expands local water supplies
By Michael Burge
April 14, 2006
– Years before a company proposed purifying ocean water to
make drinking water at a beachfront desalination plant,
Oceanside embarked on a smaller project that does something
similar with groundwater.
LAURA EMBRY /
through a tank to remove carbon dioxide at the end of
the purification process, then the water is pumped
through pipes to the distribution system.
Part of the city sits atop a
large underground basin beneath the San Luis Rey River. Water
from wells that tapped that basin supported the farms that
congregated in the valley in Oceanside's early days.
That same basin today is the
source for a project that strips minerals from brackish
groundwater at a rate of 2 million gallons a day – an amount
that is expected to triple to 6.3 million gallons by this summer
after 2 new wells come on line.
Those wells will complete a $13
million expansion that will result in the plant supplying about
20 percent of the city's average daily demand, Barry Martin, the
city's director of water utilities, said. That's enough water to
provide for 35,000 of the city's 175,000 residents.
The city's motivation for
building the Mission Basin Groundwater Purification Facility,
which initially went into operation in 1994, is the same as the
San Diego County Water Authority's reason for exploring seawater
desalination as a future source: to develop reliable local water
“This plant was in response to
the drought” of the late 1980s and early '90s, Mike McGrath,
the plant's manager, said Tuesday during a tour of the facility.
At that time, water customers
faced a possible 50 percent cutback in supplies after five dry
The drought prompted several
projects aimed at developing local sources and reducing
dependence on water imported from Northern California and the
The County Water Authority and
the Olivenhain Municipal Water District built the region's first
large dam in 50 years, for example. That project was completed
And the water authority began to
consider desalinating seawater as a possible source.
Resources Inc. has proposed building a plant that would desalt
50 million gallons of ocean water a day at the Encina Power
Station on the Carlsbad coast. Poseidon hopes to begin operating
such a plant by 2009-10. The proposal is undergoing
Oceanside had known for years that
the ground beneath the San Luis Rey River was a treasure chest
for potable water, but the development of aqueducts that carried
water from outside the area had eclipsed groundwater as a
The drought taught Oceanside and
the county a lesson: Develop local supplies or cut back in dry
But consumers' tastes had changed
when the city began to reconsider pumping the water from the
Mission Basin, as the underground stream in Oceanside is called.
“Up until 1960 this would have
been drinking water,” McGrath said. “Aqueduct water – the
quality and availability – made well water less desirable.”
So the city developed the Mission
Basin Groundwater Purification Facility, on Heritage Street just
east of the former drive-in, to pump the water from the ground
and treat it.
McGrath said the water is potable
even before the city runs it through a reverse osmosis process
to strip it of iron and manganese, minerals that stain clothing
in the laundry.
“It's an aesthetic thing, not a
health hazard,” McGrath said.
The plant's first steps in the
process are to add chemicals that suspend the iron and manganese
in a solution and to adjust the acid level, then run the water
through filters to remove suspended solids.
McGrath said the filtering is
minimal, because the water has already traveled though miles of
underground rock and sand, nature's own filters.
McGrath noted that groundwater
filtering differs from seawater filtering, because ocean water
may contain 20 times more solids.
The heart of the Oceanside process
is two “trains” of reverse osmosis membranes, which have
microscopic pores so tiny that nothing larger than a water
molecule can pass through.
The membranes are contained in 48
tubes with seven membranes in a tube, for a total of 336
membranes. The water is driven through the membranes under
“You don't want the minerals to
(stick) on the membrane's surface,” McGrath said, because that
would foul them. “You want them to be flushed away as a
concentrate or brine while the pure water passes through.”
The rejected water goes out to
the ocean through the city's outfall.
McGrath noted that the Oceanside
plant forces water through the membranes at about 120-140 pounds
of pressure per square inch. He said seawater would require
about five times the pressure, because it contains more
minerals. That also would increase energy costs.
He noted also that Oceanside's
demineralization process produces 3 gallons of pure water for
every 4 gallons that enter the system. The proposed seawater
desalination process would produce 1 gallon of pure water for
every 2 that enter the plant.
Test wells planned
McGrath said that after the water
passes through the reverse osmosis membranes, the plant trickles
it through a tank to remove carbon dioxide, and adds some
minerals to bring it to customary levels. Then it blends the
water with the rest of the city's supply.
McGrath noted that desalinating
seawater has one thing over demineralizing groundwater.
“Seawater is unlimited in
supply, whereas groundwater is limited to whatever the natural
recharge is,” from rain and runoff, McGrath said. If the city
overpumps its wells, ocean water can seep in from the west,
fouling the underground stream.
In fact, McGrath said, the city
plans to drill test wells near the mouth of the San Luis Rey
River that would probe the water beneath the underground layer
of ocean water, to see if the city can reclaim that.
The city has nine operable wells
in the basin, Martin said, and is about to add two more.
The only other facility in the
county that compares in size with Oceanside's demineralization
plant is the Sweetwater Authority's Richard A. Reynolds
Groundwater Desalination Facility in Chula Vista, which can
produce 4 million gallons a day of pure water from brackish