**3. Results**

After assessing the microbiological water-quality data collected from 35 stations during the 2007–2019 period for the middle Bogotá River basin, the following results were obtained.

For Station (No. 68) Quebrada La Tenería, the highest TC and *E. coli* levels were evidenced in 2014-02, with TC exceeding 2.4 × 108 MPN/100 mL and *E. coli* reaching 9.3 × 107 MPN/100 mL. The most recent report, for 2019-02, records 1.00 × 108 MPN/100 mL and 7.50 × 106 MPN/100 mL for TC and *E. coli*, respectively.

In the section that covers the municipality of Chía, three monitoring stations provided behavioral data. First, the upstream Chía station (No. 14) reported peak values for the first period of 2007, with a TC concentration of 2.40 × 107 MPN/100 mL and an *E. coli*

concentration of 2.00 × 107 MPN/100 mL. A third maximum peak was reported in 2019-01, with 1.70 × 106 MPN/100 mL for TC and 8.20 × 105 MPN/100 mL for *E. coli*.

The Chía municipality discharge station (No. 29) reported a maximum concentration of 2.50 × 107 MPN/100 mL for TC and 1.70 × 106 MPN/100 mL for *E. coli* in the 2008-02 period. In addition, in 2014-02, these concentrations increased from the previous year to 1.60 × 107 MPN/100 mL for TC and 4.60 × 106 MPN/100 mL for *E. coli*. The third station is the downstream Chía station (No. 3), which reports concentrations of 6.50 × 107 MPN/100 mL for TC and 1.10 × 107 MPN/100 mL for *E. coli* in 2010-01, as may be observed in Figure 2, below.

**Figure 2.** Downstream Chía concentration chart. Source: prepared by the authors, 2021.

The LG Puente la Balsa Station (No. 42) reported a significant increase in 2014-01, with TC values exceeding 2.40 × 106 MPN/100 mL and *E. coli* levels of 8.20 × 105 MPN/100 mL; in 2019-02, this station reported TC concentrations of 2.40 × 106 MPN/100 mL and *E. coli* concentrations of 2.90 × 105 MPN/100 mL.

At the next station, Rio Frio (No. 75), TC concentrations for 2011-01 were 5.80 × 107 MPN/100 mL, with *E. coli* levels of 9.60 × 106 MPN/100 mL. However, in 2019-02, TC concentrations decreased to 9.90 × 103 MPN/100 mL, as well as for *E. coli* concentrations, which showed a value of 100 MPN/100 mL.

Subsequently, at the downstream Rio Frio station (No. 10), concentrations remained high, as evidenced in 2010-01, when the station reported TC concentrations of 4.60 × 107 MPN/100 mL and *E. coli* concentrations of 1.80 × 107 MPN/100 mL; in 2016-02, TC values were observed of 1.70 × 107 MPN/100 mL, while the concentration of *E. coli* decreased (2.70 × 105 MPN/100 mL) (Figure 3).

**Figure 3.** Downstream Rio Frio concentration chart. Source: prepared by the authors, 2021.

The Cota municipality station (No. 30) reported a large number of total coliforms and *E. coli*. Below, we list the main concentration peaks recorded throughout our 12-year assessment period: in 2010-01, concentrations were reported of 2.00 × 108 MPN/100 mL for TC and 5.30 × 107 MPN/100 mL for *E. coli* and, in 2019-02, TC concentrations were reported of 1.50 × 108 NMP/100 mL and *E. coli* concentrations of 3.40 × 107 MPN/100 mL.

The next monitoring point was the Puente La Virgen station (No. 43), where maximum concentrations were reported in 2007-01, with values of 9.80 × 106 MPN/100 mL for *E. coli* and 7.30 × 107 MPN/100 mL for TC. An overall decrease in coliforms was observed at this station for 2019-01; TC concentrations were 2.40 × 106 MPN/100 mL and *E. coli* concentrations were 5.00 × 105 MPN/100 mL (Figure 4).

**Figure 4.** LG Puente La Virgen station concentration chart. Source: prepared by the authors, 2021.

The Rio Chic ú station (No. 74) reported a 2007-01 TC concentration of 2.40 × 106 MPN/100 mL, and an *E. coli* concentration of 4.30 × 105 MPN/100 mL; in 2019-02, this station reported TC concentrations of 2.40 × 103 MPN/100 mL and *E. coli* concentrations of 3.10 × 102 MPN/100 mL. Here, the maximum concentrations reported by these stations significantly decreased for total coliforms and *E. coli*.

Subsequently, the data recorded at this point reveals higher concentrations than the LM Vuelta Grande station (No. 58). In 2014-02, these concentrations increased even more, as TC concentrations were reported at >2.00 × 107 MPN/100 mL and *E. coli* concentrations were 2.00 × 107 MPN/100 mL, which denotes a lack of interest from the corporations in charge of improving water quality. However, in 2019-01, the maximum TC values (>2.40 × 106 MPN/100 mL) and *E. coli* values (5.80 × 105 MPN/100 mL) decreased from the previous peak in 2014-02, but these values still exceeded the maximum permissible limits.

