Isotope research can show source of moisture
University of Massachusetts, Amherst, hydrologist David Boutt analyzed the current severe drought in parts of New England. According to Boutt, the drought is notable, but not extreme in historical terms, and it is nowhere near the depth of dry conditions observed in the acute five-year drought years experienced from 1962 to 1967, when the water level in the Quabbin Reservoir was 20 ft lower than it is today.
“This drought is bad, but it’s not the worst,” Boutt said. “It’s probably not yet in the top five drought periods in New England historically, so we need to keep things in perspective. Drought is a normal part of the water cycle.”
Boutt, who tracks natural chemical signatures in groundwater to research the hydrology of Massachusetts watersheds, says a series of drought years also occurred in the 1980s, and a lesser dry period was noted in the early 2000s.
This year’s drought is so noticeable because it has come largely in the growing season, so everyone from farmers to homeowners to gardeners has felt its effects. It also follows an almost 15-year period of higher-than-normal precipitation. “Droughts are multi-year events; they take some years to develop. And, like the others, this one will be felt for longer than one season," Boutt said. "When soils are so dry ... it will take time for the hydrologic system to recover.”
There also is a lag time in how groundwater responds to precipitation, a concept known as groundwater residence time, or how long water remains in the groundwater portion of the hydrologic cycle. Groundwater residence time can be extremely variable, from two weeks to thousands of years, though surface water is usually resident for shorter periods.
“If you ever wondered how your local stream is still flowing even though it hasn’t rained for three months, the answer is that the stream is fed by a slow, slow draining of groundwater through the system," Boutt said. "This can take five to 25 years in Massachusetts. Most rivers’ discharge is groundwater that may have been there for months or years coming to the surface through natural processes.”
Boutt's team recently received nearly $50,000 from the U.S. Geological Survey to establish a high-resolution map and database of natural chemical signatures—hydrogen and oxygen isotopes found in surface water, precipitation and groundwater—to better understand the isotopic composition of waters in the state and how groundwater is changing as a result of human activities. The team is actively seeking water samples from interested citizens.
“Using isotopes, we can track the source of moisture coming into New England. When water evaporates into the atmosphere, it picks up a signature. Using isotopes, we can clearly tell if our rain is from the Pacific, the Gulf of Mexico, the Atlantic or the Arctic by measuring these isotopes. We can use all of this to help us understand where all the water is coming from,” Boutt said.
With the isotope project, the researchers will create a public baseline and online tool where regulators, homeowners, watershed associations and researchers can get information on a range of isotopic concentrations that tell them the residence time for each aquifer.
“At the state level, such a database can tell us how responsive a particular watershed is to natural climate variability. Right now, decisions are being made with not enough data and we hope to improve that situation,” Boutt said. “If we better understand the sources of moisture and how isotopes are moving through our local water systems we can better understand how water is being affected in a changing climate.”