Treated Wastewater Affects the Fertility and Geochemistry of Degraded Soil in the Brazilian Semi-Arid Region

Abstract

Projections for the Brazilian semi-arid (BSA) region estimate a reduction in water bodies and an increase in degraded areas. Recovering degraded soils using treated wastewater (TWW) is a strategy to increase the resilience of the local population to these climatic adversities. This study aimed to evaluate the impact of deficit irrigation with treated effluent on the (geo)chemistry of degraded soil in the BSA. An experiment with the application of TWW was conducted on soil degraded within an agroforestry system. The treatments arranged in randomized block design were WS_0.5 (water supply at 0.5 L/plant/week), TE_0.5 (treated effluent at 0.5 L/plant/week), and TE₁ (treated effluent at 1 L/plant/week). Soil samples were collected (0–15 and 15–30 cm) at the initial condition, after two years of irrigation, and two years after the end of irrigation. Analyses of chemicals and geochemicals were carried out. All treatments increased soil fertility after two years in both layers, with TE₁ resulting in higher Ca²⁺ (0–15 cm: 2.88; 15–30; 3.14; cmolc kg⁻¹), Mg²⁺ (0–15 cm: 2.13; 15–30; 2.00; cmolc kg⁻¹), and K⁺ (0–15 cm: 0.11; 15–30; 0.12; cmolc kg⁻¹), generating a residual effect two years post-irrigation suspension and no risk of salinization. However, TE₁ and mainly TE_0.5 showed an increase in sodium content, making the soil solodic (6–11%). The application of TWW changed the CaO, MgO, and K₂O contents of silt fraction, contributing to the availability of Ca, Mg, and K in soils. Future studies should monitor sodium levels and confirm K-bearing phyllosilicate (illitization) after irrigation with TWW. The application of TWW for a short period (two years) and in small volumes (0.5 L/plant/week) affects (geo)chemistry of degraded soil from the BSA.

1. Introduction

Marginal areas with degraded soil are increasing worldwide and are gaining attention for their impact on food security and environmental quality, especially in semi-arid regions of developing countries. Dryland degradation results from several factors [1]; climate variability, vegetation suppression, and long-term land use are important factors in degradation or desertification in the Brazilian semi-arid region (BSA) [2,3]. Projections for the BSA suggest a reduction in Caatinga dry forest, an increase in temperature, prolonged periods of drought, and a reduction in water bodies [4,5]. Such projections corroborate the recent identification of arid areas in the Northeast region of Brazil and the increase in degraded areas in the BSA [6].

Given this adverse scenario, there is an urgent need to establish actions that increase the resilience and adaptability of the populations of Brazilian semi-arid regions in relation to these climate extremes, which involves storing and using water to maintain small-scale agriculture. Thus, treated wastewater (TWW) emerges as an alternative to the depletion of water resources in semi-arid regions, after it undergoes primary (physical processes for separation of solids), secondary (biological processes), and tertiary (e.g., uptake organic and inorganic compounds) treatments to enhance its quality and reduce human risks [7]. The use of TWW in areas with low water supply constitutes a promising strategy as an alternative source of water to increase agricultural productivity, given its recognized capacity to improve nutrients [8,9,10], with direct environmental (i.e., soil salinization and water pollution), health (i.e., exposure to heavy metals and pathogens), and economic (improved crop production) impacts [11]. The application of the untreated sewage effluent in irrigation can also represent an alternative to preserve available good-quality water [12,13] and to recover degraded soils [14], which can be reflected in increased agricultural productivity in degraded soils. The TWW can also contribute to recycling organic matter [15,16] and to improving soil fertility and productivity [17,18], which is especially attractive for family farmers as it replaces or reduces the need for mineral fertilizers [19]. Additionally, it conserves available water and minimizes sewage discharges into water bodies, supporting environmental conservation [20].

Despite its benefits, TWW can increase the concentrations of salts, potentially toxic elements, and toxic organic compounds [21,22], resulting in environmental and human health [7,23,24] impacts. The effects of applying TWW to agriculture depend on various factors such as the water source, its level of treatment, the volume applied, climatic conditions, and soil properties [25,26].

Although irrigation with TWW constitutes an alternative source of water and nutrients for plant growth, there is still little information about its effects on the physical, chemical, and biological attributes of soils in the BSA. Studies in one such region showed that domestic sewage effluent increased the production of prickly pear cactus and improved its growth and forage production [27]. These authors also pointed out that the increase in carbon stock and N availability suggest that TWW could reduce CO₂ emissions and serve as an alternative nitrogen fertilization strategy. Irrigation with TWW significantly increased soil organic matter levels and nutrient adsorption capacity [28]. No tendency for soil salinity was observed after a 10-year period of irrigated cultivation with TWW [29], and its application actually reduced soil salinity of the naturally salt-rich soils [20]. Increase in P, Ca, K, and organic matter contents in soils irrigated with TWW and cultivated with beans was also reported [30]. However, a decrease in soil total nitrogen and an increased sodicity and salinity of BSB soils has also been reported [20,29]. These contrasting results are mainly due to the quality of the water used, the pedological diversity, and the great variation in rainfall in the BSB.

