Solute transport experiments were conducted in a decimeter scale flow cell packed with sand to study the potential for enhanced mixing of solutes in porous media and improved containment of injected plumes under multiple pumping-well driven, time-dependent oscillatory flow with respect to constant flow. Real-time imaging of the colorimetric reaction of Tiron (1,2-dihydroxybenzene-3,5-disulfonic acid) and molybdate was used to quantify mixing, whereas fluorescein was used to better examine plume size. Results from the small scale experiments clearly demonstrated the enhanced mixing of solutes under low Reynolds number oscillatory flow (a factor of 2 with respect to constant flow in homogeneous sand and a factor of 3 in layered sand), as the result of increased contact interface for solute diffusion. Further, the injected solute plume was better contained under oscillatory flow (25% less area with respect to constant flow in homogeneous sand) due to the cancellation of advective transport at each well over time. Enhanced mixing under oscillatory flow may enhance the processes of chemical and biological remediation. Furthermore, improved plume containment under oscillatory flow may require smaller amounts of chemicals to be injected during aquifer remediation.
The distribution, fate, and effects of human and veterinary antibiotics in the environment have been the subject of intense investigation for nearly two decades. Studies show that the structure and function of microbial communities in soil and sediment are modified by antibiotic exposure but the resulting impact on biogeochemical processes is poorly understood. This review summarizes the most recent data on the present use and physicochemical properties of human and veterinary antibiotics and provides an overview of their occurrence in soil and sediment. This is followed by an examination of the potential effects of antibiotics on microbial nitrogen turnover and methodological approaches to measuring the effects of antibiotics on nitrification and denitrification. Recent studies identified six major classes of antibiotics in soil and sediment, occurring at concentrations between ng·kg −1 and mg·kg −1. Among these, tetracycline and fluorquinolone antibiotics are the most resistant to degradation and leaching and may accumulate to high concentrations (mg·kg ) but their occurrence is also reported more frequently.Only 26 studies were found that investigated the effects of antibiotics on microbial nitrogen cycling. Some antibiotics had no observable effect on nitrogen redox activity in soil and sediment while others appeared to increase or decrease rates of reaction. This lack of consensus could be attributed to a number of different variables including antibiotic dose, method of antibiotic application, variations in the microbial community structure, or method of quantification. We conclude by recommending that future studies adopt a more comprehensive approach to report on changes of the microbial community structure and function as well as the short-and long-term impacts of antibiotics on the accumulation and loss of nitrogen pollutants.
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