Subsequently, data from the Juan Amarillo Bypass station (No. 22) shows that for the season of 2008-02, the TC concentrations were 1.90 × 108 NMP/mL and *E. coli* was 1.20 × 107 NMP/mL; for 2014-02, these concentrations increased even more, since the TC concentrations were 2.00 × 108 NMP/mL and *E. coli* levels were 2.20 × 107 NMP/mL, which shows great disinterest on the part of the corporations in charge of improving the quality of the water.

Station No. 59, located at the discharge (channel) of the El Salitre Wastewater Treatment Plant reported high TC concentrations of 1.50 × 108 MPN/100 mL and *E. coli* concentrations of 1.30 × 107 MPN/100 mL for 2008-02. Although the plant has been in operation since 1999, high levels of contamination at 2.0 × 103 MPN/100 mL of TC were still being reported in 2008. In fact, 6 years later, in 2014-02, concentrations had increased significantly to 1.70 × 108 MPN/100 mL for TC and 2.20 × 107 MPN/100 mL for *E. coli*, as can be observed in Figure 5.

**Figure 5.** El Salitre WWTP concentration chart. Source: prepared by the authors, 2021.

Next in line is the monitoring station of El Cortijo (No. 39), located 500 m downstream of the El Salitre WWTP discharge point. In 2009-01, this station reported a TC concentration of 2.20 × 108 MPN/100 mL and *E. coli* concentrations of 1.20 × 107 MPN/100 mL. However, in 2016-02, this station reported TC concentrations of 1.70 × 108 MPN/100 mL

and *E. coli* concentrations of 8.20 × 106 MPN/100 mL. These concentrations are extremely high compared to the concentrations reported by the Jaboque discharge station (No. 28), which are lower than the rest of the stations mentioned. For example, in 2011-01, TC concentrations only reached 4.70 × 103 MPN/100 mL and *E. coli* concentrations reached <1 × 102 MPN/100 mL, as shown in Figure 6. However, a peak was reported in 2015-01 with the presence of TC concentrations of 2.40 × 107 MPN/100 mL and *E. coli* concentrations of 9.8 × 104 MPN/100 mL.

**Figure 6.** Jaboque discharge concentration chart. Source: prepared by the authors, 2021.

Upon entering the Engativá community, the water body is further affected, as is seen in the data reported by the Engativá discharge station (No. 27) for the year 2019-02, which shows that concentrations increased even more, reaching 2.00 × 108 MPN/100 mL for TC and 4.10 × 107 MPN/100 mL for *E. coli*.

The La Ramada station (No. 53) registered a decrease in the values reported by the downstream Engativá discharge station, given that the station reported TC concentrations of 1.4 × 107 MPN/100 mL and *E. coli* concentrations of 6.7 × 106 MPN/100 mL in 2010-01; no data records are available for more recent years. However, even so, high concentrations of these coliforms, which are pathogenic to humans, are evident.

The downstream Engativá station (No. 4) reported concentrations in 2016-02 for TC of 1.1 × 108 MPN/100 mL and 2.0 × 106 MPN/100 mL for *E. coli*, reaching a significant increase; to date, these results far exceed the permissible limits provided for in Decree 1594 from 1984 (Figure 7).

At this point, the Rio Fucha station (No. 76) reported TC concentrations of 4.5 × 108 MPN/100 mL and *E. coli* concentrations of 2.0 × 107 MPN/100 mL for the first half of 2009. Likewise, in subsequent years, these concentrations reached higher values. For example, in 2019-01, the station reported TC concentrations at 1.0 × 108 MPN/100 mL and a significant increase in *E. coli* concentrations at 5.5 × 107 MPN/100 mL, representing the maximum values of fecal coliforms reported, due to domestic, commercial, and industrial wastewater discharges [34].

**Figure 7.** Downstream Engativá concentration chart. Source: prepared by the authors, 2021.

In 2009-01, the downstream Rio Fucha station (No. 11) reported TC concentrations of 1.5 × 108 MPN/100 mL and *E. coli* concentrations of 3.4 × 106 MPN/100 mL (Figure 8). The microbiological load was extremely high, showing that concentrations of TC and *E. coli* were maintained over the years.

**Figure 8.** Downstream Rio Fucha concentration chart. Source: prepared by the authors, 2021.

In 2008-01, the Rio Tunjuelo station (No. 70) reported TC concentrations of 2.4 × 108 MPN/100 mL and *E. coli* concentrations of 1.1 × 107 MPN/100 mL.

