River Water Quality Assessment Research Articles

Neutrosophic Approach to Water Quality Assessment: A Case Study of Gomati River, the Largest River in Tripura, India

This study addresses the complexity of assessing river water quality, a multifaceted process influenced by numerous water quality parameters (WQPs) characterized by inherent uncertainties and diverse judgment information from decision-makers. These uncertainties and diverse judgment information can be effectively represented and simulated using Neutrosophic sets. In this study, we propose an effective water pollution rating system, the Neutrosophic water quality index (NWQI), to derive a water pollution score (NWQI-score) for rating water pollution levels. We demonstrate the application of our methodology through an assessment of water quality indices for rating pollution in the Gomati River, the largest river in Tripura, India. Using the NWQI, we highlight the NWQI score as a pivotal indicator for evaluating the river's water quality. For assessing water quality, we consider six crucial parameters, namely Total hardness, Biochemical oxygen demand, Total suspended solids, Electrical conductivity, pH, and total dissolved solids, sampled at five strategic sites along the river. Sampling was conducted from January 2024 to July 2024, guided by the distribution of waste discharge points. Finally, we conclude the study, summarizing findings and suggesting future research directions.

Water quality assessment of Dudhganga river using water quality index and anthropogenic activities

ABSTRACT The primary goal of this study is to analyze the magnitude of water quality of Dudhganga River seasonally and temporal using physico-chemical parameters, Weighted Arithmetic Water Quality Index (WA-WQI) and anthropogenic activities of Kolhapur District, Maharashtra. Six sampling locations along the Dudhganga River (sites I to VI) were selected for sampling and analysis. In seasonal analysis, the higher values of water parameters like pH (8.2), TDS (0.42 μs/cm), turbidity (0.14 NTU), DO (6.24 mg/l), nitrate (7.56 mg/l) and phosphate (1.78mg/l) were reported in monsoon season. Temperature, Alkalinity and hardness were noted maximum values (25 °C, 66.67 mg/l and 12.67 mg/l) during summer season. However, EC was reported maximum (64 μs/cm) in winter season. Results of WA-WQI revealed that water quality index ranged from 14.90 to 24.26 all sampling sites. The values of WA-WQI were reported higher at downstream locations like IV, V, and VI, than the upstream sites I, II, and III. Along Dudhganga River substantial various human activities were significant contributor for declining the water quality of River. The acquired results suggest that water quality status is close to becoming contaminated and unsafe for domestic and human use. Such contaminated water also poses risk to plants and animals that make up the aquatic ecosystem.

Correction to: Risk assessment of river water quality using long-memory processes subject to divergence or Wasserstein uncertainty

Correction to: Risk assessment of river water quality using long-memory processes subject to divergence or Wasserstein uncertainty

Water chemical formation mechanism and water quality analysis of Shichuan River under the influence of human activities

ABSTRACT It is necessary to analyze the characteristics of the water environment and factors influencing the basin in order to determine its current ecological status. This study aims to investigate the chemical properties in the Shichuan River Basin using Gibbs diagram, Piper triangle diagram, correlation analysis, factor analysis, and main ion ratio method. The enhanced Nemerow comprehensive index method and fuzzy comprehensive evaluation method are employed for river water quality assessment. The findings reveal that runoff water has a weak alkaline nature with low salinity. The hydrochemical type of river water includes Ca2++Mg2++Na+-SO4 2-, Ca2++Mg2+-SO4 2-+Cl-+HCO3 -, Ca2++Na+-SO4 2-+Cl-+HCO3 -. Silicite and dolomite weathering are identified as primary influencing factors, while evaporite weathering also contributes to variations in chemical composition within the basin's waters. Ca2+, Mg2+, SO4 2- and HCO3 - originate from carbonate rock and silicate rock weathering processes, whereas Na+, K+ and Cl-, are primarily derived from evaporite weathering influenced by domestic sewage and agricultural activities. Agricultural production practices along with discharge of domestic sewage have an impact on Na+, K+, Cl- and NO3 -concentrations in aquatic systems. Water quality at all three monitoring sections falls under level IV classification. Phosphorus emerges as a key limiting factor for eutrophication within this basin's watersheds.

