Tuesday - April 22, 2014

Tigard’s underground ASR reservoirs store water, save money

Tigard has been buying its drinking water from Portland since incorporating as a city in 1961. In 2016, Tigard will join Lake Oswego in receiving drinking water from the Clackamas River.  The communities are working together to ensure customers have enough water when they need it, year round.

To serve water to 90,000 residents and businesses every day, Lake Oswego and Tigard will rely on high quality filtration, pipes, reservoirs and pumps that must be kept in good shape so the water is there when needed. It’s also important to consider those instances when the water supply may be reduced.  In times of low stream flow, for example, limitations can be placed on the amount of water withdrawn from the river. Since this can occur during the warmest (highest water use) times of the year, the two cities want to be ready.

Most cities, including Tigard and Lake Oswego, prepare for the worst by having connections with other water suppliers. Tigard has taken further steps to be ready for such events by building underground storage reservoirs, called aquifer storage and recovery (ASR) wells. The volcanic action of 17 million years ago created just the right conditions to store drinking water in the underground basalt aquifer beneath Tigard.  Cavities within the basalt rock were formed as lava flows cooled and solidified.  These cavities hold water much like a honeycomb contains honey.  Treated drinking water is stored in the basalt aquifer until it is needed in summer months.

Tigard developed two ASR wells in 2002 and 2005, respectively, with its third well planned to be in operation in 2017.  The two operating wells provide a total storage capacity of 500 million gallons (MG). The three wells will provide nearly  six million gallons of drinking water per day—a good buffer since each community can use up to 12 MG per day during summer peaks.

Tigard was the third city in Oregon to bring ASR reservoirs online, not only as additional supply, but to save money. Portland charges up to 2 ½ times more for water purchased in summertime.  Shaving millions of gallons per day from those purchases has meant substantial savings for Tigard water customers.  ASR also costs much less than constructing new, above ground reservoirs.  Today, a 3.5 MG above ground reservoir built nearby will cost around $7 million. To compare, Tigard’s second ASR well provides 280MG of storage water at a cost of $3 million. 

On top of these cost savings, Tigard Public Works Project Engineer Rob Murchison, also underscores the environmental and disaster prevention benefits. “ASR helps maintain in-stream flows in summer months for fish. And these underground reservoirs also give both cities an additional source of drinking water in case of disasters such as earthquakes or floods, when our primary supply might be disrupted.”

Joel Komarek, Project Director for the Lake Oswego Tigard Water Partnership concludes, “We are fortunate beneficiaries of Tigard’s foresight—and good geology.  This is great insurance.”