Characterization of groundwater dynamics and contamination in an unconfined aquifer using isotope techniques to evaluate domestic supply in an urban area

Highlights

• Groundwater for domestic supply evaluated for nitrate contamination in an urban area.

• Stable isotopes ¹⁸O_H2O and ²H_H2O were tools to identify recharge areas in well batteries.

• Stable isotopes ¹⁵N_NO3 and ¹⁸O_NO3 helped to identify nitrogen pollution processes.

• Mixing modelling using ¹⁵N_NO3 allowed to identify proportions of pollution sources.

• Lower DO in the periurban area because of OM from on-site sanitation/animal wastes.

Abstract

In the urban area of Rio Cuarto groundwater is used for domestic supply. The objective of the current research was to investigate the water dynamics and nitrate contamination in an unconfined aquifer system of Rio Cuarto River basin. Stable isotopes of water (δ¹⁸O and δ²H) and nitrate (δ¹⁵N–NO₃ and δ¹⁸O–NO₃) were used and combined with conventional chemical techniques and mixing modelling approaches to determine the recharge areas and identify the main origin of nitrate pollution. More enriched water isotope values were recorded in Rio Cuarto city than in the piedmont and mountains indicating local recharge for well batteries 2 and 3. The well battery 1 and the infiltration gallery showed more negative isotopic values, demonstrating a strong influence by the recharge from the piedmont sector (impoverished groundwater). The δ¹⁵N–NO₃ values and the Bayesian modeling showed that the dominant nitrate contamination source in the urban area is the on-site sanitation systems whereas in the peri-urban area nitrate contamination originates mostly from animal wastes. Both sources supply the aquifer with anthropogenic organic matter (>50%). The highest δ¹⁵N–NO₃ values were correlated with low dissolved oxygen values, indicating the occurrence of denitrification processes in some places. High NO₃ ions in the rural sector were attributed to the application of fertilizers and consequent nitrification processes. The samples from Rio Cuarto river located upstream of the Rio Cuarto City and close to the infiltration gallery showed the highest input from fertilizers (~40%), due to surrounding agricultural fields. The municipal well batteries showed good water quality (freshwater of low to extremely low nitrate concentration) which is linked to the high hydraulic conductivity of the aquifer that increases its capacity to dilute the contaminants. These results will be useful for local water administrators to improve water management.

Introduction

The water resources availability worldwide is decreasing due to over-abstraction, water quality degradation and climate change (Bates et al., 2008, Aznar-Sánchez et al., 2018; Bierkens and Wada, 2019). Freshwater demands are increasing rapidly and groundwater is a vital resource that represents approximately 30% of the world's freshwater resources. Nowadays, more than 50% of the world's population lives in urban areas and by 2050 this proportion is expected to increase to 66% (WHO, 2015). The significant urban population growth creates extraordinary challenges, among which provision for water is of principal attention. In large and small urban areas, the water-related problems are of various types, with the most significant ones associated to pollution caused by point or non-point sources located in the same or surrounding rural areas (Vázquez-Suñé et al., 2005; Blarasin et al., 2014, 2020a; Li et al., 2020, Giuliano Albo and Blarasin, 2014). The quantity and quality of water delivered and used for households is an important aspect of domestic water supplies, which influences hygiene and therefore public health (Li and Wu, 2019; Howard et al., 2020). The lack of access to safe water has enormous consequences in human health and well-being, leading to problems as diarrhea and cholera outbreaks, among others (Geldreich, 2020; Howard et al., 2020).

