U.S. Geological Survey for the Groundwater Ambient Monitoring and Assessment Program of the California State Water Resources Control Board), the team found that approximately 7 percent of the wells show strong evidence for producing paleowater. An additional 22 percent of wells produce mixed-age water with a component of paleowater.
“Many of these wells yielding paleowater are clustered in regions already experiencing surface and groundwater stress, so these results indicate that continued reliance on groundwater during times of drought may not be reliable, and such resources are not readily recoverable,” said LLNL hydrologist Ate Visser, co-author of a paper appearing in Quaternary International.
Wells in desert basins of southeastern California and wells in the southwestern quadrant of the Central Valley are most likely to produce paleowater that is pre-Holocene in age. Very few wells in the northwestern portion of the state, the foothills and Sierra Nevada region and coastal basins with intensive artificial recharge activities are categorized as producing paleowater.
Climate is the primary control on paleowater locations. Arid portions of the state that were wetter during the Pleistocene – referred to as the last Ice Age and lasting from 2.5 million years to about 12,000 years ago – have the largest number of wells categorized as producing paleowater. Paleowater also is found at the end of very long flow paths in confined aquifers, e.g., in the center of the northern Sacramento Valley. In contrast, paleowater may be masked in areas where unconfined or semi-confined conditions allow substantial mixing between modern recharge and paleowater. Modern, artificially recharged water has replaced very old groundwater on a large scale in urban coastal basins.
If a well produces predominantly paleowater, the conditions under which that aquifer recharges are potentially different enough that water produced cannot be sustainably replaced in the current climate. On the other hand, paleowater predates human-made contaminants and is considered less susceptible to contamination.
“The primary reason for identifying paleowater is to provide information about the sustainability of groundwater extraction and consider situations in which the rate of discharge by wells is much greater than the present rate of recharge,” Visser said.
Continued rapid extraction of pre-modern groundwater – not considered “fossil” but recharged before significant groundwater pumping started about 200 years ago – may not be sustainable either. On the other hand, large-scale managed aquifer recharge in urban areas is an example of human activity leading to a sustainable condition, if artificial recharge rates can meet pumping demands.
“Identification of paleowater in resources that are currently exploited for drinking water or for irrigation is critical for predicting the future potential of groundwater as a reliable water supply,” Visser said.