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Get to know your groundwater

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Portland's groundwater improves the reliability of our water system by providing a robust secondary drinking water source that supplements the water we get from Bull Run. Together, these two water sources ensure that we can deliver excellent water every minute of every day.
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What is groundwater?

Cross section graphic depicting the aquifers below the Columbia South Shore Well Field

With all the rivers, streams and lakes we have above ground, it's easy to forget that beneath our feet underground is a whole wide world of water that we rely on everyday, but rarely see. In fact, almost 99% of all of the liquid freshwater on Earth is in the form of groundwater! Groundwater is a common source of drinking water around the world and in the United States, where it supplies about 51 percent of the total population, and 99 percent of those living in rural areas.

Put simply, groundwater is the water found underground in the spaces between pieces of rock, like sand and gravel, and in the cracks that form in huge layers of solid rock. It usually gets there from rain and snowmelt that soaks into the ground, where it seeps through soil and other shallow rock material before landing in an aquifer—a layer of gravel, sand, or rock where all of the cracks and spaces are filled with water. In the Portland area, the groundwater we drink lives in three different aquifers: the Blue Lake Aquifer, the Troutdale Sandstone Aquifer, and the Sand and Gravel Aquifer. The Columbia South Shore Well Field (CSSWF) sits above these aquifers and draws water in multiple places from all three.

But groundwater doesn't just stay still. Over a long enough period of time, the water in an aquifer can seep into a river or lake, or even appear at the ground surface again as the source of a spring. And like the water levels in a lake or river, the level (or elevation) of groundwater in an aquifer changes over time and with the seasons, too. It's part of water's never-ending journey as it falls from the sky, moves through the earth and, eventually, is drawn up to the sky again.

How much groundwater do we have?

Large white tank several stories tall with deep purple and orange sunset sky behind it
The groundwater tank in the heart of the Columbia South Shore Well Field

In strictly unscientific terms, a lot.

The Portland area is rich in water, and while some local aquifers have lower groundwater levels now than they did before the region's population boom in the 1990s, our aquifers are not in long-term decline like those in some areas of the country. Portland's groundwater comes mainly from rainfall and from the Columbia River's water filtering slowly and deeply into the subsurface. With more than 20 wells in the well field, our groundwater supply can provide as much as 80 to 95 million gallons of drinking water per day.

The three aquifers that we pump water from are part of the Portland Basin, a 900-square-mile geologic area. The Basin was formed by converging tectonic plates and Columbia River Basalt flows, and filled in with more than 1800 feet of sediment deposited by the ancestral Columbia River and ice-age cataclysmic floods. The basin extends north to the Lewis River in Washington, south to the Clackamas River, west to the Portland Hills (Tualatin Mountains), and east to the Western Cascades, and is bisected by the Columbia River.

How do we get the water out of the ground?

A grassy knoll in the foreground with some fencing and a white box on the back side of it. Mt. Hood looms in the background.
An earth-covered well vault with Mount Hood looming to the east

Digging wells and pulling clean water from underground is a method that dates back several thousand years in human history. Most wells in historic times were dug by hand and manually drawn, often with the help of animal labor. A wide variety of techniques and styles of wells have been developed and used over the millennia, but they all follow the same basic idea: dig a hole and pull up the water. Of course, groundwater wells these days are a pretty sophisticated operation that rely on complex engineering, specialized equipment, and mechanical horsepower instead of the real thing. It's a simple concept, but one that's that's been modernized through human innovation to deliver water to large urban regions, such as ours.

Inside Portland's well field are more than 20 wells, housed in concrete underground vaults that dot the area east of the Portland airport near the Columbia River. Inside these nondescript mounds of earth is a high-tech system that tracks aquifer water levels and ensures the well is ready to go when needed. Each well pulls from one of the three aquifers at varying depths and, when powered up, pumps that water to the central Groundwater Pump Station. Huge pumps then push the water uphill for five miles to the underground reservoirs at Powell Butte. Once there, it's blended with the water from Bull Run before it enters the extensive system of pipes that delivers water to homes and businesses using the power of gravity. In an emergency situation where the Bull Run supply is entirely offline, the water leaving Powell Butte to our customers can also be 100% groundwater.