Accelerated urban river water quality assessment in Southern India using visible and near-infrared spectroradiometry

Optical indicators of water quality offer resource managers a swift and cost-effective means of monitoring water bodies. Remote sensing presents a promising avenue to overcome limitations, enabling comprehensive evaluation across geographical and temporal scales. This study presents initial findings utilizing hyperspectral sensing to analyze the water quality of the urban river Cooum in Chennai, Tamil Nadu. Lab analysis of 40 water samples included examination for key ions such as chloride, calcium, fluoride, sulphate, nitrate, and bicarbonates, as well as assessment of solids and colloidal components. Spectral analysis using a handheld Visible Near-Infrared (VNIR) spectroradiometer yielded reflectance curves, depicting the response of various water types and pollutants at different wavelengths. Integration of spectroradiometer data with water quality parameters facilitated the identification of empirical models tailored to the parameters of interest. The developed models exhibited an average R2 value of 0.7, underscoring the efficacy of hyperspectral remote sensing in rapidly assessing water quality in polluted rivers.

BaHys—A Bayesian Modeling Framework for Long‐Term Concentration‐Discharge Hysteresis: A Case Study on Chloride

AbstractImproving river water quality requires a thorough understanding of the relationship between constituent concentration and water discharge during runoff events (i.e., C‐Q hysteresis), which may be strongly non‐linear. Analysis of C‐Q hysteresis on large temporal scales provides unprecedented insights into event dynamics and long‐term concentration trends in surface and groundwater. Despite the increasing availability of time series data on water quality, there are still limited quantitative modeling frameworks that enable this analysis. Here, we combine Bayesian modeling and an existing mass balance to model long‐term C‐Q hysteresis dynamics in multi‐decade constituent concentration and water discharge time series. We focus on the case study of chloride and demonstrate that our model can simultaneously characterize the size and rotation of C‐Q hysteresis, and diffuse and low‐flow inputs to constituent concentration using only time‐series data from the river Rhine. Over 28 years of monitoring, we find that chloride exhibits a dominant clockwise dilution behavior that does not vary considerably under different hydro‐climatic conditions, hinting to similar mobilization mechanisms over time. We also show decreasing chloride concentrations in surface and groundwater, due to the cessation of mining activities in the Rhine. Our approach uses uncertainty estimates to show the range within which model parameter values lie, aiding decision‐makers in a robust assessment of river water quality. We conclude that Bayesian modeling of C‐Q hysteresis provides a powerful framework for investigating long‐term contamination dynamics that can be extended to several constituents to find factors controlling their export, ultimately suggesting mitigation measures for river contamination.

Innovative Solutions for Melaka's River Water Treatment

Melaka's rivers, essential for agriculture, drinking water, and transportation, face growing pollution challenges due to urbanization and industry, threatening their ecological balance. This study delved into the assessment of river water quality in Melaka and the design of a practical water treatment device tailored for river water purification. Extensive testing, involving Total Dissolved Solids (TDS) and pH analysis, shed light on the existing poor state of the Melaka River water. The devised water treatment device, encompassing diverse filter materials and a submersible pump, yielded noteworthy outcomes. Notably, experiment 4, employing activated carbon and cotton wool for both inlet and outlet, demonstrated the highest efficacy, reducing TDS levels by a remarkable 44.21%, surpassing other trials. The employed materials exhibited the potential to neutralize pH levels and clarify river water, with cotton wool effectively trapping residues and impurities.

Assessing river water quality for ecological risk in the context of a decaying river in India.

The decay of rivers and river water pollution are common problems worldwide. However, many works have been performed on decaying rivers in India, and the status of the water quality is still unknown in Jalangi River. To this end, the present study intends to examine the water quality of the Jalangi River to assess ecological status in both the spatial and seasonal dimensions. To depict the spatiality of ecological risks, 34 water samples were collected from the source to the sink of the Jalangi River with an interval of 10km while 119 water samples were collected from a secondary source during 2012-2022to capture the seasonal dynamics. In this work, the seasonality and spatiality of change in the river's water quality have been explored. This study used the eutrophication index (EI), organic pollution index (OPI), and overall index of pollution (OIP) to assess the ecological risk. The results illustrated that the values of OPI range from 7.17 to 588, and the values of EI exceed the standard of 1, indicating the critical situation of the ecological status of Jalangi River. The value of OIP ranges between 2.67 and 3.91 revealing the slightly polluted condition of the river water. The study signified the ecological status of the river is in a critical situation due to elevated concentrations of biological oxygen demand, chemical oxygen demand, and low concentrations of dissolved oxygen. The present study found that stagnation of water flow in the river, primarily driven by the eastward tilting of the Bengal basin, triggered water pollution and ecological risk. Moreover, anthropogenic interventions in the form of riverbed agriculture and the discharge of untreated sewage from urban areas are playing a crucial role in deteriorating the water quality of the river. This decay needs substantial attention from the various stakeholders in a participatory manner.

