What is Moment Analysis?
Moment analysis is known a powerful tool capable of determining the center of the contaminant plume and the degree of dispersion as a limited source plume migrates through porous media.
Whereas the temporal moment method has been actively utilized to characterize the breakthrough curves of soil column tests in laboratory scale, the application of spatial moment method to field data has not been highlighted as well.
Past or on-going groundwater monitoring programs implemented by local, state or federal government have accumulated tremendous information on monitoring well locations and chemical analysis data, in a form of scaled site map including boring locations, sampling logs (e.g., well locations, screen depths, field parameters) and laboratory test report containing concentrations of individual contaminants.
Moment analysis can use past submittals of field/lab reports to evaluate the site soil kinetic properties and to predict natural attenuation behaviors in future.
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The spatial moment method uses the relative x and y coordinates of well locations (well screen depth can be considered as an additional z coordinate in 3-D analysis). Total amounts of contaminants can be estimated from the 0th order moments. With the 1st order moments, the centers of plumes can be traced per each sampling event, and the connector of center points of plumes will be a site averaged migration direction of the plume.
Site averaged 2-D or 3-D dispersion coefficients of contaminants over the monitoring period can be estimated from the 2nd moment values after repositioning and rotating the coordinate system aligned with the plume migration direction. Coordinate transformation associated with 2nd moment evaluation is implemented by a matrix operation. The 3rd moment can be used to decide the adsorption rates of contaminants in response with site soils. Near zero and negative skewness indicate each equilibrium and non-equilibrium states of the chemical sorption process over the site.