Transport and biodegradation of quinoline in horizontally stratified porous media

An experimental study of the movement and biodegradation of quinoline was conducted in a saturated 2-layer system (1 m long) to identify processes that may result in increased microbial growth at hydraulic layer interfaces. The system contained two layers of contrasting hydraulic conductivity (1:12) and flow was parallel to layers. Tracer breakthrough, used to quantify interlayer mass transfer, showed that the transverse dispersivity was 0.3 cm near the interface and 0.036 cm within the low-conductivity (low-K) layer. Interlayer mass transfer resulted in arrival of substrate (quinoline) and oxygen 10's to 100's of hours sooner in the low-K layer near the interface compared to other locations within the low-K layer where substrates arrived via only advection. Early arrival of substrates near the interface resulted in biodegradation of quinoline for a longer period than within layers, yielding increased growth in a 1- to 3-cm-thick zone, as measyred by plate counts. Because biodegradation was oxygen limited in this system, microbial growth at all locations was small [log(maximum increase) ⩽ 1.0] and measured porous-medium hydraulic properties (dispersion, hydraulic gradient) were not affected by the biomass production. Although the thickness of the effected interface zone was small in this system, the effect on the overall transport of quinoline was significant; 19% of the growth (and corresponding degradation of substrates) in the low-K layer was in the relatively small interface zone. The effect of microbial biomass production at interfaces on overall solute movement is likely to be maximized in environments that have a high density of hydraulic or geochemical interfaces, particularly in settings where the interfaces serve as mixing zones between nutrient-limited waters.