A PRACTICAL BIO-ENGINEERED APPROACH TO REMEDIATION OF CONTAMINATED GROUND WATER DISCHARGING TO A TIDAL WETLAND AND STREAM

A ground water remediation strategy that combines natural attenuation and enhanced bioremediation is put forth to protect a perennial stream flowing through a tidal wetland. The tidal wetland contains ground water seeps, some of which are contaminated with chlorinated volatile organic compounds (VOCs). Thermal mapping, hydraulic gradient and seepage flux monitoring, together with geochemical analysis and ground water modeling, demonstrate that the seeps are conduits for higher velocity ground water discharges compared to the surrounding wetland sediments. The wetland sediments are shown to reduce VOC contaminant plume concentrations more effectively than the seep areas because of the longer contaminant residence times and persistent methanogenic conditions that promote biodegradation. Multiple, very old VOC sources are located in both the upland and tidal wetland areas producing diffuse (>50 acre) VOC plumes that discharge to the tidal wetland sediments and seeps. 20 years of site investigations have not identified well-defined source areas and the VOC plumes are observed to be relatively stable. Predictive model simulations indicate that the VOC plume will persist for a lengthy timeframe as VOCs will continue to diffuse out of the silt and clay matrix. The management and restoration strategy presented is designed to achieve long-term VOC mass reduction of the contaminant plumes to protect the stream environment. Bio-engineered plume treatment will be focused directly upgradient of the ground water/surface water interface in the most contaminated ground water seeps. Treatment will utilize field tested permeable reactive mat technology constructed of a mixture of sand, peat, zero valent iron, and electron donor with bioaugmentation of native microorganisms (WBC-2). The reactive mats are demonstrated to reduce over 90% of VOC mass prior to ground water discharging to the stream. Recent re-testing indicated that VOC degradation and methanogenic conditions are sustained in the reactive mat for over 3 years. Focusing bio-enhanced treatment of the VOC plumes at the seep areas provides the most effective and practical restoration strategy by providing a long-term protective remedy for an ecologically sensitive tidal stream