Polluted ground water in Central Valley caused by "fertigation"
at Washington University in St. Louis has conclusively
tied high nitrate levels in California ground water to one main source -- the
state's multibillion dollar agricultural industry.
Robert Criss, Ph.D., Professor of Earth and Planetary Sciences at Washington and his colleague Lee Davisson at Lawrence Livermore Laboratory, have been able to establish a graphic link between high nitrate concentrations in water and agricultural "fertigation" throughout the 7 million intensively irrigated acres in California's bountiful Central Valley. Fertigation, a common practice in large, irrigated farming operations, mixes nitrogen fertilizers with ground water, right at the irrigation pumps. Criss and Davisson analyzed ground water samples from water supplies in Davis, Sacramento, Brentwood and other communities.
Nitrate comes from various sources, including sodium nitrate or potassium nitrate used as fertilizer. In high doses, nitrates can be harmful, especially to infants. The technique Criss uses, stable isotope mass spectrometry, allows him to identify sources of ground water by their isotopic signatures. Maps, based on the trace oxygen-18 content of water samples taken throughout the area, graphically reveal the locations, migration and mixing paths of the different water samples -- a geochemical CAT scan of the area's water supply.
The link is proof that alarmingly high levels of nitrates in drinking water -- some well above the E.P.A. acceptable limit of 45 parts of nitrate per liter -- of nitrates in drinking water in these communities comes from "non-point source" pollution -- so-called because of its widely distributed sources. The technique they have perfected completely takes the guesswork out of ground water analysis and forces environmental officials to reconsider the nature and source of ground water contamination.
"We have established the source of nitrate, and it isn't Ma Jones' septic tank"
Davisson and Criss
presented a paper on their research last March
in Vienna, Austria, at the International Symposium on Isotopes
in Water Resource Management, sponsored by the U.N. International
Atomic Energy Agency. "We have established geochemical ties to the
source of nitrate in these samples, and it isn't Ma Jones' septic tank,"
declares Criss. "Historically, nitrate contamination of ground water has
been blamed on localized point-sources such a fertilizer plants, septic
tanks, feed lots, what have you. But we show that the high nitrate levels
of ground water in this region are primarily due to the widespread
application of fertilizers.
"Visually, it's hard to tell one glass of water from another, but we have isotopic tracers, intrinsic to water molecules themselves, that can differentiate the molecules by their origins, and then we can get a dramatic visualization of what's happening here. The message is that society should protect this ancient ground water. The quality of the supply is not only worsening with time, but it is already very bad in several areas. It's a sad, pernicious problem, and increased pumping due to population growth will only accelerate this degradation."
Criss and Davisson's work, which is ongoing throughout the Central Valley, has particular significance in light of Congressional interest to enact a new law that would loosen requirements for local governments to test ground water contamination. California is a land of Third World population growth, drought, excessive water use, turbulent water-rights battles and land subsidence from overdraft of aquifers, not to mention worsening contamination problems. Natural levels of nitrate in ground water should be less than 5 milligrams per liter, but Criss says his analysis of 30 years of data (1965 to 1995) shows that in Brentwood, for instance, practically all of the community's wells have at times greatly exceeded the EPA acceptable levels, forcing the shut-down of nearly every well and the drilling of new ones at substantial cost.
"The problem in the Brentwood region is particularly severe," Criss points out. "The community is expected to nearly triple its population in ten years, that will put extreme pressure on water tables unless other sources are found."
As is true throughout the United States and Europe, nearly one-half of California's drinking water comes from ground water. The figure is even higher in many states, which rely on ground water for as much as 90 percent of their drinking water.
Pristine ground water can be as much a 20,000 years old, and is essentially nitrate-free
spectrometry allows scientists to separate and
identify atoms (in this case, those found in ground water) by weight and
other characteristics. By comparing ratios of oxygen, hydrogen,
deuterium and carbon isotopes (heavier variations of atoms) in the
samples, Criss and Davisson have been able to determine the source and
relative age of the water. The researchers found that older ground water is
more pristine, normal in its carbon, oxygen and hydrogen isotopic ratios,
and lower in nitrates. Pristine ground water can be as much a 20,000 years
old, and is essentially nitrate-free and contains hardly any carbon-14
"Younger" ground water, which comes in contact with agricultural fertilizers, is high in nitrates, higher in carbon-14, and has a distinctive isotopic signature revealing the fact that a significant fraction of the water has been lost by evaporation, and thus can be traced to agricultural fields. The most definitive isotopic tracer is oxygen-18. An isotope of an element has at least one more neutron in its nucleus than normal, making it heavier. Normal oxygen has 8 neutrons; oxygen-18 has 10. Criss and Davisson's samples in the Central Valley show increased abundances of oxygen-18, which doesn't escape into the air upon evaporation as readily as normal oxygen. The researchers have linked the increased abundance of oxygen-18 with high levels of carbon-14 and higher nitrate contents, all of which are characteristics of irrigation water.
