Universidade Federal de Santa Maria

Ci. e Nat., Santa Maria, v. 41, e54, 2019.

DOI: http://dx.doi.org/10.5902/2179460X33188

Received: 20/06/2019 Accepted: 26/10/2019

Section Environment

Monitoring of nitrate contamination in groundwater: Case study of the Campus of UNESP, Rio Claro/SP

Elias Hideo Teramoto^I

 Pedro Paulo Bazilio da Costa^II

 Roger Dias Gonçalves^III

 Bruno Zanon Engelbrecht^IV

 Hung Kiang Chang^V

^I   Centro de Estudos Ambientais, Laboratório de Estudos de Bacias – UNESP, Campus de Rio Claro/SP, Rio Claro/SP, Brasil - teramoto@rc.unesp.br

^II  Laboratório de Estudos de Bacias – UNESP, Campus de Rio Claro/SP, Rio Claro/SP, Brasil - pedrop.bc@hotmail.com

^III Centro de Estudos Ambientais, Laboratório de Estudos de Bacias – UNESP, Campus de Rio Claro/SP, Rio Claro/SP, Brasil - rdgon@hotmail.com

^IV Centro de Estudos Ambientais, Laboratório de Estudos de Bacias – UNESP, Campus de Rio Claro/SP, Rio Claro/SP, Brasil - brunoz@rc.unesp.br

^V Departamento de Geologia Aplicada, Centro de Estudos Ambientais, Laboratório de Estudos de Bacias – UNESP, Campus de Rio Claro/SP, Rio Claro/SP, Brasil - chang@rc.unesp.br

Abstract

This study presents the results of the monitoring of nitrate concentrations in shallow groundwater at the UNESP Campus of Rio Claro/SP assumed to be sourced by septic tank leakage, which were discontinued in October 2014. The distribution of nitrate concentrations provides support to a conceptual model of contamination by multiple sources, since the concentration gradients are not observed along the flowpaths. The results of the monitoring indicate that in some monitored wells, the nitrate concentrations remain stable, while in other wells minor to strong fall trends were observed. These results provide support to the presence of other active sources, such as sewage leakage in the external and internal area of the campus. This scenario perfect fit with the maintenance of recorded high nitrate concentrations over the time. Despite the nitrate concentrations are below potability limit, additional investigations will be conducted to identify sources of contamination to ensure water quality in the future.

Keywords: Nitrate; Groundwater contamination; Hydrochemistry monitoring; Rio Claro aquifer

1 Introduction

The risk to human health from the consumption of water with nitrate occurs due to its transformation into nitrite in the digestive system, since this chemical species can oxidize the iron in hemoglobin of the blood and form methemoglobin with limited capacity of hemoglobin to oxygen transport (WEITZBERG & LUNDBERG, 2013). The deficiency of oxygen transport   creates the condition of methemoglobinemia, commonly known as "blue baby syndrome", which blood lose the ability to carry enough oxygen into the body's cells, causing the veins and skin to turn bluish. Due to potential risk to human health, the standard acceptable level for human consumption NO₃-N concentration to drinking water standard has been stablished at a maximum concentration in the 10 mg/L NO₃-N in the Brasil (Conama Resolution 396/2008).

Nitrate is the most widespread contaminant of shallow groundwater and represents a problem of increasing concern due to increasing demand of water. The contamination by nitrate have been reported in uncountable sites around the world over the last four decades, as described in Kreitler & Jones (1975), Power & Schepers (1989), Hudak (2000), Hu et al. (2005) and Vystavna et al. (2017). In Brazil, there are many study cases describing nitrate contamination in shallow groundwater used for public and private water supply, such as Castro et al. (1992), Melo et al. (1998) and Montanheiro & Chang (2016).

The leakage of sewage collection system in urban areas (JEONG, 2001; REYNOLDS & BARRET, 2003; BISHOP et al., 2008) and the use of fertilizers in the crop cultivations areas (POWERS & SCHEPERS, 1989; BIJAY-SING et al., 1995; WANG et al., 2016) represent the main sources of nitrate contamination in groundwater. In regions with absence of sanitation, the large concentration of septic tanks where the domestic sewage is disposed represent a non-point source contamination and potentially may compromise the water quality of large areas of shallow aquifers, as demonstrated by Yates (1985), Arnade (1999) and Seiler (2005).

