Groundwater provides water to public drinking supplies for over half of the Puget Sound population. A variety of chemicals, including nitrate, can pass through the soil and potentially contaminate groundwater. Nitrate comes from nitrogen, and is found in most inorganic fertilizers, animal manure, and liquid waste discharged from septic tanks. Rain or irrigation water can carry nitrate down through the soil into groundwater where it can persist for years. The detection of elevated concentrations of nitrate may also be an indication of conditions (well depth and surficial geology, for example) that make groundwater more vulnerable to contamination, beyond nitrate, from activities at the land surface (Tesoriero and Voss, 1997). 

Most groundwater in Puget Sound is not contaminated by nitrates, however, it is a concern because contaminated drinking water can cause serious health risks, especially in infants. When ingested, nitrate can cause low oxygen levels in the blood which can lead to a potentially fatal condition known as methemoglobinemia or “blue baby” disorder. For this reason, public water systems carefully monitor nitrate levels in source water and treat the water when necessary. Nitrate is not removed by conventional drinking water treatment processes; its removal requires additional, relatively expensive treatment units.

• The vast majority of Puget Sound residents get their drinking water from public water supply systems. Nearly 98% of the Puget Sound population is served by large systems known as Group A water systems, which have more than 14 connections or serve 25 or more people 60 or more days per year; over half of these people receive water sourced primarily from groundwater supplies.
• Nitrates occur naturally in groundwater but are also found in fertilizers, animal manure and waste from septic systems, which may greatly increase nitrate concentration in groundwater. If consumed in too large amount, nitrates pose a health risk. The Safe Drinking Water Act nitrate limit for delivery of water from public water systems, set to protect against adverse health effects, is 10 mg/L.
• Most groundwater in Puget Sound is not contaminated by nitrates. Across Puget Sound, the average concentration of nitrate in source groundwater (pre-treatment) remained at low levels from 2011 to 2021. Concentrations of nitrates above 2 mg/L in groundwater supplies indicate human activities are the source of the contaminant (Nolan et al., 1998). Regionally, the annual median concentration ranged from 0.25 to 0.50 mg/L meaning that the majority of groundwater sources to public water systems do not show signs of human caused nitrate pollution (see Interpretation of Results). Less than 15% of groundwater samples collected each year have nitrate concentrations above 2 mg/L.
• In most years, only 2% of groundwater samples exceed 5 mg/L. For these water sources, samples are taken more frequently to ensure concentrations stay “regularly and consistently” below 8 mg/L.
• If nitrate concentrations exceed 10 mg/L, the system must notify customers and install expensive treatment systems in order to provide clean water to the public. Five Group A water systems in Puget Sound treat water for nitrate contamination: four in Whatcom County and one in Island County.
• Whatcom and Island counties had a higher proportion of groundwater samples with nitrate concentrations above 5 mg/L relative to other Puget Sound counties. Widespread and decades-long nitrate contamination, primarily from manure applied to crops and fertilizers, has been well documented in northwestern Whatcom County groundwater. Education and outreach is ongoing in this county to address nitrate pollution. Potential areas to watch for increasing nitrates are in Clallam, Pierce, and Thurston County.

Nitrate concentrations in groundwater above 2 mg/L are generally the result of anthropogenic activities (Nolan et al., 1998). Nitrate comes from nitrogen, and is found in most inorganic fertilizers, animal manure, and liquid waste discharged from septic tanks. Rain or irrigation water can carry nitrate down through the soil into groundwater where it can persist for years. 

Across Puget Sound, the average concentration of nitrate in source groundwater (pre-treatment) remained at low levels from 2011 to 2021. Regionally, the annual median concentration ranged from 0.20 to 0.50 mg/L meaning that the majority of groundwater sources to public water systems do not show signs of human caused nitrate pollution.

The percentage of groundwater samples that exceed regulatory and health thresholds varies by county. Between 2012 and 2021, Thurston County had the highest percentage of samples with nitrate concentrations above 2 mg/L (30%), followed by Island County (20%) and Pierce County (20%). Over the 10-year period, Whatcom County had the highest occurrence of samples exceeding 10mg/L. 

Widespread and decades-long nitrate contamination, primarily from intensive agricultural practices combined with shallow depth to water and heavy rainfall that carries unused nitrate to the water table, has been well documented in northwestern Whatcom County groundwater (Carey and Cummings, 2012). The Washington Nitrate Priority Project has identified the Sumas-Blaine Aquifer in Whatcom County as a very high priority area based on the severity of nitrate contamination, population, and agricultural land use. Four Group A water systems treat for nitrate contamination and education and outreach is ongoing in this county to address nitrate pollution.

Percentage (10-year average, 2012-2021) of groundwater sources for Group A systems with mean annual nitrate concentrations below 2 mg/L and exceeding 2 mg/L, 5 mg/L, and 10 mg/L, by county. Dashed lines represent Puget Sound-wide averages; error bars represent the 95% confidence interval.

The map at the top of this page shows the location of groundwater sources to Group A systems across Puget Sound. Each source is symbolized by the average (median) nitrate concentration from samples collected between 2012 and 2021. Not all water sources reported 10 years of samples. Most sources (52%) had at least 6 years of sample results in the Sentry database over the 10-year period.

Five Group A water systems in Puget Sound treat water for nitrate contamination: four in Whatcom County and one in Island County. Three locations, two in Whatcom County and one in Pierce County, show water sources with average nitrate concentrations above 10 mg/L that are not treated. These water sources have not been sampled since 2016 and are either currently inactive or designated as emergency wells only. Emergency wells are physically disconnected from their water system and are only reconnected when an emergency arises. Under that scenario, the source water is sampled for water quality, including nitrates, prior to reconnection and delivery.

DOH information on nitrate in drinking water

USGS Nutrients National Synthesis Project: Probability of Nitrate Contamination

Carey, B. and R. Cummings, 2012. Sumas-Blaine Aquifer Nitrate Contamination Summary. Washington State Department of Ecology, Olympia, WA. Publication No. 12-03-026. 57 pages. 

Nolan, B.T., B.C. Ruddy, K.J. Hitt, and D.R. Helsel, 1998, A National look at nitrate contamination in ground water: Water Conditioning and Purification, v. 39, no. 12, p. 76–79. 

Tesoriero, A.J, and F.D. Voss, 1997. Predicting the probability of elevated nitrate concentrations in the Puget Sound Basin: implications for aquifer susceptibility and vulnerability. Groundwater, v. 35, no. 6, p. 1029-1039.