Why do we use groundwater?

Old photo of two men - one is wearing amazing red and white plaid pants - standing in a muddy area looking at pipes coming out of a groundwater pump in the ground. Water is splashing out of the pipes and onto the ground.
Testing an exploratory well in the Columbia South Shore Well Field in 1976. Also being tested: unironic plaid. 

Although Portland has relied on the Bull Run Watershed as its primary source of drinking water since 1895, beginning in the 1960s a number of events spurred interest in developing a supplemental source of water for the region.

A series of natural disasters in the Bull Run Watershed, including landslides and floods, as well as a concern that persistent population growth would outpace water supplies, underscored the need for action. Starting in the early 1980s, the City of Portland invested $30 million to develop the Columbia South Shore Well Field to respond to this need.

Having a supplemental source of drinking water is key to maintaining the resilience of our water system, a major component of our Strategic Plan. Resilience means that our water system can withstand unforeseen events without an interruption to water service, which is critical to protecting public health. For example, if a landslide occurs in the Bull Run Watershed and causes muddy waters in the reservoirs (known as a turbidity event), we can immediately activate our groundwater wells to provide clean water while sediment flushes out of the unfiltered Bull Run supply.

Building and maintaining infrastructure for resilience is how our region will grow and prosper in the coming decades, especially as the effects of climate change increase the likelihood of extreme natural events and more hot, dry summers. By making investments in the present, we can ensure that future generations will be able to rely on the safe, consistent (and delicious!) water resources that we enjoy today.

When do we use groundwater?

A picture of Bull Run dam holding water back with bare ground in the foreground
Groundwater provides some of our water supply during the dry season when Bull Run reservoir water levels go down. The reservoirs refill every winter when the rains return.

In a given year, the Water Bureau uses Bull Run water most of the time and groundwater as needed. On average, the bureau uses groundwater for about a month each year. During long, dry summers, the Water Bureau can blend groundwater with Bull Run water to make sure there's enough for all during Portland's dry season. This means looking out for the fish that rely on the Bull Run River, too! Check out our Habitat Conservation Plan and other Bull Run projects for more info on how Portland manages flow and temperature in the river to help endangered species. As climate change makes summers longer and drier, the city will likely rely on groundwater more often.

If Bull Run water is unavailable for any other reason, such as a storm, flood, fire, or natural disaster, groundwater can temporarily become Portland's sole source of drinking water. Our healthy groundwater supply means that our water system will continue to deliver water every minute of every day.

How do we protect groundwater?

map of the columbia south shore well field

In 1987, the city adopted a well field protection plan in response to concerns about potential contamination from industrial pollutants. This plan was one of the first of its kind in the US and relied on land use restrictions to prevent groundwater degradation.

The discovery of legacy groundwater contamination sites led to the 2003 adoption of a new and improved well field protection program. The protection area boundary was expanded to ensure consistent safeguards are in place everywhere they are needed, and the the city implemented additional measures to manage threats to groundwater quality.

At 100 to 600 feet underground, Portland's drinking water aquifers are naturally protected from urban pollutants and contamination by layers of silt and clay. But over time, some chemicals still have the potential to contaminate groundwater. To keep our water supply safe, the Water Bureau:

  • Collaborates with the cities of Gresham and Fairview to implement the Columbia South Shore Well Field Groundwater Protection Program beyond Portland's city limits
  • Partners with the Oregon Department of Environmental Quality to ensure legacy contamination does not impact the public's drinking water wells
  • Regularly tests water from drinking water wells and monitoring wells. Portland's groundwater supply meets all safe drinking water quality standards.

How can I help?

Illustration of outdoor, residential chemical uses including landscaping, lawn care, and car fluids.

Take care with chemicals. How you use, store, and dispose of chemicals can affect groundwater. Certain chemicals called volatile organic compounds (VOCs), like paint thinner, metal degreaser, furniture stripper, spot remover, and lighter fluid, pose the greatest risks to groundwater.

