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Posts Tagged: aquifer
Study offers insights on reducing nitrate contamination from groundwater recharge
Light irrigation before flooding stimulates microbes to remove nitrates from soil
With California enduring record-breaking rain and snow and Gov. Gavin Newsom recently easing restrictions on groundwater recharge, interest in “managed aquifer recharge” has never been higher. This process – by which floodwater is routed to sites such as farm fields so that it percolates into the aquifer – holds great promise as a tool to replenish depleted groundwater stores across the state.
But one concern, in the agricultural context, is how recharge might push nitrates from fertilizer into the groundwater supply. Consumption of well water contaminated with nitrates has been linked to increased risk of cancers, birth defects and other health impacts.
“Many growers want to provide farmland to help recharge groundwater, but they don't want to contribute to nitrate contamination of the groundwater, and they need to know how on-farm recharge practices might affect their crops,” said Matthew Fidelibus, a University of California Cooperative Extension specialist in the UC Davis Department of Viticulture and Enology.
A recently published study by UC scientists sheds new light on how nitrates move through an agricultural recharge site and how growers might reduce potential leaching. Researchers analyzed data from two grapevine vineyards at Kearney Agricultural Research and Extension Center in Fresno County – one flooded for two weeks, and other for four.
Understanding initial nitrate levels crucial
A key factor in mitigating contamination is understanding how much nitrate is in the soil at the outset, said study author Helen Dahlke, a UC Davis hydrologist and leader of UC Agriculture and Natural Resources' strategic initiative on water. In areas with little precipitation and cropping systems that require greater amounts of synthetic fertilizer, the accumulation of residual nitrate – resulting from nitrogen in the fertilizer not taken up by the plants – can be quite high.
“The percentage of nitrates in some soils can really increase over the years, particularly if you have many dry years in a row where you don't have access to irrigation water or natural precipitation flushing some of those nitrates out of the soil,” Dahlke said.
While intense rains in recent weeks have helped dilute nitrate concentrations naturally, farmers looking to participate in recharge during the dry years ahead should consider flooding their fields with greater volumes of water.
“If you're doing this for the first time – on-farm recharge in the winter – check your residual soil nitrate levels because if they're very high, you should apply a lot of water in order to make sure that the residual nitrate is diluted down,” said Dahlke, who also added that growers should check their soil properties for suitability of recharge projects.
She recommended using, as a “good first approximation,” the online Soil Agricultural Groundwater Banking Index map, a project led by Toby O'Geen, a UC Cooperative Extension soil resource specialist.
Researchers looking at other ways to reduce nitrates
Even before flooding the fields for recharge, there are several practices that can lower initial nitrate levels and risk of leaching. Cover crops such as alfalfa and triticale, for example, can help take up residual nitrates that accumulate from fertilizing a main crop over time.
Dahlke and Fidelibus – a co-author of the San Joaquin Valley vineyard study – both pointed to pre-flooding irrigation that encourages denitrification, a process in which soil microbes transform nitrates into gaseous forms of nitrogen.
“Those denitrifying microbes need to be stimulated to do the work,” said Dahlke. “What we have found is that if you do a little bit of irrigation before you start the flooding, increasing the soil moisture can get those microbes started and they can take out more nitrate from the soil.”
The timing and quantity of fertilizer applications are also major factors in reducing leaching. Although more growers are following high-frequency, low-concentration practices to maximize uptake by crops, Dahlke said there needs to be more emphasis on incorporating nitrogen transformation processes – such as denitrification – in the nutrient management guidelines that farmers follow.
“Implementing thoughtful nutrient management plans will play a particularly important role in participating farms,” Fidelibus added.
A more holistic view of groundwater recharge
In short, choices made during the growing season can affect those in the winter recharge season – and vice versa. For example, applying compost or other organic amendments to soil can give microbes the “fuel” they need for sustained denitrification.
“What we have found is that our denitrifying bacteria often run out of steam because they don't have enough carbon to do the work,” Dahlke said. “Like us, microbes need energy to do the work, and for microbes this energy comes from soil carbon.”
Then, adding moisture via recharge to that field with high organic content can stimulate mineralization and nitrification, processes in which microbes transform the organic nitrogen into ammonium – and subsequently nitrates – that the plants can then take up. Those naturally occurring nitrates would thus reduce the need for the grower to apply synthetic fertilizer.
