Eutrophication Index
Zhou et al. (1983) developed an Eutrophication Index method for preliminary assessment of water quality in Bohai Bay, which accounts for the effects of chemical oxygen demand (COD), dissolved inorganic nitrogen (DIN), and dissolved inorganic phosphorus (DIP). The Eutrophication Index above or equal to 1 in a water body indicates the eutrophication condition because of excessive nutrients.
Since most of the studies have paid attention to the nutrients especially DIN and PO4-P in seawater, a general relationship between the nutrient concentration and the level of eutrophication had been established (Arhonditsis et al., 2000; Moncheva et al., 2001).
Subsequently, Costa et al. (1992) developed a Water Quality Eutrophication Index as a tool to summarize water quality monitoring data collected in the Citizen’s monitoring program in Buzzards Bay, USA (http://www.buzzardsbay.org/eutroindex.htm), which was later adopted by Hillsborough County in Florida to evaluate changes of water quality in Tampa Bay. Costa’ index classifies water quality into 0–35 (for eutrophication condition), 35–65 (fair condition), and 65–100 (good to excellent condition).
Uses and Limitation:
This index method is used to detect the eutrophication in surface water.
Categorization Table
The eutrophication assessment was based on whether the eutrophication index was greater than or equal to 1 in the nutritional status index method.
Standards Required
Sea water was analyzed by the multiple effects of COD, DIN, and DIP on eutrophication.
Variables Selection
The water quality parameters, for those any national and international standard is assigned could be used in this method.
Calculation of Eutrophication Index:
Referring to Quan et al. (2005) Eutrophication Index is given below.
\begin{eqnarray*}
E =\frac{COD * DIN * DIP}{4500} * {10_{6}}
\end{eqnarray*}
where COD = the concentration of chemical oxygen demand; DIN = the concentration of dissolved inorganic nitrogen; DIP = the concentration of dissolved inorganic phosphate.
DIN concentration is calculated by the total concentration volume of nitrate, nitrite, and ammonium, whereas the DIP is calculated by the concentration of phosphate in water (Son et al., 2020).
Case Studies based on Eutrophication Index
Wang et al., (2009) collected water samples were from 120 stations in the Bohai Sea of China to analyze the distribution of dissolved nutrients and assess the degree of eutrophication in August 2002. The result shows that the average concentration of DIN increased and the PO4-P concentration sharply decreased compared to the previous data of corresponding period. The high concentrations of DIN and PO4-P occurred in coastal waters, especially in the bays and some river estuaries, while the high concentrations of SiO3-Si in the surface and middle depth occurred in the central area of the Bohai Sea. The average ratio of DIN/ PO4-P was much higher than the Redfield Ratio (16:1). Apparently, PO4-P was one of the limiting nutrient for phytoplankton growing in the sea. The average concentrations of DON and DOP were higher than their inorganic forms. The results of eutrophication assessment show that 22.1% of all stations were classified as violating the concentration levels of the National Seawater Quality Standard (GB 3097-1997) for DIN and only 3.9% for PO4-P. The average eutrophication index in the overall area was 0.21±0.22 and the high values occurred in Bohai Bay, Liaodong Bay and near the Yellow River estuary. This means that the state of eutrophication was generally mesotrophic in the Bohai Sea, but relatively worse in the bays, especially some river estuaries.
Sources of pollution discharges and water quality samples at 27 stations in 2006 in the coastal waters of Hebei Province, western Bohai Sea, were analyzed by Liu et al., (2011). Pollutant loads from industrial sewages have shown stronger impact on the water environment than those from the general sewages. Analysis indicated that pollution of COD is mainly resulted from land-based point pollutant sources. For phosphate concentration, non-point source pollution from coastal ocean (fishing and harbor areas) plays an important role. To assess the water quality conditions, Organic Pollution Index and Eutrophication Index have been used to quantify the level of water pollution and eutrophication conditions. Results show that pollution was much heavier in the dry season than flood season in 2006. Based on COD and phosphate concentrations, results show that waters near Shahe River, Douhe River, Yanghe River, and Luanhe River were heavily polluted. Water quality in the Qinhuangdao area was better than those in the Tangshan and Cangzhou areas.
A combined water quality index (WQI) and pollution index (PI) to assess and characterize river water quality of Cau River which is one of the longest rivers in the north of Vietnam. Five different water quality and water pollution indices were used including the Water Quality Index (WQI), Comprehensive Pollution Index (CPI), Organic Pollution Index (OPI), Eutrophication Index (EI), and Trace Metal Pollution Index (TPI). The combined water pollution indices show more serious pollution towards the river downstream. In particular, CPI and OPI reveal a high
risk of eutrophication. Cluster analysis was applied to classify water monitoring points into different quality groups in order to provide a better understanding of the water status in the river. This study indicates that a combined water quality analysis could be an option for decision making water use purposes while its single index shows the current situation of water quality (Son et al., 2020).
References
Wang, X.L., Cui, Z.G., Guo, Q., Han, X.R., 2009. Distribution of nutrients and eutrophication assessment in the Bohai Sea of China. Chinese Journal of Oceanology and Limnology 27 (1), 177–183.
Liu, S., Lou, S., Kuang, C., Huang, W., Chen, W., Zhang, J. & Zhong, G. 2011. Water quality assessment by pollution index method in the coastal waters of Hebei Province in western Bohai Sea, China. Mar. Pollut. Bull. 62, 2220–2229.
Cao Truong Son, Nguyen Thị Huong Giang, Trieu Phuong Thao, Nguyen Hai Nui, Nguyen Thanh Lam, Vo Huu Cong; Assessment of Cau River water quality assessment using a combination of water quality and pollution indices. Journal of Water Supply: Research and Technology-Aqua 1 March 2020; 69 (2): 160–172. https://doi.org/10.2166/aqua.2020.122