Appellants press for groundwater study as engineer says stormwater model meets county code

Tyler Stanich, a civil engineer retained by the applicant, told the San Juan County hearing examiner that the project’s drainage analysis used the Department of Ecology’s Stormwater Management Manual for Western Washington and the Western Washington hydrology model and that the site-level inputs (including geotechnical data) showed the post‑development flows would not exceed predevelopment flows.

Stanich said his team dug five geotechnical test pits to about 9 feet, found two pits with perched groundwater, and used those observations, standard hydrology modeling, and a PV‑SMART permeability coefficient to size detention and conveyance features. “Two of the five test pits experienced perched groundwater conditions,” he testified, and later said, “A subsurface groundwater model is not done for this project.” He added the design includes a southern detention pond (approximately 2,400 square feet bottom area) with orifices and outlet control, and swales and a riprap pad that discharge to an existing mapped wetland.

Appellants pressed a different point. Charlie Conway, one of the appellants, said topographic maps and on‑site soil data create a plausible pathway for subsurface interflow from the project site toward his property and the airfield downhill of the project. “There is at least a reasonable possibility that interflow from the site goes from the site to the area West‑Southwest of the property,” Conway said while showing USGS contour exhibits. He argued the shift from forest canopy to a meadow‑type post‑development condition could increase infiltration, and that increased infiltration combined with local soil stratigraphy could drive subsurface flows onto neighboring parcels and affect septic drainfields.

Stanich responded that the modeling accounts for evapotranspiration and percolation and that, under the code and the Western Washington manual, his team modeled pre‑ and post‑development conditions and applied accepted best management practices. He repeated that a subsurface groundwater model was not performed because it is not required for this type of land‑development project and that the adopted modeling approach is the standard the county requires and has reviewed. He also described using PB/“PV‑SMART” research and the hydrology model to represent how solar arrays influence infiltration and runoff.

County counsel Jeff Morgan told the examiner that county staff reviewed the applicant’s drainage analysis and found it compliant with the applicable stormwater manual and county code. Appellants and their counsel argued that compliance with code‑required modeling does not answer the specific concern about interflow and potential subsurface impacts on neighboring properties, and they urged the examiner and county to require further study or clearer documentation showing how subsurface flow paths were evaluated.

The hearing continued with additional witnesses scheduled; the examiner reserved time for closing arguments from all parties after testimony concluded.