The lower basin of the Tunjuelo River covers an area of 390 km2 from the Cantarrana dam to the mouth of the Bogotá River [35]. In the downstream Rio Tunjuelo station (No. 13), in 2008-02, TC concentrations reached a maximum value of 1.7 × 108 MPN/100 mL and *E. coli* concentrations reached 1.2 × 107 MPN/100 mL, which indicates that there was a high contribution of wastewater and industrial waste throughout its course. In 2019-01, the same level of contamination was observed, with TC concentrations of 3.9 × 107 MPN/100 mL and *E. coli* concentrations of 8.6 × 106 MPN/100 mL. This station is affected by anthropic influences from the urban periphery (Figure 9).

**Figure 9.** Downstream Rio Tunjuelo concentration chart. Source: prepared by the authors, 2021.

At the Rio Balsillas station (No. 72) in 2019-01, concentrations had decreased with respect to previous stations, yielding TC concentrations of 1.6 × 107 MPN/100 mL and *E. coli* values of 1.9 × 106 MPN/100 mL.

At the Rio Soacha station (No. 79), the 2009-01 microbiological results revealed that TC concentrations were at 1.6 × 108 MPN/100 mL and *E. coli* concentrations were at 1.1 × 107 MPN/100 mL. However, in 2019-02, TC concentrations had decreased to 9.2 × 107 MPN/100 mL but *E. coli* concentrations had increased to 2.6 × 107 MPN/100 mL.

At the Puente Variante Mondoñedo station (No. 60), the 2008-02 microbiological results revealed that TC concentrations were at 8.7 × 107 MPN/100 mL and *E. coli* concentrations were at 2.7 × 106 MPN/100 mL; for 2016-02, TC concentrations were 1.1 × 108 MPN/100 mL, and *E. coli* concentrations were 7.2 × 106 MPN/100 mL. At this point, the body of water had a very high level of biological oxygen demand (BOD), with a value of 320 mg BOD/L in 2012, which was directly related to the high TC and *E. coli* concentrations measured at this station.

At the station before reaching Tequendama Falls (the upstream Salto de Tequendama Station No. 18), a significant number of coliforms was recorded. In 2009-01, TC concentrations were at 2.4 × 108 MPN/100 mL and *E. coli* concentrations were at 1.5 × 105 MPN/100 mL (see Figure 10).

**Figure 10.** Upstream Salto de Tequendama concentration chart. Source: prepared by the authors, 2021.

At the Quebrada La Cuy station (No. 31), San Antonio Tequendama municipal discharge, an all-time-high percentage of coliforms was again recorded, In 2018-01, TC concentrations were at 1.6 × 108 MPN/100 mL and *E. coli* concentrations were at 2.0 × 107 MPN/100 mL (see Figure 11).

**Figure 11.** San Antonio Tequendama Quebrada, La Cuy Municipio. Discharge concentration chart. Source: prepared by the authors, 2021.

At the point when the Bogotá River reaches the Quebrada Honda station (No. 67) and these two water bodies merge, TC levels remain elevated and *E. coli* decreases. The concentrations recorded in 2019-02 of TC were 9.8 × 104 MPN/100 mL and those of *E. coli*

were 8.4 × 102 MPN/100 mL. However, during the 12 years of study, it is evident that the highest peak was in 2007-01, with a total coliform value of 6.5 × 106 MPN/100 mL and *E. coli* value of 1.4 × 106 MPN/100 mL. Station Nos. 9, 21, 23, 54, 55, 56 and 57 also present maximum values of TC and *E. coli* in the different periods of the last 12 years; these data are shown in Table 1.

**Table 1.** Description of the 35 stations in the middle basin of the Bogotá River during the years 2007–2019, with their main characteristics, such as maximum values for TC and *E. coli*, the most representative of the study. Source: prepared by the authors, 2021.

## **4. Discussion**

The above results allow us to develop an analysis of the possible causes and triggers affecting the state of the basin. In the current investigation, most of the 35 stations located along the middle basin of the Bogotá River showed high levels of total coliforms and *E. coli*, exceeding the limits allowed and established at the national and international levels. For example, the WHO guidelines recommend that the amount of total and fecal coliforms should be 0 UFC (colony forming units)/100 mL for water supply sources [36]. Similarly, the results presented by the World Water Quality Monitoring Program show contamination by these pathogenic microorganisms affecting approximately one-third of the river sections in Asia, Africa, Asia and Latin America [37].

These results coincide with the study carried out by Ochoa-Herrera et al. (2020), who analyzed the water quality of 18 rivers located in Quito, the capital province of Pichincha, Ecuador, by means of microbiological parameters, where high levels of contamination by total coliforms and *Escherichia coli* are evidenced [38]. The maximum values of these enterobacteria, reported in the current study, in the different water bodies are shown in Table 1.

The initial station of the river system is Quebrada La Tenería No. 68, which reports a high degree of contamination from fecal coliforms since it receives a large part of the sewage waters from the municipality of Cajicá [39].

