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UCSC scientists study groundwater renewal

Susan L. Young, The Californian
2/25/2011

http://www.thecalifornian.com/article/20110226/NEWS01/102260321/A-pond-ponder?odyssey=tab

UCSC scientists study groundwater renewal

Monterey County is overdrawn.

In the Pajaro and Salinas valleys, thirsty municipalities and farms have used underground water faster than it can be replenished, leaving the systems in overdraft.

Groundwater levels are low in many parts of the region as a result, but farmers along the coast feel a particular symptom of the overdraft when their wells go salty, the result of seawater invading the empty pockets of rock that once held freshwater.

Water management agencies in the valleys are working to balance the underground water supplies with redistribution programs that reduce the need to pump from heavily used wells, and encourage rain water to find its way from the surface down into the water-bearing rock.

But no management program alone is able to balance the high demand for water in Monterey County, which draws up to 90 percent of its water from underground aquifers.

Experts say they must continue to research how to improve the health of groundwater systems while supporting the water needs of the industries that sustain the community: agriculture and tourism.
A pond to ponder

With big issues, sometimes it's best to start small.

In a 7.5-acre pond tucked in the middle of Pajaro Valley's berry farms and decorative plant nurseries in southwest Watsonville, scientists are studying the speed and quality of water filtering into the underground aquifer. Their findings could help water management agencies choose the best methods for balancing supply and demand in the huge underground water systems that stretch below the Salinas and Pajaro valleys.

Bouncing along in a black university pickup truck, University of California, Santa Cruz, graduate students Calla Schmidt and Andrew Racz drive through a web of rough, dirt roads off west of Highway 1. The truck stops and Schmidt hops out to unlock the chain-link gate that leads to their outdoor water laboratory.

Each winter, the pond is filled with water diverted from Harkins Slough, a tributary of the Watsonville Slough complex that flows eastward under Highway 1, according to the Watsonville Wetlands Watch. As water fills the pond, its sandy bottom allows the liquid to filter down into pockets of rock below. The water is stored during the wet winter and then distributed to coastal farmers in the dry summers.

"It's kind of like a bank," Schmidt said. "You put savings in during the winter and pull them out during the summer when you need it."

The saved water builds up on a clay layer about 100 feet below the base of the pond.

Ten sky-blue "recovery" wells ring the pond, all within 100 yards of the pond's edge. These wells pull water out from the ground below and send it to the Coastal Distribution System, a network of pipes that delivers water to farmers along the coast.
An invading force

The Pajaro Valley Water Management Agency built the 20 miles of pipeline that stretch from Moss Landing north to Sunset Beach as a means to distribute water to coastal farmers whose wells have been contaminated by seawater. The agency also built and operates the Harkins Slough recharge project.

"Some ranchers that are connected to our coastal distribution system say they can only get their water that way because their wells are intruded with seawater," said Brian Lockwood, hydrologist for the agency.

Dale Huss, general manager of Castroville-based Sea Mist Farms, is familiar with the saltwater problem.

Back in the mid-1990s, Huss was having some mysterious trouble with one of his coastal crops.

"I couldn't figure out what was going on with our artichokes until I happened to get a shot of sprinkler water in the face, and I could taste the salt. It became very clear at that point," he said.

Huss' problem has been the bane of many farmers along the coast, dating back to 1953.

When groundwater is pulled out faster than it can be replaced by freshwater, seawater begins to seep in, a process known as "intrusion."

The Pajaro Valley ranches of Sea Mist farms now use the Coastal Distribution System as a water source.

"If we were still using well water along the coast there's a very good likelihood we would not be able to farm because of the high salts that the wells were putting out," Huss said.

The Coastal Distribution system also helps keep precious freshwater in the ground.

"The best way to reduce intrusion into the basin is to stop or greatly reduce pumping along the coast," Lockwood said.

It only takes 1 gallon of saltwater in 100 gallons of freshwater to render the water undrinkable and unsuitable for crops, according to Andrew Fisher, professor of Earth and planetary sciences at UC Santa Cruz.
Outdoor laboratory

Fisher, who leads the UC Santa Cruz studies on the recharge pond, has been studying the groundwater systems of Pajaro Valley for more than 10 years.

