Active venting

Control of ga.s movement by recovery. The movement of gases in landfills can also be controlled by instadhng gas-recovery wells in completed landfills (see Fig. 25-74b). This is considered an active venting system. Clay and other hners are used when landfill gas is to be recovered. In some gas-recovery systems, leachate is collected and recycled to the top of the landfill and reinjected through perforated lines located in drainage trenches. Typically, the rate of gas production is greater in leachate-recirculation systems. 

Kusakabe, M., Mayeda, S. and Nakamura, E. (1990) S, O and Sr isotope systematics of active vent materials from the Mariana basin spreading axis at I8°N. Earth Planet. Sci. Lett., 100, 275-282. 

Sites 1248-1250 characterize the ridge crest with active venting... 

Because seawater uptake dominates the REE content of metalliferous sediment, neodymium isotopic analysis of metalliferous carbonate can provide a reliable proxy for contemporaneous seawater, away from input of near-vent sulfide detritus (Mills et al., 1993). Osmium also exhibits a similar behavior and seawater dominates the isotopic composition of metalliferous sediments even close to active vent sites (Ravizza et al., 1996). Consequently, analysis of preserved metalliferous carbonate sediments has proven extremely useful in determining the past osmium isotopic composition of the oceans, both from modem marine sediments (e.g., Ravizza, 1993 Peucker-Ehrenbrink et al., 1995) and those preserved in ophiolites (e.g., Ravizza et al., 2001). Only in sediments close to an ultramafic-hosted hydrothermal system, have perturbations from a purely seawater osmium isotopic composition been observed (Cave et al., 2003, in press). 

Manus Basin is a back-arc basin located north of the New Britain island arc that consists of NE-SW trending ridge segments offset by transform faults. On the Manus Spreading Center in central Manus Basin the Vienna Woods hydrothermal field consists of a 300 m-diameter sulfide mound with an extensive foresf of active and inactive sphalerite-barite-rich, sulfide-sulfate-oxide chimneys up to 15 m-long atop it. Vent fluids from one of the active chimneys reach temperatures as high as 302°C. In the eastern Manus Basin, two important hydrothermal fields are known. The PACMANUS field includes discontinuous sulfide occurrences over a 3 by 0.8 km area and active venting of unknown nature. Volcanic rocks are andesitic to rhyodacitic. 

Most chimneys have growth rates that are rapid in comparison to the half-life of Pb (22.3 years), and radioisotope ages of several days or less for precipitates of some active vents are consistent with growth rates observed from submer-sibles (5-10 cm per day Hekinian et al. 1983 Johnson and Tunnicliffe 1985). Larger vent complexes commonly have measured ages on the order of decades (Koski et al. 1994), but the data for entire vent fields may span several thousands of years (Lalou et al. 1993). 

Feely, R.A., Lewison, M., Massoth, G.J., Rohert-Galdo, G, Lavelle, J.W., Byrne, R.H., von Damm, K.L. and Curl, H.C.(jr.), 1987. Composition and dissolution of black smoker particulates from active vents on the Juan de Fuca Ridge. Journal of Geophysical Research, 92 11347-11363. 

Above consideration concerns only with the vertical settling of mineral particles. Feely et al. (1987) considered the effect of dispersion of black smoker mineral particles from active vents on the Juan de Fuca ridge in addition to the settling and obtained the distribution of the size of mineral particles settled onto the seafloor from the equation... 

Releases to the environment do not include releases that are wholly contained inside a closed containment structure, such as a building or an enclosed vehicle. Hazardous substances discharged in buildings or vehicles with active vents or openings, however, may become releases into the environment. 

Positive pressurisation below floor slabs (a particular form of active venting)... 

Active venting uses fans to enhance the airflow through underfloor voids. The number of fans provided depends on the required air flow. It is best if fans that allow passive air flow when they are not operating are used and that a gas monitoring system is installed in the underfloor void. The monitoring system activates the fans to increase air flow when the gas levels exceed set values. 

Underfloor natural (passive) venting wherever possible (or car park). Where passive ventilation is not feasible or cost effective use active venting or positive pressurisation... 

For larger buildings the use of passive venting becomes more difficult and active venting or pressurisation may be required... 

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