O'ahu water quality could drop to salty
By Jan TenBruggencate
Advertiser Science Writer
Extended dry weather and increasing demand for water have worked some O'ahu wells so hard that they're starting to go salty.
Honolulu's pumping of underground water is nearing the maximum amount the island can sustain. Engineers are considering options to meet the increasing demand, from irrigating parks with recycled water as a conservation measure to building reverse-osmosis desalination plants.
The water isn't running out, but continued high levels of pumping will reduce its quality by raising its salt content, water officials say.
"These past five years of extended drought have driven pumpage into the high 170s and up to 180 million gallons per day, resulting in stress to some of our sources. By 'stress' we mean high chloride levels, pumping water from lower in the lens where water is saltier," said a statement from the Board of Water Supply water resources staff.
Cliff Jamile, manager and chief engineer of the Honolulu Board of Water Supply, said the island's residents have done well when asked to conserve. But conservation will only go so far as more and more people hook up to water mains.
"Whatever slack we had in the system has finally been used up," Jamile said. "We need a more revolutionary approach to water conservation. Old-fashioned approaches are no longer enough to protect our water resources."
O'ahu's domestic water, virtually all of which is pumped from underground, averages 155 million gallons a day, dropping below that during the wet months and jumping 10-or-more million gallons higher in August.
Jamile said the department is authorized by the state Water Commission to pump as much as 200 million gallons a day from underground but can't draw water at that level for long.
"You can't maintain that level without damage to the aquifer," he said.
When a well starts to get salty, engineers must allow it to "rest" and be recharged from rainwater. Meanwhile, the difference must be made up by pumping other areas more heavily.
The island's main underground water reservoir, or aquifer, is a lens-shaped body of fresh water, which floats on the salt water that permeates the rock below. The lens is thicker toward the middle of the island and thinner toward the edges.
The aquifer is recharged from rainfall on the island's surface, percolating down through the soil and rock.
Damage to the aquifer occurs when it is pumped faster than it is being recharged. The lens can narrow, meaning the water level in wells drops. And when that happens, salt water can contaminate the water supply.
One of the simplest techniques the department is promoting is planting trees in Hawaiian forests. Re-establishing forests with dense understories of ferns and mosses can prevent water from evaporating or quickly flowing off to the sea. The longer the water stays, the more of it percolates toward the aquifer, he said.
The Food and Agriculture Organization of the United Nations will stress the forest-water relationship at its World Forestry Conference in Quebec in September, saying that well-managed forests directly affect water yields.
"We are asking forest scientists to demonstrate more clearly the role of forests in influencing water balances. At the same time, we are asking foresters to make water management a prominent feature of their forest plans," said FAO forestry director R. Michael Martin.
Jamile said he is convinced of the link between dense, complex forests and water.
"We have to make sure that the maximum amount of water is, in fact, recharging our aquifer all the time, so that Hawai'i retains healthy water stocks," Jamile said.
The department is also experimenting with irrigating parks with recycled water — which is extensively treated sewage — in hopes the water will be purified by the natural filtration through topsoil and rock, and will recharge the aquifer more quickly than if the city waited for rain. Jamile said one such experiment is being conducted at the Central O'ahu Regional Park. Officials will monitor the ground water in the vicinity to be certain any contaminants have been filtered out before considering using the water.
The department is studying using recycled water instead of potable water where possible, to reduce the demand on drinkable supplies. Many areas are using treated sewage effluent to irrigate landscaping.
The city is studying several techniques for creating drinkable water from the ocean. One is reverse osmosis, in which the salt is essentially filtered out of the seawater. It is an energy-expensive process, but new designs are making it more efficient, and less costly, Jamile said.
Another water production technology is Ocean Thermal Energy Conversion, a process studied extensively at the state's Keahole Point energy laboratory on the Big Island. It was developed as a way to use the difference in temperature between deep cold water and warm shallow water to make electricity. But the condensation on the cold water pipes can be a good source of drinking water, Jamile said.
Each of these technologies will be considerably more expensive than sinking a well and pumping, but that's no longer an option for most regions.
"In the old days, any time you need more water, you just drill another well. But I think we have taken it to the point where we would not want to force the aquifer into yielding any more than it is yielding," he said.
"The era of cheap water is a thing of the past," Jamile said.
As the summer drought progresses, the Department of Water is again asking residents to conserve. Conservation has done well in the past, with annual per-capita consumption dropping 6 percent in the past 10 years. But the O'ahu population grew more than 8 percent to 881,000 in the same period, pushing demand ever higher.
Reach Jan TenBruggencate at jant@honoluluadvertiser.com or (808) 245-3074.
