Minimizing Contaminated Groundwater Flow to Surface Water Bodies: Lab Data and Model Evaluation of Alternative Active Cap Designs

Contaminated groundwater flow to surface water bodies, whether from localized seepage or regional impacts, can be minimized by use of active caps on surface water banks or beds. These approaches reduce the “risk-of-remedy" often created by conventional sediment removal and ex situ treatment alternatives and avoid the unsustainable practice of moving the problem from one media to another (i.e. from sediment to upland landfill). Recent advances in delivery and placement systems for sediment cap treatment materials have created a wide range of potential designs that may be used to implement an “active cap” or treatment system to accomplish in situ reduction of contaminant levels in sediments and to protect receiving water bodies. 

This paper presents an analysis of two alternative cap designs that utilized data collected on permeability and removal efficiencies of reactive granular capping materials in combination with a low permeability AquaBlok^® capping material.  Modeling data are presented to illustrate the impact of the addition of reactive materials to the composite aggregate to treat contaminants and sensitivity to variables such as cap permeabilities, groundwater discharge rates, steady-state/declining contaminant source levels, and other design parameters for system performance, including treatment efficiencies and active system life.  The two primary configurations evaluated are active caps and funnel and gate systems.

The modeling demonstrates that effective long-term performance can be achieved by combinations of active in situ treatment (chemical fixation and adsorption) in practical application configurations.  Using this approach immediate conformance to eco-risk goals can be achieved in the short-term and can be sustained over long time periods sufficient to exceed source depletion rates and/or function effectively until supplanted by other natural attenuation processes.  Additional evaluations of other configurations would likely identify lower overall costs, especially for model assumptions that account for depleted contaminant sources and long-term deposition of new clean sediment.