The municipality of Cajicá currently uses a combined sewer system, wherein only 48% of the wastewater and rainwater that collect are treated before being discharged into the river. Another important factor is that the Calahorra WWTP, which receives most of the municipal wastewater, can only treat ~50% of the wastewater received [40]; this has prompted strategies for extending the capacity of the plant. Likewise, hydraulic adaptation work has been conducted in the river, which may account for the lower values that have been reported recently.

Following this water along its trajectory through the municipality of Chía, the values remain above the provisions established in Executive Order 1594 of 1984 and Agreement 43 of 2006, which sought to establish measures for improving the quality of water in the Bogotá River basin by 2020. This failure may be due to deficiencies in wastewater treatment, ultimately leading to direct discharges into water bodies. The municipality of Chía built a treatment plant in 1989, but the plant was not sufficient for the 68% population increase experienced since 2000, 78% of which corresponds to the urban population. Even when the treatment plan covers 83.6% of the sewer system [41], it only manages to properly treat 30–40% of the wastewater produced. In particular, due to the excessive increase in urban population, wastewater flow rates have also increased, exceeding by far the hydraulic capacity of the plant. Currently, a large part of the urban area is covered by the Chía I WWTP, which can only treat 2.36 m3/s; this has caused a negative impact on the environment and the population at large [42].

In addition to the discharges from the WWTP into the Bogotá River, 11 domestic discharge points, five rainwater discharge points, and one industrial discharge point have also been identified [41]. To mitigate this issue, the municipal government built the Chía II treatment plant, seeking to treat 70% of the wastewater that reaches the Frio River, a body of water that is also deeply affected by municipal discharges and that flows into the Bogotá River. The Chía II plant began operation testing in February 2020 [43].

The next station at Puente de la Balsa (No. 42) is highly impacted by pollutants, since it receives significant discharges, including discharges from the Chía I WWTP. In addition, industrial and rural domestic discharges have been identified in this section, a situation worsened by the high number of water hyacinths found in this area of the river, which causes greater retention of suspended particles [44].

In the next section of the river system, the Rio Frio and downstream and Rio Frio stations No. 75 and No. 10, which were built to evaluate the behavior of the Frio River, an effluent of the Bogotá River, reported lower pollutant concentrations in some periods, especially for the first station. This is commonly caused by the dynamics between the hydraulic, morphological, and water quality characteristics of the water body [45], which, in the case of the Rio Frio, constitutes a fluvial and alluvial valley. For this reason, the area usually becomes flooded during the rainy season [46]. This process can help to dilute pollutants, as is supported by the fauna still present in the area. In addition, another influencing factor that may affect the first section of the river is that it flows through the municipality of Chía, where the river system does not receive significant urban discharges. Here, some of the wastewater produced in this municipality is treated by the Chía I WWTP, which mitigates its impact in this area. However, as evidenced by the data, low contamination concentrations are not constant. This may be because the river receives domestic and industrial discharges as it passes through the municipality of Cajicá and the entrance to Chía. Nevertheless, the downstream Rio Frio station (No. 10) exhibits an opposite dynamic, generated by the multiple discharges it receives. In fact, in 2018, 35 wastewater discharge points were identified—three were agricultural, 15 were rainwater, 15 were domestic water, and two were industrial [41]. The construction of the Chía II WWTP is supposed to mitigate this situation, but we will not know how efficient it is until the plant becomes operational.

The Frio River originates in the Páramo de Guerrero in the northeastern area of Zipaquirá and constitutes a structural axis for agricultural development and growth, since it flows through several irrigation districts and provides continuity to agricultural activities [47]. According to reports, this water source has not yet been considered as an alternative source of drinking water because it exhibits high concentrations of lead and heavy metals. In fact, the 2016 Quality Index issued by the Regional Autonomous Corporation (CAR) states that near the river mouth between the Frio and the Bogotá rivers, the water quality decreases by 20%, which is a level considered to be unpleasant for developing the ecosystem and aquatic life in said body of water [41]. Likewise, researchers have identified issues in the area, such as deforestation, eucalyptus trees, pasturelands, single-crop farming, e.g., potatoes and flowers, invasive acacia species, and inadequate canalization, which also trigger contamination through phytosanitary products, chemicals, and agricultural waste runoffs [47]. Although a significant decrease in pollutant concentrations is expected, the

projections from the Chía development plan remain uncertain, due to the conurbation of the municipalities surrounding the city of Bogotá. Furthermore, to foster municipal urban development, the construction of 6–12-story buildings and an area of ~300 ha have been approved for urban expansion [48]. This will undoubtedly boost population growth and, in turn, increase wastewater generation, thus affecting the new plant's effectiveness.

