In accordance with Article 4 of the Japanese Waterworks Act, tap water must conform to the water quality standards stipulated in "ministerial ordinances related to water quality standards." There are 51 stipulations, including one regarding pH. A pH of 8 or higher decreases the effectiveness of chlorine sterilization, and a pH of 6.5 or lower decreases the aggregating effect of the water treatment process.
(For reference)pH standard of Tap water
| Japan | 5.8～8.6 |
| European Union | 6.5～9.5 |
| U.S. Environmental Protection Agency | 6.5～8.5 |
As for water from rivers, lakes, and the ocean, Japan's Basic Environment Law, which sets out environmental standards for preserving citizens' quality of life, establishes guidelines for water quality and soil. There is a set typology based on the purpose that the water is used for.
The scale ranges from AA, which is the cleanest water, to E, which is the most polluted water. Rivers are covered by types AA to E, lakes are covered by types AA to C, and ocean water is covered by types A to C.
(For reference)
| River types AA–C | 6.5～8.5 |
| River types D–E | 6.5～8.5 |
| Lake types AA–B | 6.5～8.5 |
| Lake type C | 6.0～8.5 |
| Ocean types A–B | 7.8～8.3 | Ocean type C | 7.0～8.3 |
[Reference materials]
Standards for irrigation water used in agriculture present ideal indexes for normal cultivation of rice plants in paddies. The criteria was established in 1970, and the acceptable concentrations for preventing harm for each type of contaminant are discussed (not a legally binding standard).
The prescribed pH range is 6.0 to 7.5. If the pH is too low, it can lead to inhibited growth of the roots and deterioration of the soil through basic soil erosion. If the pH is too high, it can cause discoloration of the leaves due to iron deficiency.
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