Soil Chemistry and Soil Physics
Domain scope
This domain tests how soil chemistry and physics control contaminant fate, water movement, and treatment feasibility. Expect questions on texture, cation exchange capacity (CEC), pH-dependent charge, bulk density, porosity, water content, and hydraulic conductivity.
Core rules and concepts
- Texture (sand/silt/clay) sets surface area; clay and organic matter dominate CEC. CEC is reported in cmol(+)/kg, numerically equal to meq/100 g.
- Porosity n = 1 − ρb/ρs, with particle density ρs ≈ 2.65 g/cm3 for mineral soil. Volumetric water content θ = gravimetric w × ρb/ρw.
- Darcy flux q = −K i; K falls by orders of magnitude from sand to clay and depends on saturation.
- Low pH generally increases Cd, Zn, Ni, and Pb solubility; arsenic can become more mobile at high pH. Redox status controls Fe/Mn oxides and reductive dechlorination.
Worked example
A 100 cm3 core has 150 g dry soil and 20 g water. ρb = 150/100 = 1.5 g/cm3. Porosity = 1 − 1.5/2.65 = 0.434, or 43.4%. Gravimetric water content = 20/150 = 0.133. Volumetric water content = 0.133 × 1.5 = 0.20, so 20% of total volume is water. Pore volume = 43.4 cm3.
Traps that cost marks
Using wet mass to compute bulk density; treating CEC as fixed when pH changes; mixing cmol/kg with mg/kg; and reporting gravimetric water content as volumetric.