Using this technique, the researchers have analyzed hundreds of water samples and have been able to trace the source and age of ground water from supplies in Davis, Woodland, Winters, Brentwood and Sacramento. In Davis, for instance, they have analyzed samples from each of the city's 20 water wells and discovered nitrate-laden doses infiltrating the wells in summer. In Sacramento, they have been able to determine that ground water levels north and south of the American River are depressed and filling up with river water.
"In the past, ground water analysis has relied on expensive computer modeling of water tables and permeability data that creates a lot of gray areas in analysis," says Davisson. "But this method bridges a gap between an engineering perspective and a chemical one. It yields lots more information and gives water resource managers better predictive capabilities."
In the 1990s, water is competing with earthquakes as the chief geological and environmental hazard in California. The process of ground water use and recharge, where the water table is replenished after precipitation, in the Central Valley has been likened to mining of ore minerals. One problem is simple over-draft -- too much water is pumped out and the water table drops, forcing communities to lift the water out from aquifers as much as 1,000 feet into the ground. This is a particularly severe problem in the arid parts of Arizona and in irrigated farming regions of the West where farmers rely on deep aquifers, such as the Ogallala in the Great Plains, to irrigate crops. That aquifer is expected to dry up permanently within a few decades.
"The principle of water mining is very simple -- basically, you pump out the water and it recharges so slowly that it's effectively gone," explains Criss. "It's the same idea as digging out a coal seam until it's depleted. But we've identified a different kind of water-mining here in the Central Valley. It's a process of taking ancient, pristine water, using it for irrigation, mainly, and returning water of lower quality to the aquifer. Conceptually, this is not much different than digging gold ore out of the ground, extracting it, and then stuffing the processed tailings back into the hole."
"This is a global problem that has to be addressed"
the problem of degraded and diminished ground water
in the Central Valley is masked by the apparent abundance of ground
water on irrigated fields that grow as many as three crops of vegetables or
fruit during the long California growing season. There are 7 million
irrigated acres in the Central Valley. "Under natural conditions, you only
have winter storms to recharge the groundwater supplies, but today the
fields are flood-irrigated all summer long. It's wet all the time out there.
Modern recharge rates are ten times larger than natural rates in the Central
Valley. This recharge often is loaded with nitrates from fertilizers and
By Criss's calculations, beneath every acre of Davis, a community of about 50,000, there are nearly 7,000 pounds of nitrate in the first few hundred feet of ground water. That amount, he says, "is the same amount of nitrate you would get if you'd crash a tank truck of concentrated nitric acid at every intersection in the city and let the nitrate spill into the ground. These are gross quantities."
Moreover, he says fertilizer applications average 525 pounds of nitrate annually to every cultivated acre in California. "California has over 10 million cultivated acres, and this is what's being done to every single one of them," Criss says. "We've seen that you can get tons of nitrate in ground water in only a decade. You can't get tons from chicken coops or other point sources, located miles away. This is a global problem that has to be addressed."
Criss and Davisson plan to map the entire Central Valley with their isotopic tracers to better analyze the scope of the problem. Criss also is interested in the Midwest, where nitrates have been documented in ground water. In addition, each spring midwestern farmers apply more than 300 million pounds of herbicides alone to agricultural fields.
"Historically, every society that has relied too heavily on irrigated agriculture has had problems ranging from loss of ground water, to land subsidence, to salinization and waterlogging of the soil," Criss points out. "The answer to these problems in California has been to shut down existing wells and drill new and deeper ones, but that is a short-term mentality. The ground water degradation we are seeing there is insidious, like a cancer that will only be amplified by the water mining process."
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