Original Research ArticleAnalysis of the Suspended solids transport in a stream located in the southwest of the Buenos Aires province, Argentina
Introduction
The content of suspended solids is a relevant parameter for stream management and streamflow quality characterization (Ahn and Steinschneider, 2018; Bieroza et al., 2018). The amount of total suspended solids (TSS) comprises the nutrients and pollutants transported by the water in the form of organic and inorganic particles (Cieśla et al., 2020). The concentration of solid particles has a direct effect on the penetration of light and, consequently, on the ecological habitats that are prone to develop within the stream (Bilotta and Brazier, 2008; Vercruysse et al., 2017). Likewise, streams or rivers with significant seasonal regime variability in combination with a large supply of solids in suspension are prone to undergo relevant geomorphological changes. In this regard, the content of TSS is useful to derive a balance of solids which, at the same time, can be employed in an inference framework to predict what kind of physical changes are plausible to expect in the channel (Dean et al., 2016). The geomorphology of the deltas is also sensitive to the contents of sediments in the streamflow. In effect, the interruption of the natural streamflow caused by the construction of a dam could generate a radical change both in the geomorphology of the downstream channel as well as in the delta itself (Mueller et al., 2017; Słowik et al., 2018). In this sense, periodical data acquisition on the TSS content acquires greater relevance: knowing the usual solid supply allows to estimate its effect on the useful capacity of the reservoir and, furthermore, to quantify possible variations in the shape of the channel downstream of the dam.
The variables that intervene in the TSS concentration are diverse and complex (Vercruysse et al., 2017). Indeed, the concentration of suspended particles in a particular spot along a stream at a particular time, is a sensitive random variable (RV) which needs to be registered at the higher possible frequency (Jones et al., 2012). However, the frequent measurement of TSS through discrete water sampling is neither straightforward nor economically convenient: each campaign requires accessing the sampling site and performing the corresponding laboratory tests. On the other side, turbidity has proven to be a reliable and easy-to-measure surrogate parameter for TSS concentration (Gippel, 1995; Göransson et al., 2013; Jones et al., 2011; Steffy and Shank, 2018). In this approach, the first step consists of conducting paired field determinations of turbidity and TSS concentration. The gathered data are then employed in a statistical regression scheme to derive an expression relating turbidity and TSS concentration (Lannergård et al., 2019; Oliveira et al., 2018; Rügner et al., 2013; Williamson and Crawford, 2011). Even though a series of parameters such as the color of the water, the shape and size of the suspended particles, affect the relation between turbidity and TSS, the correspondence between both registers is accepted as long as a statistically significant correlation is established (Downing, 2006; Nasrabadi et al., 2016).
Based on the evidence regarding the potential value of turbidity as a proxy for TSS concentration, the present investigation is aimed at deriving an analytical expression that allows to relate the content of solids and the turbidity in the Napostá Grande stream, in the peri-urban zone of the city of Bahía Blanca (Buenos Aires, Argentina). The water supply system for the city of Bahía Blanca and its influence area is currently in a critical state. Recent hydric crises reveal the urgency for the evaluation of potential complementary water sources, in order to guarantee the water supply for more than 450,000 inhabitants, and for the Petrochemical Complex installed in the Ingeniero White Industrial Park, which is responsible for 60% of the national petrochemical production (López et al., 2012). During the 1990 decade, a series of studies was conducted in order to evaluate potential sources of water for human and industrial consumption in Bahía Blanca and its influence area up to the year 2050 (Schefer et al., 2012). Among the alternatives, the construction of a dam in the Napostá Grande stream is highlighted by the authors. This project would also contribute to control the frequent overflows of the stream, which causes frequent floods in Bahía Blanca (López et al., 2012). Moreover, the local Water Supply Agency (ABSA) is currently making use of the Napostá Grande flow through three separate superficial intakes located along the stream in the lower drainage basin. However, these sources frequently become unusable when the water turbidity surpasses the maximum capacity of 10 Nephelometric Turbidity units (NTU) supported by the local Water Treatment Plant. An investigation undertaken from 2013 to 2014 (López et al., 2015) reports that 55% of the water samples taken at Puente Canessa, and 80% of the samples taken on a nearby site which is placed 20 km downstream, named Paso Vanoli (Fig. 1), would require additional treatment for human consumption, due to high values of turbidity. In this regard, the construction of a dam in Puente Canessa would promote the natural decantation of solid particles, thus reducing the turbidity both at the reservoir and downstream.
In brief, the Napostá Grande stream stands as a positive alternative solution to the serious water supply problem that affects the Bahía Blanca region. However, the natural resource has not been studied or characterized in depth so far. In this context, the general objective of the present investigation is to develop statistically based relations between the parameters involved in the suspended solids transport, in order to characterize this phenomenon through the measurement of variables of simple and straightforward field determination. In this regard, the following particular objectives are settled:
To evaluate the statistical correlation amongst turbidity and TSS concentration in the Napostá Grande stream at the sampling site Puente Canessa.
To analyze the existence of a statistically significant correlation between the total discharge and the solid flow rate in the Napostá Grande stream at Puente Canessa.
To analyze the variation of the TSS concentration and the solid flow rate in the Napostá Grande stream between the closure of the middle stream drainage basin and the sub-urban area of Bahía Blanca City.
Section snippets
Study area
The Napostá Grande stream drainage basin is located in the southwest of the Buenos Aires province, Argentina. The basin extends over an area of 1237 km2, where the climate is classified as subhumid-humid, mesothermal. Winds are predominant from the NW and N directions, with an estimated 9 km h−1 annual mean velocity. Rainfall represents the only water income into the stream drainage basin, with 666 mm average annual precipitation, and a decreasing gradient of 2.4 mm km−1 towards the South. The
Methodology
The turbidity and TSS concentration sampling campaigns in the Napostá Grande stream at the site Puente Canessa were carried out in two separate temporal periods. The first one started in May 2012 and the campaigns were conducted on a monthly basis (approximately every 30 days) until August 2013, except for March and May 2013 when the data collection was temporarily interrupted. The second period of sampling started in May 2018 and continued monthly until December of the same year. The campaigns
Results and Discussion
The results of the investigation are presented in this section. The dates and campaign records are informed in Table 1 together with relevant statistics —i.e. median, sample mean, first and third quartiles, and standard deviation (SD). The values of turbidity (T), TSS concentration, QL and QS measured at Puente Canessa are identified along the article with the subscript “C”. The QL data which are identified with an adjacent asterisk (• *) correspond to discharges estimated by the implementation
Conclusions
The Napostá Grande stream constitutes a potentially valuable resource that could improve the deteriorated water supply system of the city of Bahía Blanca and its area of influence. Motivated by this, a statistical analysis of relevant variables that characterize the Napostá Grande stream water, such as discharge, turbidity, TSS concentration and solid flow rate, was performed in this investigation. In particular, analytical expressions have been developed by statistical correlation, which allow
Ethical Statement
The research was done according to ethical standards.
Declaration of Competing Interest
None declared.
Acknowledgment
The authors acknowledge Eng. J. C. Schefer y and Dr. E. R. Albouy, directors of the Research Projects that funded this investigation. Likewise, the authors acknowledge the support provided by the Laboratory of the Provincial Water Authority (ADA); as well as the Laboratory of Hydraulics and the Environmental Chemistry Laboratory, both dependent of the Universidad Nacional del Sur.
Funding Body
The authors acknowledge Eng. J. C. Schefer y and Dr. E. R. Albouy, directors of the Research Projects that funded this investigation.
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