Several studies have shown the effect of TWW on soil attributes [8,10,16,17,18,19]; however, there are few studies that have shown its effects on soil geochemistry and mineralogy [19]. According to [31], applying potassium-rich wastewater to 2:1 clay soil can lead to mineralogical changes, such as illitization, which can occur quickly. On the other hand, some studies have found no significant effects on soil mineralogy composition due to long-term irrigation with treated wastewater [32,33].

Numerous studies have shown the effects of TWW long-term reuse on soil properties [19,34]. However, no studies have yet documented the effect of TWW on the geochemistry of soils in the semi-arid region of Brazil. Therefore, it is important to generate this information to support decision-makers in formulating public policies on the use of treated effluent. Our result can contribute to future studies aimed at developing regulations with specifications and guarantees of adequate levels of nutrients and tolerable levels of elements in soils, which can be monitored based on soil (geo)chemical indicators.

Given the recent concern about the effects of climate change on the reduction in water bodies and an increase in degraded areas in the BSA, the National Semi-Arid Institute (INSA) has conducted an experiment to evaluate the efficiency of TWW in recovering degraded soils. The experiments consist of forage palm (Opuntia stricta) intercropped with native species from the Caatinga (agroforestry system) irrigated with different amount of TWW. The water used comes from sewage generated at INSA, which undergoes primary and secondary treatment in the institute’s treatment station (filtering tanks and septic tanks for sedimentation). Our hypothesis is that even with a deficit application of treated sewage effluent, the contribution of nutrients and organic matter will improve fertility and promote geochemical changes in degraded soil. This research aimed to evaluate the effect of deficit irrigation with treated effluent applied for two consecutive years and then suspended for two years on the chemistry and geochemistry of degraded soil in the semi-arid region.

2. Materials and Methods

2.1. Description of the Study Area

The study was conducted in the experimental area of the Instituto Nacional do Semiárido (INSA) in Campina Grande, State of Paraíba, Brazil (7°15′11″ S to 7°15′13″ S and 35°56′49″ W to 35°56′51″ W; 556 m) (Figure 1). The region’s climate is classified as As-type by the Köppen classification, characterized as hot and humid with irregular rainfall and a long dry season [35]. The air temperature ranges between an annual maximum of 28.6 °C and a minimum of 19.5 °C, with the relative humidity averaging around 80%.

2.2. Soil Characterization

The geology belongs to the São Caetano complex, consisting of gneiss, metagraywacke, felsic, and intermediate metavolcanic rocks [36], and composed of muscovite-biotite gneiss, garnetiferous, biotite gneiss, and muscovite schist, including crystalline limestone, quartzite, and metavolcaniclastic rocks [37]. The gently undulating local relief contributes to the predominance of Dystric Gleiyc Planosol (Loamic, Ochric) [38,39,40], which presents a sequence of horizons A₁-A₂-Btg₁-Btg₂-Cr₁-Cr₂. In the experiment area, the surface and diagnostic horizons of the soil (Btg) were removed by civil construction, undergoing a truncation process that exposed the subsurface horizons (Cr₁ and Cr₂; saprolite). The main characteristics of the soil are stoniness and shallowness. The particle size analysis was conducted according to Teixeira et al. (2017) [41], and the texture was classified as sandy loam (sand = 716 g kg⁻¹; silt = 150 g kg⁻¹; clay = 134 g kg⁻¹).

2.3. Experimental Description

The experimental area and all field materials, reagents and equipment for analytical determinations were provided by INSA (Campina Grande, Paraíba, Brazil). The experiment was conducted on an area of 780 m² in a randomized block design with three irrigation treatments and 10 replications. The treatments were as follows: WS_0.5—water supply at 0.5 L/plant/week, representing 10% of the reference evapotranspiration (ET0), TE_0.5 (treated effluent at 0.5 L/plant/week, representing 10% of the reference evapotranspiration), and TE₁ (treated effluent at 1 L/plant/week, representing 20% of the reference evapotranspiration). Water from the INSA supply system was used, derived from rainwater harvesting and reused water from sewage generated at the INSA administrative headquarters, which includes toilets and pantries (Table 1). TWW chemical parameters were performed periodically in accordance with Apha (2012) [42].

The sewage underwent primary and secondary treatment at the INSA effluent treatment plant, consisting of a sequence of filtering tanks followed by a septic tank for sedimentation (Figure 2), and was then pumped using a self-compensating drip irrigation system. The nutrient input to the soil via irrigation treatments was estimated based on the chemical characteristics of the two types of water used in the experiment.

In the study area, cladodes of forage palm (Opuntia stricta) cultivar “Orelha de elefante mexicana” were planted in a double row at a spacing of 0.5 × 0.5 × 1.5 m, resulting in a density of 20,000 plants per hectare, with cuts made annually. The palm was grown intercropped with seedlings of two native Caatinga species with timber potential, namely Sabiá (Mimosa caesalpinifolia) and Aroeira Branca (Astronium urundeuva Allemão), arranged alternately at 2 × 2 m spacing between the palm rows, forming an agroforestry system. The blocks are 45 m² and consist of three double rows of forage palm and three rows of trees, each corresponding to an irrigation treatment. The irrigation treatments were applied between June 2013 and June 2015, with a total irrigation depth of 52.8 mm for WS_0.5 and TE_0.5 and 76 mm for TE₁, with a total rainfall of 748 mm (Figure 3). The treatments were suspended in the following two years (June 2015 to July 2017), with a total rainfall of 934 mm (Figure 3).

