Water is generally used in many ways, such as a raw water source for tap water, for agriculture, for recreation,

and for the survival of aquatic life, etc. The water quality requirements vary depending on how the water is used. The general water quality index mentioned refers to the overall condition of a river, without directly specifying its uses.

Similarly, describing a person’s physical condition—whether they are healthy or sick—doesn’t directly indicate what they can do.

(A person with minor illness can still perform some tasks.) Therefore, we call it the General Water Quality Index (GQI) to indicate

the level of water quality—whether it’s very good, acceptable, or poor. This allows us to determine what control and management measures are needed for the river. Similarly,

if someone is sick (due to low water quality), they need to see a doctor (immediate action is required). The extent of the solution depends on the severity of the illness and

the cause, which can be natural, such as turbidity, or human-induced, such as wastewater discharge.

The Unweighted Multiplicative River Water Quality Index is a method used to disseminate water quality knowledge to the public using simple language. A concise and easy-to-understand method, not overly complex, used in the United States and one of the methods used in preparing reports for the House of Representatives of the United States (developed by Brown, R.M., et al. 1970).

The general water quality index (WQI) mentioned is a score ranging from 0 to 100 points (think of it like an exam). 91–100 points are considered

very good water quality; 71-90 points are good water quality; 61-70 points are fair water quality; 31-60 points are poor water quality; and 0-30 points are very poor water quality.

These scores are usually generated by summing existing scores. The nine water quality indices are: pH, Dissolved Oxygen (DO),

Total Solids (TS), Fecal Coliform Bacteria (FCB), Nitrate (NO₃⁻), Phosphate (PO₄⁻),

Turbidity, Temperature, and Biological Oxygen Demand (BOD). These are combined to form a single total score using the equation WQI = [(pH) (DO) (TS) (FCB) (NO₃⁻) (PO₄⁻) (Turbid) (Temp) (BOD)]^1/9.

The origin of these nine indices and their associated scores comes from surveys conducted among hundreds of experts (a method similar to the development of the atomic bomb).

The objective was for these experts to determine which indices should be considered in general water quality assessments. And if we were to rate different concentration levels, for example,

what score would be given for an oxygen level of 3 milligrams per liter? The combination of expert opinions led to the development of a general water quality index (WQI), which has been proven

to be acceptable when comparing the water quality scores obtained using this method with the experts’ perceptions.

The Pollution Control Department tested this method on water quality data from 45 rivers for one year and modified it for practical use. In reporting,

the general water quality index uses 8 water quality indices, excluding temperature, to ensure the WQI is reasonably sensitive to changes in water quality. This is because

water and air temperatures in Thailand do not fluctuate significantly (temperature is only used when thermal pollution is detected). Experiments using the Modified Water Quality Index (WQI)

with water quality data from various rivers in Thailand showed that the analysis results are practically effective. They can be used to describe the overall water quality in water sources,

and to compare water quality levels between rivers, making it easier to understand the overall picture for administrators and the public, even those without scientific knowledge, to grasp the water quality problems.

The scores obtained from each parameter are presented below. This can be tested using the proposed rating curve integrated with the program. After calculating each parameter, all scores are added together again to find the final score, using the formula described above. The Pollution Control Department has adjusted the rating curve to develop a WQI calculation formula suitable for rivers in Thailand.

This formula is also comparable to surface water standards.