Statistical Characterization of Hyporheic Zone Properties Over Three Years

Hydraulic and solute transport properties of streambed sediment were determined using in-stream piezometer measurements in a 263m reach each year during a three consecutive year period in the mercury (Hg) contaminated East Fork Poplar Creek, Tennessee. Key hydrologic variables (e.g., hydraulic conductivity, hydraulic head gradients, seepage) and dissolved Hg concentration were quantified. Statistical analyses were used to characterize the measurements. The mean hydraulic conductivity, seepage flux, Hg concentrations, and Hg-flux decreased each of the three years. For each attribute, goodness of fit was assessed for various probability distributions including Normal (i.e., Gaussian), Log-normal, and Gamma distribution. Results confirmed that both untransformed and log transformed hydraulic conductivity from 2017 and 2018 were consistent with Gaussian distributions, whereas seepage flux from 2017 and 2018 were consistent with Gamma distributions. Log transformed Hg-flux distributions each year were consistent with sampling from Gaussian distributions. A statistical mixed model was also used to characterize and compare the streambed attributes, and examine whether the data varied over time. Results of this model indicated that means of log transformed hydraulic conductivity and differential hydraulic head were significantly different over three years. However, the means of log transformed seepage flux, inverse square root of Hg concentrations and log Hg-flux were not significantly different over the three-years period. The inter-annual changes in hydraulic and geochemical properties are attributed to sediment transport processes that accompanied with anomalously high streamflow events. Hydraulic properties changed year to year whereas changes in seepage and Hg flux were not detected even though these fluxes depend on gradient and hydraulic conductivity. This might be due to opposing and offsetting changes in hydraulic gradient and hydraulic conductivity that either allowed for no net change or increased variability in flux.