Introduction

More than 50% of the world population is currently living in urban areas. Due to the stressful city lifestyle, the possibilities of entertaining and rest in a natural environment are the most valued ecosystem services of the urban green areas, required for a life of good quality and important for public health (Bolund and Hunhammar 1999; Niemela et al. 2010). Many social, cultural and sportive activities in the city are performed thanks to the existence of this green infrastructure (McPhearson et al. 2014). However, as a result of overpopulation, the green spaces in the large cities are exposed to many negative impacts that affect the resilience of the urban environment and deteriorate its quality.

Water is usually part of the landscape of green urban areas. Aquatic ecosystems (rivers, lakes, groundwater, coastal waters and seas) support the delivery of crucial ecosystem services (Grizzetti et al. 2016). The analysis of recreational water quality is usually focused on the detection of bacteria. According to Wade et al. (2003), the association between this quality and human health outcomes has been studied for almost 70 years. Contact with water also exposes human health to diverse physical and chemical hazards (WHO 2003). The analysis of recreational water quality must take into consideration the health of bathers without leaving aside the rest of potential impacts of recreational water quality over the environment. Water quality has impacts on the entire local ecosystem: soil, vegetation, fish, birds, insects and other animals. It is fundamental for many of the biogeochemical processes of the ecosystem and determinant for the aesthetic landscape value of the area (González Castelain 2013).

Water quality is composed of multiple related variables. A change in one of them can affect the others. In some cases, these changes have a negative impact over several of the ecosystem services associated with water (Keeler et al. 2012). Monitoring this quality is important to obtain scientific information that functions as an indicator of the urban environment state.

This research is focused on the study of the quality of the recreational water at the main green urban areas of Buenos Aires City, Argentina. In order to apply a broader vision than public health to determine the quality of recreational water, several analytical methodologies were simultaneously used.

Methodology

The sampling period was from March 2015 to March 2017. Samples were collected on a seasonal basis, four times a year for each site. The parameters analyzed were pH, temperature, dissolved oxygen, 5-day biological oxygen demand, dissolved solids, nitrates, total phosphates, turbidity, transparency, fecal coliforms, arsenic and chromium VI. Results were compared with water quality standards established in national legislation and were ranked by the National Sanitation Foundation Water Quality Index (NSF-WQI). The transparency of the water was measured by a Secchi disk and used as an indicator of the impact on aquatic life.

Sample collection, stabilization, transportation and storage were made according to the Standard Methods for the Examination of Water and Wastewater (APHA 2012).

These measurements were made in situ with a portable meter: pH (S.M. 4500 B), electrical conductivity (Con) (S.M. 2510 B), and dissolved oxygen (DO) (S.M. 4500 O B). Transparency (Tr) of the water was semiquantitatively estimated with a Secchi disk (ISO 7027), and water temperature with an infrared thermometer (NIST Traceable, ISO 17025). Triplicate samples were taken into sterilized borosilicate glass bottles (S.M. 9060) of 500 mL each and were transferred to the laboratory for further analysis. The samples were transported at 4 °C inside an isolated container using ice packs and gel packs. At the laboratory nitrates (NO3−) were determined by reduction of cadmium (S.M. 4500 NO3− E), phosphates (PO43−) by ascorbic acid method (S.M. 4500 PO43− E), chromium VI (Cr6+) by diphenylcarbohydrazide method (ASTM D 1687 Test Method A), arsenic (As) by semiquantitative method, Gutzeit modified (Luong et al. 2007), 5-day biological oxygen demand (BOD5) by dilution method (S. M. 5210 B), turbidity (Tur) by formazin method (S.M. 2130 B), dissolved solids (DS) estimated by conductivity measurements (Carlson 2005), and fecal coliforms (FC) by multiple tube fermentation technique (S.M. 9221 E). The temperature change of the water (∆T) was calculated with respect to the average value of the surface waters temperature in the Metropolitan Area of Buenos Aires (AMBA) region. According to the Environmental Atlas of Buenos Aires (2010), the annual mean value is 20 °C. The water quality was rated by NSF-WQI calculation, was compared with established regulations based on international water quality standards and with Secchi depth recommendations. Table 1 summarizes the parameters analyzed and the used methods.

Table 1 Parameters analyzed
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The NSF-WQI was developed in 1970 with the objective of providing a tool for evaluating water quality.

This index was selected because is one of the most frequently used for water quality assessment across the world (Lumb et al. 2011; Poonam et al. 2013) and it has great credibility since more than a hundred experts contributed to its development (Sutadian et al. 2016). The index allows studying the evolution of water quality for a specific site in a timeline, or for different sites of the same body of water at a given time in a reproducible way (Phadatare and Gawande 2016). It also allows a standardized comparison between different bodies of water. (Brown et al. 1970; Poonam et al. 2013).

