Hydrogeologic Controls on Bioactive Zone Development in Biostimulated Aquifers

Abstract

This study investigates the hydrogeological factors that control the feedback between bioactive zone formation and groundwater flow, as well as developing and assessing tools useful for making these investigations. The interaction between groundwater flow and bioactive zone development was investigated in both a subsurface biostimulation study and a laboratory experiments. Prior to the investigation, necessary advances were made in point velocity probe (PVP) technology to ensure successful observation in the field. Advances in these areas allowed for high resolution site characterization of an aquifer to undergo biostimulation for the removal of nitrate contamination. Heterotrophic denitrification was stimulated through daily pulses of acetate into the aquifer. With the onset of acetate delivery, decreases in groundwater velocity magnitude and flow redirection were correlated with permeable zones of relatively finer grain-size and poorer-sorting. In contrast, adjacent well-sorted and relatively coarse zones with initially high groundwater velocities indicated relatively little or no change in groundwater velocity after biostimulation. Superimposed onto these results, greater changes in groundwater velocity were noted where individual acetate pulses mixed through dispersive processes. In contrast, locations up-gradient of acetate pulse mixing indicated a higher variability in measured groundwater velocity. In total, changes in groundwater velocity in the biostimulated zone were in excess of those anticipated by analysis of the hydraulic gradient. Sediment-attached viable cell populations were interpreted to be partially responsible for the measured changes in velocity. Variables of grain-size and initial groundwater velocity were tested in controlled laboratory column experiments to evaluate the most favorable conditions for a bioactive zone to develop, subsequently leading to preferential bioclogging. Results from these experiments supported field observations that transition zones where relatively fine-grained, permeable sediments that lie adjacent to discontinuous coarser sediments - where the high velocities would lead to the highest flux of injected nutrients - represents the likely starting place for bioactive zone development, but may also be at greater risk for bioclogging. Further work was conducted to show that ground penetrating radar could noninvasively detect zones of relatively high bioactivity in granular material.