Assessment of the changes in contributions from water sources to streamflow induced by urbanization in a small‑sized catchment in Southeastern Brazil using the dual stable isotopes of water ( 18 O and 2 H)

Avaliação das mudanças nas contribuições das fontes de água para o fluxo de água induzidas pela urbanização em uma pequena bacia hidrográfica no sudeste do Brasil usando os isótopos estáveis ​​duais da água (¹⁸ O e ² H)

ABSTRACT

Urban growth often results in changes in the urban hydrological cycle, causing impacts on water availability in densely populated regions. The water isotopologues can provide relevant information about the origin of water under different hydrogeological scenarios, aiding to implement better strategies for water conservation in coupled natural-urbanized environments. In this study, the isotopic compositions of multiple water sources were assessed in a pristine (Ipanema National Forest, FLONA) and an urbanized (Lavapés catchment, SOR) watershed located in the Sorocaba River basin (State of S.o Paulo, Southeastern Brazil), seeking to understand the causes of isotopic variability and to determine the relative contribution from different sources to streamflow, using the Bayesian mixing model approach. Differences in isotopic composition were observed, as FLONA yielded the most depleted water (ca.−7.5 ‰  ¹⁸O for surface and groundwater and ca.+11.0 ‰ d-excess), while SOR yielded the most enriched water (ca.−5.5‰ ¹⁸O for surface and roundwater and−3.8‰  ¹⁸O for the water supply system), with evidence of evaporation (ca.+8.2 ‰ d-excess). The differences observed in isotopic compositions are related to a combination of different factors, such as geological framework, groundwater recharge, and evaporation associated with the Itupararanga water reservoir. Bothin FLONA and SOR, groundwater discharge is the most important factor that regulates streamflow. However, in SOR, losses from the water supply system were almost constant along the year, representing an important contribution. The results presented here highlight the use of isotope hydrology techniques to solve problems related to urban hydrology.

Keywords: Climate change, Land Use and Land Cover changes, GLM-AED2, Water quality

RESUMO

O crescimento urbano frequentemente resulta em mudanças no ciclo hidrológico urbano, causando impactos na disponibilidade hídrica em regiões densamente povoadas. Os isotópicos da água podem fornecer informações relevantes sobre a origem da água em diferentes cenários hidrogeológicos, auxiliando na implementação de melhores estratégias para a conservação da água em ambientes naturais e urbanizados acoplados. Neste estudo, as composições isotópicas de múltiplas fontes de água foram avaliadas em uma bacia hidrográfica intocada (Floresta Nacional de Ipanema, FLONA) e urbanizada (bacia hidrográfica de Lavapés, SOR) localizada na bacia do rio Sorocaba (Estado de São Paulo, Sudeste do Brasil), buscando compreender as causas da variabilidade isotópica e determinar a contribuição relativa de diferentes fontes para o fluxo de água, usando a abordagem do modelo de mistura bayesiano. Diferenças na composição isotópica foram observadas, pois a FLONA produziu a água mais esgotada ( ca. −7,5 ‰  ¹⁸ O para águas superficiais e subterrâneas e ca. + 11,0 ‰ d -excesso), enquanto o SOR produziu a água mais enriquecida ( ca. −5,5‰  ¹⁸ O para águas superficiais e subterrâneas e −3,8‰  ¹⁸ O para o sistema de abastecimento de água), com evidências de evaporação ( ca. + 8,2 ‰ d -excesso). As diferenças observadas nas composições isotópicas estão relacionadas a uma combinação de diferentes fatores, como estrutura geológica, recarga de águas subterrâneas e evaporação associada ao reservatório de água de Itupararanga. Tanto na FLONA quanto no SOR, a descarga de águas subterrâneas é o fator mais importante que regula o fluxo do rio. No entanto, em SOR, as perdas do sistema de abastecimento de água foram quase constantes ao longo do ano, representando uma contribuição importante. Os resultados aqui apresentados destacam o uso de técnicas de hidrologia isotópica para solucionar problemas relacionados à hidrologia urbana.

