Regional Water Quality

Regional Water Quality

Our health, safety, welfare, education, and economic and employment opportunities, as well as preservation of cultural and natural resources, are all connected to how we use and interact with our water.

Water quality is improving, and we have more to do.

In the 1960s, Lake Washington was too polluted for swimming. Now, it is one of the cleanest urban lakes in the country.

Photo of a sign warning about pollution in Lake Washington.
Current photo of children jumping into Lake Washington.

Data from the last 40 years shows:

Past improvements have improved water quality.

  • Less bacteria that can make people sick.
  • Fewer nutrients that can cause toxic algae blooms.
  • More dissolved oxygen for fish to breathe.

There is more to do to achieve water quality goals.

  • Water temperature is getting warmer, which is worse for fish.
  • Water does not always meet state standards for bacteria and dissolved oxygen.
  • Historically contaminated sediments need to be cleaned up or contained.

How do pollutants get into water?

These pathways transport pollutants into Puget Sound, rivers, and lakes in our region and negatively impact water quality.


Stormwater is rain that runs off streets, parking lots, driveways, lawns, and sidewalks and picks up pollutants. Stormwater can also run off fields and other lands in rural areas.


  • Suspended Solids
  • Bacteria
  • Nutrients
  • Metals
  • Organic Chemicals
  • CECs

Wastewater treatment plant discharge

Wastewater treatment plants treat sewage from homes and businesses then “discharge” it or return it to the environment


  • Nutrients
  • CECs

Overflows of untreated wastewater and stormwater

Some pipes carry both sewage and stormwater to a treatment plant. When heavy rains fill these pipes they can overflow into local water bodies to prevent backups into homes and businesses.


  • Bacteria
  • Suspended Solids
  • Nutrients
  • Metals
  • Organic Chemicals
  • CECs

Contaminated sediments

Sediment is the soil or sand on the bottom of a water body. In some parts of the region, sediments are contaminated from past industrial and commercial activities


  • Organic Chemicals
  • Metals

Air pollution that settles on waterbodies

Air pollution from cars, industry, and even forest fires is made of tiny particles that can float down from the air onto the water.


  • Metals

Climate change

In the Northwest, climate change is bringing drier summers, warmer winters, bigger winter storms, and sea-level rise.


  • High Water Temperature
Illustration of Puget Sound with key showing pathways that transport pollutants into Puget Sound, rivers, and lakes in our region.

Click image to enlarge.

Pollutants can have harmful impacts on our waters, aquatic life, and health.


Bacteria from animals or humans can make people sick.


Nutrients, like nitrogen and phosphorus, are fertilizers for plants, but can cause algae blooms that decrease dissolved oxygen and harm fish, shellfish, and aquatic life as well as make waters unswimmable.

Suspended Solids

Suspended solids, like sediment, silt, and sand, can carry pollution, settle to the bottom, and smother habitat for fish, shellfish, aquatic life, and small animals.


Heavy metals, like arsenic, copper, lead, and mercury, can harm fish, shellfish, and aquatic life and cause human health problems.

Organic Chemicals

Organic chemicals, like phthalates from plastics, PBDEs from flame retardants, chlorinated pesticides, and banned industrial chemicals called PCBs, can sicken people, fish, shellfish, and aquatic life.

Contaminants of Emerging Concern (CECs)

Little information is available on the potential impact to people, fish, shellfish, aquatic life, or wildlife from contaminants of emerging concern, including medicines, personal care products, and plastic microfibers.

High Water Temperature

Warm water has lower levels of dissolved oxygen for fish, shellfish, and aquatic life to breathe and can result in an increase in algal blooms along with a number of other ecological effects.

Who protects water quality in the region?

King County and other local governments help manage:

  • Wastewater facilities that transport and treat sewage from homes and businesses.
  • Stormwater facilities that protect waterways from pollutants that run off streets, parking lots and other paved surfaces.

State agencies and local governments manage initiatives that achieve other goals related to water safety and quality.