As was observed at the station in the municipality of Cota, No. 30, in the first two years, large total coliform concentrations were reported. However, these concentrations somewhat decreased in 2012-01 and 2014-01, before increasing again and maintaining similar values. For fecal coliforms, such as *E. coli*, a significant increase in concentrations was observed. At this point, this behavior was related to different situations occurring in this municipality. Cota has an active Wastewater Treatment Plant (WWTP) located in Vereda El Rozo. The municipal sewer system covers 97.65% of its urban area and 68.56% of its rural area, using a combined system. However, there are still several natural drains and spillways (fences) in the rural areas [49]. Despite this sewer network coverage, wastewater treatment is still not optimal. For example, the El Rozo WWTP can only treat 13.96% of the wastewater produced in the area. In addition, this plant underwent an optimization process in 2016, during which time it remained nonoperational and did not treat any wastewater at all. However, even after these enhancements, it still has not been able to effectively meet the demand [50]. According to the Environmental Report issued by the municipality, 85.94% of the area discharges wastewater directly into the Bogotá River from the Pueblo Viejo discharge point. For this reason, the construction of a new WWTP has been planned. This new treatment plant was expected to start operations at the end of 2019. At the time of writing, no reports of its commissioning have yet been received. In addition, 73 discharge points have been identified, of which only 5 have been officially permitted [49]. One of the conflicts that also significantly impact water source contamination is the location of the municipality of Cota, which has been experiencing accelerated urban and industrial expansion. For example, this municipality hosts numerous industrial parks, such as Siberia [51]. Furthermore, their industrial discharges have not been characterized and, according to the latest analysis conducted in 2014, they constitute the largest pollutant load in the river due to the lack of a treatment system and the mixing that takes place within the sewer system, wherein domestic wastewater, industrial wastewater, and rainwater are mixed before being directly discharged into the Bogotá River [49]. According to Agreement 43 of 2006, this section of the river is expected to reach Class-IV quality parameters, which includes restricted agricultural and livestock use, especially considering that this section is used to supply irrigation and drainage to the La Ramada district. Still, according to a 2016 report, only 20% of this objective is being fulfilled [49].

Next in the watercourse, the Puente La Virgen station, No. 43, is located within the municipality of Cota. According to the study conducted, contamination at this point in the river is due to an invasion from the riparian buffer zone, which the municipal Territorial Organization Plan (POT) sets at 100 m for the Bogotá River. During this study period, both fillings and droughts were evident in areas surrounding the water body. In addition, this area has become a dumping ground for wastewater, garbage, and chemical waste from industrial establishments dedicated to grazing, farming, and some secondary sector industries [52]. This decrease may be due to the fact that the Regional Autonomous Corporation (CAR) has recently established a protection and conservation treatment area under the Central Savannah Association of Municipalities. Nevertheless, since this basin constantly suffers from moderate water stress, it still remains under conditions of contamination [53].

The performance of the Rio Chic ú station, No. 74, is noteworthy, given that, in fact, this is one of the stations with the lowest concentrations. This may be due to the fact that the Chic ú river is located in the municipality of Tabio, which has a wastewater treatment plant (WWTP) that reduces the sludge and microorganisms found in this body of water, thus complying with the maximum permissible values. In addition, since 2016, the municipal government managed the Tabio Territorial Organization Scheme before the

relevant authorities, in accordance with the new Basin Organization and Management Plan (POMCA) guidelines for the Bogotá River [54].

In the case of the LM Vuelta Grande station No. 58, it continues to exceed the values allowed by the current regulations. According to an assessment conducted in 2015, the urban municipality of Tenjo has a wastewater treatment plant and uses two main purification processes: an anaerobic piston-flow reactor (RAP) and an oxidation pond. As these treatment units lack sufficient capacity to treat the total amount of wastewater generated throughout the year, all excess wastewater is left untreated and is discharged directly into the Bogotá River, where it eventually reaches the LM Vuelta Grande station No. 58. In addition, the oxidation pond presents some deformities caused by gases emanating from the decomposing organic matter due to poor maintenance, which reduces the hydraulic capacity of the pond [55].

In the case of the Juan Amarillo station No. 22, this receives its name from the eponymous wetlands; these wetlands work as a buffer pond and are used to prevent flooding of the Juan Amarillo River. The outlet or discharge from this river to the Bogotá River is located downstream. The increase in concentrations at the monitoring point is due to multiple erroneous connections making direct contributions to the wetlands, as well as to the amount of bovine and hog feces generated in the reserve [56]. This section of the river is afflicted by a lack of environmental awareness, illegal settlements and dwellings, fraudulent actions by people seeking to avoid paying utility bills, the growth and development of neighboring buildings, and total ignorance of the applicable environmental regulations [57].

Another station that shows unusual circumstances is the one located at the El Salitre WWTP, given the values recorded at the WWTP. Overall, these values are not justifiable as they should be in compliance with the regulations, since the effluent discharges are of water that is previously treated by the WWTP. Hence, this treatment plant is not playing its part properly. This may be due to the fact that the Bogotá urban drainage system (Salitre Channel–Salitre WWTP–Bogotá River) lacks a comprehensive scheme, which destabilizes the plant during contamination peaks. In addition, the Salitre channel exhibits low flowrate speeds due to backwater and water storage effects, thus leading to the sedimentation of solids and organic matter. However, the structure and functionality of the plant are also inefficient, since it is not able to treat water received during the first few minutes of rainfall, which contains large pollutant loads from garbage and waste [58]. Currently, the El Salitre WWTP is being expanded and optimized to treat 7.0 m3/s and prevent at least 450 t/month of garbage from reaching the Bogotá River. If this is successful, the pollution levels reported in previous years are expected to decrease [59].