Palavras-chave: Climate change, Land Use and Land Cover changes, GLM-AED2, Water quality

Introduction

Despite the importance of pristine environments and natural landscapes to understand natural conditions of water circulation along the water cycle, most of the challenges resulting from changes in water circulation are related to increasing urban demands. The use of stable isotopes as a tracer of water movement can be helpful to solve problems associated with water supply and water dynamics in urban environments (Ehleringer et al., 2016; Jameel et al., 2016; Li et al., 2019).

Urbanization creates impervious surfaces by sealing soils and expanding surface runoff, therefore decreasing evapotranspiration and infiltration in urban catchments (Shuster et al., 2005). The combination of these processes has led to important modifications in urban streamflow, directing most of the surface runoff to urban river channels with an apparent decrease in the contribution from groundwater, due to the reduced recharge of aquifers, whereas contributions from utilities (e.g., drainage, water supply and sewage systems) seem to be more relevant to urban catchments (Barron et al., 2013; Bonneau et al., 2018; Gabor et al., 2017; Lerner, 2002; Li et al., 2019; Shuster et al., 2005). A better understanding of these processes in tropical urban regions is required, in view of the impacts of extreme events associated with climate changes.

Contrarily to urban environments, streamflow in natural catchments has been largely studied, taking into account different climatic scenarios (Batista et al., 2018; Gan & Luo, 2013; Lachniet & Patterson, 2009; S.nchez-Murillo et al., 2015; Santarosa et al., 2021; Schulte et al., 2011; Soulsby et al., 2011; Tetzlaff & Soulsby, 2008; Zhang et al., 2017). The contribution of groundwater to streamflow has been found to be variable and directly related to environmental parameters, such as geological and geomorphological frameworks and soil types (Berhanu et al., 2015; Betancur et al., 2020; Carlier et al., 2018; Cheng et al., 2012; Price, 2011; Soulsby et al., 2004, 2011; Zhang et al., 2017).

The use of stable isotopes as tracers of the water origin and flow represents an important tool to understand the hydro‐climatic factors that affect water resources at distinct spatial and temporal scales, including water flow in watersheds and the connections between precipitation, surface, and groundwater (Aggarwal et al., 2012, 2016; Jouzel et al., 2013; Kendall & Coplen, 2001; Rozanski et al., 1993), as well as the modification induced by anthropogenic activities, such as irrigation in agricultural areas (Cherry et al., 2020).

Urbanization promotes important changes in urban streamflow, having direct impacts on the water isotopic composition. These impacts are mostly related to modifications implemented in the urban subsurface that can result in adjustments of the processes related to water flow (infiltration, surface runoff, and evaporation), as well as changes in the contributions from different sources (including losses from the water supply system and sewage discharge). In addition

to the vast literature focusing on isotope hydrology applied to the investigation of catchment processes and water balance in quasi-undisturbed or undisturbed basins (Bonneau et al., 2018; Darling, 2011; Li et al., 2019; Santarosa et al., 2021; Unnikrishnan & Praveen, 2012), urban hydrology studies have recently incorporated the dual stable isotopes of water to evaluate the effects of urbanization on hydrological processes, such as runoff distribution and transit times (Dimitrova‐Petrova et al., 2020; Soulsby et al., 2014), the contribution of different water sources to urban streamflow (Jasechko, 2019; Jefferson et al., 2015), and the origin of the water distributed by an underground water supply network (Ehleringer et al., 2016).

The middle Sorocaba River basin (hereafter SRb) is located in the central-eastern portion of the State of S.o Paulo, which is one of the most developed regions of the state, being in a very advanced stage of land occupation and containing few natural and preserved areas. At present, the estimated SRb population is 1.5 million inhabitants, mostly living in Sorocaba, the largest SRb city that encompasses an 3 57 Page 2 of 15 Environ Monit Assess ¨2022© 194º 357:important industrial park and has reached high levels of urbanization and sanitization (FAT & FABHSMT, 2016).

