Which runoff sources near a wetland could lead to eutrophication: I) runoff from crop fields treated with large doses of fertilizer, II) oil runoff from roads, III) leached metals from a mine?

What the question is really testing

You are being asked which listed pollutants add enough plant nutrients (especially nitrogen and phosphorus) to a wetland to cause algal blooms and oxygen depletion, which is eutrophication.

Check each option for nutrient loading

• I. Fertilizer runoff from crop fields: Fertilizers contain high levels of $NO_3^-$ (nitrate) and $PO_4^{3-}$ (phosphate). When these wash into wetlands, they can rapidly increase primary productivity (algal growth), starting eutrophication.
• II. Oil runoff from roads: Oil is mainly hydrocarbons. It can create surface films, reduce gas exchange, and be toxic, but it is not a major source of nitrogen or phosphorus, so it does not usually cause eutrophication.
• III. Leached metals from a mine: Metals (for example, $Pb^{2+}$, $Hg^{2+}$, $Cu^{2+}$) can be toxic and can disrupt ecosystems, but they are not the nutrient inputs that drive eutrophication.

Choose the best answer

Only statement I describes a source that directly increases nutrient concentrations enough to trigger eutrophication, so the correct choice is A) I only.