“The winter on-farm recharge experiments have shown that altering the moisture regime in the winter has consequences for the nitrogen budget in the summer growing season,” Dahlke explained. “Theoretically, what we need to be doing is better integrating both seasons by keeping an eye on the soil-nitrogen balance across the whole year so that we can ensure, at the end of the growing season, the residual nitrate in the soil is minimized.”
The study, published in the journal Science of The Total Environment, was part of the post-doctoral work of former UC Davis researcher Elad Levintal. In addition to Fidelibus and Dahlke, other authors are Laibin Huang, Cristina Prieto García, Adolfo Coyotl, William Horwath and Jorge Rodrigues, all in the Department of Land, Air and Water Resources at UC Davis.
El Niño isn't filling Monterey County lakes as quickly as other state reservoirs
In December, Lake Nacimiento was at 16 to 17 percent of capacity. It has now risen to 22 percent. Lake San Antonio, which dropped to 3 percent of capacity last summer, is still at 3 percent now. It is so low that engineers refer to it as a "dead pool" because gravity cannot pull water out of the reservoir when it is at that level.
The Monterey County lakes don't fill as quickly as other lakes - such as Shasta, Folsom and Oroville - because they are fed by relatively small watersheds. Nacimiento and San Antonio were built in the 1950s and '60s for flood control and to recharge aquifers. With dropping aquifer water levels, farmers have had problems with their wells, the story said.
“Some growers' wells pull in as much air as water, so that they need repairs or lose the wells entirely. I've seen well drillers around, which indicates re-drilling,” said Michael Cahn, a UC Agriculture and Natural Resources Cooperative Extension advisor in Monterey County. “This is a cost for agriculture.”
As they drill deeper, farmers also risk more seawater flooding in, contaminating the limited water supply. It was seawater intrusion that originally led to the construction of the Nacimiento and San Antonio dams.
Cahn was quoted at the end of the story with a positive message.
“The aquifers are currently at the lowest levels ever recorded, but they can go back up,” he said.
Researchers study farm field flooding for aquifer recharge
UC ANR Cooperative Extension specialist Toby O'Geen was the lead author of research published in California Agriculture journal that identified agricultural lands in California suitable for flooding in order to bank groundwater. He has created an app that allows landowners across the state to assess the suitability of their property for groundwater banking.
The Modesto project will determine what impact winter flooding will have on the health of almond trees and almond yield. UC ANR Cooperative Extension advisor Roger Duncan was quoted in the Los Angeles Times about the potential advantages and disadvantages of flooding crops in the winter. He said water could spur more fungal diseases, but could also drown out worms and mites that damage crops.
The Almond Board of California is funding the project, anticipating that certain almond orchards will be good candidates for groundwater recharge.
"Almond orchards have good soil characteristics, and water delivery systems are already in place,” said Bob Curtis, director of agriculture affairs for the almond board. “Winter flooding should actually benefit the trees while replenishing groundwater to benefit us all."
Following are recent articles about the project:
Researchers test a possible drought solution by flooding an almond farm
Geoffrey Mohan, The Los Angeles Times, Jan. 20, 2016
(Reprinted in Daily News 24/7)
Scientists flood almond orchards to restore groundwater in California
Capital Public Radio, Jan. 20, 2016
Stormwater floods Modesto almond orchard in experiment to restore aquifer
San Jose Mercury News, Jan. 19, 2016
(Reprinted in the Contra Costa Times)
Researchers show off groundwater recharge near Modesto
Modesto Bee, Jan. 20, 2016
(Reprinted in the Fresno Bee and Bloomberg Business)
UC Davis scientists flood Modesto orchards in hopes of finding way to restore groundwater
CBS13, Sacramento and Modesto affiliates, Jan. 20, 2016
Researchers test a possible drought solution by flooding an almond farm
KTLA News 5, Jan. 20, 2016
(Rebroadcast on KRQE News 13)
Orchard tries experiment to restore aquifer
Morning Ag Clips, Jan. 20, 2016
Almond orchard key to water banking experiment
AgraNet, Jan. 20, 2016
