Changes in Concentrations over Time

In place of distance measurements, concentration can be used to determine the age of a plume. Assuming that the plume is derived from a single slug-type release of a contaminant which is only partially soluble in groundwater, and not present as an NAPL, then the date upon which the contaminant enters the water table or aquifer is equivalent to that point in time when the concentration of the contaminant approaches its solubility limit  

This method is best solved graphically by plotting the maximum concentration measured at the center of the plume on different dates, then extrapolating to determine the date on which the concentrations of the contaminant in question approaches its solubility limit. The solubility limit, corrected for the temperature of the aquifer, for a particular contaminant can be obtained from the literature. 

It is assumed that the contaminant enters the water table or aquifer at a concentration near its solubility limit, although there is no practical means to verify this. This method is more favorable when the release occurred as a single, short-term episode. A long-term release from a continuing source would result in a date that more closely represents the last date upon which the contaminant entered the aquifer at or near its solubility limit. Should the contaminant enter the aquifer below its solubility limit, then a date earlier than the actual event would result. Conversely, should the contaminant enter the aquifer as NAPL for a period of time, a date in which all the NAPL dissolved in groundwater would result. If NAPL was present when measurements were obtained, then the zone of highest concentration would 

Changes in concentration gradient between the center of the plume and its leading edge over time can be expressed as  

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The actual movement of a specific deep-well-injected hazardous substance depends on the types of processes that act on the waste and on the ways in which different processes interact. Figure 20.3 shows the expected change in concentration over time of a deep-well-injected organic compound in an observation well at an unspecified distance from the original point of injection. 

To learn more about reaction rates expressed as changes in concentration over time, go to the Chemistry 12 Electronic Learning Partner. 

Reactions are usually described in terms of changes in concentration over time. In the hypothetical reaction... 

The average rate of travel for these activities is based on the change in distance over time. Similarly, the reaction rate is based on the change in concentration over time. 

Diffusion coefficients in aqueous environments are usually measured by observing changes in concentration oVer time. 

Because we are looking at change in concentration over time, we need the first-order integrated rate law. Equation... 

The distribution does not take place instantaneously upon the chemicals emission, but requires some time that depends both on the xenobiotic and on the constitution of the multiphase system concerned. Until the establishment of a steady state, the parallel processes - uptake, distribution and elimination into and from the individual compartments - yield changes in the concentrations of the chemicals in each of the respective phases. Although the increase or decrease in the concentrations of the compounds due to the uptake, distribution, transformation and elimination may result from a variety of processes, the change in concentration over time can generally be described by a few differential equations according to zero-, first- or second-order kinetics (Figure 2.4). 

The change in concentration over time for the first-order rearrangement of gaseous methyl isonitrile at 199 °C is graphed in Figure 14.11. Because the concentration of this gas is directly proportional to its pressure during the reaction, we have chosen to plot pressure rather than concentration in this graph. The first half-life occurs at 13,600 s (3.78 h). At a time 13,600 s later, the methyl isonitrile pressure (and therefore. 

The First-Older Process movie eChapter 14.3) illustrates the change in concentration over time for a process that is first order overall, (a) Determine the rate constant for... 

To determine a rate of reaction, we need to measure changes in concentration over time. A change in time can be measured with a stopwatch or other timing device, but how do we measure concentration changes during a chemical reaction Also, why is the term average used in referring to a rate of reaction These are two of the questions that are answered in this section. 

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