Central Maui's groundwater has gotten saltier over the past four decades due to increased pumping of surrounding wells and a decreased amount of rainfall, experts said.
Salinity levels, or the amount of chlorine, potassium and other elements in saltwater, have been increasing at the Waiehu deep monitor well, according to the latest U.S. Geological Survey report commissioned by the state Commission on Water Resource Management and the county Department of Water Supply.
The Waiehu well is used to monitor the Wailuku area, where most of Central and South Maui's groundwater is pumped from and then used for fire protection, domestic and irrigation purposes.
Salinity is measured at two points in the well - the "midpoint," or where the water consists of 50 percent seawater and 50 percent fresh water; and the "freshwater lens," or the depth at which there is only 2 percent saltwater.
Currently, the midpoint of the groundwater in the 1,020-feet deep Waiehu well is at 626 feet. In 1985, when USGS hydrologists first started monitoring the well, the midpoint was nearly 200 feet deeper, at 822 feet.
Because freshwater floats on top of saltwater, the shallower the midpoint, the saltier the groundwater, explained USGS research hydrologist Stephen Gingerich, who is based on Oahu. He is in charge of gathering data at the monitoring well in Waiehu, as well as five other wells on Maui.
Hydrologists also look at the depth at which fresh water is no longer safe for drinking - the "freshwater lens" - at which salinity levels are at 2 percent.
Currently, the freshwater lens rests at 495 feet. In 1985, the freshwater lens was at 690 feet deep.
Salinity levels "have been rising at the same trend now for almost the past 20 years," Gingerich said. "A lot of it's due to pumping. Once you start pumping from a well, the freshwater lens is going to shrink, saltwater's going to move up."
He added that the last 10 years of "below-level rainfall" have also affected the increased salinity of the well's water.
"There is a serious problem," said Kihei resident Buck Joiner, who was alarmed when he noticed the report's finding of continued rise in salinity levels, in an email. "It is pretty obvious we are overpumping, and the ocean water is creeping up . . . This looks like a long-term trend, and that is not good."
Joiner also noted that his tap water has been tasting "weird" lately, though he couldn't say for sure if that is caused by the increased salinity.
Gingerich said he wouldn't be too worried because it would be at least another 100 years, if current pumping rates and rainfall levels continue, before the groundwater in the area becomes too salty to sustain itself, and the county would most likely take action "long before it gets to that."
"The county is tasked with providing fresh water. That's why they pay us to do these monitoring studies," Gingerich said.
The county has an obligation to provide safe drinking water under the federal Safe Drinking Water Act, and county water officials are unlikely to violate their duties, according to county Water Director Dave Taylor.
"There are regulatory agencies - the state Department of Health for water quality, the (state) Commission of Water Resource Management for water quantity, and ourselves - to regulate our operations within parameters of law," Taylor said.
For example, the state allows the county water department to pump a "sustainable yield" of 20 million gallons of water a day from the Iao aquifer, the most-pumped aquifer in West and Central Maui. On average, the county pumps 16.2 million gallons per day, according to Taylor.
As for why residents' tap water may be tasting unusual or salty, he said that could stem from a number of factors ranging from the water purification process to old pipelines. Residents with concerns should call the department at 270-7633 so field officers can be dispatched to collect water samples, he said.
* Eileen Chao can be reached at firstname.lastname@example.org.