Answer
$0.0306\text{ mg/L}$
Work Step by Step
Determine nitrogen mass entering the stream:
$1500\text{ kg}\times 0.10=150\text{ kg}$
Runoff into stream: $15\%$:
$150\text{ kg}\times 0.15=22.5\text{ kg}$
Convert to milligrams:
$22.5\text{ kg}=22.5\times 10^6\text{ mg}$
Calculate total stream volume per year:
$365\times 24\times 60=525,600\text{ min/year}$
The total volume is:
$1.4\text{ m}^3\text{/min}\times 525,000\text{ min}=735,840\text{ m}^3$
Convert to liters:
$735,840\text{ m}^3\times 1000=7.3584\times 10^8\text{ L}$
Calculate concentration:
$\frac{22.5\times 10^6\text{ mg}}{7.3584\times 10^8\text{ L}}\approx 0.0306\text{ mg/L}$