Risk assessment of river water quality using long-memory processes subject to divergence or Wasserstein uncertainty

Risk assessment of river water quality using long-memory processes subject to divergence or Wasserstein uncertainty

Klang River Water Quality Assessment and Its Effects on Human Health Using Chemometric Analysis

Introduction: River water pollution has been a significant hazard to human health and is associated with severe health risks. This study evaluates water quality and heavy metal levels in the Klang River, analyzing their health risks through chemometric analysis. Methods: Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) was used to analyse the heavy metal contents in river water samples obtained from 9 sampling stations. Chemometric statistical techniques (principal component analysis (PCA) and hierarchical cluster analysis (HCA)) are employed to identify the sources of physicochemical properties and heavy metals. The human health risk was evaluated using statistical analysis, apart from hazard quotient (HQ), hazard index (HI), and carcinogenic risk (CR). Results and Discussion: Results showed that the physicochemical parameters were within acceptable limits. The concentration of heavy metals was found to follow a decreasing order of As > Ni below permissible levels, except at P9 and P8. PCA and HCA showed important connections among parameters, emphasizing that COD, NH3N, and TDS are key factors affecting Klang River water quality. Conclusion: The study assesses pollution risks in the Klang River, offering crucial insights for sustainable estuary management. It highlights significant changes in temperature, pH, TDS, BOD, DO, and NH3N levels, along with specific trends in heavy metal concentrations. The Health Risk Assessment indicates acceptable HQ and Target Cancer Risk values. However, the study's limited sample sites and focused timeframe might hinder understanding long-term patterns and regional differences. Extended data collection and additional information are necessary to improve water quality management and protect public health

Assessment of coastal river water quality in Bangladesh: Implications for drinking and irrigation purposes.

Saltwater intrusion in the coastal areas of Bangladesh is a prevalent phenomenon. However, it is not conducive to activities such as irrigation, navigation, fish spawning and shelter, and industrial usage. The present study analyzed 45 water samples collected from 15 locations in coastal areas during three seasons: monsoon, pre-monsoon, and post-monsoon. The aim was to comprehend the seasonal variation in physicochemical parameters, including water temperature, pH, electrical conductivity (EC), salinity, total dissolved solids (TDS), hardness, and concentrations of Na+, K+, Mg2+, Ca2+, Fe2+, HCO3-, PO43-, SO42-, and Cl-. Additionally, parameters essential for agriculture, such as soluble sodium percentage (SSP), sodium absorption ratio (SAR), magnesium absorption ratio (MAR), residual sodium carbonate (RSC), Kelly's ratio (KR), and permeability index (PI), were examined. Their respective values were found to be 63%, 16.83 mg/L, 34.92 mg/L, 145.44 mg/L, 1.28 mg/L, and 89.29%. The integrated water quality index was determined using entropy theory and principal component analysis (PCA). The resulting entropy water quality index (EWQI) and SAR of 49.56% and 63%, respectively, indicated that the samples are suitable for drinking but unsuitable for irrigation. These findings can assist policymakers in implementing the Bangladesh Deltaplan-2100, focusing on sustainable land management, fish cultivation, agricultural production, environmental preservation, water resource management, and environmental protection in the deltaic areas of Bangladesh. This research contributes to a deeper understanding of seasonal variations in the hydrochemistry and water quality of coastal rivers, aiding in the comprehension of salinity intrusion origins, mechanisms, and causes.

Evaluation and monitoring of the water quality of an Argentinian urban river applying multivariate statistics.

