A Devil of a Site

At the Red Devil site in Mt. Vernon, New York, paint-related liquids were released and seeped into the subsurface, where they accumulated as a light non-aqueous phase liquid (LNAPL) on the water table. As a plume developed it seeped into the Bronx River, resulting in noticeable discharge of LNAPL, causing the site to be listed by New York State Department of Environmental Conservation (NYSDEC) for remediation. A numerical flow model of the site was developed to assess LNAPL movement and evaluate a proposed hydraulic barrier. Complexities at the site include: the LNAPL, which increases in viscosity as the lighter fraction evaporates and solidifies when exposed to air; limited access, including steep banks and an active railroad; an urban river, which periodically inundates the aquifer; and a century old retaining wall. LNAPL simulation was performed with DYNSWIM, a finite-element numerical flow model, which allows for simulation of two-phase flow. As the properties of the LNAPL were difficult to measure and varied spatially and temporarily, the model was used to simulate movement of the LNAPL with varying viscosity, dispersivity and residual saturation values. Model simulations were used to estimate mass in locations where it was difficult to collect data and to estimate changes in LNAPL under different remediation scenarios. Simulations showed that a proposed flow barrier could stop LNAPL migration to the river, but it would be costly and have a substantial impact on the local ecology. Construction of the barrier would require fill to be placed in the river and removal of existing trees. Instead, a more cost-efficient and sustainable approach was selected, which included limiting movement of LNAPL under the railway by pumping at the source, periodical removal of LNAPL through bailing of wells, and collection of material which enters the river and is trapped by a series of booms.