Alkalinity is not a pollutant. It is a total measure of the substances in water that have "acid-neutralizing" ability.

Alkalinity is not a pollutant. It is a total measure of the substances in water that have "acid-neutralizing" ability. Don’t confuse alkalinity with pH. pH measures the strength of an acid or base; alkalinity indicates a solution’s power to react with acid and "buffer" its pH — that is, the power to keep its pH from changing.
To illustrate, we will compare two samples of pure water and buffered water. Absolutely pure water has a pH of exactly 7.0. It contains no acids, no bases, and no (zero) alkalinity. The buffered water, with a pH of 6.0, can have high alkalinity. If you add a small amount of weak acid to both water samples, the pH of the pure water will change instantly (become more acid). But the buffered water’s pH won’t change easily because the Alka-Seltzer-like buffers absorb the acid and keep it from "expressing itself."
Alkalinity is important for fish and aquatic life because it protects or buffers against pH changes (keeps the pH fairly constant) and makes water less vulnerable to acid rain. The main sources of natural alkalinity are rocks, which contain carbonate, bicarbonate, and hydroxide compounds. Borates, silicates, and phosphates may also contribute to alkalinity.
Limestone is rich in carbonates, so waters flowing through limestone regions generally high alkalinity — hence its good buffering capacity. Conversely, granite does not have minerals that contribute to alkalinity. Therefore, areas rich in granite have low alkalinity and poor buffering capacity.

Table 1. Some recommended alkalinity values
Industry and Process	Recommended Maximum Total Alkalinity (in mg/L CaCO3)
Carbonated beverages	85
Food products (canning)	300
Fruit juice	100
Washing diapers	60
Pulp and paper making(ground-wood process)	150
Rayon manufacture	50
Tanning hides	135
Textile mill products	50-200
Petroleum refining	500