In this work, we present the water quality assessment of an urban river, the San Luis River, located in San Luis Province, Argentina. The San Luis River flows through two developing cities; hence, urban anthropic activities affect its water quality. The river was sampled spatially and temporally, evaluating ten physicochemical variables on each water sample. These data were used to calculate a Simplified Index of Water Quality in order to estimate river water quality and infer possible contamination sources. Data were statistically analyzed with the opensource software R, 4.1.0 version. Principal component analysis, cluster analysis, correlation matrices, and heatmap analysis were performed. Results indicated that water quality decreases in areas where anthropogenic activities take place. Robust inferential statistical analysis was performed, employing an alternative of multivariate analysis of variance (MANOVA), MANOVA.wide function. The most statistically relevant physicochemical variables associated with water quality decrease were used to develop a multiple linear regression model to estimate organic matter, reducing the variables necessary for continuous monitoring of the river and, hence, reducing costs. Given the limited information available in the region about the characteristics and recovery of this specific river category, the model developed is of vital importance since it can quickly detect anthropic alterations and contribute to the environmental management of the rivers. This model was also used to estimate organic matter at sites located in other similar rivers, obtaining satisfactory results.

Water quality assessment of river based on phytoplankton biological integrity index in rural areas of the upstream Citarum River, West Java, Indonesia

Abstract. Pratiwi D, Oktavia D, Sumiarsa D, Sunardi S. 2024. Water quality assessment of river based on phytoplankton biological integrity index in rural areas of the upstream Citarum River, West Java, Indonesia. Biodiversitas 25: 881-889. The deterioration of water quality in the upstream Citarum River is one of the consequences of agricultural practices characterized by fertilizer and pesticide runoff that disrupts the ecological balance of aquatic ecosystems. The research area was on agricultural land use in the Cihawuk segment, Kertasari, Bandung District. The present study attempts to community structure and assess the water quality using the biological indexes of rural areas in upstream Citarum River through phytoplankton studies. A purposive sampling technique was used to collect data on physico-chemical parameters and phytoplankton community structure from stations selected on the basis of agricultural land use. Furthermore, biological indexes such as pollution levels based on the Shannon Wienner diversity index (H'), Palmer algae index, and Diatomeae index are examined using the data community structure of phytoplankton. The findings show that the abundance of phytoplankton in rural areas is 4,105 cells/L. The community structure of phytoplankton was dominated by Coelastrum sp. and Trachellomonas sp. The Palmer algae index ranges from 23-26, indicating that the water was a highly organic pollutant. The pollution level of upstream Citarum River based on the diversity index shows moderately polluted water. Meanwhile, the diatomeae index showed less than 0.2, which sites water bodies were mesotrophic. Phytoplankton and their indexes serve as valuable tools for evaluating the quality of the water environment.

Spatio-temporal dynamics of ecological, bacteriological, and overall water quality of the Damodar River, India.

Assessing river water quality is crucial for human and ecological needs because of the deterioration of the river and escalated water pollution under the threats of anthropogenic activities. In order to assess river water quality, the Damodar River water was evaluated from the perspectives of spatio-temporal dynamics of ecological (organic pollution index or OPI and eutrophication index or EI), bacteriological (coliform count and comprehensive bathing water quality index or CBWQI), and overall water quality assessments (water quality index or WQI and comprehensive pollution index or CPI). The OPI reveals that 44.66% of water samples have heavy organic pollution; however, EI depicts that almost all water samples of Damodar River have severe eutrophication, especially in the pre- and post-monsoon seasons. Moreover, the fecal coliform count and CBWQI indicate the unsuitability of river water for bathing. The overall WQI portrays that 21.56%, 33.59%, and 22.47% of water samples have heavy pollution in pre-monsoon, monsoon, and post-monsoon, respectively. Moreover, 73.39% of water samples have low CPI indicating slight comprehensive pollution. This study also reveals that the pollution level in the Damodar River downstream of the Durgapur barrage is high among the other stations. The major reasons behind the severe contamination of Damodar River water are urban-industrial and agricultural effluents mixing into the river that lead to higher concentrations of BOD, DO, fecal coliform, COD, fluoride TSS, and turbidity in the river water. Thus, this study carries appreciated information on policy recommendations for the different stakeholders of the Damodar River basin including regional planners, agri-engineers, and ecological river engineers for sustainable river management.