The Cortijo station No. 39 was evaluated; it can be determined that here, the concentrations were caused by a number of cement industries operating in the area that use water to wash their aggregates and clean their equipment and plants. The waters are discharged completely untreated, or with low-quality treatment, into the Bogotá River, thus increasing the presence of sludge and, in turn, significantly increasing organic matter. Given the high load of organic matter transported by this tributary and the inefficient treatment of the El Salitre WWTP, the Rio Bogotá reach high levels of contamination [44].

The dynamics of the Jaboque station No. 28 show that concentrations did not undergo massive changes over time. This is because this station is located in the Jaboque wetlands, which are separated from the Bogotá River by dams to prevent river waters from flowing into the wetlands. This way, the wetlands discharge into the river but not the other way around, thus preventing the wetlands from functioning as buffer zones. In addition, at least 95% of the sewer wastewater was reduced due to the creation of the Capital District Wetlands Policy, as per Executive Order 624 of 2007 [60], wherein these wetlands were declared a wildlife sanctuary. For this reason, their contamination rates are minimal and fully comply with the corresponding regulations [61].

As it passes through the town of Engativá, the river system is faced with a problem; this section of the river flows through a community with no environmental awareness, in addition to the wastewater discharges from domestic, commercial, and industrial activ-

ities [27], coupled with inadequate solid waste management. Hence, a large part of this waste ends up on the Bogotá riverbed, causing the proliferation of disease vectors, offensive odors, and general deterioration around the area [62].

The community of Engativá has a significant hydrological system consisting of either the Salitre or Juan Amarillo river, in addition to three wetlands—Jaboque, Santa María, and Juan Amarillo. Near these buffer zones, there are hazardous areas that are prone to landslides and flooding, as well as to sewer, garbage, and excrement discharges produced by the surrounding population [63]. This improper management of liquid and solid waste (burning and agglomeration) produced by anthropic activities leads to favorable environments for the spread of pests, mosquitoes, rodents, bad odors, vector-borne diseases, and the continuous deterioration of the environment [62].

The population of the community of Engativá is ~797,000 inhabitants (11.6% of the total population of Bogotá), representing the third-largest region in terms of population. In addition, 20,579 Bogotá businesses operate in this area, representing 9% of all city businesses. The businesses operating in this community are mostly service-based (73%), industry (19%), and construction (5%) businesses [64].

In the case of the Ramada station, No. 53, this is an irrigation and management station for swamps and ponds within the Bogotá River basin program. Hence, this agricultural area is being contaminated with water from the Bogotá River, thus negatively impacting the environment due to the contamination levels reported in its middle basin [65]. Considering that Executive Order 1594 from 1984, which still remains in effect, establishes that the most probable number (MPN) should not exceed 5000 for total coliforms and 1000 MPN/100 mL for fecal coliforms when the water resource is used for irrigating fruits that are consumed unpeeled and for short-stemmed vegetables. The water quality is now expected to be in a better condition even when there are no records because, since 2019, the Regional Autonomous Corporation has been working on the recovery of the Bogotá River to prevent its degradation, especially in agricultural areas [66].

The Fucha River Basin, which originates in the El Delirio forest reserve in the Cruz Verde Páramo, flows into the Bogotá River, covering an area of 12,991 urban and 4545 rural ha, corresponding to the eastern hills of the city [34]. This basin includes the communities of San Cristóbal, Antonio Nariño, Los Mártires, and Rafael Uribe. This is the apparent reason for the maximum values of fecal coliforms obtained due to domestic, commercial, and industrial wastewater discharges. Many of these populations are illegal settlements, Strata 1 and 2, with a serious shortage of infrastructure and public spaces [67].

The Fucha River collects all the sewer wastewater from downtown Bogotá, and, from the industrial zone (Américas, Calle13), the wastewater from numerous slaughterhouses and the community of Fontibón is collected, comprising a total area of 16,390 ha that discharges directly into this river [34].

Due to the aforementioned issues, the Fucha River currently ranks as the second most polluted river basin in the city. The discharges from this river into the Bogotá River are calculated as being 65% industrial waste and 35% domestic waste. On a daily basis, the Fucha River discharges at least 590 t of solids and 274 t of biological waste into the Bogotá River, of which 107 t comes from dwellings and the rest comes from businesses and industries [34].

Our analysis confirmed the degradation of water quality for agricultural use. In fact, the concentrations of coliforms follow an exponential trend, which is related to the behavior of the Fucha River index [35]. However, the Territorial Organization Plan for the Fucha River comprises an ecological corridor that seeks to preserve natural channels within the city. However, this micro-basin has few green areas due to its high level of urbanization and industrialization, which fact has also brought about complex usage conflicts that have hindered the proper management and recovery of the river [68].