2.4. Soil Sampling and Analysis

Soil samples were collected at baseline (before the start of the experiment), at the end of two years of irrigation (June 2015), and at the end of two years after the irrigation treatments had stopped (June 2017). A total of 60 soil samples were collected, 30 at the 0–15 cm soil layers and 30 at the 15–30 cm soil layer. These depths coincide with the thickness of the Cr₁ and Cr₂ horizons that were exhumed by the removal of the surface horizons. This approach aimed to support other studies that evaluate the ecological functions and ecosystem services of these soils, with an emphasis on the pedogenetic processes. The soils were sampled according to Santos et al. (2015) [43]. The samples were collected just below the drippers in the double rows of forage palm, always after the forage palm had been harvested. All soil samples were air-dried, sieved through a 2 mm mesh, and analyzed for chemical attributes according to the standards in Teixeira et al. (2017) [41]. The soil pH was determined in water at a ratio of 1:2.5 (solid:liquid). The electrical conductivity (EC) of the soil was measured in saturated paste extract. Ca²⁺ and Mg²⁺ were extracted with potassium chloride solution (1M KCl) and determined by an atomic absorption spectrophotometer (AAS); P, K⁺, and Na⁺ were extracted with Mehlich-1 solution. Available P was determined by colorimetry, while exchangeable K⁺ and Na⁺ were determined by flame photometry. Al³⁺ and H⁺Al were determined from potassium chloride (KCl) extracts and extraction with calcium acetate (1 mol L⁻¹ at pH 7.0), respectively, and then determined by titration with NaOH (0.025 mol L⁻¹). Subsequently, exchangeable bases (EB), cation exchange capacity (CEC), base saturation (BS), aluminum saturation (AS), and the exchangeable sodium percentage (ESP) were calculated as recommended by Teixeira et al. (2017) [41]. Total carbon was determined according to the methodology based on Yeomans and Bremner (1988) [44].

2.5. X-Ray Fluorescence

Soil samples from the treatment with TWW at the volume of 1 L/plant/week (TE₁) were separated into their granulometric fractions at different periods (initial condition, after two years of application of treated effluent, and after two years of stopping irrigation) considering the 0–30 cm soil layer. Clay was separated using a pipette, sand by sieving, and silt by sedimentation [45]. The total contents of major elements in the sand, silt, and clay fractions were obtained using energy-dispersive X-ray fluorescence spectrometry (EDXRF). The samples were pressed with boric acid in a Vaneox Pressing Technology press. The elements were then determined as oxides in the S2 Ranger equipment with an X Flash silicon detector with Peltier cooling.

2.6. Statistical Analysis

The chemical attributes of the soil were subjected to analysis of variance (F-test), and the means were compared using the Tukey test for the irrigation treatment factors with repeated measures over time. The Dunnet test was used to compare the chemical attributes in the two periods (irrigated period and non-irrigated period) with the initial condition in the 0–15 and 15–30 cm layers. SISVAR 5.6 software [46] was used for all statistical analyses.

3. Results

3.1. Soil Chemical Attributes

The chemical attributes of the soil over the period studied are shown in Table 2. The soil reaction was initially moderately acidic at both layers (pH 0–15 cm: 6.07; pH 15–30 cm: 6.37). The Ca²⁺ content was 0.27 cmol_c kg⁻¹ (0–15 cm) and 0.28 cmol_c kg⁻¹ (15–30 cm), while the Mg²⁺ content was 0.23 cmol_c kg⁻¹ (0–15 cm) and 0.18 cmol_c kg⁻¹ (15–30 cm), and the Na⁺ content was 0.21 cmol_c kg⁻¹ (0–15 cm) and 0.16 cmol_c kg⁻¹ (15–30 cm). The levels of K⁺ (0–15 cm: 0.02; 15–30 cm: 0.01 cmol_c kg⁻¹) and Al³⁺ (0–15 and 15–30 cm: 0.1 cmol_c kg⁻¹) were similar. The P contents were 8.14 mg kg⁻¹ (0–15 cm) and 4.07 mg kg⁻¹ (15–30 cm). Thus, the soils had CEC values of 4.0 cmol_c kg⁻¹ (0–15 cm) and 3.5 cmol_c kg⁻¹ (15–30 cm), with both layers being dystrophic (0–15 cm: 19.05%; 15–30 cm: 18.60%). The carbon content was 7.6 g kg⁻¹ (0–15 cm) and 5.3 g kg⁻¹ (15–30 cm).