Fungal communities as dual indicators of river biodiversity and water quality assessment

Given their ecological importance, bioindicators are used for the assessment of the health of river ecosystems. This study explored the fungal compositions and the potential of fungal taxa as bioindicators for indicating the water quality of the Mekong River, as the use of fungal indicators of the Mekong River was not previously well characterized. The Mekong River exhibited dynamic variations in both physicochemical/hydrochemical properties and fungal communities according to seasons and locations. The results revealed the dominance of alkaline earth metal ions and weak acids in the water. The magnesium-bicarbonate water type was found in the dry season, but the water became the chloride-calcium type or mixed type of magnesium-bicarbonate and chloride-calcium in the rainy season at downstream sites. Fungal composition analysis revealed the dominance of Chytridiomycota in the dry season and intermediate periods, and Ascomycota and Basidiomycota in the rainy season. The fungal communities were influenced by stochastic and deterministic assembly processes, mainly homogeneous selection, heterogeneous selection, and dispersal limitation. The extent of environmental filtering implied that some fungal taxa were affected by environmental conditions, suggesting the possibility of identifying certain fungal taxa suitable for being bioindicators of water quality. Subsequently, machine learning with recursive feature elimination identified specific fungal bins mostly consisting of Agaricomycetes (mainly Polyporales, Agaricales, and Auriculariales), Dothideomycetes (mainly Pleosporales), Saccharomycetes (mainly Saccharomycetales), Chytridiomycota, and Rozellomycota as bioindicators that could predict ambient and irrigation water quality with high selectivity and sensitivity. These results thus promote the use of fungal indicators to assess the health of the river.

Assessment of River Water Quality as a Raw Water Source by Regional Drinking Water Companies

This study examines the water quality of the Sampit River in East Kotawaringin Regency, which serves as a source of raw water for the production of drinking water. Increased human activity results in various problems for the environment, in this case, water quality. Currently, the Sampit River is a source of raw water for the Regional Drinking Water Company (PDAM) Bagendang Unit, where the quality of the raw water also affects the quality of clean water produced and distributed by the PDAM Bagendang Unit to the community, especially in Sampit City. Therefore, the objective of this study is to assess the quality of the Sampit River water over the 3-year period from 2021 to 2023. In addition, the study analyzes activities in the vicinity of the Sampit River based on secondary data obtained from relevant agencies. The sampling method in this research was purposive, considering aspects of the field and season (March) and the presence of other activities around the Sampit River. Water quality test includes measurements of turbidity, color, TDS, temperature, odor, pH, Fe, hardness, Mn, NO3-N, NO2-N, organic substances, total coliform, and E. coli, which are then compared with PP Number 22 of 2021. Based on the results of the analysis carried out on the chemical and physical parameters of the Sampit River water, it was found that the water color, water temperature, Fe, Mn, total coliform, and E. coli exceeded the quality standards according to PP Number 22 of 2021. Meanwhile, the TDS, pH, NO3-, and NO2- are still below the quality standards according to PP Number 22 of 2021. These observations indicate that the condition of the Sampit River, which serves as the raw water source for the PDAM Bagendang Unit, is experiencing fluctuating changes, particularly during the rainy season (March 2021, 2022, and 2023). This will become a concern later in the processing of raw water into clean water conducted by the PDAM Bagendang Unit.

Investigation of Deoxygenation Rate Determination in Cikakembang River, West Java, Indonesia

Cikakembang River, a tributary of Citarum River, is situated in the densely populated Majalaya District, renowned for textile production. Direct discharges of domestic and industrial pollutants into the river contribute to substantial pollution, making it crucial to manage pollution levels. This implies that controlling pollution is crucial, as it significantly impacts the condition of Citarum River, already infamous as one of the world most polluted rivers. A key indicator for assessing river water quality is Biological Oxygen Demand (BOD), representing the oxygen required for microorganism-mediated decomposition. This parameter is influenced by deoxygenation rate, denoted as kd. Therefore, this study aimed to analyze the most suitable kd value for Cikakembang River using various empirical methods, including Simple, Fujimoto, Sawyer, Thomas, Fair, and Hydroscience. The result showed that Thomas method provided the most accurate prediction for BOD concentration of the river. In rainy season, Root mean square error (RMSE), mean absolute percentage error (MAPE), and coefficient of determination (R2) values were 0.542, 0.035, and 0.981, respectively, and in dry season, the values were 0.117, 0.009, and 0.999. Additionally, kd value effectively simulated the river water quality using HEC-RAS, yielding satisfactory results. RMSE, MAPE, and R2 values for BOD concentration were 3.551, 0.162, and 0.331 in rainy season and 1.071, 0.100, and 0.812 in dry season. Finally, the modeling result showed that Cikakembang River did not meet the Class 2 Water Quality Standard during both rainy and dry seasons. This finding is critical, as it underscores the severity of the pollution problem in the river and the urgent need for comprehensive and effective management strategies to improve its water quality.

