THREE-DIMENSIONAL ANALYSIS OF GROUNDWATER CONTAMINATION SUSCEPTIBILITY WITHIN A SEQUENCE STRATIGRAPHIC FRAMEWORK

A backward travel time probability density (BTTPD) approach proposed by Fogg et al. [1999] is developed to assess 3D, deep groundwater susceptibility to non-point source contamination within a sequence stratigraphic framework observed in Kings River fluvial fan (KRFF) aquifer. The BTTPD, which represents complete age distributions within a single groundwater sample, is used as a quantitative, transient measure of aquifer susceptibility. The resultant 3D imaging of susceptibility using simulated BTTPDs in KRFF reveals the strong influence of regional-scale heterogeneity on susceptibility. The regional-scale incised-valley fill deposits increase the susceptibility of aquifers by enhancing rapid downward solute movement and displaying relatively narrow and young age distributions. Conversely, the regional-scale sequence-bounding paleosols within the open-fan deposits “protect” deep aquifers by slowing downward solute movement and displaying a relatively broad and old age distribution. Further comparison of the simulated susceptibility maps to known contaminant distributions shows that these maps are generally consistent with the high concentration and quick evolution of the contaminant 1,2-Dibromo-3-Chloropropane in groundwater around the incised-valley fill since the 1970s'. This application demonstrates that the BTTPDs, affected significantly by the subsurface heterogeneity, can be used as transient and relatively reliable measures of deep aquifer susceptibility.