Water Quality Assessment and Eutrophic Classification of Hanoi Lakes Using Different Indices

Nguyen Thi Thu Ha,Nguyen Ngoc Tu,Trinh Quang Huy,Dinh Tien Dzung,Ho Thi Thuy Hang

doi:10.31817/vjas.2021.4.4.03

Nguyen Thi Thu Ha, Nguyen Ngoc Tu + Show 3 more

Open Access

https://doi.org/10.31817/vjas.2021.4.4.03

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Abstract

The trophic levels in urban lakes are typically based on the forms of nutrition and phytoplankton communities in the lakes. In this study, comparisons between eutrophication indices and the water quality index (VN-WQI) were used to classify the water quality of 20 lakes in Hanoi, Vietnam. The results showed that the water quality ranged from very bad to poor. High levels of the eutrophication phenomenon in terms of total N and P were observed in all sampling sites. Cyanobacteria was the dominate algae phylum making up 65.78% of the total population, whereas Chlorophyta (including 19 genera) was the most diverse phylum. The density of the algae was 5,000-14,000 cells mL-1 and the chlorophyll-a level was 10-40 µg L-1. Based on this information, the water quality levels in the lakes were classified from eutrophic to polytrophic. The Trophic Status Index (TSI) and Trophic Level Index (TLI) values were 66.9-86.0 and 54.4-76.0 points, respectively, corresponding from eutrophic to extreme-trophic. Classifications based on algae community structure indices were from oligotrophic to eutrophic, similar and consistent with the TLI classifications. According to the results in this study, the TLI was found to be more accurate and precise than the other indices. It is recommended that the TLI is an applicable tool to classify eutrophication in urban lakes.

Highlights

Eutrophication in closed lakes had been widely studied all over the world since the nineteenth century
Samples were analyzed for the Secchi depth (SD), pH, dissolved oxygen (DO), total suspended solids (TSS), biochemical oxygen demand (BOD5), chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP), amount of orthophosphate as phosphorus (P-PO43-), amount of ammonium as nitrogen (N-NH4+), amount of nitrate as nitrogen (N-NO3-), and total coliforms according to the methods guided by the National Technical Regulations 08-MT: 2015/MONRE
The Vietnam water quality index (VN-Water Quality Index (WQI)) in the lakes ranged widely from 13.5 to 70.5 points, 20% of which were determined as polluted (0-25 points), namely the lakes of Van Chuong, Van Quan, Kim Quan, and Thien Quang; another 25% of the total were rated as having low quality water (25-50 points), namely the lakes of Hoan Kiem, Yen So, Ben, Thanh Nhan, and Gia Lam Park; the remaining 55% had a poor water quality level (50-75 points); and none of the case lakes had good quality water (75-100 points)

Summary

Introduction

Eutrophication in closed lakes had been widely studied all over the world since the nineteenth century. According to the Center for Environmental and Community Research (CECR, 2015), most of the 112 lakes in Hanoi are under bad hygienic conditions because of receiving domestic wastewater and urban runoff. This may lead to serious pollution of organic matter, increased turbidity, loss of nutrients, increased numbers of microorganisms, and the depletion of dissolved oxygen (DO) in some lakes such as Thien Quang, Truc Bach, and Thanh Nhan. The pollution causes negative impacts on the aesthetics of the landscape and ecological value of these lakes (Scholten et al, 2005)

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