After two consecutive years of irrigation, the treatments showed no significant differences (p > 0.05) for the macronutrients at both layers (Table 2). However, base saturation in TE₁ (effluent treated at a volume of 1 L/plant/week) was higher than the other treatments at both layers. Except for pH, TOC, P, and Al³⁺, the other attributes differed significantly (p < 0.05) in the 0–15 cm layer after two years of irrigation (Table 2). During this period, there was an increase in the levels of Ca²⁺ (2.88 cmol_c kg⁻¹), Mg²⁺ (2.13 cmol_c kg⁻¹), K⁺ (0.11 cmol_c kg⁻¹), and Na⁺ (0.92 cmol_c kg⁻¹) in TE₁, which represents an increase of 10, 9, 5, and 4 times the initial levels in the surface layer, respectively. As a result, nutrient levels were considered good (Ca²⁺), very good (Mg²⁺), medium (K⁺), and very high (Na⁺), while base saturation (BS) went from very low to good (19.05% to 71%) [47].

At 15–30 cm, only Al³⁺ did not differ significantly from the initial condition in irrigation treatments. In the subsurface layer, there was a significant increase (p < 0.05) in TOC in the TE_0.5 treatment and P in WS_0.5 and TE₁ compared to the initial condition.

Two years after the irrigation treatments were stopped, there was a significant decrease (p < 0.05) in Ca²⁺ for all treatments, as well as in Na⁺ and fertility indexes (EB, CEC, ECEC, and BS) in the TE₁ treatment, and BS% in WS_0.5 compared to the end of the irrigated period. However, there was an increase in Ca²⁺ and K⁺ levels compared to the initial condition at 0–15 cm (Table 2). In the subsurface layer, there were differences for all the basic cations except for Mg²⁺ and Na⁺ in the TE_0.5 and WS_0.5 treatments, respectively. At both soil layers, there was a significant difference for all the fertility indexes (EB, CEC, ECEC, BS, and AS), which were higher compared to the initial condition.

Given the significant amounts of Ca, Mg, Na, K, and P in the effluent, these nutrient inputs were naturally higher than those from the water supply (Table 3). Considering the nutrient levels in the soil at the start and end of the experiment (Table 2), subtracting the contributions from the treated effluent (Table 3), in the TE₁ treatment, there was a deficit at 0–15 cm of 14.85, 6.53, 15.55, 20.51, and 10.46 kg of Ca, Mg, Na, K, and P, respectively, while at 15–30 cm the deficit was 15.83, 6.17, 75.57, 20.55, and 10.57 kg for Ca, Mg, Na, K, and P. This difference between the amount of nutrients in the soil and the amount contributed represents the amount that has left the system, mainly through crop extraction or leaching.

3.2. Electrical Conductivity and Na⁺ in the Exchange Complex

After two years of irrigation, there was no significant difference (p > 0.05) in the soil electrical conductivity (EC) at both layers evaluated (Figure 4A,C). However, there was a trend towards higher average EC values in TE₁, which, compared to the initial condition, was significantly (p < 0.01) higher in the surface layer, representing an increase of 67% (Figure 4A).

There were no significant differences in the salt levels two years after the irrigation suspension at both soil layers (Figure 4B,D) and a downward trend in EC was observed for all treatments compared to the end of the irrigated period. This decrease was significant in the surface layer only for TE₁ (−46%) and in the subsurface layer for all treatments (−53%, −40%, and −41% for WS_0.5, TE_0.5, and TE₁, respectively). There was also a trend towards average values below those found in the initial condition for all treatments at both soil layers at the end of two years after suspension of irrigation treatments, with a significant decrease (p < 0.01) for WS_0.5 in the 15–30 cm layer.

After two years of irrigation, the soil ESP showed no significant differences between the treatments in the layers evaluated (Figure 5A,C). Concerning the initial condition, there was a trend towards higher averages in the treatments with irrigation, with a significant difference (p < 0.05) only in TE₁ at 0–15 cm. There was no significant difference in ESP after the suspension of irrigation and at the end of the irrigated period (p > 0.05) in both soil layers evaluated (Figure 5B,D). However, there was a downward trend in the surface layer compared to the irrigated period, especially in the TE₁ treatment, which saw a 40% reduction. Except for the WS_0.5 treatment in the subsurface layer, in the TE_0.5 and TE₁ treatments, there was an average increase in ESP compared to the surface layer, with TE_0.5 obtaining a significant difference (p < 0.01) concerning the ESP of the initial condition. TE_0.5 reached an ESP value of 11%, indicating a solodic character (6–15%) [48].

3.3. Soil Geochemistry

The total contents of the major elements in the sand, silt, and clay fractions are shown in Table 4. SiO₂ predominates in all fractions, followed by Al₂O₃, Fe₂O₃, Cl⁻, MgO, K₂O, and CaO. The sand fraction showed the smallest changes in the major oxides in the periods with and without irrigation, while in the period with effluent application, there were significant geochemical changes in the silt fraction with an increase in the levels of K₂O, CaO, MgO, Cl⁻, and SO₃. Similarly, the increase in the K₂O, MgO, and CaO content in the silt fraction followed the trend of an increase in the content of these cations in the soil CEC. Finally, two years after irrigation was suspended, there was a decrease in K₂O, CaO, and MgO in the silt fraction.

