Based on the results of a 2004 study by geotechnical and environmental consultant Shannon and Wilson, Inc. (S&W), DPD will be reviewing new development projects in the former peat bog area of Greenwood with the goal of limiting the withdrawal of groundwater both during construction and from permanent development.

Commissioned by Seattle Public Utilities in 2003, the S&W study responds to neighborhood concerns that property settlement in the area was increasing. During the study S&W developed a map that delineates the former peat bog area and identifies the variable depth and thickness of the peat. They also tested the peat for the ability to re-introduce water into the substrata and placed monitoring devices in several locations for a more long-term assessment of groundwater flows and levels in the area.

The S&W study indicates that the withdrawal of ground water from the peat area should be avoided, and water should be re-introduced where feasible to maintain current water levels.

One outcome of this study is that DPD is now requiring new projects subject to the State Environmental Policy Act (SEPA) to show how their design will result in no net loss of groundwater. In the study area, SEPA applies to developments above certain thresholds on properties zoned commercial or multifamily along portions of North 85th Street and Greenwood Avenue North.

Several options for meeting this requirement have been identified, which include eliminating basements, developing water-tight basements, or providing the re-introduction of water via such methods as infiltration and newly permeable surfaces that replace water lost through traditional drainage facilities.

DPD is researching whether environmentally critical area (ECA) regulations might also apply, which would extend the new groundwater requirement to single family zoned properties, which constitute the majority of the peat area.

In addition, Seattle Public Utilities plans to inspect and clean out perforated catch basins that were placed in the area in the past to re-introduce water into the substrata. They are also investigating opportunities for other water re-introduction features in other components of the utility, where replacement of utilities is required and such a goal is feasible, based on soils conditions.

Questions?
For more information about the S&W study, or on DPD's review of projects affected, please contact:

Cliff Portman, DPD Principal Planner
cliff.portman@seattle.gov, (206) 684-5593