Water Pollution Index Calculator
Compute water pollution index using validated scientific equations. See step-by-step derivations, unit analysis, and reference values.
Formula
WPI = Sum(wi x qi) / Sum(wi)
Weighted pollution index equals the sum of weight times sub-index for each parameter divided by sum of weights.
Worked Examples
Example 1: Clean River
Problem: BOD=2, DO=8, pH=7.2, NO3=5, PO4=0.1.
Solution: qBOD=33.3, qDO=0, qpH=10, qNO3=10, qPO4=5 WPI = (4x33.3+4x0+3x10+2x10+2x5)/15 = 12.9
Result: WPI = 12.9 (Excellent)
Example 2: Polluted Stream
Problem: BOD=8, DO=4, pH=6.0, NO3=30, PO4=1.5.
Solution: qBOD=100, qDO=50, qpH=50, qNO3=60, qPO4=75 WPI = (400+200+150+120+150)/15 = 68
Result: WPI = 68 (Marginal)
Frequently Asked Questions
What is a water pollution index?
A water pollution index combines multiple water quality parameters into a single number for easy comparison and communication. Each parameter is converted to a dimensionless sub-index on a 0-100 scale and then weighted by importance. Higher index values indicate worse water quality while lower values indicate cleaner conditions.
Why is BOD an important water quality parameter?
Biochemical Oxygen Demand measures the oxygen consumed by microorganisms decomposing organic matter in water over 5 days. High BOD indicates heavy organic pollution that depletes dissolved oxygen harming fish and aquatic life. Clean rivers typically have BOD below 2 mg/L while heavily polluted water exceeds 10 mg/L.
What does dissolved oxygen indicate about water quality?
Dissolved oxygen is essential for aquatic life with most fish requiring at least 5 mg/L for survival and reproduction. DO levels depend on temperature, atmospheric pressure, and the balance between reaeration and oxygen consumption. Values near saturation (8-10 mg/L at typical temperatures) indicate healthy water while low DO signals organic pollution.
How does pH affect water quality?
Most aquatic organisms thrive in pH 6.5-8.5 with values outside this range causing stress and mortality. Acid rain, mine drainage, and industrial discharge can shift pH dramatically in affected water bodies. The sub-index penalizes deviations from neutral pH 7.0 in either acidic or alkaline direction.
What are acceptable nitrate levels in water?
The EPA drinking water standard for nitrate is 10 mg/L as nitrogen to prevent methemoglobinemia in infants. Natural background levels in pristine streams are typically below 1 mg/L of nitrate. Agricultural runoff carrying fertilizer is the primary source of elevated nitrate in surface and groundwater.
What role does phosphate play in water pollution?
Phosphate is often the limiting nutrient for algal growth in freshwater systems making it a key eutrophication driver. Concentrations above 0.1 mg/L can trigger algal blooms that deplete oxygen and produce toxins when they decompose. Sources include municipal wastewater, agricultural runoff, and detergent discharge.