4. Discussion

4.1. Effects of Treated Wastewater on Soil Chemical Attributes

Initially, the soil exhibited a moderate acid reaction [48], with values considered high from an agronomic perspective [47]. The contents of TOC, available P, and Mg²⁺ were low, while the contents of Ca²⁺ and K⁺ were very low, and the content of Na⁺ was considered high [47]. The significant increase in soil fertility after two years of irrigation can likely be attributed to the influence of the growth of forest species (Sabiá and Aroeira) and forage species (Palma Orelha de Elefante) in the area, contributing to nutrient recycling through leaf deposition and root development. According to Salton and Tomazi (2014) [49], the root system can significantly improve soil structure and organic matter accumulation. TWW irrigation increased CEC by the second year under secondary and tertiary treatment in potato and also enhanced soil fertility [18].

Comparing the period when irrigation was interrupted with the previous period under irrigation, the reduction in TOC levels and the sum of bases, especially Ca²⁺ levels, with a consequent reduction in CEC and base saturation, reflects the suspension of TWW input and the role of soil biota in decomposition of the SOM. These results indicate a tendency for nutrient levels and fertility to balance between treatments due to the suspension of irrigation inputs and nutrient cycling combined with consumption and export, mainly by the forage palm crop. This also confirms the need for organic inputs, including active crop roots, for maintaining organic matter content, as well as to enhance microbial activity to improve soil structure and protect organic matter.

On the other hand, the increase in exchangeable bases and fertility indices (EB, CEC, ECEC, and BS) compared to the initial condition shows that despite the decrease in nutrient levels due to the interruption of irrigation and crop extraction, there was a residual effect of the applications in the first years of irrigation. Thus, the application of treated effluent at 1 L/plant/week represents a viable alternative to improve the chemical quality of degraded soils in the BSA. Several studies have reported an increase in macronutrients and CEC after TWW irrigation [8,18,28]. The slight increase in P levels (0–15 cm) after the interruption of irrigation suggests that the regular decrease in water availability likely contributed to a reduction in the mineralization of organic P and plant absorption, leading to nutrient accumulation in the soil. Since TWW contains forms of phosphorus most readily accessed by plants can be [50], its application can be crucial for the supply of phosphorus, assuming importance in the BSA soil that presents a low content of phosphates in the parental material. Thus, the TWW irrigation can release phosphorus to plants in the medium and long term, reducing the need for high-cost chemical fertilizers in the BSA. Orthophosphate content was also higher in plots with TWW irrigation compared to the control [18]. Pinheiro Júnior et al. (2018) [51] found higher levels of available P in degraded soil in the Brazilian semi-arid region due to little or no vegetation, resulting in its accumulation in the soil. According to Ahmad et al. (2020) [52], microbial activity regulating mineralization levels affects available P in sandy soils in Kazakh semi-desert with wastewater application. Therefore, our results demonstrate that the application of TWW, even in small quantities, increases soil fertility and maintains higher nutrient concentrations than initial levels.

4.2. Soil Salinity and Sodicity

Our data confirmed the contribution of salts to the soil from irrigation with treated effluents, widely reported in the literature, and attributed to the salts dissolved in the wastewater [53,54]. However, despite the increase observed with effluent irrigation, the values found are well below the levels required for the soil to be saline (EC ≥ 4000 µS cm⁻¹) [48]. Thus, irrigation with effluent applied regularly for two consecutive years in small quantities did not cause soil salinization, which does not harm the production and yield of most crops of agronomic interest in the region. After two years of applying TWW to sandy loam soil in an arid region of Texas, Chaganti et al. (2020) [54] reported that EC did not exceed acceptable levels. The research by Oliveira et al. (2016) [20] on sandy soil in the Brazilian semi-arid region with short-term irrigation with treated effluent also showed no soil salinization. Similar results were also found in potato and corn growth irrigated with treated wastewater reuse [18].

After irrigation was stopped, there was no salinity in the soil, conforming to the action of the leaching process throughout the rainfall events that occurred during the non-irrigated period (934 mm between 2015 and 2017) (Figure 2). This hypothesis is supported by the fact that the sandy loam texture of these soils allows greater water infiltration due to their higher permeability, favoring the removal of soluble salts [55]. This behavior was also observed by Adrover et al. (2017) [53], with a decrease in salts in soils irrigated with wastewater during the rainy season without irrigation. Hussain et al. (2019) [14] state that irrigation with effluent causes a consistent increase in soil salinity unless it is leached by clean water, excessive irrigation, or rain, which corroborates our data.

Initially, the soils were classified as “non-sodic” (ESP < 6) [48]. After two years of application, the TE₁ treatment showed a 90% increase in ESP, making the soil solodic (ESP of 7–10%). Similar results were obtained by Jaoude et al. (2025) [18] and Ofori et al. (2021) [56]. Additionally, the tendency for ESP to decrease in the surface layer compared to the irrigated period, with an average increase in ESP in the subsurface layer compared to the initial condition, indicates probable leaching, with a decrease in contents in the surface layer and an accumulation of sodium in the deeper layer (solodic character) in the treatments with treated effluent. These values are below the 15% level accepted by the FAO (2000) [57] as the limit for severely negative impacts on the soil that would affect crop productivity with the dispersion and possible illuviation of clays and loss of hydraulic conductivity.

