Question 3 of 5: Estimation of Contaminant Effects – Golf-Course Pond 2,4-D Spill

Question 3: Estimation of Contaminant Effects – Golf-Course Pond 2,4-D Spill (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given.

Find. (a) meaning of BCF; (b) which EPIWIN datum drives the BCF estimate; (c) BCF; (d) trout LC50 via the guppy correlation; (e) whether the pond concentration exceeds LC50; (f) mass of 2,4-D ingested per 0.25 kg fish meal.

Approach. Use Eq. 3-2 with the given log K_ow for BCF, Eq. 3-1 for the guppy/trout surrogate LC50, a simple dilution mass balance for the pond concentration, then BCF×C_water×mass eaten for the ingested dose.

1. (a) Meaning of bioaccumulation potential / BCF. The bioconcentration factor is the equilibrium ratio of a chemical's concentration in an organism's tissue to its concentration in the surrounding water, $BCF=C_{biota}/C_{water}$ (units L/kg, since it converts a mass-per-volume-water concentration into a mass-per-mass-tissue concentration). It quantifies the tendency of a lipophilic, poorly-metabolized/poorly-excreted compound to concentrate in fatty tissue faster than it is eliminated, so that the organism's internal concentration can substantially exceed the ambient water concentration — the mechanism that allows a contaminant present at a sub-toxic water concentration to still pose a dietary exposure risk to anything that eats the organism (directly relevant to part (f)).
2. (b) Which EPIWIN datum drives the estimate, and why. The octanol-water partition coefficient, log K_ow = 2.41, is the controlling datum. K_ow measures a compound's relative affinity for a lipid-like (octanol) versus aqueous phase, which is the same partitioning behaviour that governs uptake into an organism's lipid-rich tissue; BCF correlates strongly with K_ow for non-polar, non-ionizable, non-metabolized organics (Eq. 3-2 is exactly this correlation), so no separate biological measurement is required to obtain a screening-level BCF estimate.
3. (c) Bioconcentration factor, Eq. 3-2. $$\log BCF=0.79(2.41)-0.40=1.9039-0.40=1.5039$$ $$BCF=10^{1.5039}=\boxed{31.9\ \text{L/kg}}$$
4. (d) Trout LC50 via guppy surrogate, Eq. 3-1. $$\log(1/LC_{50})=0.871(2.41)-4.87=2.0991-4.87=-2.7709$$ $$1/LC_{50}=10^{-2.7709}=1.695\times10^{-3}\ \Rightarrow\ LC_{50}=590\ \text{mmol/L}=5.90\times10^5\ \mu\text{mol/L}$$ $$\boxed{LC_{50,\text{trout}}\approx 590\ \text{mmol/L} = 5.90\times10^{5}\ \mu\text{mol/L}\ \text{(guppy-correlation, surrogate for trout)}}$$ This is roughly 670× larger than the 880 µmol/L reference value EPIWIN reports directly (itself likely a different test species/endpoint); Eq. 3-1 is a baseline narcosis-mechanism QSAR, and 2,4-D esters are known to act through a more specific (non-narcotic) toxic mechanism in fish, so this correlation is expected to underestimate the compound's true acute toxicity (i.e. predict a higher, less conservative LC50) — a limitation to flag explicitly (see part (f) assumptions and the concept aside below).
5. (e) Was the LC50 exceeded? Pond concentration from a simple dilution mass balance: $$C_{water}=\frac{100\ \text{g}}{1.5\times10^{6}\ \text{L}}=6.667\times10^{-5}\ \text{g/L}=0.0667\ \text{mg/L}=\frac{66.7\ \mu\text{g/L}}{221.04\ \text{g/mol}}=\boxed{0.302\ \mu\text{mol/L}}$$ Comparing to both LC50 estimates: $0.302\ \mu\text{mol/L}\ll5.90\times10^{5}\ \mu\text{mol/L}$ (Eq. 3-1 value, a factor of $\approx2\times10^{6}$ below) and $0.302\ \mu\text{mol/L}\ll880\ \mu\text{mol/L}$ (EPIWIN reference value, a factor of $\approx2900$ below). The LC50 was NOT exceeded by either estimate — the pond concentration is also well below the 98 mg/L (443 µmol/L) maximum water solubility, confirming the spilled mass is fully dissolvable in the pond volume with no free-phase residue expected.
6. (f) Dietary ingestion from trout fish tacos. Fish tissue concentration from the BCF (part c) and pond concentration (part e): $$C_{fish}=BCF\times C_{water}=31.9\ \text{L/kg}\times0.0667\ \text{mg/L}=\boxed{2.13\ \text{mg/kg fish (wet weight)}}$$ Ingested mass for a 0.25 kg meal: $$m_{ingested}=C_{fish}\times m_{meal}=2.13\ \frac{\text{mg}}{\text{kg}}\times0.25\ \text{kg}=\boxed{0.532\ \text{mg 2,4-D per meal}}$$ Assumptions: instantaneous equilibrium partitioning between water and fish tissue (BCF is an equilibrium, not kinetic, ratio — a conservative simplification if the golfer's meal follows soon after the spill, since real bioaccumulation takes time to reach equilibrium and the true concentration could be lower); uniform, fully-mixed pond concentration with no sediment or biota compartment removing mass from solution (the question states significant sediment is present, which this simple 2-compartment water/fish balance conservatively ignores — a 3-compartment water/sediment/biota balance would partition some mass out of the water column and modestly reduce this estimate); the entire 0.25 kg fish portion is edible tissue at the same concentration as whole-body BCF predicts (no allowance for lower-fat edible fillet vs. whole-body lipid content); and no cooking-loss or degradation of 2,4-D during food preparation.