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Studying nitrogen impacts from septic tanks and alder trees in Hood Canal


By Amelia Apfel, for the Puget Sound Institute

As part of a series of long-term studies, Puget Sound Institute scientist Andy James is collaborating with Julie Horowitz of the Hood Canal Coordinating Council and Mike Brett and Mark Benjamin at the University of Washington to complete an evaluation of the fate of various human-associated nitrogen sources in the areas surrounding the Hood Canal.

Hypoxia and eutrophication are widely considered to be serious threats to the Hood Canal, and have been key action items for the Puget Sound Partnership and state and federal agencies around the region. Dissolved oxygen may cycle with seasons and tides, but low levels in Hood Canal have resulted in dramatic fish-kill events. Researchers say chronic hypoxia has the further potential to cause long-lasting damage to Hood Canal biota.

Poor mixing (a function of the canal’s geography), stratification, and varying amounts of sunlight, are recognized as contributing factors to hypoxic conditions, but scientists are also interested in the role of potential anthropogenic contributions of nitrogen, which can lead to eutrophication. Nitrogen is an important naturally occurring nutrient, but excess amounts can choke the water with an overgrowth of organic material.

To examine possible impacts, the scientists have been looking at two potentially significant sources of human-caused nitrogen loading: septic tanks and red alder trees that colonize disturbed or heavily logged areas around the canal.

Much of Hood Canal’s population lives along the shoreline, placing septic tanks close to the water and making them an obvious suspect. But measuring direct impacts can be difficult. Researchers are using a combination of approaches to model the amount of nitrogen entering the canal, including estimates based on census data and field measurements around septic tanks, as well as data collected by Kitsap and Mason county at the estuarine fringe. They are also looking at the effects of denitrification in the soil.

Red alder, although naturally occurring, may also be potentially significant. Red alder (Alnus rubra) is one of the first species to colonize disturbed areas, and often comes back after land is cleared or logged. The tree’s roots play host to bacteria that fix nitrogen from the air and put it into the soil in biologically available forms. In Hood Canal, logging has led to recolonization in many of the lower reaches, which are now dominated by red alder. A study by Steinberg et al. in 2011 found that red alder stands were responsible for 51 ± 15% of total dissolved nitrogen (TDN) load from watershed runoff.

At Oregon State University, scientists developed a method for estimating red alder land cover using a combination of aerial photos, surface maps, and GIS. The UW study uses their techniques, along with soil measurements, to estimate the amount of nitrogen red alder stands contribute to the watershed.

James says that although studies are ongoing, the data suggests that approximately 70% of nitrogen put into the watershed is lost prior to reaching sensitive waters of the Hood Canal, although this is highly variable. Research continues in an effort to determine if human contributions of nitrogen are driving a shift in dissolved oxygen that exceeds the Washington State Department of Ecology standard of 0.2mg/L.

Amelia Apfel is a freelance writer and Editorial Assistant at the Puget Sound Institute.