Despite not exceeding the limit value, this accumulation trend deserves attention as it may indicate sodification problems, especially with the decrease in electrical conductivity (Figure 3) and exchangeable cations (Table 2), which potentiates sodium’s harmful effects. This fact also shows that the application of TWW followed by a period of free drainage has led to the leaching of salts from the system (desalination) and may contribute to an increase in the concentration of Na⁺ ions in the cation exchange complex, although at levels not sufficient to characterize them as sodic (sodification). This confirms that applying effluent can trigger the pedogenetic process of solonization (sodification and desalination).

When the sodium content rises, the problem becomes more serious as sodium generally persists after soluble salts have been eliminated [58]. The accumulation of sodium in the soil can cause serious damage by promoting the dispersion and possible illuviation of clays, leading to a loss of hydraulic conductivity, sealing subsurface horizons, and resulting in increased surface runoff carrying soil, which can even lead to environmental damage with the eutrophication of water bodies [25,54,59,60]. Soils in semi-arid regions tend to have a higher content of salts such as Na⁺ due to their mineralogical nature, combined with low rainfall and high natural evapotranspiration, making this a critical aspect when evaluating management with TWW, which is commonly rich in Na⁺ [20]. Despite this, Bedbabis et al. (2014) [61] observed in sandy soil in an arid region of Tunisia that after four years of application of treated effluent, the sodium adsorption ratio (SAR) did not pose a risk and decreased due to leaching caused by rainfall in recent years.

4.3. Tretated Wastewater and the Geochemical Environment

The higher SiO₂ contents mainly reflect the occurrence of quartz in the sand fraction and kaolinite and 2:1 mineral in the clay, while the considerable Al₂O₃ contents followed by K₂O and CaO in the sand fraction and notably in the silt fraction indicate the occurrence of feldspars and plagioclase. The also considerable levels of Fe₂O₃ indicate the presence of biotites. This mineralogy corroborates mineralogical studies on Planosols, Leptosols, and Regosols in the region [38,62]. Biotite (flakes > 2 mm; light brown color) and plagioclase (perthitic lamellae of orthoclase exsolution) are the main primary minerals responsible for clay formation in Planosols of the BSA (clay formation in situ) [63]. The preferential alteration of these minerals favors the release of Ca²⁺, Mg²⁺, Fe²⁺_, K⁺, and Na⁺ [64,65].

The sand fraction showed the smallest changes in the major oxides in the periods with and without irrigation, while in the period with effluent application, there were significant geochemical changes in the silt fraction with an increase in the levels of K₂O, CaO, MgO, Cl⁻, and SO₃. Similarly, the increase in the K₂O, MgO, and CaO content in the silt fraction followed the trend of an increase in the content of these cations in the soil CEC. These results confirm that the silt-sized particles in the soils studied contribute considerably to the balance between the solid phase and the soil solution, likely related to the mineralogical assemblage still enriched in easily weathered minerals, which are reserves of these elements common to both the reuse water and the soil solution.

The reduction in the K₂O, CaO, and MgO contents of the silt fraction with the suspension of irrigation is due to the lower input of these nutrients via fertigation and because tree species, especially forage palm with a high demand for K⁺ and Ca²⁺ [66], continued to remove these elements from the soil solution. These reductions are evidenced by the decrease in the concentration of these nutrients in the soil CEC over the same period (Table 2). These losses imply a system rebalancing through the weathering of mineral sources of these nutrients, which will be released into the exchangeable phase. In addition, our experiment was conducted in a degraded area (saprolite) formed by gneiss weathering. Previous studies showed that the weathering evolution of gneiss in the BSA occurs in four stages, with a loss of Na, K, Ca, and Mg to saprolite layers due weathering of biotite and plagioclase, such as labradorite and albite, and a decrease in the amount of orthoclase and the consequent increase in K mainly at the upper saprolite [67]. Eventually, muscovite can weather if the K activity in the solution decreases below the stability line, also contributing to the release of K [68]. Thus, likely mineral alteration must also have contributed to the reduction in the silt fraction in the saprolite evaluated.

