Answer
(c)
Work Step by Step
To find the concentration of aluminum in the water that leaches from the landfill, we first need to calculate the total amount of aluminum leached in a year. Given that $1\%$ of the total mass of aluminum ($100,000$ kg) is leached each year, the amount of aluminum leached is
$0.01 \cdot 100,000\text{ kg} = 1,000 \text{ kg}$.
Next, we need to find the total volume of water that infiltrates and leaches through the soil. $20\%$ of the annual precipitation ($200$ million L) infiltrates, so the volume of water leaching is
$0.2 \cdot 200\text{ million L} = 40 \text{ million L}$.
Finally, we can calculate the concentration of aluminum in the leachate by dividing the amount of aluminum leached ($1,000$ kg) by the volume of leachate ($40$ million L):
$\text{Concentration} = \frac{1,000,000\text{ g}}{40,000,000\text{ L}} = 0.025 \text{ g/L} = 25\text{ mg/L}$.
The answer is (c).
