A spatial life cycle cost assessment of stormwater management systems

Abstract

Although widely implemented, the research and understanding of the economic impacts and benefits of green infrastructure (GI) systems remain limited. Currently, few studies have investigated the economics of GI systems from a spatial perspective and typically opportunity costs related to land and property tax were ignored. This study aims at bridging these gaps by investigating both the equivalent annual costs (EAC) and cost effectiveness of seven GI systems and compare them against local wastewater treatment facilities in five different US cities. To do this, we utilized capital and maintenance cost data obtained from GI systems that are currently installed at the University of New Hampshire. The costing data were then extrapolated across five different cities considering reported local material, land, tax, and labor rates. A system dynamics model was utilized to calculate the total stormwater reduction as well as the amounts of nitrogen and phosphorous removed by each GI system over its life cycle under a certain city setting. Based upon these outcomes, the cost effectiveness (CE) in terms of stormwater reduction, nitrogen treatment, and phosphorous treatment of the GI systems was calculated. Land and tax costs were found to be a significant component of the EAC for GI systems with larger footprints in cities with higher property values, accounting for up to 78% in some cities. The rankings of the GI systems differ significantly when different types of cost effectiveness are under consideration. The tree filter performs the best when the CE is calculated based on stormwater reductions, while the subsurface gravel wetland performs the best considering nitrogen treatment, and either the subsurface gravel wetland or the sand filter performs the best considering phosphorous treatment. Our study suggests recommendations of GI systems need to be made based on local needs and issues to achieve the most cost-effective solution.