Chapter 18

Discuss the extent and types of water contamination problems that are possible when septic tanks for sewage disposal are located in (a) sandy or (b) clayey soils.

Explain why clay-rich soils have desirable physical and chemical properties for use as liners for landfill sites.

Radiocaesium is one of the nuclides arising from the fallout from the Chernobyl reactor incident in 1986. Some of it rained out over the United Kingdom. In the soil, caesium is either immobilized or is taken up by plants that are then eaten by grazing animals. In the UK, controls come into force where levels in animal flesh are greater than $1000\mathrm{Bq}{\mathrm{kg}}^{-1}$. In 1987 , levels as high as four times this value were measured in some sheep. Highest animal concentrations were measured where pasture was developed on upland soils, high in organic matter; lower levels were found where the sheep grazed on lowland soils, rich in clays. Suggest a reasonable explanation for these observations.

Consider the following data for a forest soil: $$\text{Unknown environment 'tabular'}$$ Comment on the significance of these values in terms of porosity and permeability in each horizon.

With reference to an ion exchange medium, selectivity refers to the thermodynamic tendency to retain a particular species. The order of selectivity for alkali metal cations by most clays is $${\mathrm{Cs}}^{+}>{\mathrm{K}}^{+}>{\mathrm{Na}}^{+}>{\mathrm{Li}}^{+}$$ Explain this in terms of the aqueous solution chemistry of these ions.

Logging of a forest by removing all the mature trees is a controversial forestry practice. Aside from issues such as the effect on species biodiversity and erosion, clear-cutting can alter chemical processes in the soil and even in the global environment. Explain how this practice could lead to increased nitrification and denitrification-and how this may affect soil acid-base properties and the stratospheric ozone concentration.

The following are chemical properties of two Venezuelan surface soils. Predict their relative sensitivity to acidic inputs from rain or fertilizer and give reasons for your prediction. | | pH | OC | N | Clay | Ca | Mg | K | Na | Al | CEC//cmolkg^(-1) | BS//S | | | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :---: | | | | | | | | | | | | | | | | Machiques Barinas | 6.0 | 0.75 | 0.08 | 7.2 | 0.3 | 1.4 | 0 | 0.01 | 0.1 | 3.7 | 44 | |

In a waterlogged soil, carbon dioxide from decomposition processes may increase, aqueous oxygen levels may decline to near zero due to impeded gas exchange, and the soil solution may accumulate significant concentrations of hydrogen sulfide and methane. Discuss possible effects of these properties on the behaviour of aluminium, iron, and manganese in the soil.