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API Bull 24-2006 pdf free download

API Bull 24-2006 pdf free download.Downward Solute Plume Migration: Assessment, Significance, and Implications for Characterization and Monitoring of “Diving Plumes”.
Direct-push technologies can be a cost-effective means for characterizing the subsurface lithology and the vertical and horizontal extent of a plume. Installation of multiple characterization points can be conducted in a relatively short time period and can initially define the plume and identify the number and location of permanent depth-discrete monitoring wells. Many ways to construct depth-discrete monitoring wells include individual completions of wells at different depths, completion of short screened wells within a single borehole, or a single well with multi-level ports or sampling chambers. Available guidance documents from state and federal agencies can assist in site characterization strategies and methods. Biodegradation Characterization Diving plume occurrence can be identified by analyzing depth-discrete shallow groundwater samples for dissolved oxygen, iron, methane, or other parameters that may indicate the presence of overlying unaffected groundwater versus groundwater affected by hydrocarbons or other contaminants. Clean recharge water will generally contain concentrations of dissolved oxygen greater than 1 milligram per liter (mg/l), iron concentrations less than 0.5 mg/l, and methane concentrations less than 0.1 mg/l. For hydrocarbon-affected groundwater, dissolved oxygen will be depleted and iron and methane concentrations will generally be above 1 mg/l. Depending on the age of the source, this electron-acceptor depleted “shadow” zone may extend downgradient of the current extent of the BTEX or MTBE plume. Detecting this zone can increase confidence that the monitoring network is located in the probable path of the plume. Dissolved organic carbon will also tend to be low (less than 2 mg/l) for unaffected water, and will be elevated (often above 10 mg/l) in hydrocarbon plumes (Weaver and Wilson 2000). Care must be taken when collecting groundwater samples for dissolved oxygen analyses and other redox- sensitive species.
This relationship assumes uniform recharge and uniform velocity in the aquifer, and is strictly accurate only if the aquifer is infinitely thick, and the hydraulic gradient is insensitive to the rate of recharge. For aquifers that are bounded from below by a relatively impermeable aquitard, this relationship will tend to overestimate the rate of plume dive. More detailed methods that relax some of these simplifying assumptions are discussed below. In addition, this relationship does not address in-well dilution effects that may occur if the well screen intercepts a mixture of contaminated and uncontaminated groundwater. For example, if the distance from a source to a downgradient monitoring well is 1,000 feet, and the downgradient monitoring well screen extends 10 feet below the water table, the recharge accretion would need to be greater than 1 percent of the ambient horizontal groundwater velocity for the plume to fully dive below the well screen, resulting in a false negative analytical result from the monitoring well. If the well were only 500 feet downgradient, the accretion rate would have to be greater than 2 percent of the recharge rate to produce a false negative result. For the case where the well is 500 feet downgradient of the source, if the horizontal velocity were 100 feet per year, then the accretion rate would need to exceed 2 feet per year for the plume to dive below the monitoring well. Assuming a total porosity of 25 percent, this condition would require the net recharge exceed 6 inches per year. Data requirements for this analysis include estimates of groundwater velocity, which is calculated from hydraulic conductivity, hydraulic gradient, and effective porosity. Hydraulic gradients can be calculated from monitoring well water level data, or in the absence of any site-specific data, they can be estimated from regional groundwater potentiometric maps (if available).

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