Discipline Report Water Quality Prepared by: KPFF Consulting Engineers Shannon and Wilson, Inc. April 2005 Draft EIS Magnolia Bridge Replacement City of Seattle Contents Contents ......................................................................................................................... i Purpose and Need ........................................................................................................ 1 Purpose .................................................................................................... 1 Need.......................................................................................................... 1 Structural Deficiencies...........................................................................................1 System Linkage .....................................................................................................1 Traffic Capacity......................................................................................................4 Modal Interrelationships ........................................................................................4 Transportation Demand.........................................................................................4 Legislation .............................................................................................................5 Description of Alternatives .......................................................................................... 7 No-Build Alternative................................................................................ 7 Alternative A ............................................................................................ 8 Alternative C ............................................................................................ 8 Alternative D ............................................................................................ 8 Methods ....................................................................................................................... 17 Applicable Regulations and Policies................................................... 17 Federal ................................................................................................................17 State of Washington ............................................................................................17 City of Seattle ......................................................................................................18 Surface Water Methods......................................................................... 18 Groundwater Methods .......................................................................... 19 Affected Environment................................................................................................. 21 Historic Environment ............................................................................ 21 Existing Built Environment................................................................... 22 Topography .........................................................................................................22 Climate ................................................................................................................22 Drainage Basins and Watersheds .......................................................................22 Drainage Pathways .............................................................................................25 Surface Water Resources ...................................................................................30 Elliott Bay Water Quality......................................................................................30 Elliott Bay Sediment Quality ................................................................................33 Water Quality Discipline Report Contents Page i Magnolia Bridge Replacement Sediment Dynamics ............................................................................................35 Structural Nearshore Limiting Factors.................................................................35 Aquatic ESA Status.............................................................................................36 Public Water Supply and Wastewater Systems ..................................................36 Groundwater...........................................................................................36 Aquifer Systems ..................................................................................................37 Shallow Unconfined Aquifer System ...................................................................37 Upland Aquifer System........................................................................................38 Deep Artesian Aquifer System ............................................................................38 Groundwater Rights ............................................................................................39 Public Drinking Water Wells................................................................................39 Sole Source Aquifers...........................................................................................39 Wellhead Protection Areas..................................................................................39 Critical Aquifer Recharge Areas ..........................................................................39 Studies and Coordination ...........................................................................................41 Studies ....................................................................................................41 Data Sources ..........................................................................................41 Impacts.........................................................................................................................43 Surface Water Impacts...........................................................................43 Operational Impacts ............................................................................................43 No-Build Alternative.............................................................................................43 Alternatives A, C, and D......................................................................................43 Pollutant Loading Estimate..................................................................................44 Existing Outfall Capacity .....................................................................................45 Shoreline Impacts................................................................................................46 No-Build Alternative.............................................................................................47 Alternative A........................................................................................................47 Alternative C........................................................................................................47 Alternative D........................................................................................................47 Secondary and Cumulative Impacts....................................................................51 Groundwater...........................................................................................51 Operational Impacts ............................................................................................51 No-Build Alternative.............................................................................................51 Alternatives A, C, and D......................................................................................51 Secondary and Cumulative Impacts....................................................................51 Mitigation Measures ....................................................................................................53 Surface Water .........................................................................................53 Stormwater Management Requirements.............................................................53 Operational Mitigation .........................................................................................54 No-Build Alternative.............................................................................................54 Alternatives A, C and D.......................................................................................54 Shoreline Mitigation.............................................................................................55 Page ii Contents Water Quality Discipline Report Magnolia Bridge Replacement Alternative A ........................................................................................................55 Groundwater .......................................................................................... 55 Operational Mitigation..........................................................................................55 No-Build Alternative.............................................................................................55 Alternatives A, C and D .......................................................................................55 Construction Impacts ................................................................................................. 57 Surface Water ........................................................................................ 57 Impacts ................................................................................................................57 No-Build Alternative.............................................................................................57