In this framework, the use of isotopes constitutes a reliable tool for the assessment of hydrological problems in urban and surrounding rural areas to improve the sustainable management of water resources. The isotopic studies conducted over different spatial and/or temporal scales can provide powerful insights into natural ecosystems’ function and the effects of anthropogenic disturbances (Kendall and Aravena, 2000; Kendall et al., 2010; Kaushal et al., 2011; Matiatos, 2016; Gallagher and Gergel, 2017; Blarasin et al., 2020a). The use of water for domestic supply in urban areas is complex and usually obtained from many sources (i.e. groundwater, surface water, precipitation harvesting and in some cases seawater) with different isotopic signatures that can be used to better define the origin, the pathways and the interactions between the water bodies (Zhan et al., 2016). For example, stable water isotopes can be used to understand relations between water consumers and sources to develop strategies that ensure a long-term sustainability of domestic water supplies. On a local scale, the spatio-temporal distribution of stable water isotope ratios across an urban area can be used to understand and monitor the function of the municipal water systems, which is of major importance for water resources managers. The use of environmental isotopes also provides information about leakages in the distribution systems used for domestic supply as well as faulty sewage systems leaching to urban aquifers (Grimmeisen et al., 2017).

Water pollution problems caused by point or non-point sources in urban areas can be readily identified by the use of multi-stable isotope approaches combining conventional hydrochemical data with stable water isotopes, such as δ¹⁸O and δ²H of H₂O, δ¹⁵N and δ¹⁸O of NO₃⁻, δ¹³C, δ³⁴S and δ¹⁸O of SO₄²⁻ (Kendall et al., 2007, Clark, 2015; Sanci et al., 2016; Ehleringer et al., 2016; Grimmeisen et al., 2017, among others). Nitrate isotopes have been widely applied to separate different pollution sources in urban areas. For example, Grimmeisen et al. (2017) used dual water isotope end member mixing calculations to identify that city effluents from leaky networks and sewage systems contribute 30–64% to groundwaters, which have become polluted. Adebowale et al. (2019) describe contaminant sources and denitrification processes using isotopes of nitrate near a wastewater treatment plant in peri-urban settings. By using nitrate isotopes, Zendehbad et al. (2019) showed that sewage is the primary source of nitrate contamination in an urban groundwater system.

Regarding nitrate source tracing, isotope mass-balance mixing models (e.g. SIAR, MixSIAR) based on δ¹⁵N–NO₃ and δ¹⁸O–NO₃ approach have been used to determine the relative contribution of NO₃ sources in freshwater systems (Phillips and Koch, 2002; Deutsch et al., 2006; Torres-Martínez et al., 2020). For example, Xue et al. (2010) implemented a Bayesian isotope mixing model (SIAR) to determine the proportional contribution of nitrate source to six different surface waters in Belgium affected by agriculture, greenhouses and households. Matiatos (2016) showed that the relative contribution of nitrate in groundwater, originating from sewage-related organic matter, is higher in urban areas than in agricultural. By applying MixSIAR model, Romanelli et al. (2020) found that nitrified ammonium from soil and fertilizers is the most significant contributor to the dissolved nitrate in three groundwater-dependent shallow lakes in Argentina. Luu et al. (2020) applied a δ¹⁵N–NO₃ - δ¹⁸O–NO₃ mass-balance mixing model in a tropical river system to demonstrate that NO₃ derived from nitrified urea sources is higher during wet fertilization seasons.

The Argentina Pampean Plain hosts cities of various sizes, which use surface or groundwater resources for domestic and other related supplies. In the case of Rio Cuarto city, which is located in the South of the province of Córdoba (Argentina), the domestic supply is fully covered by groundwater abstracted from an unconfined aquifer system susceptible to contamination. In this framework and accordingly to the expected problems the objective of this study was to investigate groundwater dynamics and nitrate contamination in this urban environment and its surroundings, by combining stable isotopes with conventional chemical techniques and modeling approaches (water mixing models). The study attempts to highlight the need for better identification of nitrogen pollution sources and processes in groundwater systems in urban areas that can lead to preserve water quality through their protection and the implementation of targeted remediation strategies (Fig. 1).

The study area lies in an urban and peri-urban setting of sub-humide climate type. The distribution of precipitation exhibits a seasonality, with 74% of precipitation allocated from November to March (spring-summer), with an average annual precipitation of 799 mm. This area is characterized by the existence of on-site sanitation systems in some peripheral sectors, animal breeding facilities, and crops fertilized mostly with urea. In Rio Cuarto 100,000 inhabitants are supplied (300 L/inhab/day) exclusively with groundwater through an infiltration gallery that was built in 1931 and used to collect water from the unconfined aquifer (500 m³ h⁻¹).

