Dear City Council Members, Mayor Harrell, and the Office of Planning and Community Development (OPCD),

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I am writing to comment on the anticipated impacts to Green Lake from the recent zoning code updates. I have been studying and managing water quality of Green Lake since 1990 for Seattle Parks and Recreation as an aquatic scientist with Herrera Enviornmental Consultants, and my love of the lake has led to my current position as president of the Friends of Green Lake. The City has spent over $2M to control toxic algae blooms in the lake and keep it swimmable and fishable by treating the lake with aluminum sulfate (alum) in 1991, 2004, and 2016. These alum treatments of the entire lake are needed because phosphorus inputs from stormwater runoff within the lake watershed are not adequately controlled. Stormwater treatment is desperately needed in the Nearshore Basin within the park and in the large Densmore Basin extending north to the City limits.

The zoning code updates will increase impervious surfaces in the watershed, which will increase stormwater runoff and phosphorus loading to Green Lake, will increase the frequency and intensity of toxic algae blooms, and will result in closure of the lake to all recreational uses to protect human health and prevent dog deaths. The City can continue to pour aluminum sulfate into the lake to mitigate the stormwater pollution, or decide to invest that money into stormwater treatment specifically designed to remove phosphorus.

Contrary to what is stated in the One Seattle Draft EIS, Green Lake is designated by Ecology's 303(d) (draft 2022) list in Category 5 (impaired and requiring a TMDL or other improvement) for harmful algae blooms (HABs), total phosphorus in the water, and several organic chemicals in fish tissue (due to pesticides, PCBs, and mercury in lake sediments primarily from historical inputs). Haller Lake is listed in Category 2 (concern but not enough data to require improvement) for HABs and total phosphorus. Total phosphorus inputs cause the HABs, which is the greatest water quality and human health concern for both of these urban lakes, and are continuing to increase due to increasing rainfall intensity caused by climate change. The importance of phosphorus and the increasing impacts of HABs in Green Lake is why the City had me prepare the 2016 Phosphorus Managment Plan for the most recent alum treatment.

The loss of trees from this action will likely impair water quality in both lakes. Loss of trees and increased density will increase inputs of warm and polluted stormwater runoff to these lakes. Increased water temperatures increase the duration and intensity of HABs, but increased phosphorus inputs from the increased runoff inputs is of most concern. In most cases, new development triggered by the new zoning will be required to treat stormwater runoff that is currently untreated. The required treatment may reduce phosphorus concentrations in the runoff and could have a positive effect of reducing phosphorus inputs and HABs if the treatment benefit is not outweighed by the increased runoff volume. The increased runoff will decrease infiltration to ground water, decrease the shallow water table, lower groundwater inputs to the lake, and lower lake levels further. Although further study is needed, water levels in Green Lake appear to be dropping in recent years to critical levels in late summer/fall lake levels as a result of decreased groundwater input from both 1) the reduced infiltration and 2) the pumping of groundwater to the sewer to keep underground parking garages dry at high density developments. 

The most important lake water quality mitigation measure is to require all new development in the lake basins to provide phosphorus enhanced treatment of stormwater rather than just the standard basic treatment for removing suspended solids. City stormwater code needs to designate Green Lake as phosphorus impaired in accordance with the 2016 Phosphorus Management Plan and 2022 Water Quality Assessment. City drainage engineers need to recognize the need for phosphorus enhanced treatment in the basins to reduce public health impacts of toxic algae blooms. The City recently built a $12M crew house on Green Lake and did not spend a dime to treat any of the untreated stormwater inputs from adjacent park land using an approved phosphorus enhanced treatment method, and stormwater monitoring data has shown this drainage to be loaded with phosphorus. When I ask City drainage engineers to construct phosphorus treatment facilities in the park to reduce algae blooms, they say those phosphorus inputs are controlled by the alum treatments of the lake, which is contrary to state water quality policy requiring watershed controls first and foremost. Phosphorus enhanced treatment of stormwater from all new development alone is not likely to prevent HABs in Green Lake altogether, but it will extend the effectiveness longevity and overall cost of the alum treatments. Also, enhanced stormwater treatment has an important benefit of reducing toxic pollutants in the lake sediments that are bioaccumulating in the fish eaten by humans and wildlife.

Rob Zisette