Residues and Environmental Chemistry
Questions test how pesticide residues dissipate and move: half-life arithmetic, adsorption and leaching indices, bioaccumulation, and the meaning of an MRL.
Core rules
- Dissipation is usually approximated as first order. After n half-lives the fraction remaining is 0.5^n; time to 90% loss is about 3.32 half-lives, to 99% loss about 6.6 half-lives.
- DT50 is the time for 50% loss under defined conditions; it differs between soil, water and plants, so a residue question must specify the compartment.
- Degradation pathways are microbial, hydrolytic and photolytic; adsorption to organic matter and clay reduces bioavailability and leaching.
- Koc normalises adsorption to organic carbon; chemicals with low Koc and long DT50 are the leachers. The GUS index combines both.
- Bioaccumulation is assessed by bioconcentration factors and correlates with lipophilicity (log Kow). Organochlorines such as DDT accumulate; metabolites like DDE can persist longer than the parent.
- MRL is a trading and compliance standard, not a toxicological threshold; residues below MRL are still residues.
Worked example
A compound has a soil DT50 of 15 days. To reach 90% loss: 0.5^n = 0.10, so n = log(0.10)/log(0.5) = 3.32. Time = 3.32 × 15 ≈ 50 days. If the labelled PHI were 30 days, residues would be roughly 25% of the initial value at harvest - about 2 × DT50 - which is why PHI and DT50 are read together.
Traps that cost marks
- Reporting time to 90% loss as two half-lives instead of 3.32.
- Quoting DT50 without stating the compartment.
- Assuming a metabolite is safer or shorter-lived than its parent compound.