Water Quality Analyzer

The WQI is a single number (0-100) that summarizes multiple water quality parameters into one score. Each parameter is converted to a sub-index on a 0-100 scale, then combined using a weighted average.

where $w_i$ is the weight of parameter $i$ and $q_i$ is its sub-index score. Dissolved oxygen carries the highest weight (28%) because it is critical to aquatic life.

• 90-100 Excellent, safe for all uses
• 70-89 Good, minor threats possible
• 50-69 Fair, some impairment
• 25-49 Poor, significant pollution
• 0-24 Very Poor, unsuitable for most uses

Dissolved oxygen (DO) is the amount of oxygen gas dissolved in water. Fish and aquatic invertebrates need at least 5 mg/L to survive. Cold water holds more oxygen than warm water.

This Benson-Krause equation gives the maximum DO (in mg/L) at temperature $T$ (in °C) at 1 atm. At 0°C, water can hold about 14.6 mg/L. At 25°C, this drops to about 8.3 mg/L.

Percent saturation ($\%\mathrm{sat} = 100 \times \mathrm{DO} / \mathrm{DO}_{\mathrm{sat}}$) is often a better indicator than raw DO because it accounts for temperature effects.

Eutrophication is the process where excess nutrients (primarily nitrogen and phosphorus) trigger explosive algal growth. The cycle follows a predictable pattern.

1. Nutrient loading from fertilizer runoff, sewage, or industrial discharge increases nitrogen and phosphorus levels.
2. Algal bloom develops as algae grow rapidly with abundant nutrients and sunlight.
3. Oxygen depletion occurs when dead algae sink and decompose. Bacteria consume oxygen during decomposition.
4. Hypoxia (DO below 2 mg/L) creates dead zones where fish and invertebrates cannot survive.

The simulation uses logistic growth for algae with Monod kinetics for nutrient uptake and a reaeration term for oxygen recovery.

The U.S. EPA sets standards for water quality parameters to protect human health and aquatic ecosystems.

Note that these are general guidelines. Specific criteria vary by water body type, designated use (drinking water, recreation, aquatic habitat), and state regulations.