WHEN you drink a glass of water, the
first question that might come to mind is not how old is this
water? but whats in this water? A study
at Lawrence Livermore is demonstrating that, in many cases, answering
the first question can help answer the second.
California residents have
been asking a lot of questions about their drinking water, in particular
about their groundwaterthe source for 50 percent of the states
drinking water. The closing of several public drinking wells because
of contamination has concerned citizens so much that in 1999, the
state mandated the Groundwater Ambient Monitoring and Assessment
(GAMA) Program to investigate to what degree groundwater is susceptible
The GAMA Program calls for testing every one of the approximately
16,000 public drinking wells scattered throughout the state. It
is a daunting task, to be sure, but Livermore is a natural choice
for tackling the challenge. Livermore has a history of working
with the state on groundwater issues and providing solutions,
according to environmental scientist Dave Rice. In 1999, for example,
the Laboratory was part of an investigation that looked at leaking
underground fuel tanks (LUFTs) and their effects on the states
groundwater. (See S&TR, April
MTBE with Statistical Data.) The Laboratory developed GeoTracker,
a geographic information system that provides online access to data
such as LUFT sites and all public well sites in California. Maintained
by the state, GeoTracker is providing a repository for data generated
by the GAMA Program.
According to physicist Bryant
Hudson, Livermore scientists have partnered with the California
State Water Resources Control Board and the U.S. Geological Survey
to test 1,200 wells so far. Two years into the project, the scientists
have analyzed wells in the Los AngelesOrange County basin,
the Santa Clara Valley, the LivermoreNiles area, and the counties
of Sacramento, Butte, Fresno, and San Joaquin. During this testing,
they have also been educating the public and students in schools
An important part of the Groundwater Ambient
Monitoring and Assessment Program is public education. Here,
at the 2002 Edward Teller Science and Technology Education
Symposium, Bryant Hudson uses a groundwater model to show
students how water moves in a simulated aquifer.
Young and Vulnerable
With a suite of analytical tools at their disposal, Laboratory scientists
are developing a comprehensive picture of the states groundwater
resources. They are determining where contamination has occurred,
what the groundwater flow pattern is, and from where the groundwater
originates. The work begins with age-dating water from municipal
drinking water wells. For groundwater geologists, the age of water
is a good indicator of its probability of contamination.
Geochemist Jean Moran says using age to assess the vulnerability
of groundwater to contamination is based on a simple concept: younger
water has been in the aquifer for a shorter time, so it has more
recent contact with ground surfaces where contaminants are present.
Older water that has been in the subsurface for hundreds or thousands
of years will have stayed relatively isolated and protected from
any pollutants on the surface.
To determine how long the water has been out of contact with the
atmosphere, Livermore scientists use the tritiumhelium-3 method,
a capability available only in a handful of laboratories worldwide.
Tritium, a radioactive hydrogen isotope, occurs naturally at very
low levels in Earths upper atmosphere, but it was produced
in much greater amounts during atmospheric nuclear testing in the
1950s and 1960s. Thus, it is an excellent tool for age-dating water
that has entered an aquifer in the past 50 years. Tritium has a
half-life of 12.3 years; it decays into helium-3, a stable noble
That helium-3 remains in solution once water containing tritium
enters an aquifer. As tritium continues its decay over time, the
amount of helium-3 in the water grows and the amount of tritium
declines; the sum of both stays constant. By measuring both the
remaining tritium and the decay product helium-3, scientists can
determine the time at which the water entered the aquifer.
To measure tritium, the scientists remove all of the helium from
a large water sample so they can see the decay of tritium into helium-3
over a two-week period. The beauty of all of this is in the
sensitivity of the measurements, Moran notes. Noble gas mass
spectrometry allows measurements sensitive to just a few thousand
helium-3 atoms. With such precision, the scientists can determine
the age of a water sample to within about one year. These age determinations
allow them to directly infer the direction and rate that the groundwater
in the aquifer is flowing.
of tritium–helium-3 ages measured in drinking water wells
from Los Angeles and Orange counties. A general pattern of
increasing age away from the artificial recharge areas (where
water entering the aquifer reaches saturation) is observed.
The red line shows the boundary between younger groundwater
and groundwater more than 50 years old. The measured age shows
the direction and rate the groundwater in the aquifer is flowing.
Tracking the Contamination
To test the principle
that younger water is more likely to be contaminated, Livermore
perform several types of analyses to get a
larger picture about groundwater resources in the state. One
analysis detects actual contamination at ultratrace levels, using
in combination with mass spectrometry to track volatile organic
compounds (VOCs) such as the gasoline compound methyl tertiary-butyl
ether (MTBE) and the dry cleaning solvent perchloroethylene (PCE).
Such VOCs are ubiquitous and can be detected at very low concentrations
nearly anywhere on Earths surface. The analyses require
that the mass spectrometer used to detect VOCs in groundwater
clean; the responsibility for keeping it that way falls to Livermore
scientist Roald Leif, who makes it possible to detect VOCs to
parts-per-trillion sensitivity. The detection results indicate
where groundwater has
been contaminated and also verify the age-dating results.
In another analysis to
round out their information about the states
groundwater, the scientists measure two stable oxygen isotopes
(oxygen-16 and oxygen-18) to determine the origin and flow patterns
water. The ratio of oxygen-16 to oxygen-18 in the water varies,
depending on where the water precipitated, the elevation at which
it was found, and its distance from the ocean. For example, water
from the Sierra Nevada has a different ratio of these oxygen
isotopes than that found in precipitation in the San Francisco
Using a mass spectrometer, the team can measure the isotope ratio
and determine the source of the water.
Older May Be Better
Some of the data analysis shows that older water can remain relatively
contaminant-free, despite its location. For example, the Silicon
Valley, with its large number of contaminated waste disposal sites
and more Superfund sites than anywhere else in the nation, has remarkably
uncontaminated drinking water wells. Because most of the Silicon
Valley water has been resident in underground aquifers for longer
than the contaminants have been present, the water has been relatively
protected, and very few VOCs have been detected. A similar example
is in the Los Angeles and Orange County basins. Despite their urban
location, 59 of 176 wells tested were devoid of tritium, and those
same wells were free of VOCs. The pathway of water into the aquifer
is blocked by thick layers of clay, and that has kept most of the
water underneath the basin protected from surface contaminants over
the past 50 years.
The vulnerability of younger water to contamination can be seen
in the Livermore Valley. The east side of the water basin (under
the city of Livermore) has widespread PCE occurrence and younger
groundwater ages than the west side. Water located on the west side
of the basin, under the city of Pleasanton, approximately 10 kilometers
away, is protected by a confining layer that prevents the direct
transport of water from the surface to the aquifer.
Next year, the group hopes to take the age data accumulated so far
and study nitrate contamination, which is the most frequent cause
for shutting down a drinking water well in California. In the meantime,
Livermores data showing that newer isnt better will
help the state make informed decisions to protect wells and plan
future development. Moran says California intends to increase its
use of groundwater for drinking purposes, and these data could keep
the state from making costly mistakes.
If you know the age of the groundwater in a basin, it can
tell you how fast water is moving, where it is being replenished
in the ground, how much storage you have, how fast water turns overa
complete picture, Moran says. Water is a huge issue
in California, so the more data you have, the better.
Acknowledgment: This work is managed by the Environmental Protection
Department in collaboration with the Chemistry and Materials Science
and the Energy and Environment directorates.
Key Words: age-dating, groundwater contamination, stable oxygen
isotopes, tritiumhelium-3 analysis.
For further information contact Jean Moran (925) 423-1478 (firstname.lastname@example.org).
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