In an oxidizing environment, the nitrogen that makes up the organic matter (N_org) of the domestic effluent is hydrolyzed to the ammonium ion (NH₄⁺) by the ammonification reaction (THAMDRUP, 2012). During the advective movement of contaminated groundwater, ammonium is oxidized to nitrite (Equation 1) by bacteria of the genus Nitrosomas and, later, nitrite is oxidized to nitrate (Equation 2) by bacteria of the genus Nitrobacter (THAMDRUP, 2012).  

                                     (1)

                                           (2)

Large portion of the Rio Claro aquifer is located in the urban area of the municipality of Rio Claro (SP), which makes it strongly susceptible to contamination by sewage leaking. The water that supplies UNESP's Campus in the city is mostly from the Rio Claro Aquifer, and supplemented by water from Itararé Aquifer. The septic tanks, possible sources of nitrate contamination, were deactivated in October 2014, after continuous monitoring of nitrate concentrations was implemented in the area. In order to improve the current understanding concerning the behavior of nitrate in shallow aquifers, this work aimed to present the results of the continuous and systematic monitoring of nitrate concentrations in groundwater.

2 Material and Methods

2.1 Study site

The study area is located in UNESP Campus of Rio Claro (Figure 1), where several hydrogeological studies (e.g., OLIVA et al., 2005; OLIVA, 2006, NETO et al., 2016; GONÇALVES & CHANG, 2018; GONÇALVES et al., 2019) allowed the characterization of the hydraulic properties of the Rio Claro Aquifer and the geologic framework. The Rio Claro Aquifer in the studied area is mainly composed by sandy sediments with high hydraulic conductivity values (typically above 2 x 10^-5 m/s), as demonstrated by OLIVA (2006) and Gonçalves & Chang (2018). However, in some portions of the aquifer there are relatively thick clayey lenses, as indicated by drilled borehole. The thickness of the Rio Claro Aquifer varies within 14.4 m and 21 m and the groundwater flow has direction NE-SW with average hydraulic gradient of 0.0075.

Figure 1 - a) Study area showing the distribution of the monitoring wells, supply wells and septic tanks deactivated, as well as the potentiometric map.

2.2 Groundwater use history

The demand for water supply at UNESP, Campus in Rio Claro, was provided by a single well, installed in 1997, with a depth of over 500 m and extracting water from the Itararé Aquifer System. The increase in demand, accompanied by the continuous drop in the flow rate of this well, led to the installation of new supply wells in shallow Rio Claro Aquifer. Between the years of 2002 and 2006, nine monitoring wells were installed in UNESP Campus for academic purposes (PM-01 to PM-09). The studies conducted from these wells, presented in the work of Oliva (2006), demonstrated that the Rio Claro Aquifer holds excellent potential for water supply. The chemical analyzes of groundwater of the wells installed in the Campus indicated, at that moment, excellent water quality for consumption purposes. Moreover, pumping tests in 4" monitoring wells provided flow rates above 10 m³/h, adequate to supply the Campus water demand. In order to supply the water demand of the campus, three of the 4 "monitoring wells (PM-05, PM-08 and PM-09) were equipped with pumps for groundwater extraction since 2004.

Sporadic analyzes were conducted prior to the year 2010 in these wells not detecting nitrate concentrations above 1 mg/L. However, measurements of the electrical conductivity of water, since 2010, showed continuous growth in well PM-08, starting from an initial value of 12 µS/cm and reaching 168 µS/cm. In order to investigate the source of the electrical conductivity increase sampling was carried out in four monitoring wells and in the three supply wells on the campus in July 2014. The results verified that the operating wells had high concentrations of nitrate, particularly PM-08, with concentrations of 11.02 mg/L of N-NO₃, above the potability limit of 10 mg/L of N -NO₃ (nearly 44 mg/L of NO₃^-). Because the elevated concentrations identified in well supply PM-08, its operation was immediately discontinuous.

The UNESP campus was implemented in the 1970s, when the surrounding area was sparsely urbanized and the sanitation network had not been implanted, causing all sewage to be destined for septic tanks distributed on the campus. Although they had impermeable walls and floors and the effluent was periodically removed, cracks or ruptures in the walls of these tanks led to the effluent percolation in the unsaturated zone. The percolating effluent eventually reaches the saturated zone, contaminating the groundwater. The septic tanks, supposed to be the contamination source recognized in June 2014, were cleaned and buried in September 2014 (Figure 2) and the sewage collection system was implemented across the campus.

