Advective flux measurement

Advective flux measurements can be conducted using seepage meter, piezometer, dye tracers, and radium or radon isotopes. 

In Illustrative Example 19.2 we discussed the flux of trichloroethene (TCE) from a contaminated aquifer through the unsaturated zone into the atmosphere. The example was based on a real case of a polluted aquifer in New Jersey (Smith et al., 1996). These authors compared the diffusive fluxes, calculated from measured TCE vapor concentration gradients, with total fluxes measured with a vertical flux chamber. They found that the measured fluxes were often several orders of magnitude larger than the fluxes calculated from Fick s first law. In these situations the vapor profiles across the unsaturated zone were not always linear. The authors attributed this to the influence of advective transport through the unsaturated zone. In order to test this hypothesis you are asked to make the following checks ... 

Failing to incorporate soil-gas advection induced by barometric pumping into gas-phase subsurface transport models may, under certain conditions, under predict contaminant flux to the atmosphere. As previously described, Smith et al. (1996) compared TCE vapor fluxes measured with a chamber device to TCE in groundwater being removed by a pump-and-treat system and discharge into a surface-water receiving body at the same site. These researchers found VOC removal rates by flux to the atmosphere comparable in magnitude to both of the other attenuation pathways. 

The first case can be quite easily assessed quantitatively on the basis of porosity measurements one might imagine a fresh and water-rich deposited sediment column and observe the compaction as a further descent of the solid phase relative to the water that remains at the same place. Hence, the upwards directed advective flux results as the movement of water relative to the further sinking sediment. If the sediment exhibits a water content of approximately 0.9 at its surface, and even a value of 0.5 in several meters depth, and provided that the boundary between sediment and bottom water moves npwards due to the accumulation of new sediment, then it will inevitably follow that the compaction indnces an advective flnx which will not exceed values similar to the rate of sedimentation. As for deep sea... 

Some examples for the second case were provided by Schultheiss and McPhail (1986). By using an analytical instrument that freely sinks to the ocean floor, they were able to measure pressure differences between pore water, located 4 m below the sediment surface, and the bottom water directly. At some locations, deep sea sediments of the Madeira Abyssal Plain displayed pressure differences of about 0 Pa, at other locations, however, the pressure in pore water 4 m below the sediment surface was significantly lower (120 or 450 Pa) than in bottom water. With regard to the porosity (([)) and the permeability coefficient of the sediment (k), the advective flux is calculated according to the following equation ... 

In this equation, known as the Darcy Equation, and which is applied in hydrogeology for calculating advective fluxes in groundwater, vjm s ] denotes the velocity with which a particle/solute crosses a definite distance in aqueous sediments. Ap refers to the pressure altitude measured in meters of water column (10 Pa = 1 bar 750 mm Hg 10.2 m water column) and Ax [m] is the distance across which the pressure difference is measured. In the example shown above ((]) = 0.77, k = 7-10 m s ), this distance amounts to 4 m. Insertion into Equation 3.30 yields ... 

At flow rates of about 10 cm s over an uneven sediment surface (mounds up to 1 cm high), the oxygen measured by means of microelectrodes had penetrated to a maximum depth of 40 mm, whereas a penetration depth of only 4 mm was measured under comparable conditions when the sediment surface was even (Fig 3.27). Huettel et al. (1996) were able to show in similar flume experiments that not only solutes, but, in the uppermost centimeters, even fine particulate matter was likewise transported into the pore water of coarsely grained sediments. Similar processes with marked advective fluxes are, however, not to be expected in the finely grained sediments predominant in the deep sea. 

Advective fluxes can be measured using indirect and direct methods ... 

A variety of methods exist for quantifying advective flux in wetlands. A direct approach involves use of seepage meters, which are the enclosures placed on the sediment surface to measure the flow over a small area. 

Several methods are now available to measure accretion, advection, flux, and diffusion flux in wetland. 

The overall advective flux equation combining dry and wet deposition is defined using the vapor concentration in the air, Ca, in mg/m. This air concentration measurement is typically performed to assess the chemical mass source strength in the atmosphere. The equation is ... 

Vxh can then be compared to Pph to assess the magnitude of lateral °Thxs advection, or with the measured sediment-trap °Thxs flux to assess the trapping efficiency. 

Fig. 10.4 Measured breakthrough curve of bromide with CTRW and advection-dispersion equation (ADE) fits. Here, the quantity j represents the normafized, flux-averaged concentration (top) Complete breakthrough curve, (bottom) Region identified by the bold-framed rectangle in the top plot. Note the difference in scale units between the plots. Pressure head h=-10cm water velocity v=2.82 cm/h. The dashed tine is the best advection-dispersion equation solution fit. The soUd line is the best CTRW fit. (Cortis and Berkowitz 2004)...

In Illustrative Example 22.4 we reanalyze the case of the flux of trichloroethene (TCE) from a contaminated aquifer through the unsaturated zone into the atmosphere. As pointed out in Illustrative Example 19.2, measurements of TCE in the soil indicate that vertical advection of air may influence the TCE profile and the flux. 

Compared to this idealized model, the actual flux of Rn may be diminished by the saturation of pore space by water (the mean length of Rn diffusion in water is on the order of a milhmeter, so saturation diminishes the flux by up to a factor of 1,000) and decreases in porosity with depth. Advection of gas through soil in response to barometric pressure change, soil gas convection, and transpiration of Rn saturated soil solution will increase the radon escape rate. All of these processes are difficult to model accurately, so the determination of Rn fluxes rehes on measurements. 

---