Comparative Analysis of Water Quality Indices and Their Relationship with Anthropogenic Activities. Case Study: Bogotá River

13 The Water Quality Index is the numerical value that qualifies in categories, the water quality of a 14 surface stream, based on the measurements obtained for a set of physicochemical and 15 microbiological parameters registered in a water quality monitoring network. This indicator allows 16 knowing the physical-chemical and microbiological quality conditions of a body of water and 17 identifies contamination problems at a certain point. In the present study, the water quality was 18 studied in 43 stations located in the Bogota river, considering different methodologies for 19 calculating quality indices such as NSF, Oregon, IDEAM, Prati, and diffuse logic to establish the 20 differences, advantages, and disadvantages of each method. Similarly, a correlation analysis was 21 carried out between the parameters, and the land uses in the basin. The results indicated that all 22 the indices showed regular or poor water quality for more than 80% of the stations for the temporal 23 window studied. However, the methodology used in Colombia (ICA water quality index, in 24 Spanish) has eclipsing problems in most of the evaluation points since where the water quality 25 qualifies as good, other indices such as the fuzzy logic show a degree of contamination by any of 26 the parameters mainly manganese or total suspended solids. 27

qualifies as good, other indices such as the fuzzy logic show a degree of contamination by any of 26 the parameters mainly manganese or total suspended solids.

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Keywords: Bogotá River, Water quality index, water pollution, spatial and temporal evaluation. Water is a development engine for communities, so knowledge about this resource, distribution, 30 and pressure on quantity and quality constitutes the support for territorial planning and decision-31 making (Fu et al., 2020). Likewise, social dynamics and population growth lead to increased use, 32 alterations to natural conditions, and effects on water quality that must be evaluated to define 33 comprehensive strategies for the conservation of water systems and disaster prevention (Mester et 34 al., 2020; Walsh and Milon, 2016). 35 One way to assess these pressures on water quality is to apply water quality indices. Water Quality The Bogotá River, which is in the Cundiboyacense highlands, from northeast to southeast of 48 Cundinamarca, is one of the water bodies of great interest in Colombia. Although it is not a 49 navigable or fast-flowing river, the Bogotá River is significant for Colombia because, in its round 50 and the surrounding areas, economic activities are developed, representing 26% of the total 51 national level (CAR, 2018). Among them, agricultural, livestock, and industrial production stand 52 out. One of the activities that generate the most significant pollution in the river is the tannery 53 sector. The basin receives wastewater from more than 100 tannery companies between the 54 municipalities of Villapizon and Chocontá. These discharges are characterized by having a high municipalities that contribute about 32% of the national Gross Domestic Product (GDP), but that 58 each second discharges more than 20,000 liters per second of wastewater per second (CAR, 2006). 59 Although studies have been carried out on the determination of water quality indices of the Bogotá 60 river, these have left aside studying the problems mentioned above. Various authors have 61 formulated quality indices according to the permissible limits in the environmental standard, the 62 criteria for the specific use of water and its potential, and a reduced number of parameters with 63 significance that generate information on the change in water quality and its impact (Lozada et al., chemical oxygen demand, Electrical conductivity, and pH). In the Castro et al. report (Castro 67 Fernández et al., 2015), information for 2008 -2015 was collected for the same five parameters 68 listed above. However, when they were calculating the WQI, importance weights were given 69 equally to the five parameters, which goes against the recommendations for most approaches to calculating water quality indices where the weight of dissolved oxygen must be greater than the 71 total suspended solids or pH due to its importance in the degradation of organic matter and other 72 processes. Additionally, the existing correlations between the parameters found, the relationship 73 of the calculated indices with the land uses were not studied, nor did they discuss in profound the 74 possible implications of the calculation methodologies on the index, such as the aggregation 75 function.

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For this reason, the present study aims to evaluate the water quality index on 43 stations on the 77 Bogotá River, considering different approaches, using 12 physicochemical parameters, assessing 78 the effect of the aggregation function used, the correlations between the physicochemical 79 parameters/land use, and a comparative analysis between different models of water quality index 80 for its application as a water resource management tool. range temperature between 6 and 30°C. The demand for this stream is given for agricultural, 94 livestock, and industrial services, so that along its route, it receives water from rivers that cross 95 large, populated centers, such as Bogotá, which leads to an increase in the polluting loads to the 96 current principal.    Pearson, Spearman, and Kendall's correlation coefficients were used to calculate the degree of 120 association between the water quality variables, where there will be a force of proportion that is 121 measured in a degree from 0 to 1 and a direction already be it positive or negative.

Calculation of quality indices.
Water quality indices were proposed to evaluate the degree of contamination and the main 124 riverbed's general quality in the basin. The indexes provide a simple, objective, and rapid method 125 that allows the integration of the essential parameters to quantify the degree of alteration of the 126 water resource's natural qualities. The analysis was performed by calculating quality indices that 127 differ in the aggregation function and evaluating the parameters.

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The following formula defines the second index of Dinius: Where IQW, is the water quality index, Ii corresponds to the sub-index of the parameter that arises 152 from the parameter curves and Wi, is the weighting factor for the sub-index. This index includes an additive aggregation of a modified unweighted average, which also 155 transforms the parameters that affect the aquatic ecosystem. Among the parameters, the presence 156 of heavy metals such as copper, chromium, lead, and nickel stands out. It is developed by: Where n is the number of parameters and qi is the characteristic function of the indicator • if the DO is "low" and the BOD is "very low", then the water quality is "moderate."

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• if the pH is "low" and the temperature is "moderate", then the water is "moderate."

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• if the fecal coliform is "very low" and the total coliform is "very low", then the water 189 quality is "excellent."  When comparing the concentrations with the respective quality objective required for that section, 210 it is observed that a large part of the points does not meet these quality objectives (Figure 2). The 211 only variable that remains within the range required by the environmental authority is the pH. in its production process and susceptible to oxidation processes. Also, the agricultural activity has 219 adverse effects on river quality, particularly the potato crops in the area and agrochemicals, 220 fertilizers, and pesticides that reach the water body through secondary tributaries (Bautista, 2004).

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The water body improves its quality with the discharge of water from the Sisga reservoir, causing   in the middle and/or lower part (for example, CANOAS wastewater treatment plant), which is 291 unfortunate since it is much easier to carry out treatments due to the amount of water to be treated 292 and according to the quality indices calculated, the impacts will be more significant for the 293 ecological benefit of the basin.

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Since the source of the river in the Guacheneque moor (water quality stations 5 to 10), the quality   Figure 1 Study Area, Bogotá river basin. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 2
The behavior of water quality along the river.

Figure 3
Comparison of water quality indices in the Bogotá River.

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