Case Study: Stormwater Infiltration Impact Analysis

Client: Quadrant Corporation Location: King County (DuPont), WA Contact for water resource projects: Tim Flynn

Project Background & Challenges

Infiltration is required for managing stormwater within the City of DuPont, the site of Northwest Landing, a major mixed-used development. Under the SEPA review process, developers must assess potential impacts to the groundwater system, wetlands, and/or streams resulting from the proposed infiltration. Two factors posed several challenges for minimizing the potential impacts associated with locating and designing these infiltration facilities: a set of unique hydrologic conditions, and the presence of a large environmental cleanup site within the development footprint.

Solution

For this project, Aspect conducted a series of assessments that successfully demonstrated no adverse impact to local groundwater or surface water resources. At one location, we assessed potential impacts to wetlands due to changes in stormwater runoff patterns. Our approach involved demonstrating not only that the wetland’s hydroperiod was controlled by fluctuations in a highly transmissive shallow aquifer but also that stormwater contribution was negligible compared to the total water budget. Other assessments entailed using a numerical groundwater model to simulate the cumulative effects to the aquifer when operating multiple infiltration ponds under 100-year storm events. The model predicted that the groundwater mound would grow and dissipate with time, and that local groundwater flow directions would change in response to the cumulative infiltration. We also quantified the potential impacts to both surface water and to wells in the deeper aquifer. Another study addressed a regional infiltration pond designed to handle stormwater flows from 218 acres — four times larger than the State’s 50-acre recommended limit (at that time) for a single pond. In all cases, the City of DuPont agreed with our conclusions and approved the facility designs.