The NSF-WQI is based on nine parameters selected by Delphi method. A subindex Qi is obtained for each parameter introducing the experimental value into the respective rating curve (Tyagi et al. 2013). Each parameter has been assigned a weight, as seen in Table 2. The index is calculated as a weighted average, in accordance with the equation:

Table 2 NSF-WQI (weight of the parameters)
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$$ {\text{WQI}} = \mathop \sum \limits_{i = 1}^{n} W_{\text{i}} *Q_{\text{i}} $$

Its score ranges from 0 to 100, with 100 being the maximum score. According to the score, the NSF-WQI determines the status of the water body into five categories, namely excellent (91 to 100), good (71 to 90), medium (51 to 70), bad (26 to 50) and very bad (0 to 25) (Alexakis et al. 2016).

The obtained data were also compared with the parametric standard values proposed in the regulation issued by the Water Resources Secretariat of the Ministry of Interior (SSRH). This regulation defines the values associated with each category of use of the water, according to Tables 3 and 4 (Ministerio del Interior 2003).

Table 3 Uses of the water
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Table 4 Standard values of the analyzed parameters associated with each category of use
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In relation to public health, the fecal coliforms are accepted as fecal indicator bacteria, because they are more easily measured than other pathogens and more abundant in fecal material. This increases the likelihood of detection (Griffith et al. 2016).

Arsenic and hexavalent chromium are other important pollutants to consider when studying the water quality in relation to human health. The analysis of arsenic levels in Argentina is important because its frequent presence is responsible for chronic endemic regional hydroarsenicism (HACRE). In many regions of the country, including certain areas of Buenos Aires, the concentration of arsenic exceeds the level of 10 μg/L admitted by the World Health Organization (Litter 2010). The high degree of industrialization along the coast of the Río de la Plata generates a chromium hexavalent input (Licursi and Gomez 2013). According to Rocchetta et al. (2007), this pollutant is also present in the Riachuelo waters and the main discharge comes from tanning industry. Certain chromium hexavalent compounds are carcinogenic in humans. It can cause a wide range of other health effects such as allergic reactions, ulcers, liver damage, convulsions or even death. (Pellerin and Booker 2000).

Photosynthesis is possible only within a thin layer of the superficial waters in which at least 1% of the photosynthetically active radiation can penetrate. This zone is referred as euphotic depth. A simple way to assess the penetration of light into water is by using a Secchi disk (Padial and Thomaz 2008; Luhtala and Tolvanen 2013). Water absorbs or scatters light due to different factors like the color of the water, the presence of phytoplankton and presence of erosional materials suspended (Lee et al. 1995). The optimum Secchi depth for fish at lakes and lagoons varies in relation to the species of fish and the causes of the turbidity. According to Bhatnagar and Devi (2013), a range from 30 to 80 cm is good for fish health. Low transparency of the water causes fish stress. The Food and Agriculture Organization of the United Nations (FAO 2005) recommends a range from 40 to 60 cm. A value less than 40 cm indicates the presence of an amount of plankton that endangers the survival of the fish during the night when there is no production of oxygen due to photosynthesis but respiration of plankton increases consumption. A transparency greater than 60 cm indicates low amounts of plankton and the fish may not have enough natural food.

The city and the sampling sites

The Autonomous City of Buenos Aires, known by the acronym CABA, is the capital city of Argentina. It has an area of 203.3 km2 (78.5 square miles) and according to the last national census, it has 2.89 million inhabitants. Buenos Aires is surrounded by an urban cluster known as Great Buenos Aires (GBA). This urban conglomerate is one of the largest in the southern hemisphere. The urban zone formed by GBA and CABA is called the Metropolitan Area of Buenos Aires (AMBA) and has an area of 2681 km2 (1035 square miles). According to the census of 2010, the population estimate is 14.28 million inhabitants and the population density is 5.3 thousand people per km2 (13.8 thousand people per square mile). Buenos Aires AMBA is at the 21st place among the 1000 most populated cities in the world (Demographia 2017). It is estimated that 12% of the GBA inhabitants enter into CABA daily for work, study or for other reasons (Diario Clarín 2013). The city has 1055 green spaces that cover an area of 1924.5 ha (7.43 square miles). The relationship between this area and the population indicates that it corresponds to 6.3 m2 of green space per inhabitant (9765 square inches per capita) (Buenos Aires Ciudad 2017a).

Water is an important presence in the green spaces of Buenos Aires City. The east city limit is the Río de la Plata coast, surrounded by “La Costanera,” an area of walks along the river. The south city limit is the Riachuelo (formerly Matanza River). Next to it, “Caminito,” a picturesque tango corner of the city and a favorite place for tourists. The city also has another 19 important green urban areas of which 14 have ponds and lakes, where citizens and tourists run, walk, row and relax.

