Henderson Inlet multi-parameter TMDL

Henderson Inlet is one of five inlets that form the southern end of Puget Sound. A study revealed that the inlet has water quality problems due to dissolved oxygen, fecal coliform bacteria, pH, and temperature. We are working with local stakeholders to address these impairments.

Water quality issues

We conducted a water quality study in Henderson Inlet watershed during 2002-2005. The study evaluated water quality impairments.

Dissolved oxygen and pH issues in freshwater were largely found to be related to natural causes, such as discharge from groundwater or wetlands. However, some dissolved oxygen issues appeared to be related to too many nutrients (nitrogen and phosphorus) from human sources entering the water bodies. Our hydrologic analysis was not able to explain why the water is too warm in parts of Woodland Creek, one of the larger tributaries to Henderson Inlet. Other likely human sources of incoming pollution include failing septic systems, pet waste, and poor livestock management practices.

The study also found that fecal coliform bacteria (FC) concentrations were too high throughout the watershed. The water quality study found several stormwater outfalls carrying significant bacteria loads. Stormwater, while not itself a source of bacteria, contributes to bacteria problems by carrying pollution into waterways, even from some distance away.

Wildlife, too, can contribute bacteria. Wildlife is generally considered part of the “natural background.” However, there may be management opportunities to reduce bacteria where human activities encourage unusual numbers of wildlife, for instance waste management in commercial areas, or feeding waterfowl along shorelines.

What we have done

The total maximum daily load (TMDL) report was developed and submitted to the Environmental Protection Agency (EPA) for approval in 2006, after a public review and comment period. EPA approved it in 2007. The study set goals for reducing bacteria at important locations. The report's implementation strategy made recommendations for reducing nutrients in the watershed. We sent the final water quality implementation plan to EPA on July 31, 2008.

Status of the project

We completed water quality sampling for an effectiveness monitoring project in 2015 to evaluate the progress of water quality cleanup. We finalized the report summarizing findings in early 2017.

This study found statistically significant FC declines across the watershed. Water quality improvements are largely due to the hard work of local partners. After more than 20 years of work the results of the Henderson Inlet watershed partnership are among the most positive results in any Washington watershed. Water quality trend data suggest stormwater retrofits, septic-to-sewer projects, and land acquisition for preservation are likely responsible for most of the decline in FC. The increasing trend in FC has been stopped in spite of an increasing population. In 2017 the approved shellfish growing area has expanded southward in Henderson Inlet. Data show that the most significant water quality improvements were seen in areas where the most money was invested. 

Although water quality has improved, state standards are still not being met. Cleanup work needs to continue and includes:

  • Improved management of stormwater discharges
  • Implementation of an on-site septic system operations and maintenance program
  • Source investigation including septic surveys, water quality monitoring, and visual surveys of land use and management practices
  • Technical assistance to landowners
  • Informational workshops and other outreach aimed at encouraging landowners to improve land use practices
  • Oversight of sources with discharge permits
  • Enforcement
  • Water quality monitoring to guide cleanup; assess effectiveness of cleanup actions; monitor progress toward water quality goals; and, ultimately, demonstrate compliance with water quality standards

Why this matters

Oxygen dissolved in healthy water is vital for fish and aquatic life to “breathe” to survive. It is more difficult to transfer oxygen from water to blood than it is to transfer oxygen from air to blood. Therefore, it is critical that an adequate amount of oxygen is maintained in the water for this transfer to take place efficiently and sustain aquatic life. Oxygen is also necessary to help decompose organic matter in the water and bottom sediments as well as for other biological and chemical processes.

Fecal coliform is a type of “bacteria” common in human and animal waste. It indicates that sewage or manure is entering a water body. As the level of fecal coliform increases the risk of people getting sick from playing or working in the water increases. Bacteria can get into water from untreated or partially treated discharges from wastewater treatment plants, from improperly functioning septic systems, and from livestock, pets, and wildlife.

People can help keep bacteria out of the water. Properly collect, bag, and trash dog poop. Check your onsite sewage system to make sure it is maintained and working properly. Ensure livestock and manure are kept away from the water.

pH is a measure of the water's acidity or alkalinity. The pH is measured on a scale of 0-14, with the lower numbers indicating acidic conditions and higher numbers alkaline conditions. Optimal pH levels to support fish and wildlife should range from 6.5 to 9.0. A pH of 7 is neutral.

pH can affect the solubility of nutrients and metal compounds. By affecting the solubility of nutrients, it can change the amount of nutrients available for plant growth. If too many nutrients are available, aquatic plants can grow out of control. When these plants decompose, they can deplete the water of oxygen. The solubility of many metal compounds also changes greatly with pH. Generally, a reduction in pH (more acidic) increases the solubility of heavy metals. When more metals are dissolved in the water, aquatic animals may absorb them faster. Therefore, a lower pH (more acidic) may make these metals more toxic to aquatic life.

Water temperature influences what types of organisms can live in a water body. Cooler water can hold more dissolved oxygen that fish and other aquatic life need to breathe. Warmer water holds less dissolved oxygen. Many fish need cold, clean water to survive.

One way to cool water temperature is to shade the water body by adding or retaining streamside vegetation.