Water quality assessment of Elgo river in Ethiopia using CCME, WQI and IWQI for domestic and agricultural usage

The increasing demand for water due to the escalation in population and aggressive agricultural activities for drinking and irrigation purposes in the rural areas of Ethiopia has put tremendous stress on water requirements. The Elgo River in southern Ethiopia is deteriorating due to sedimentation, soil erosion, stormwater runoff, and anthropogenic activities. Elgo village faces water shortages and a lack of safe drinking water. The purpose of this research was to identify the extent of pollution in Elgo River water using the Canadian Council of Ministers of the Environment (CCME), Water Quality Index (WQI), and Irrigation Water Quality Index (IWQI). A total of 12 water samples were collected from 3 river sampling sites for the dry and wet seasons to test the physicochemical and biological parameters. Results obtained were: turbidity (46.5–156) NTU, colour (103.65–606.5) TCU, EC (182–268) μS/cm, TDS (192.5–275.5) mg/l, TSS (680–2774) mg/l, Ca2+ (22–45) mg/l, Mg2+ (19.5–23.5) mg/l, Cl- (10.5–16.65) mg/l, and SO42- (17.18–47) mg/l for both the dry and wet seasons, respectively. The CCME WQI revealed that the overall results were 38.38 for the dry season and 36.6 for the wet season for drinking water parameters. The CCME WQI categorization indicates that the Elgo River water is classified as poor, with results ranging from 0 to 44. For irrigation purposes 10, parameters such as SAR, PS, PI, MAR, KI, RSC, EC, SSP, TH, and %Na were examined to compute indices using the IWQI model. The overall result of water quality indicated that IWQIs of 81.4 and 62.14 are good for the dry season and poor for the wet season, respectively. This research provides a thorough analysis through modelling to determine the suitability of water for different purposes for the tribal and backward communities of the area.

Integrating fecal pollution markers and fluorescence analysis for water quality assessment of urban river

Human fecal contamination in urban rivers poses significant health risks, but their potential connections with other substances like dissolved organic matter (DOM) remain underexplored. In this study, five fecal pollution markers related to fecal Bacteroides or human fecal contamination (AllBac, HF183, BacH, Hum2, and Hum163) and DOM along an urban river were analyzed using quantitative polymerase chain reaction (qPCR) and three-dimensional excitation-emission (3D EEM) fluorescence spectrometry. All five markers were detected with average absolute abundance ranging from 2.51 to 6.28 lg gene copies/100 mL, showing a progressive increase along the river (R2 = 0.29–0.92, p < 0.05). Parallel factor analysis identified three dominant DOM components (humic acid-like, fulvic acid-like, and protein-like), with strong positive correlations between protein-like components and all fecal markers (R2 = 0.59–0.66, p < 0.001). Both fecal and DOM distributions consistently showed significant differences between upstream and downstream areas (p < 0.001), suggesting their complementary assessment. While DOM was more sensitive to environmental variables such as rainfall, rubber dam, and tidal dynamic, the combination of fecal pollution markers and 3D EEM analysis allowed a more comprehensive assessment of contamination levels, mitigating potential biases caused by the influence of multiple factors on a single method. Furthermore, due to the strong correlation between protein-like and fecal markers in the DOM, 3D EEM can be used as a pre-detection means for qPCR detection, reducing testing time and costs.

Assessment of River Water Quality during Summer Season in Prayagraj, Uttar Pradesh, India

The present study is focused on assessment of river water quality during summer season in Prayagraj, Uttar Pradesh was experimentally investigated. The purpose of this study was to analyzed the physio-chemical properties of the river Ganga and Yamuna in Prayagraj. Water parameters such as pH, EC, DO, BOD, Alkalinity, Chloride, TDS and Hardness were analyzed. The results reveals that water was suitable for irrigation on purpose as the values were found to be within the permissible limit except DO whose values were found to be slightly less than the permissible limit.