DELINEATING BURIED PEAT BOGS IN SEATTLE, WASHINGTON USING A BOREHOLE DATABASE

Groundwater withdrawal from thick peat deposits has caused damaging settlement in overlying and nearby structures in the Greenwood bog of Seattle. Bogs and glacial recessional lake deposits frequently contain compressible deposits of peat and organic-rich silt. These deposits are present in relatively isolated but widespread and buried locations throughout the City of Seattle. Most of these bogs began growing at the close of the last glacial period but are now covered by fill, in some cases 18.3 m (60 ft) thick.

Peat deposits are easily found using a simple query of a geotechnical database containing subsurface layer data from over 35,000 boreholes. Using borehole data, 63 deposits were mapped in Seattle and 5 types of former bogs are recognized based on their formation: 1) those formed at the heads of or along modern streams, 2) those formed at the heads of or along recessional streams, 3) those formed in apparently isolated upland depressions, probably kettles 4) those formed in the Duwamish Valley floodplain and marine estuaries, and 5) those formed adjacent to Lake WA and related to the artificial lowering of the lake in 1916. The thickest [18.3 m (60 ft)] and most continuous peat deposits occur adjacent to Lake Washington. However, saturated continuous peat deposits on the order of 4.6 m (15 ft) thick are present in all types. Most peat deposits are generally 0.6 to 2.4 m (2 to 8 ft) thick. Type 4 deposits are the least continuous and tend to be the thinnest of the 5 types.

Peat occurs in bogs and as discontinuous lenses in lacustrine and fluvial deposits, in the latter case, mapping discrete peat lenses is not plausible. For these deposits with discontinuous lenses of peat, maps show boreholes where peat was encountered and conversely where peat was not encountered. Where peat occurs in lacustrine deposits, soft, thick, organic-rich silt is often present.

The City of Seattle is considering adding peat bogs to the list of critical areas because of the damage that can result to adjacent properties from groundwater withdrawal during construction and from long-term pumping. Regulations may follow similar regulations for sensitive areas within the City of Seattle: permit applications for development on land within mapped bogs will trigger restrictions regarding groundwater and impacts to the hydrologic regime.