Sorocaba presents one of the highest rates of population growth of the State of S.o Paulo, starting from ca. 70 thousand inhabitants in the 1940s to almost 700 thousand inhabitants nowadays (Gon.alves et al., 2014; IBGE, 2021). This increase in population has led to fast urbanization that, despite land-use planning, has taken place in a disordered way, leading to hydrological and hydrogeological problems (FABH-SMT, 2020). About 65% of the SRb population’s water demand is supplied by the Itupararanga reservoir, located 20 km southeast of Sorocaba city (Fernandes et al., 2016).

Contrasting with this intense urbanization, the Ipanema National Forest (FLONA), located about 20 km northwest of Sorocaba, is one of the few preserved areas in SRb (Fig. 1). FLONA is an environmental conservation unit established in 1992, with the purpose of preserving fragments of the semideciduous forest (Atlantic Forest) and the savannah (Cerrado), as well as the rich historical site located in the area, where the first iron foundry was installed in Brazil in the 1800s (de Silva & Mazine, 2016).

This study adopted the paired-catchment approach, focusing on two small watersheds of SRb: (i) the Ferro micro-basin (hereafter FLONA), located in a pristine region of the Ipanema National Forest, and (ii) the Lavap.s catchment (hereafter SOR), located in the Sorocaba urban zone. Our main purpose was to investigate the origin of water and the influence of the urban water supply network on the urbanized watershed, in order to quantify the contributions of different water sources using stable isotopes (²H and ¹⁸O) as tracers and the Bayesian mixing model approach. The following questions were made to guide this study: (i) How does urbanization impact streamflow in urban catchments? (ii) Can river water isotopic compositions be used to identify and quantify different water sources in urban and pristine watersheds? (iii) Are the water isotopic compositions in different watersheds related to the geological framework? The answers to these questions will (i) inform us about the temporal and spatial variations of the water sources in both watersheds and (ii) provide the basis for the use of the dual stable isotopes of water to identify losses in the water supply network, as monitoring programs can help the water management agencies implement programs to reduce such water losses.

General settings

The two watersheds selected for this study (SOR and FLONA) are located in SRb, which is in turn situated in the central-southern portion of the State of S.o Paulo. SRb is part of the Middle Tiet.-Sorocaba UGRHI-10, which is one of the 21 water resources management units of the state. Draining an area of about 5,300 km2, SRb totally or partially comprises 18 municipalities, supplying water for about 1.5 million inhabitants (FAT & FABH-SMT, 2016).

Two climate types predominate in SRb, according to the K.ppen–Geiger climate classification (Peel et al., 2007): subtropical (Cwa) and humid subtropical (Cfa). The mean temperature is 21.4 ‹C and the mean annual rainfall is about 1,300 mm and mostly concentrated in the summer, which lasts from October to March, while the period from April to September is characterized by a dry and cold climate (Silva & Silva, 2016). SRb drains two distinct geological compartments.

The eastern portion of the basin, where Sorocaba city is located, is dominated by phyllites, quartzites, and metasedimentary rocks of the S.o Roque Group and granitic rocks of the Embu Complex (Godoy et al., 1996). The western portion, where FLONA is located, is inserted in the Paran. Sedimentary Basin, predominating outcrops of the Itarar. Group and

basic and alkaline igneous rocks related to the Serra Geral Formation (DAEE, 2005; Petri et al., 1996) (Fig. 1). The two main aquifers of the study area are the fractured aquifer or Precambrian Aquifer System (hereafter PAS), developed in crystalline rocks of the SRb eastern portion, and the porous aquifer or the Tubar.o Aquifer System (hereafter TAS), encompassed by the Itarar. Group.

The average thickness of the TAS in outcrop area is of about 300 m that increases significantly westwards (Ezaki et al., 2020). Due to the multi-layering and interfingering of the sedimentary layers, the TAS is one of the most complex

hydrogeological units of the Paran. Basin. Despite groundwater flow is preferentially controlled by the primary porosity of the sedimentary rocks that constitute TAS, structural discontinuities can also influence water movement throughout the unit.

These discontinuities are associated with lithological heterogeneity and layering, which attribute anisotropy characteristics to TAS and create semi-confinement and/or confinement conditions (DAEE, 2005; Ezaki et al., 2020).