REMEDIATION OF GROUNDWATER PLUMES DISCHARGING TO SURFACE WATER: FUNDAMENTAL QUESTIONS, CHALLENGES, AND WHERE DO WE GO FROM HERE?

Plumes of contaminated groundwater that discharge to surface water have the potential to adversely impact the aquatic life and water quality of the receiving water. Discharging plumes contaminate the sediment, porewater, and surface water and pose unique, complex, and difficult problems with respect to remediation and mitigating risks. Although over the last two decades progress has been made with respect to locating and characterizing these discharges, many fundamental questions and challenges still remain regarding how to deal with these plumes, and the purpose of this presentation is to assess what our goals should be and where we should be focusing our research and decision making efforts. The groundwater/surface-water interface is a unique ecological habitat that has demonstrated the ability to biodegrade and attenuate discharging plumes, so we need to decide is the interface a valuable ecosystem we need to protect or is it a treatment opportunity? Are we characterizing these plumes in ways that truly inform and help ecological risk assessments, remedial decision making, and the long-term success of the chosen remedy? Several remedial options such as dredging, natural attenuation, subaqueous caps, cutoff walls, reactive walls, and pump and treat systems are being implemented, but each have their own advantages, disadvantages, and inherent uncertainties. Some of the key challenges we face include: adequately characterizing heterogeneity of these aquatic systems; our ability to predict system response to natural events or to remedial actions; adequate monitoring of remedy performance; and the implications of climate change on chosen remedies. The key to moving forward and achieving desired outcomes is to: critically evaluate project goals and desired outcomes; make sure ecological risk assessors are integrated into the investigation and remedial design teams from the beginning; identify and candidly acknowledge knowledge gaps (e.g., lack of ecological, ecotoxicological, hydrogeological, and biogeochemical information); and design new monitoring techniques and strategies to adequately address data gaps, reduce uncertainty, and inform scientific decision making.