Solution to Exercise 3
Using the same procedure as in the solution for Exercise 2, the calculated water and contaminant velocities and proportions are listed in the following tables.
The exponential decline in hydraulic conductivity with depth creates a preferential flux of water (0.78) and contaminant (0.85) through the upper 10 cm of the peatland under the wet condition. Even under dry conditions, it is the upper 10 cm of the saturated peat that transports the majority of both water (0.67) and contaminant (0.95). In the uppermost saturated peat layer, the proportion of contaminant flux is higher than the proportion of water flow; below this upper layer, the proportion of water flow is higher than the proportion of contaminant flux. This is driven by the increase in retardation coefficient with depth that is common in peatlands, highlighting the complex interaction between solute transport with the hydrophysical and geochemical properties of peatlands. In short, the exponential decline in hydraulic conductivity has a very strong control on the distribution of contaminants within a peat profile.
Relative to the peatland in Exercise 2 that has a more monotonic decline in hydraulic conductivity, there is more mass of contaminant exported to the in the stream (1066 g d-1 versus 267 g d-1) in the exponential decline peatland under the wet conditions because of the substantially higher Ksat in the top layer and smaller Rf in the top two layers. Assuming the same Rf profile, the difference remains large between the exponential and monotonic Ksat profiles (1066 g d-1 versus 396 g d-1). However, under dry conditions, the contaminant mass exported to the stream is reduced by ~40% between the exponential (11 g d-1) and monotonic (18 g d-1) peatlands due to the 2 times larger sorption in the monotonic case. These two examples and their comparison highlight the necessity of understanding both the hydrological and geochemical properties of peatlands to properly predict, remediate, and/or mitigate potential impacts from disturbances.