Continuing the analysis, the Tunjuelo River basin represents the largest of the three rivers flowing through the capital city. It originates in Los Tunjos Lake in the Sumapaz Páramo, at an altitude of ~3450 masl. The river basin is formed by three main channels—the

Mugroso, Chisacá, and Curubital rivers—with a length of 73 km. It is located in the southeastern part of the city of Bogotá and runs through the municipality of Soacha. The river basin covers an area of 41,534 ha, including the communities of Usme, Ciudad Bolívar, Kennedy, Tunjuelito, Rafael Uribe, San Cristóbal, Puente Aranda, Antonio Nariño, Bosa, and the municipality of Soacha [69]. The Tunjuelo River basin plays a critical role in supplying water to the southern areas of the city of Bogotá [70]. This basin is classified into the upper, middle, and lower basins, with the upper and middle areas being rural, while the lower basin is mostly urban. The upper basin is located between Lake Tunjos in the Sumapaz Páramo and the La Regadera dam. The middle basin extends from the La Regadera dam to the Cantarrana dam, very close to Quebrada Yomasa. The lower basin covers the area from the Cantarrana dam to the mouth of the Bogotá River. This area reports the greatest anthropic incidence, leading to higher contamination rates [69].

The Tunjuelo River faces various socio-environmental conflicts, such as the application of pesticides in crop areas, contamination by tanneries, mining activities, contamination due to its proximity to the Doña Juana sanitary landfills, agricultural activities in the conservation areas, danger due to hydraulic phenomena, land use issues, legal and illegal settlements, and extraction of construction materials, among others [70].

According to reports from the downstream Tunjuelo River station No. 13, the pollutant load at this point is mainly generated by the pollutants provided by the Tunjuelo River as it flows into the Bogotá River. A large percentage of the domestic, industrial, and commercial wastewater from the city's sewer system is discharged into the Tunjuelo River [71]. Alternatively, a large part of the leachate produced by the Doña Juana landfill eventually falls into the Tunjuelo River, a situation that worsens whenever the river flow decreases at times of drought and due to anthropic actions in the upper river basin. In addition, the pollution load carries mining wastewater and solid waste, which has turned this river into a complete sewer [72]. Although there is a leachate treatment system in the landfill, it can only treat 15 L per second, which falls short of the 25 L per second actually generated in 2017. This means that many pollutants that remain untreated or that are partially treated are discharged directly into the Tunjuelo River, which, starting from this point, runs through four other communities before flowing into the Bogotá River [73].

On the other hand, at the Balsillas River Station, No. 72, another contributor to the contamination of the Bogotá River basin is the Balsillas River sub-basin, which is part of its middle river basin. This body of water is a contributor of high-impact pollutants associated with the presence of coliforms. In addition, heavy metals are also an important factor because most of the activities developed in the area use raw materials with a high content of elements such as cadmium and lead in their production processes [74].

After this point, the municipality of Soacha is the main contributor of pollutants to this river. The municipality is located in the southern part of the Bogotá savannah, bordering the city of Bogotá to the west, at an altitude of 2600 masl. The municipal territory has a total area of 184.45 km2, with an urban area of 19 km2, a rural area of 165.45 km2 and a population of approximately 535,000 inhabitants [75]. The Soacha River is highly polluted, as seen in the results from the Río Soacha station, No. 79, due to the presence of TC in its alluvial aquifers and fans. This is mainly due to biological and industrial factors and an average concentration of biodegradable organic matter that is outside the limits established by the regulations [76].

Next is the station of Puente Variante Mondoñedo Bridge, No. 60; in this area, the Bogotá river has already passed through several communities in the upper and middle basin, including the city of Bogotá DC and the municipality of Soacha, among others, receiving large amounts of domestic and industrial wastewater [77]. These values far exceed the water quality objectives established by the Regional Autonomous Corporation (CAR) for the year 2020 in Agreement 43 of 17 October 2006, "by which the water quality objectives for the Bogotá river basin to be achieved by the year 2020 are established", thus setting a maximum value of 7 mg BOD/L and a maximum permissible TC concentration value of 2.0 × 104 (MNP/100 mL) for this Class-II zone [78].

When evaluating the upstream Salto Tequendama station, No. 18, upon reaching the Tequendama Falls, due to the geomorphology of the area, which forms a natural 257-m-high waterfall [79], and the hydraulic characteristics of the water body at this point, the flow rate of the river increases, dragging garbage, colloidal solids, and dissolved solids from the upstream basin. This may account for the high levels of coliforms recorded in this section of the river [80]. Agreement 43 of 2006, issued by the Regional Autonomous Corporation (CAR), sets forth a maximum limit of 2.0 × 104 (MPN/100 mL) for Class IV. However, the values historically recorded by this station well exceed these objectives, as defined by the Corporation [78]. At Tequendama Falls, the Bogotá River leaves the savannah and enters the Tequendama archeological site in the province of Cundinamarca. The oxygenation it receives through this waterfall allows the river to recover part of its macrobiotic life [81].