Previous studies have shown that the occurrence of illite in the clay fraction of semi-arid soils is due to neoformation from the weathering of plagioclase [69], illitization of orthoclase [63] or transformation from primary micas due to iron oxidation and loss of structural charge [68,70,71]. Neoformation from solubilization of easily weathered biotite has also been reported, especially in pedoevironments with low concentrations of available Al³⁺ [63]. Other studies also pointed out that TWW increased the smectites or illite (illitization) in soils [19,31,72]. Despite the soils mica, kaolinite, goethite, and 2:1 phyllosilicates [73], our results showed an increase in the Si:Al ratio after two years of no irrigation, indicating a more intense secondary mineral formation route. This bissialitization process is widely reported as one of the mechanisms of clay formation in the BSA [63,64,65,69]. The higher levels of Fe and Mg after irrigation also suggest that these silicate phases may be illite ((K,H₃O)(Al,Mg,Fe)₂(Si, Al)₄O₁₀[(OH)₂,(H₂O)]), since there is a gain in K, Mg, and Fe, probably during the illitization from plagioclase weathering [69]. However, our results do not allow us to confirm this hypothesis, since the mineralogical data are not conclusive [73], requiring more detailed studies, which may include micromorphological analyses and scanning electron microscopy/energy dispersive X-ray spectrometry (SEM-EDS), which can effectively confirm the illitization process of feldspars or from the transformation of biotite/muscovite. The evidence of illite formation can contribute to increasing CEC, adsorption of essential plant nutrition (e.g., Ca²⁺, Mg²⁺, and K⁺), and water retention in soil from the BSA. Regarding the soils of the semi-arid region, this issue is even more important given the widespread occurrence of sandy soils with high permeability and low retention of cations and water, degraded soils that have had the fertility of the horizons altered due to anthropic activity, and soils with considerable levels of easily weathered minerals, which can release elements from interlayers (e.g., structural K) for absorption by plants.

Thus, our research shows that even the application of treated wastewater for a short period and in small volumes led to changes in the geochemistry of degraded soil, affecting soil chemical characteristics and fertility.

5. Conclusions

The effluent treated at a rate of 1 L/plant/week resulted in higher fertility at the end of two years of application, with maintenance of this status even after irrigation ended. Soil salinity increased during the irrigation period and decreased after irrigation was suspended, without presenting any risk of degradation. However, an incipient solodization process was observed at the end of the experiment.

The application of TWW changed total contents of the major elements, mainly in the silt fraction, directly contributing to the availability of basic cations, particularly Ca, Mg, and K. This process may involve an incipient process of illitization, which should be confirmed in future studies. Lastly, the TTW requires more in-depth geochemical and mineralogical studies, taking into account the effects of different rates and periods of application and the effects on different soils and crop types important for the Brazilian semi-arid region.

Author Contributions

Conceptualization, V.J.L.F., S.d.S.M. and R.S.M.; methodology, C.d.S.S., R.F.d.S.S. and R.V.d.S.; software, R.F.d.S.S.; validation, V.J.L.F., S.d.S.M. and R.S.M.; formal analysis, V.d.S.F., A.P.B. and M.C.C.C.; investigation, V.J.L.F.; resources, S.d.S.M. and R.S.M.; data curation, B.d.O.D. and V.d.S.F.; writing—original draft preparation, V.J.L.F. and S.d.S.M.; writing—review and editing, R.F.d.S.S., A.P.B., M.C.C.C. and R.V.d.S.; supervision, S.d.S.M.; project administration, S.d.S.M. and R.S.M.; funding acquisition, S.d.S.M. and R.S.M. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The original contributions presented in the study are included in the article; further inquiries can be directed to the first author (Victor Junior Lima Félix).

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Tables

Figure 1. Location map of the study municipality (Campina Grande) in the semi-arid region of Paraíba, Brazil.

Figure 2. Panoramic view of the primary and secondary sewage treatment plants at the National Semi-Arid Institute headquarters, Campina Grande, Paraíba. Source: INSA.

Figure 3. The volume of rainfall and temperature in Campina Grande, PB, during the experiment.

Figure 4. Electrical conductivity (EC) of the soil in the three irrigation treatments (WS0.5: water supply at 0.5 L/plant/week; TE0.5: treated effluent at 0.5 L/plant/week; TE1: treated effluent at 1 L/plant/week) at the initial condition (IC), after two years with irrigation at 0–15 cm (A) and 15–30 cm (C) soil layers, and after two years of end of irrigation at 0–15 cm (B) and 15–30 cm (D) soil layers. Means followed by the same lowercase letter for soil layers and uppercase letter for periods do not differ according to the Tukey test at the 5% significance level. Means followed by ns and **: non-significant and significant at 1% probability, respectively, concerning the reference treatment (Initial Condition, IC) according to the Dunnett test for each period and soil layer.

Figure 5. Exchangeable sodium percentage (ESP) of the soil in the three irrigation treatments (WS0.5: water supply at 0.5 L/plant/week; TE0.5: treated effluent at 0.5 L/plant/week; TE1: treated effluent at 1 L/plant/week) at the initial condition (IC), after two years with irrigation at 0–15 cm (A) and 15–30 cm (C) soil layers, and after two years of stopping irrigation at 0–15 cm (B) and 15–30 cm (D) soil layers. Means followed by the same lowercase letter for soil layers and uppercase letter for periods do not differ according to the Tukey test at the 5% significance level. Means followed by ns, *, and **: non-significant and significant at 5% and 1% probability, respectively, concerning the reference treatment (Initial Condition, IC) according to the Dunnett test for each period and soil layer.

Table 1

Chemical characterization of the water supply and treated wastewater used to irrigate degraded soil in Campina Grande, PB.

