The vapor intrusion impacts associated with the presence of chlorinated volatile organic contaminant plumes in the ground water beneath residential areas in Colorado and New York have been the subject of extensive site investigations and structure sampling efforts. Large data sets of ground water and indoor air monitoring data collected over a decade-long monitoring program at the Redfield, Colorado, site and monthly ground water and structure monitoring data collected over a 19-month period from structures in New York State are analyzed to illustrate the temporal and spatial distributions in the concentration of volatile organic compounds that one may encounter when evaluating the potential for exposures due to vapor intrusion. The analysis of these data demonstrates that although the areal extent of structures impacted by vapor intrusion mirrors the areal extent of chlorinated volatile organic compounds in the ground water, not all structures above the plume will be impacted. It also highlights the fact that measured concentrations of volatile organic compounds in the indoor air and subslab vapor can vary considerably from month to month and season to season. Sampling results from any one location at any given point in time cannot be expected to represent the range of conditions that may exist at neighboring locations or at other times. Recognition of this variability is important when designing sampling plans and risk management programs to address the vapor intrusion pathway.

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The area surrounding the Colorado Department of Transportation Materials Testing Laboratory in Denver was the subject of intense investigation, involving the collection of thousands of ground water, soil-gas, and indoor air samples in order to investigate indoor air impacts associated with a subsurface release of chlorinated solvents. The preremediation portion of that data set is analyzed and reduced in this work to ground water-to-indoor air attenuation factors (a gw ¼ the ratio of the measured indoor air concentration to the soil-gas concentration predicted to be in equilibrium with the local ground water concentration). The empirical a gw values for this site range from about 10 -6 to 10 -4 with an overall average of 3 3 10 -5 (lg/L indoor air)/(lg/L soil gas). The analysis of this data set highlights the need for a thorough data review and data screening when using large data sets to derive empirical relationships between subsurface concentrations and indoor air. More specifically, it is necessary to identify those parts of the data that contain a strong vapor intrusion pathway signal, which generally will require concentrations well above reported detection levels combined with spatial or temporal correlation of subsurface and indoor concentrations.

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Introduction Acid mine drainage impacts on water quality from historic mining districts are well documented in Colorado's mountainous areas. It is less well known that natural acid rock drainage (ARD) adversely affects water quality in eleven different headwater areas of the state. The geology of these areas is characterized by hydrothermal alteration, commonly related to volcanic activity, post-volcanic mineralization, or igneous stock emplacement. Areas of Colorado known to exhibit natural acid rock drainage include the