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Urban Soils & Seattle Examples ESRM 311 / SEFS 507 Week 4

Urban Soils Outline What are urban soils? Hazards How they differ from unmodified soils Physical properties Hydrologic cycle Hazards Denny re-grade Earthquake hazards Example of Urban Soil Magnuson Park Soil Pits

How would you define an Urban Soil?

Urban Soil Bockheim (1974) gives an appropriate and useful definition of urban soil: “A soil material having a non-agricultural, man-made surface layer more than 50 cm thick, that has been produced by mixing, filling, or by contamination of land surfaces in urban and suburban areas”

Soil Physical Properties Horizons Structure Texture Color Moisture Temperature Single Grained Granular Blocky Prismatic Columnar Platy

Idealized Soil Profile Layered characteristics Example forest Litter layer A horizon Organisms or gravity Plant root growth E horizon Organic acids through leaching B horizon Clays accumulate C horizon Parent material

Urban Soil Characteristics Urban Soil Great vertical and spatial variability Modified soil structure: compaction Waste materials and other contaminants Restricted aeration and water drainage Interrupted nutrient cycling and modified soil organism activity Surface crust; usually hydrophobic Modified temperature High pH

Soil Mixing Slope that was back filled Original profile covered Organic layers covered (13) Fill materials used Refuse, Clayey fill Would you build a house on the fill? Cold weather Organic matter

Structure and Compaction Disturbed, displaced or compacted Reduce pore space and increase bulk density Low organic matter Soil organisms activity less Low frequency wet-dry or freeze-thaw Less aggregation and structure formation Compressive forces Vehicle, water, human or paving Reduced vegetative cover Bare soil, erosion, crust formation

Crusting and pH Crusting Precipitation on bare or exposed soil sorts the finer matter and fills small pores Impervious surface Barrier for plant growth pH Ca increase soil pH Sources: De-icing compounds Weathering building rubble Calcium-rich water in gardens

Water movement and Aeration Urban environment limits aeration and infiltration Water movement restriction Mixing of layers Impervious surface

Hydrologic Cycle in Urban Areas Relationship between impervious cover and surface runoff. Impervious cover in a watershed results in increased surface runoff. As little as 10 percent impervious cover in a watershed can result in stream degradation. 40% evapotranspiration 30% evapotranspiration 55% runoff 10% runoff 10% shallow infiltration 25% shallow infiltration 25% deep infiltration 5% deep infiltration 75-100% Impervious Cover Natural Ground Cover http://water.epa.gov/polwaste/nps/urban_facts.cfm

Decomposition Water movement is restricted Little aeration Limited root zone Less favorable for organisms

Denny Re-grade Removal of Denny Hill

Denny Re-grade

Hazards Earthquake hazards Liquefaction: Water-saturated sandy soils that lose strength during earth shaking

Hazards Earthquake hazards Site Class National Earthquake Hazards Reduction Program (NEHRP), site class where soils amplify ground shaking

Urban environment affects soil? Covering of soil Disturbed water cycle Nutrient and decomposition limited Horizon mixing

Example Urban Soil Magnuson Park 1855: surveyed alder grove, Douglas fir forest with trees 2-6 feet, Cedar, streams and wetlands Mid 1930’s: conversion to military base, trucked in hundreds of loads of fill, Mud lake, the marsh and Pontiac Bay covered Current Day: restoration of the park

Magnuson Park Site Characteristics 1953 1927 2011

Soil Pits Dug three soil pits Forested Area Cottonwoods Grassland Characteristics Horizons Bulk density pH Structure

Sites and Pits Site 1 Site 2 Site 3

Horizon Depth (inches) Survey Results Site Horizon Type Horizon Depth (inches) Texture Moisture Content pH Structure 1 A 29 - 25.5 Clay Loam 29.7% 5.41 Crumb Structure (small rock or shot Fe) Bts 25.5 – 14 Sandy Clay 21.2% 5.00 Clay Crumb Bts2 14 – 6 82.1% 4.75 Saturated Clay (plastic limit) 2 28 – 25 Silt Clay 46.0% 5.32 Organic with rocks B1 25 – 17 Sandy Loam 25.2% 5.45 Fill with Fingering B2 17 – 12 26.6% 5.80 Clay with Moddles B3 12 – 0 Loamy Sand 28.0% 6.07 Sand with Organic Matter 3 23 – 17 31.0% 5.37 Crumb Structure 17-0 27.9% 5.95 Blocky Bulk Density   Horizon Moisture Content Volume (cm3) Bulk Density (g/cm3) 26.5% 177.11 1.39 13.1% 68.70 2.45 16.2% 2.10

Magnuson Park Vegetation indictor of underlying soils Horizon development Natural horizon, moved? (site 1) Water logged (site 2) Hill area (site 3) Poor drainage, site 2 water table 2 feet High bulk density Trees dying off around site 2

Urban Soils Urban soils vs. idealized profile Characteristics Variable horizons Modified soil structure Compaction Waste materials mixed in Interrupted nutrient cycling Low soil organism activity High pH Local examples Denny Hill re-grade Magnuson Park