"Peering into the black box: quantifying transient storage processes 
in groundwater and stream systems with geophysics"

Abstract:
Solute-transport behavior not described by traditional 
advection-dispersion processes has been observed at research and 
aquifer-remediation sites in diverse geologic settings. Anomalous 
behavior such as concentration rebound, long breakthrough tailing, and 
poor pump-and-treat efficiency have been explained by rate-limited 
mass transfer, where the pore space is treated as a continuum 
comprising a mobile domain, which consists of well-connected pore 
space, and a less-mobile domain, which consists of poorly connected 
pores. Despite recognition of the importance of such non-Fickian 
transport, verification of its occurrence and inference of controlling 
parameters remain problematic. Conventional geochemical measurements 
preferentially sample from the mobile domain and thus provide only 
indirect information for the less-mobile domain and exchange between 
domains. Here, we present a petrophysical framework, experimental 
methodology, and analytical expressions that can be used to infer 
mass-transfer parameters from co-located breakthrough curves of mobile 
concentration and bulk conductivity from geoelectrical measurements.  
We present field-experimental geoelectrical data (1) from an 
aquifer-storage recovery site showing evidence of mass transfer in 
Charleston, South Carolina, and (2) during hyporheic exchange in a 
steep, narrow headwater catchment at the H. J. Andrews Watershed in 
Oregon.  Temporal moments of solute and ER data are used to compress 
trends into descriptive statistics and identify the distribution of 
dominant solute transport processes (e.g., transient storage 
dominated, advection dominated) in the subsurface. These data are an 
improvement over traditional methods, which would otherwise provide 
only reach-averaged values, or single observations in space.