Figure 2 - Septic tanks L1 and M2, during the deactivation

In order to evaluate possible decrease of the trend in the nitrate concentration after the removal of the supposed contamination sources, periodic measurements of nitrate concentration were carried out. Additionally, investigations were carried out to identify other possible sources of contamination, with the installation of additional monitoring wells (PM-10 to PM-25), in which two were multilevel PM-10 and PM-11.

2.3 Water quality monitoring

            To monitor the evolution of groundwater quality in the monitoring and supply wells in the study area, periodic sampling campaigns were carried out. Groundwater sampling was performed under low flow or using disposable bailers. In the case of low flow sampling, the samples were collected immediately after the parameters (i.e. electrical conductivity, pH, temperature, dissolved oxygen and oxide-reduction potential) were shown to be stable. Sampling in the supply wells was done by collecting water from existing taps near these wells.

Aliquots of collected groundwater samples intended for major cations and anions analysis were filtered with 0.45 μm membrane in the field, in agreement with the requirements and norms to determine the parameters to be analyzed. Once collected, the samples were immediately sent to the LEBAC / RAIH laboratory, UNESP, for analysis. The alkalinity was determined in the laboratory by potentiometric titration method. The anions were determined by ion chromatography (IC), while the cations were determined by optical emission of inductive argon plasma (ICP-OES) spectrometry. All the methodologies used followed the procedures described in the technical reference APHA (2005).

3. Results

3.1 Chemical analysis of the septic tank effluents

            In order to investigate the chemical composition of the effluent contained in the septic tanks of the campus, particularly the concentrations of nitrogenous compounds, in September 2014 samples of two different septic tanks were collected, prior to their deactivation. The results of the physico-chemical parameters are presented in Table 1; the major anions and cations are shown in Table 2.

Table 1 - Physical-chemical parameters of effluent collected from two septic tanks

Septic tank	Physical-chemical parameters
	CE (mS/cm)	pH	Eh (mV)	OD (mg/L)
1	1280	8.17	-286.1	0.6
2	2640	0.63	316	6.2

Table 2 - Concentration of cations and larger anions determined in samples from two septic tanks

Septic tank	Ca^2+(mg/L)	Na^+ (mg/L)	K^+(mg/L)	Mg^2+(mg/L)	NO3^-(mg/L)	NO2^-(mg/L)	NH3^+(mg/L)	SO4^2-(mg/L)	F^-(mg/L)	Cl^-(mg/L)	HCO3^-(mg/L)
1	13.1	46.3	26.2	2.32	<5	<5	104	19.3	<0.5	86.2	444
2	19.2	102	50.2	4.47	<5	<5	170	47.4	<0.5	168	882

3.2 Groundwater analysis

The sampling campaign of October 2014 represents the highest recorded nitrate concentration in the study area (Figure 3). The distribution of ammonium (Figure 3a), nitrite (Figure 3b) and nitrate concentration (Figure 3c) support to the assumption of a multi-source contamination conceptual model, since they do not indicate a concentration gradient along the flow lines.

Figure 3 – Distribution of nitrogen species in October 2015: a) ammonium; b) nitrite; c) nitrate

Due to the use of sodium chloride, the Na⁺ and Cl^- are higher in the domestic effluent, as shown in Table 2, and the contamination of sewers is also characterized by high concentrations of these chemical species. Figure 4 illustrates the isovalues of Cl^- concentration, which show a good agreement with nitrate distribution in the studied site. Contrary to Na⁺, Cl^- is not subject to sorption by clay minerals of aquifer and represents an excellent indicator of sewage contamination.

Figura 4 – Cl^- plume as in October 2014

Figures 5 and 6 illustrate respectively the time-series of nitrate concentration in the supply wells and monitoring wells.

Figure 5 - Variations of nitrate concentrations in three supply wells at UNESP's Campus

Figure 6 - Nitrate concentrations in three monitoring wells installed in UNESP Campus

4 Discussion

Shallow aquifers represent an important available source of potable water for public and private water supply. However, such aquifers are highly susceptible to contamination from anthropogenic activities, particularly nitrate, the main groundwater contaminant. The monitoring of its quality and the implementation of preventive actions to prevent contamination are essential measures to assure its potability and use in the future.

