Researchers at Scripps Institution of Oceanography at UC San Diego have found that the water table in a San Diego aquifer system dropped by nearly 20 meters at the end of the last ice age.
Groundwater refers to water below the Earth’s surface, and the water table defines the top level of groundwater; below the water table, the soil is saturated with water. Wells and other water management systems tap into groundwater. Today, approximately five percent of San Diego County’s drinking water is supplied from local groundwater, much of which originally fell as rain and percolated into groundwater during the last ice age.
The findings from this NSF-funded study are consistent with other evidence that the San Diego area was once a place of many lakes and lush forests. Additionally, the newly-developed tools used in this study allow researchers and resource managers to understand regional groundwater levels before the advent of modern instrumental records and sets the stage for potential future work to quantify how human activities (i.e. pumping of groundwater) have lowered regional water tables.
Alan Seltzer, who led the research while a PhD student at Scripps Oceanography and is now a postdoctoral scholar at Woods Hole Oceanographic Institution, developed a new method to precisely measure tiny changes in the stable isotopes of noble gases dissolved in groundwater. Noble gases – which lie in the far right column of the periodic table of elements and include elements such as helium and xenon – are inert, meaning they do not undergo chemical reactions, and their concentrations in the atmosphere remain the same over 10,000-year timescales. Stable isotopes are forms of the same element that have different atomic masses and do not radioactively decay. Since they are unaffected by chemistry, biology, and radioactivity, stable isotopes of noble gases are prime subjects for studying physical processes that happen over long periods of time.
The findings from this study were published Dec.16 in the journal Nature Communications. In the study, Scripps researchers, along with colleagues from the United States Geological Survey and Columbia University, measured the composition of noble gases dissolved in old groundwater, much of which had not been in contact with air since the last ice age – over 20,000 years ago. The new technique makes use of processes originally set by gravity in ancient soil air to determine past water table levels. The ultimate goal of this technique is to better quantify and understand past changes in water availability linked to major climatic shifts.
“Prior research has shown that San Diego was once a place of large lakes and lush vegetation, so it is not surprising that water tables were higher in the past. But this is the first finding that quantifies the extra water availability in the last glacial period,” said senior scientist and Scripps paleoclimatologist Jeffrey Severinghaus. “This finding paves the way for quantitative tests of climate models that are employed to predict our future climate, and how California water resources will change in response to fossil-fuel burning.”
Gravity causes heavier gases in soil air to preferentially settle towards the bottom relative to lighter gases. Because the dissolved gases in groundwater originated from soil air at some time in the past, the ratio of heavy-to-light gases in old groundwater can be used to determine ancient water table depth. That is, a higher heavy-to-light noble gas isotope ratio indicates a deeper water table.
“This effect is called gravitational settling, and if not for vigorous mixing, it would actually happen in our lower atmosphere – and increase the concentration of oxygen at the Earth’s surface, since oxygen is heavier than nitrogen,” said Seltzer. “However, in soil air, which is stagnant, gravitational settling does in fact occur, and the ratio of heavy-to-light noble gases increases with depth below the surface. In this study, we were able to demonstrate for the first time that this gravitational signal is transferred to dissolved gases in groundwater.”
Seltzer compared the ratios of krypton and xenon isotopes in groundwater to their ratios in the atmosphere and was able to attribute small changes of these ratios to the influence of gravity in order to estimate past water-table depths. The researchers took samples from wells across the state to test their new analytical technique.
To verify their method, Seltzer and colleagues traveled to Fresno in the Central Valley, an area that has maintained records of groundwater level since before the explosive growth in population and agriculture activity in the late 20th century. They were able to precisely date groundwater that “recharged,” or last exchanged dissolved gases with air, during the 1980s. The researchers found close agreement between their isotope-based water level estimate and historical records.
This study had a particular focus on the San Diego region, where samples were collected in or near Chula Vista, National City, Balboa Park, North Park, and San Diego County Credit Union (formerly Qualcomm) Stadium. Noble gas isotope measurements in these samples indicated that the regional water table fell an average of nearly 20 meters during the transition out of the last ice age, from roughly twenty to ten thousand years ago.
“By developing this new measurement technique at Scripps in San Diego, we were really fortunate to have easy access to groundwater from the last ice age that sits below much of the city,” said Seltzer. “The next step for us is to couple our results with models to understand how a 20-meter shallower water table during the last ice age translates to past rates of precipitation and evaporation.”
Seltzer speculates that the drop in groundwater level at the end of the last ice age was due to redistribution of rainfall along the West Coast. During the last ice age, the jet stream and storm tracks – which deliver precipitation – are thought to have been displaced further south over western North America, possibly due to the interaction with the large ice sheet covering much of northern North America. As the glaciers retreated, the storm tracks may have shifted back north, delivering less rainfall to Southern California and causing the San Diego water table to fall.
Co-authors include Jeff Severinghaus and Jessica Ng of Scripps; Martin Stute of Columbia University; and Wes Danskin, Justin Kulongoski, and Riley Gannon of USGS. This study was funded by an NSF Earth Science grant and an NSF Graduate Student Fellowship.
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