Check vehicles for leaks. Leaks can contribute to groundwater contamination.

Avoid or cut back on fertilizer and pesticide use. Most yards don't need as much fertilizer or pesticide as people think, and you can help protect groundwater by using less.

Never pour anything into a storm drain. Anything poured into a storm drain could eventually affect groundwater.

Properly dispose of used batteries and other chemical wastes. The heavy metals in batteries can contaminate groundwater. Metro holds neighborhood collection events for household hazardous wastes so you shouldn't have to go far.

Check underground storage tanks for leaks. Many older homes have underground heating oil tanks.

Cool groundwater lingo

Cross-sectional diagram of a well causing drawdown of the water table and the resulting cone of depression

Maybe you didn't know, but all the cool kids are using obscure technical lingo about water to communicate with each other these days. Okay, maybe that's not true, but it doesn't mean you won't benefit from having a few of these in your pocket:

Air sparging: A cleanup method in which pressurized air is injected beneath the water table to promote mass transfer of volatile organic compounds out of the groundwater and mass transfer of oxygen into the groundwater.

Aquitard: A layer of clay, silt, or rock that acts as a barrier to groundwater. Aquitards have low permeability, separate aquifers, and partially disconnect the flow of water underground. Also a type of confining unit, aquitards limit and direct the surface water that seeps down and replenishes aquifers and provide some protection from contamination from surface sources.

Aquiclude: A layer of rock, sediment, or soil through which ground water cannot flow. Also a type of confining unit, but generally more impenetrable to water than an aquitard.

Cone of depression: The zone around a well that is depressurized as a well is pumped (see radius of influence below), leaving an area where the potentiometric surface dips down to form a cone shape. The shape of the cone is influenced by aquifer composition and the water yield or pumping rate of the well, and the difference between the original potentiometric surface elevation and the bottom of the cone is referred to as drawdown. When a well is turned off, the cone of depression goes away as the pressure level rebounds to its original elevation.

Cross-sectional diagram of confined and unconfined aquifers

Confined and unconfined Aquifers: An unconfined aquifer is open to infiltration directly from the ground surface. The top of the aquifer can still be at great depth below the ground, and it can also be directly connected to a river or lake bed. A confined aquifer has an aquitard or aquiclude above it, which allows for greater pressures to build up and also protects the aquifer from the downward movement of contaminants.

Discharge and recharge: Discharge refers to the water that leaves an aquifer, sometimes through a natural pathway like a river bed and sometimes through a built pathway like a well. Recharge refers to the way water enters an aquifer, most commonly infiltration through natural pathways—but it can also be through built infiltration facilities meant to manage stormwater or specifically to increase the water available in an aquifer for human use.

Piezometric or potentiometric surface: An imaginary surface where groundwater pressure is equal to atmospheric pressure, and defined by the level to which water will rise in a well. Reported as an elevation relative to sea level or a similar known reference elevation. In an unconfined aquifer, this is also known as the water table. In a confined aquifer, where pressure can build up as water is pushed deep below the confining layer, the potentiometric surface can actually be higher than the water table or even the level of the ground surface! Wells like this can produce water even without a pump and are called artesian wells.

Radius of influence: The area over which a pumping well affects groundwater levels, measured as the distance from the center of a well to the point where groundwater pressure levels remain unchanged during pumping at the well. It can be visualized as the land surface overlying the cone of depression during drawdown from a pumping well.

Turbidity: The measure of relative clarity of a liquid. It is an expression of the amount of light that is scattered by material in the water when a light is shined through the water sample. The higher the intensity of scattered light, the higher the turbidity. Turbidity is caused by the suspension of clay, silt, organic matter, algae, soluble colored organic compounds, and other microscopic organisms. Reported in nephelometric turbidity units (NTU).

Volcanism: A phenomenon of eruption of molten rock (magma) onto the surface of the Earth, where lava, pyroclasts (a type of volcanic rock), and volcanic gases erupt through a break in the surface called a vent. It includes all phenomena resulting from, and causing, magma within the crust or mantle to rise through the crust and form volcanic rocks on the surface.