The gallery is located at the margins of the Cuarto River, 15 km NW of the city of Rio Cuarto. From this gallery, the groundwater is transported by gravity through an aqueduct towards a storage tank to be further distributed. As a result of population increase, the first wells were made in 1965 in the fluvial hydrogeological environment in the middle of the city. Numerous wells in the same fluvial environment have been constructed since then, including those built next to the gallery to enhance its performance given its partial obsolescence. To date, the EMOS (Municipal Sanitary Company) is responsible for more than 30 wells used for domestic supply. The well batteries were named WB1 (gallery and adjacent perforations), WB2 (North margin of the river) and WB3 (South margin of the river). At present, and for the current population of 170,000 inhabitants, the EMOS aims to undertake new facilities to collect groundwater in the same territory where the gallery is located. To fulfill those tasks different hydrogeological aspects related to water quantity and quality need to be addressed.

The sedimentary profiles in this area allow us to interpret climatic variations corresponding to the Quaternary period (Iriondo, 1999). The humid cycles are related to interglacial periods, when fluvial sediments of Rio Cuarto River were deposited (Blarasin et al., 2014; Carignano et al., 2014). The dry cycles coincide with cold glacial periods characterized by the deposition of loess sediments that cover part of the study area. Under the current humid climate conditions (1100 BC), the fluvial belt of the Rio Cuarto River was developed. Based in these climatic and geological changes, the geomorphological and lithological characteristics of the area allowed defining three large environments: Aeolian, fluvial and transitional fluvio-Aeolian one. The first one is composed of very fine silty sands dunes mainly of Upper Holocene age (“Laguna Oscura” Formation). The second environment was derived from the Rio Cuarto River activity during the Quaternary period, and is characterized by fluvial geomorphologic features associated with different hydrodynamic stages (different levels of terraces, meandering paleochannels of different size, meander migrations, channel bars, spills, among others). The transitional environment is composed of Aeolian formations superimposed on the fluvial deposits.

The unconfined aquifer system under study is hosted in Quaternary sediments and has a thickness of approximately 80 m. The depth of the water table varies between 1 and 20 m. Existing borehole profiles showed significant matrix heterogeneity (Blarasin et al., 2014, 2019, 2019). Thus, the aquifer is hosted in a sedimentary complex with variable hydraulic conductivity (K) between the fluvial (sands and gravels, K = 50 m d⁻¹) and Aeolian (very fine sands, K = 1 m d⁻¹) being the fluvio-Aeolian a transitional environment, with intermediate K values (Blarasin et al., 2014, 2019, 2019). The differences in the hydraulic properties of different sectors is also due to variable carbonate cementation among the sediments’ matrix, especially in the aeolian deposits. Fluvial sediments are dominated by quartz grains (potassium feldspar and micas subordinated). Aeolian regional sediments are composed of volcanic glass and minerals, such as Fe-oxides, pyroxenes, amphiboles, feldspars and illites (Matteoda, 2014).

According to Blarasin et al. (2019) and Giacobone et al. (2018), the annual unconfined aquifer recharge from precipitation is in the order of 10–15% in the aeolian sediments, and almost 30% in the fluvial materials, which is consider enough to replenish regular or fluctuating aquifer reserves. Tritium data from the same unconfined piedmont aquifer in neighboring-to-the-study-area basins exhibited tritium levels of 5–9 TU in 2011, indicative of water of modern age (Giuliano Albo, 2013).

The base of the unconfined aquifer consists of a clayed layer of up to 50 m of thickness. Below the impermeable layer, the confined aquifer system (CAS), specifically CAS A1, is present (Blarasin et al., 2014), with the most permeable layers located between 120 and 160 m of depth. The groundwater of the CAS system shows low salinity (EC ranges between 300 and 823 μS cm⁻¹) and is of sodium-bicarbonate water type. The low tritium values (1, 5 ± 0.2 to 2.5 ± 0.2 TU) recorded in the CAS A1 aquifer is because of a mixture of modern with old groundwater (Cabrera et al., 2017; Maldonado et al., 2018).