As reported at the Quebrada La Cuy municipal discharge point, at this stage, high levels of TC and *E. coli* concentrations are recorded, mostly due to fecal contamination from animals. In 2010, the Regional Autonomous Corporation of Cundinamarca conducted a hog farm census of the area. The results revealed that 17,921 hogs were kept in 117 farms, none of which evidenced optimum waste-disposal systems. The discharges generated by hog farms in the area, in addition to producing bad odors, also directly affect the water in the middle basin of the Bogotá River and the waters near this section, which are used for animal consumption [82].

Finally, the Quebrada Honda No. 67 station is located in the last sections of the middle Bogotá River basin; the Quebrada Honda stream acts as a dissolution system for the pollutants that reach this point, considerably reducing both TC and *E. coli* levels. The Quebrada Honda stream is located within the Tequendama province in the department of Cundinamarca, mainly encompassing the municipalities of Tena and Bojacá. This sub-basin covers an area of ~1979.56 ha and runs at a height of 800–2550 masl [83]. These ameliorating effects are mainly due to the fact that a large part of the Quebrada Honda sub-basin is located in a protected forest reservation [84]; consequently, the anthropic effects on this body of water are minimal.

These agreements and measures were implemented to protect the Guacheneque Páramo, where the Bogotá River originates, as well as to manage the different industrial, agricultural, and livestock discharges, repair the river and its riverbed, and commission WWTPs [85]. As the consequences from river basin contamination increase, legal actions have become the go-to mechanisms for protecting environmental rights, even leading to work aimed at cleaning up the Bogotá River [85].

Throughout the entire middle basin of the Bogotá River, higher levels of coliforms and *E. coli* than the ones provisioned in Executive Order 1594 from 1984 on water and wastewater have been recorded. These standards establish a limit of 2.0 × 104 MPN/100 mL for TC and 2.0 × 103 MPN/100 mL for *E. coli*. All assessments included in this study are compared against these benchmark values. As evidenced in the aforementioned regulations, none of the sampling points comply with maximum permissible limits, except the Rio Frio station, No. 75, from 2009-02 to 2019-02, the downstream Jaboque station, from 2008-02 to 2019-02, the Quebrada Honda station, No. 67, from 2011-02 to 2019-02, and the Rio Chicú station, No. 74, from 2011-01 to 2019-02, where a decrease in fecal coliforms (*E. Coli*) is evidenced. The latter case is probably due to the conditions of the rivers that flow into this section of the river basin, as well as to a decrease in the contributions from the different municipal activities. For this reason, the water body becomes re-aerated, increasing by small amounts the presence of dissolved oxygen, especially after 109 km of the Bogotá River, exactly where it meets the Frio River [86]. Oxygenation helps the aerobic microorganisms in the water to breathe, which can then fulfill their function of naturally decontaminating the water. For this reason, the physicochemical conditions and the biological activity of the tributaries and effluents of this water resource are extremely significant [87].


**Investigations on the Unsteady Aerodynamic Characteristics of a Horizontal-Axis Wind Turbine during Dynamic Yaw Processes**

#### **Xiaodong Wang 1,\*, Zhaoliang Ye 1,2, Shun Kang 1 and Hui Hu 3**

Received: 11 June 2019; Accepted: 7 August 2019; Published: 14 August 2019

**Abstract:** Wind turbines inevitably experience yawed flows, resulting in fluctuations of the angle of attack (AOA) of airfoils, which can considerably impact the aerodynamic characteristics of the turbine blades. In this paper, a horizontal-axis wind turbine (HAWT) was modeled using a structured grid with multiple blocks. Then, the aerodynamic characteristics of the wind turbine were investigated under static and dynamic yawed conditions using the Unsteady Reynolds Averaged Navier-Stokes (URANS) method. In addition, start-stop yawing rotations at two di fferent velocities were studied. The results suggest that AOA fluctuation under yawing conditions is caused by two separate e ffects: blade advancing & retreating and upwind & downwind yawing. At a positive yaw angle, the blade advancing & retreating e ffect causes a maximum AOA at an azimuth angle of 0◦. Moreover, the e ffect is more dominant in inboard airfoils compared to outboard airfoils. The upwind & downwind yawing effect occurs when the wind turbine experiences dynamic yawing motion. The e ffect increases the AOA when the blade is yawing upwind and vice versa. The phenomena become more dominant with the increase of yawing rate. The torque of the blade in the forward yawing condition is much higher than in backward yawing, owing to the reversal of the yaw velocity.

**Keywords:** HAWT; aerodynamic characteristics; dynamic yawing process; near wake; start-stop yaw velocity