FATE OF CONTAMINANTS IN UNCONSOLIDATED AQUIFERS: 30 YEARS OF ADVANCES IN UNDERSTANDING

Protection of drinking water supplies and ecosystem health requires spatial and temporal information on contaminant distribution to minimize the exposure of humans and other organisms to toxic substances. Over the past 30 years, substantial advances have been made in understanding the processes controlling the transport and fate of contaminants in the subsurface. Early studies on contaminant transport processes focused on measurements made in existing contaminant plumes. Introduction of natural gradient tracer experiments provided increased control of source functions and quantification of mass recoveries. These studies also provided an opportunity to select experimental conditions to evaluate the impacts of aquifer characteristics and geochemical conditions on the transport of solutes. This transition coincided with rapid advances in analytical techniques associated with computerization of analytical instrumentation, thus providing the opportunity for analysis and management of thousands of samples collected from controlled tracer experiments. Detailed studies were conducted on contaminant fate, including studies on the advancing and receding stages of plume migration. These studies provided insight into the complexities of (bio)geochemical processes controlling contaminant fate. As analytical tools became more available, studies which incorporated multifaceted chemical analyses allowed for an even greater understanding of natural attenuation processes and the duration of aquifer recovery. More recently, the development of a new generation of analytical tools (e.g. tandem electrospray mass spectrometry) has allowed for studies to focus on new contaminant classes such as pharmaceutical compounds. Advances in molecular biological tools have provided additional information on microbial processes influencing the transport of both inorganic and organic contaminants. Integrated numerical models have allowed for more complex analysis of large data sets from both contaminated and experimental tracer sites. This presentation will provide an overview of the state-of-art knowledge of contaminant fate in unconsolidated aquifers 30 years ago, major advances to date, and potential directions for future research.