Impacts of Stormwater Infiltration on Urban Watershed Hydrology and Water Quality: Spanaway Watershed-Scale Stormwater Management Plan

Pierce County (WA) developed the Spanaway watershed-scale stormwater management plan to meet MS4 permit requirements. Nearly all stormwater in the drainage area infiltrates and strong surface water and groundwater interaction plays an important role in stormwater planning. We built an HSPF model and calibrated it using streamflow, surface water, and groundwater quality monitoring data collected during 2014-16. We re-discretized a regional MODFLOW model to simulate groundwater flow in the study area with a finer resolution. We simulated existing, historic, and future streamflow, temperature, bacteria, and metals. Model simulation results were used to assess hydrologic metrics and estimate B-IBI scores based on regional regression equations. The study found that extensive stormwater infiltration effectively mitigated MS4 stormwater impacts on water quality and hydrology. Nevertheless, observed B-IBI scores are low. The study identified elevated water temperature and lack of habitat complexity as likely stressors for benthic macroinvertebrates. Four stormwater management strategies were evaluated.