Field study of dispersion in a heterogeneous aquifer: 4. Investigation of adsorption and sampling bias

Abstract

The declining mass balance trend observed for bromide and three fluorobenzoate tracers during a natural gradient experiment at a site near Columbus, Mississippi, prompted investigations related to the reactivity of the tracers and the representativeness of the tracer samples obtained from multilevel samplers. A laboratory soil column study indicated that adsorption of bromide during the field experiment was of the order of 20% and that up to 10% of the three fluorobenzoates was adsorbed. The presence of iron oxides and kaolinitc in the alluvial aquifer combined with the low groundwater pH of 4.8 produced geochemical conditions conducive to adsorption of the artionic tracers. Multilevel samplers (MLS) used in the field experiment were evaluated by comparing bromide concentrations from the MLS with water samples extracted from adjacent soft cores. Vertically averaged bromide concentrations for the MLS were 21% lower than those for the cores. A matrix diffusion process in conjunction with a natural tendency for preferential sampling from permeable regions in the heterogeneous alluvial sediments is proposed as an explanation for the apparent bias in the MLS sample concentrations. This process is shown to be qualitatively consistent with the tracer mass balance observed during the natural gradient experiment. (PFBA), trifluoromethylbenzoic acid (TFBA), and 2,6difluorobenzoic acid (DFBA). Transport of the tracers was monitored over a period of approximately 20 months using an extensive three-dimensional array of multilevel sampling points. The analysis of the spatial moments of the tracer concentration measurements reported by Adams and Gelhar [this issue] indicated the anomalous trend in the tracer mass balances shown in Figure 1. The mass recoveries for all tracers generally showed declining trends over time, beginning with initial overestimates of mass followed by mass underestimates during the later stages of the experiment. Several possible causes for the mass balance trend were investigated, including (1) analytical error associated with tracer concentration measurements, (2) tracer retention in the vadose zone resulting from water table fluctuations, (3) the method of tracer mass estimation, (4) variations in aquifer porosity, (5) incomplete spatial sampling of the plume, (6) tracer adsorption, and (7) sampling bias associated with the multilevel samplers. The contributions of analytical error and vadose zone retention of tracers to the Copyfight 1992 by the American Geophysical Union. Paper number 92WR01759. 0043-1397/92/92WR-01759505.00 mass imbalance were found to be negligible by Boggs et al. [this issue]. Questions regarding tracer mass estimation methods, porosity variations, and plume truncation are addressed by Adams and Gelhar [this issue].In this paper we describe a series of investigations designed to determine the extent to which tracer adsorption and sampling bias contributed to the anomalous tracer mass balance trends observed during the natural gradient test. A laboratory column experiment was per...

Changes in Subsurface Catabolic Gene Frequencies during Natural Attenuation of Petroleum Hydrocarbons

“…Data from previous field investigations, as well as laboratory experiments using aquifer material collected from the study site, demonstrated a potential for the natural attenuation of hydrocarbons within the Columbus aquifer (12,(14)(15)(16)(17)(18)(19). Subsequent samples collected from the NAT study site successfully linked changes in microbial metabolic activity and community structure with exposure to hydrocarbons.…”

“…Aquifer material ranges from sand (fine granular) to gravelly sand (coarse granular). Background hydrological and microbiological information have been previously described (12,(14)(15)(16)(17)(18)(19). Subsurface core samples were recovered from locations in close proximity to multilevel groundwater sampling wells and processed onsite using aseptic technique as previously described (12).…”

“…These observations directly linked molecular and physiological changes in the microbial community to field observations of contaminant attenuation in a transient system. Previous field experiments at Columbus AFB provided detailed knowledge of the aquifer's hydraulic features (14)(15)(16)(17)(18)(19) as well as the natural biodegradative potential (20). After preliminary characterization of the site's microbiology (12) and geochemistry, the study aquifer was contaminated with a "model" jet fuel consisting of BTEX, naphthalene, and decane.…”

Influence of conceptual model uncertainty on contaminant transport forecasting in braided river aquifers

“…The mean is porosity l n ¼ 0:20. These values are of course different from a mean lnðKÞ of À11.6 and variance of 6.6 obtained by direct-push injection logger measures (Bohling et al, 2012) and from a mean porosity of 0.31 as reported by Boggs and Adams (1992). These differences are the consequence of the data paucity assumption and they do not prevent pursuing the aims of the papers.…”

Radionuclide Transport Models Under Ambient Conditions

“…The edge sites on clays are also reported to contain positive sites. Under low pH conditions (e.g., pH = 5), significant sorption of anionic tracers occurs in systems with high concentrations of iron oxides and kaolinite (Boggs and Adams 1992;Seaman 1998 . Because of competition, their effective sorption is minimal.…”