Flux Measurements. Most of the chemicals detected during flux sampling were also present in the groundwater samples. In a few instances, chemicals that were not detected in the groundwater sampling were detected during the flux sampling events. For a site with uniform lithology and a single VOC groundwater contaminant, some level of correlation between the groundwater concentration and the measured flux can be expected. However, for a site as large as NRP, with complicated lithology, shallow groundwater, and its density of underground utilities, correlation between VOC groundwater concentrations and measured flux will be poor, as observed at other South Bay sites. Other factors that will affect the correlation between groundwater and flux are the presence of small soil contaminant sources located in close proximity to the flux sample sites, and variation in capillary fringe (capillary rise) thickness due to local variation in soil properties. Variation in soil lithology can include the presence of clay layers that can retard and alter the vapor transport pathway, or soil lenses with high organic carbon content that can strongly adsorb VOCs. Use of fill, if present during construction activities, can also have a large local influence on soil vapor transport. Perturbations in lithology, whether due to natural variation or human efforts (fill or utilities), can result in either enhanced or reduced measured flux values depending on the location of the perturbation with respect to the groundwater and the site of the flux measurement. In addition to soil- and site-specific factors, differences in chemical physical properties and the interaction of the chemicals with soil and groundwater also affect flux. Some of these factors include the chemical s Xxxxx’x constant, diffusion coefficient in air and water, solubility in water, and organic carbon absorption coefficient (Koc). All of these factors, plus others, results in a complex relationship between these physical factors and the resulting surface soil flux values.