The dissimilatory reduction represents the main natural mechanism for attenuation of nitrate contamination in groundwater. In this reaction, NO₃^- is reduced to N₂ by the enzymatic action of denitrifying bacteria, when oxygen is absent and reduced chemical species are present in the water. In oxidizing environments, however, nitrate is chemically stable and denitrification is not very expressive. In addition to its chemical stability, nitrate is not subject to sorption and its solubility is high, allowing plumes of this contaminant to have large lengths and remain in the groundwater for unlimited periods.

The effluent samples from the wells show different compositions (Table 2), reflecting different nature of use. The domestic sewage contamination in the studied site can be diagnosticated by the presence of high concentrations of Cl^- (Figure 5), SO₄^2-, F^- and Na⁺. Since the composition of the septic tank effluents are highly variable, it is expected that the contaminations generated by the different tank leakage have distinct impacts on the chemical composition of the contaminated aquifer.

Most likely, the well PM-07 (Figure 6), with nitrate concentrations typically below 1 mg/L and electrical conductivity below 15 µS/cm, can be considered as representative of nitrate background level. The work of Stradioto & Chang (2010) supports this finding, since the chemical monitoring of rainwater presented by these authors indicates that the nitrate concentration in the rainwater collected at the UNESP Campus of Rio Claro varies between 0.012 and 3.25 mg/L, with an average value of 0.88 mg/L. Hence, nitrate concentrations below 3.25 mg/L can be natural and come from the water that enters by recharging the aquifer.

The UNESP Campus of Rio Claro is located in a recharge area of the Rio Claro Aquifer, along an important groundwater flow divide and the scenario of contamination by upgradient source should be excluded. The conceptual model of contamination previously elaborated predicted that the nitrate contamination originated from non-point source of contamination, represented by the diverse septic tanks present in the campus. However, the distribution of nitrate values (Figure 3c) indicates that the highest concentrations are observed in the supply wells. The the distribution of nitrate concentrations, as illustrated in Figure 3c, does not provide evidences for the high concentrations downstream of the supply wells, hampering the identification of leakage sources.

The supply well PM-08, at the beginning of water extraction, had values of electrical conductivity lower than 15 µS /cm and nitrate concentrations below 1 mg/L. The pumping rate may have created a capture zone that promoted changes in the local flow directions, favoring the advective movement of existing nitrate plumes towards the supply wells. This scenario may explain the gradual increase in the values of electrical conductivity, which reached 168 µS/cm in June 2014. This behavior suggests that the contaminant distribution in the aquifer may be driven by the regime of pumping of the supply wells.

The comparison of the multilevel wells with 1.0 m filter sections indicates that the higher concentrations are observed in the wells with deeper screen sections. Most likely, the high groundwater recharge rates  determined by Neto et al. (2016) promotes strong dilution of the nitrate concentrations in the upper portion of the saturated zone and represents the main mechanism acting upon its attenuation.

The PM-08 well is 19 m deep, which the pump is positioned in the deepest portions of the well and where the highest concentration of nitrate was recorded. In March 2016 a test was carried out after pumping the sampling wells for three to seven minutes before sampling. The first sample provided a nitrate concentration of 1.78 mg/L, whereas the second 42.3 mg / L. Since the pump is positioned close to the bottom of the well, it is expected that at early stage of pumping the produced water will be from the deeper portions of the aquifer and free of contaminant. In the later stage of pumping, however, the contribution of contaminated water from the upper portion of the aquifer is increased and the nitrate concentration strongly increases. Since the screen section of the supply wells is longer than 10 m, the concentrations of nitrate probably is higher in the uppermost portion of saturated zone.

5 Conclusions

            The time-series of nitrate concentration on the UNESP campus in Rio Claro provides support for a conceptual model with multiple sources of contamination. It can be concluded from the monitoring of nitrate concentrations that concentrations remain stable in some wells and show a downward trend in others. These finding suggests that septic tanks are not the only sources of groundwater contamination and that there are other active sources of contamination. Most likely, the contamination from leaks from the sewage collection system inside the campus contribute to the nitrate contamination.

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