Water Quality Standards
River Pollution Index (RPI) The current assessment of river quality by the E.P.A is based on a comprehensive index known as “River Pollution Index”, or RPI for short. RPI is an integrated indicator used to determine the level of pollution of a river. The index value is calculated using the concentration of 4 parameters in water quality: Dissolved Oxygen (DO), Biochemical Oxygen Demand (BOD5), Suspended Solids (SS) and Ammonia Nitrogen (NH3-N). The calculation and comparison baselines for RPI are shown below:
Water Quality/Item Non(Slightly)-polluted Lightly-polluted Moderately-polluted Severely-polluted
Dissolved Oxygen(DO)mg/L DO≧6.5 6.5>DO≧4.6 4.5≧DO≧2.0 DO<2.0
Biochemical Oxygen Demand(BOD5)mg/L BOD5≦3.0 3.0<BOD5≦4.9 5.0≦BOD5≦15.0 BOD5>15.0
Suspended Solids(SS) mg/L SS≦20.0 20.0<SS≦49.9 50.0≦SS≦100 SS>100
Ammonia Nitrogen(NH3-N)mg/L NH3-N≦0.50 0.50<NH3-N≦0.99 1.00≦NH3-N≦3.00 NH3-N>3.00
Point Scores 1 3 6 10
Pollution Index Integral Value S≦2.0 2.0<S≦3.0 3.1≦S≦6.0 S>6.0
Remark: According to the resolution of “River pollution index (RPI) standard and calculation revision” meeting on May 30, 2013, RPI classification standard is adjusted with reference to monitoring report decimal rules published by Environmental Analysis Laboratory, starting from 2013. Carlson’s Trophic State Index For assessment of eutrophication levels of the water in reservoirs, the E.P.A uses “Carlson’s Trophic State Index” (CTSI), which is calculated based on the concentration of three independent water quality variables: Transparency (SD), Chlorophyll-a (Chl-a), and Total Phosphate (TP). The calculation method and comparison baselines for CTSI are shown below:
Carlson’s Trophic State Index CTSI = ﹝TSI(SD)+TSI(Chl)+TSI(TP)﹞
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TSI (SD) = 60 - 14.41 x ln SD
TSI (Chl-a) = 9.81 x ln Chl-a+30.6
TSI (TP) = 14.42 x ln TP+4.15

Where
SD = Transparency(m)
Chl-a = Chlorophyll-a Concentration(μg/L)
TP = Total Phosphate(μg/L)
CTSI index value Eutrophication Level of a Reservoir
TSI<40 Oligotrophic
40≦CTSI≦50 Mesotrophic
CTSI>50 Eutrophic
Example calculation of RPI. Take Danshuei River Mouth station as an example, monitoring data in Dec. 2003 consisting of Dissolved Oxygen (DO) 3mg/L, Biochemical Oxygen Demand (BOD5) 2.2mg/L, Suspended Solids (SS) 13.6mg/L, and Ammonia Nitrogen (NH3-N) 1.54mg/L. The scores are obtained as following:
Water Quality/Item Monitoring Data Point Numbers
Dissolved Oxygen(DO)mg/L 3 6
Biochemical Oxygen Demand(BOD5)mg/L 2.2 1
Suspended Solids(SS) mg/L 13.6 1
Ammonia Nitrogens(NH3-N)mg/L 1.54 6
Total Point Score   14
The total point score is then divided by the number of items for determining water quality, i.e. 14/4 = 3.5. The obtained value 3.5 is then compared to the above “Pollution Index Integral Value”, which falls between 3.1~6.0, and therefore is determined as moderately-polluted. Calculation example of CTSI. In this example, CTSI is calculated according to the Feitsui Reservoir No.6 monitoring station, with monitoring data from Nov. 3, 2003, consisting of Transparency (SD) of 5.1 m, Chlorophyll-a (Chl-a) of 4.48 μg/L, and Total Phosphate (TP) of 11 μg/L. What is the condition of this water? First, from the following equations:
TSI (SD) = 60 - 14.41 x ln SD
TSI (Chl-a) = 9.81 x ln Chl-a + 30.6
TSI (TP) = 14.42 x ln TP + 4.15
CTSI = ﹝TSI(SD)+TSI(Chl)+TSI(TP)﹞
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Therefore at the monitoring station:
CTSI = [ ( 60-14.41 x ln5.1 ) + ( 9.81 x ln4.48+30.6 ) + ( 14.42 x ln11+4.15 ) ] / 3 = 40.19

2. As the value is compared to the table below:
CTSI Index Value Eutrophication Level of Reservoir
CTSI<40 Oligotrophic
40≦CTSI≦50 Mesotrophic
CTSI>50 Eutrophic
The water condition at the monitoring station is “Mesotrophic”.
UPDATE:2019/09/27