To assess the quality of the recreational water in Buenos Aires City, samples were collected at eight sites on the main green areas of the city. The sampling sites are marked over the map as shown in Fig. 1.

Fig. 1
figure 1

Map of Buenos Aires City and the sampling sites (Mapoteca Educar)

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The description of the sampling sites and the activities performed in every location are shown in Table 5.

Table 5 Description of the sampling sites
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Results and discussion

Table 6 summarizes the results obtained for the different parameters analyzed. Due to the minimum seasonal variation observed, the values presented in this study correspond to the average result for each sampling site.

Table 6 Parameters values for each sampling site
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Table 7 shows the subindex (Qi) for the nine parameters (factors) required by the NSF-WQI, obtained by transferring each parameter value to the corresponding rating curve.

Table 7 Value of the subindex Qi for each factor
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Table 8 shows the results for the NSF-WQI at each sampling site.

Table 8 NSF-WQI results for the sampling sites
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In accordance to the NSF-WQI ranking, the eight sampling sites have bad or medium water quality.

In the sites covered by this study, people engage in outdoor activities such as sporting or recreational rowing, biking, skating, running and walking around the water. Comparing the obtained results with the regulations of the SSRH, as seen in Table 9, sampling site #1 allows primary, secondary and passive recreational uses of the water, and sampling sites #2 and #6 only admits passive recreational activities. The water quality of the other five sampling sites does not meet the requirements of the SSRH regulations for the performance of recreational activities.

Table 9 Possible water uses at the sampling sites
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Secchi disk depth is less than 30 cm in sampling sites #2, #3, #5 and #6 (see Table 3). The low transparency on these sites may cause stress to some fish species.

The explanation of the presence of Cr6+ and the elevated fecal coliform count in some of the lakes that do not receive directly sewage or industrial effluents can be found by the analysis of the hydrological setting of the city. Under Buenos Aires run several piped streams. The most important of them are Medrano stream, Vega stream, Maldonado stream and Radio Antiguo Basin (see Fig. 1). These streams were channeled to prevent city floods. All of them are connected to the highly contaminated Río de la Plata. The waters of this river are polluted due to the dumping of effluents from different sources without a correct treatment. These wastewaters come from the two main cities of the country: Buenos Aires and La Plata (Colombo et al. 1989). The presence of strong winds from the south-southeast quadrant coinciding with the axis of the river is a common meteorological phenomenon known as Sudestada. This wind drags on the waters of the river, which produces the increase in the level of the waters on the coast of Buenos Aires. According to the National Meteorological Service (SMN 2000), it makes difficult the drainage of smaller channels and brings the Riachuelo waters up to the city. This indirect contact with the Río de la Plata, polluted by the Riachuelo waters, through the piped streams can explain the level of contamination with Cr6+ of the ornamental lakes on sampling sites #1, #6, #7 and #8.

Table 10 shows a summary of results for each sampling site. Sampling site #1 has the best water quality status, according to the ranking of the NSF. The explanation of this result may be related to the origin of the water. This pond is connected to several natural springs but its level is also maintained thanks to a clean water pumping system. The stream Cildáñez connects the Riachuelo with sampling site #2 (Lago Lugano) to prevent floods over the city. This connection with the fourth most polluted watershed in South America explains the poor water quality of the lake. Sampling site #3 is at the Riachuelo riverside and has the poorest quality of all the sites analyzed. The pollution of this river is one of the biggest environmental issues of Buenos Aires. The lack of planning for land use, with no urban planning nor sanitation infrastructure, and the disorderly location of industries are some of the historical causes of the pollution of this basin. Its remediation represents a challenge that different governments have not yet been able to solve (Faggi et al. 2015). Sampling sites #4 and #5 are on the Río de la Plata riverside. The results reflect the serious environmental status of this river. Sampling sites #6, #7 and #8 are located in a park at one of the most beautiful, important and elegant neighborhoods of the city. These lakes are connected to the channeled stream Vega. This connection and the periodical occurrence of the Sudestada phenomenon explain the low quality of its waters and the presence of Cr6+, a clearly industrial pollutant.

Table 10 Summary of results
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Conclusion

The study of recreational water quality with a broad point of view than public health allows identifying potential risks over the different environmental components of the urban green areas. To prevent the deterioration of the ecosystem services provided by these areas, the city government must consider including the monitoring, improvement and maintenance of the quality of the recreational water of the city in its public policies. As an outcome of this study, recommendations for the implementation of protection tasks, as well as the promotion of a law stating the sanitary and environmental emergency of the studied areas were given to the city government.