Per- and polyfluoroalkyl substances (PFASs) are persistent contaminants detected in groundwater at closed municipal landfills in rural Minnesota, where legacy waste continues to release PFASs decades after closure. In this setting, contaminated groundwater used for irrigation creates a direct pathway for PFAS transfer into the food supply. This study evaluates remediation at the groundwater–irrigation interface through a unified techno-economic assessment comparing granular activated carbon (GAC) adsorption and phytoremediation with Festuca rubra (F. rubra). Design influent concentrations were set at the 90th percentile (P90) of five representative PFASs measured in rural Minnesota landfill groundwater. Adsorption was simulated using a pore and surface diffusion model for single-bed and lead–lag configurations under fouling and non-fouling conditions. Phytoremediation was modeled using three complementary scenarios: a greenhouse-derived upper bound, a bioconcentration factor (BCF)-derived mechanistic case, and a decomposed scaling formulation. Unit costs were anchored to Minnesota Pollution Control Agency 2023 guidance and University of Minnesota (UMN) Extension data. Adsorption achieved compliance within weeks at $5.24–$8.00 m⁻³; phytoremediation showed lower per-volume cost ($0.43 m⁻³) but required centuries to multi-millennial timescales for long-chain PFASs. Cumulative cost was dominated by treatment duration rather than per-volume intensity, with adsorption more cost-effective when compliance timeframes are constrained. These findings highlight that remediation feasibility depends on alignment between treatment timescale and risk-management objectives rather than levelized cost alone.