"We'd like to know if there is a way to get more water into the ground," Fisher said, "and whether this knowledge could be used to create similar projects in different parts of the valley."

To Fisher, the pond is a semi-natural, semi-engineered laboratory that benefits from a large number of sampling wells and the ability to monitor the amounts of water going in and being pulled out.

Tire tracks at one end of the pond serve as reminders for the cyclical nature of the outdoor lab. Each summer, the pond is drained so that the fine grain silt that builds up and eventually clogs the recharge pond can be scraped up and removed. When winter rains fill Harkins Slough, the excess water is used to refill the pond. As water seeps down into the sediments below, more water is diverted from the slough.

When the pond is dry and empty, the scientists can drill holes for instruments that measure the moisture content of the underlying soil and the temperature and pressure of the water that seeps through it.

Many of graduate student Racz's instruments operate independently. Once the pond is filled, they get to work, storing information until the instruments can be collected at the end of the operating year. By monitoring how the temperature of the water changes at different depths, Racz can infer the speed at which it percolates down through the sand.

To the eye, the earth that lines the bottom of the pond may look the same from one end to the other, but Racz finds that water flows through it at surprisingly different rates. Even though the area of the pond is small, water may filter down quickly in one location and dribble slowly in another.

This variation could be due to the sediments that wash in from the slough or the land around the pond, Racz said, and one location might receive more sediment than another.

Although large particles of dirt are filtered out of the water at the site of the slough, small bits do get through. Eventually, those small bits build up and block the water's downward movement. At that point, the pond has to be drained and scraped.

"It's teaching us how to better augment our water resources," Racz said of the study. By learning how these ponds operate and which conditions allow water to filter into the groundwater systems, the research program can help residents of the Pajaro Valley and other regional stakeholders get more water into the ground.
Filtration findings

When she was still doing field work, graduate student Schmidt would also take advantage of the pond's dry season to set up instrumentation in its base. After three years of taking measurements and pumping water samples from the bottom of the filled pond, she is now analyzing her data.

Schmidt has been studying the quality of the water as it moves through the bottom of the recharge pond. In particular, she analyzed the removal of nitrate, a form of nitrogen.

"In agricultural areas, nitrogen pollution is a common problem," Schmidt said. Nitrate leaches into water from fertilizer runoff from farms and from septic systems.

Infants who drink nitrate-contaminated water can become seriously ill and can die if left untreated, according to the Environmental Protection Agency. A byproduct of nitrate made in the digestive system prevents blood cells from carrying oxygen and can cause shortness of breath and blue baby syndrome.

She finds that about half the nitrate is removed from the water as it travels just three feet or so through the pond's sandy bottom. Naturally occurring bacteria convert the nitrate into harmless nitrogen gas, the main component of Earth's atmosphere.

"The ultimate goal is to get a lot of water into the ground, but we are also achieving a water-quality improvement," Schmidt said.

That level of reduction is on par with forcing water through patches of vegetation, such as those found in wetlands, a common strategy for management of contaminants like nitrates, she said.

The results of Schmidt's nitrogen study may help the Pajaro Valley Water Management Agency decide whether to use recycled water in the recharge pond, said Lockwood, the principal geoscientist for the agency.

Recycled water is an important source of alternative water for Monterey County. Since 1998, the Castroville Seawater Intrusion Project has sent about 45 billion gallons of recycled water to farmlands in the Castroville area, according to Keith Israel, general manager of the Monterey Regional Water Pollution Control Agency.

The UC Santa Cruz study could also help the agency understand the physical processes driving recharge within the basin, Lockwood said.

In 2012, the agency will form a new basin management plan, or BMP, which guides the major projects and programs the agency pursues, with the goal of solving the overdraft problem in the basin, he said.

"Hopefully, our data can help them during the drafting of the BMP," UCSC's Fisher said. "We want to provide them with information they can use."

For more information:

UCSC Fisher lab recharge pond project: http://es.ucsc.edu/~afisher/Research/ResDesc.htm#land

Harkin's Slough Project, Pajaro Valley Water Management Agency: http://www.pvwma.dst.ca.us/basin_management_plan/harkins_slough.shtml

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