Parameter	Unit	Water
		Supply	Residual
pH	-	7.5	8.3
EC	dS m−1	0.79	1.35
TOC	mg L−1	1.72	3.7
N	mg L−1	0.28	26.3
NH4+	mg L−1	-	22.3
NO3−	mg L−1	-	4.5
P	mg L−1	1.68	14
PO43−	mg L−1	-	9.4
K+	mg L−1	5.4	27.6
Ca2+	mg L−1	11.2	24.5
Mg2+	mg L−1	6.4	10.7
SO43−	mg L−1	-	51.9
Na+	mg L−1	9.1	22.3
Cl−	mg L−1	178	270

EC, electrical conductivity; TOC, total organic carbon; N, total nitrogen; NH₄⁺, ammoniacal nitrogen; NO₃⁻, nitrate; P, total phosphorus; PO₄³⁻, phosphate; K⁺, potassium; Ca²⁺, calcium; Mg²⁺, magnesium; SO₄³⁻, sulfate, Na⁺, sodium, and Cl⁻, chlorine.

Table 2

Macro and micronutrient content and soil fertility in the initial condition, after two consecutive years of irrigation with treated effluent and water supply, and after two years of stopping irrigation in degraded soil in the semi-arid region.

Trea.	pH1:2.5	C	P	Ca2+	Mg2+	Na+	K+	Al3+	H+Al	EB	CEC	ECEC	BS	AS
	H2O	g kg−1	mg kg−1	---------------------------------------------------cmolc kg−1-------------------------------------------------------									--------%------
		Initial condition
0–15	6.07	7.6	8.14	0.27	0.23	0.21	0.02	0.1	3.29	0.74	4.0	0.84	19.05	12.45
15–30	6.37	5.3	4.07	0.28	0.18	0.16	0.01	0.1	2.87	0.64	3.5	0.74	18.60	16.04
		After two years of irrigation
		0–15 cm soil layer
WS0.5	5.94 aA ns	9.4 aA ns	6.3 aA ns	2.84 aA **	1.11 aA ns	0.52 aA *	0.08 aA **	0.11 aA ns	2.45 aA **	4.55 aA **	7.00 aA **	4.66 aA **	66 bA **	2.51 aB **
TE0.5	5.85 aA ns	9.7 aA ns	6.8 aA ns	2.87 aA **	1.59 aA *	0.52 aA *	0.09 aA **	0.11 aA ns	2.93 aA **	4.56 aA **	7.20 aA **	4.67 aA **	61 bA **	2.58 aA **
TE1	6.02 aA ns	10.9 aA ns	9.5 aA ns	2.88 aA **	2.13 aA **	0.92 aA **	0.11 aA **	0.10 aB ns	2.32 aA **	6.04 aA **	8.36 aA **	6.14 aA **	71 aA **	1.78 aB **
		15–30 cm soil layer
WS0.5	5.89 aA *	8.1 aA ns	7.2 aA *	3.03 aA **	1.05 aA ns	0.56 aA ns	0.12 aA **	0.10 aA ns	2.48 aA **	4.76 aA **	7.24 aA **	4.86 aA **	68 abA **	2.19 aA **
TE0.5	5.79 aA *	11.5 aA *	6.2 aA ns	2.74 aA **	1.40 aA *	0.77 aA **	0.11 aA **	0.13 aA ns	3.23 aA ns	4.61 aA **	7.84 aA **	4.75 aA **	58 bA **	3.26 aA **
TE1	5.89 aA *	10.3 aA ns	7.4 aA *	3.14 aA **	2.00 aA **	0.82 aA **	0.12 aA **	0.10 aA ns	2.08 aA **	6.08 aA **	8.16 aA **	6.18 aA **	73 aA **	1.83 aB **
		After two years the end of irrigation
		0–15 cm soil layer
WS0.5	5.72 aA ns	7.9 aA ns	7.9 aA ns	1.62 aB **	1.06 aA ns	0.37 aA ns	0.10 aA **	0.14 aA ns	3.05 aA ns	3.12 aA **	6.17 aA *	3.26 aA **	51 aB **	5.26 aA **
TE0.5	5.62 aA *	10.9 aA ns	8.1 aA ns	1.70 aB **	1.05 aA ns	0.40 aA ns	0.09 aA **	0.13 aA ns	3.29 aA ns	3.24 aA **	6.53 aA **	3.37 aA **	50 aA **	4.45 aA **
TE1	5.65 aA ns	9.6 aA ns	9.8 aA ns	1.61 aB **	1.09 aA ns	0.40 aB ns	0.10 aA **	0.19 aA *	3.23 aA ns	3.20 aB **	6.43 aB *	3.39 aB **	48 aB **	6.71 aA **
		15–30 cm soil layer
WS0.5	6.02 aA ns	5.7 aA ns	2.5 aB ns	1.36 aB **	1.66 aA *	0.34 aA ns	0.09 aA *	0.10 aA ns	2.29 aA ns	3.45 aA **	5.74 aA *	3.55 aA **	59 aA **	3.30 aA **
TE0.5	6.03 aA ns	7.1 aA ns	4.0 aA ns	1.29 aB **	1.21 aA ns	0.55 aA *	0.10 aA **	0.14 aA ns	2.00 aB ns	3.15 aA **	5.15 aB ns	3.28 aB **	61 aA **	4.81 aA **
TE1	6.08 aA ns	6.2 aA ns	3.1 aB ns	1.39 aB **	1.61 aA *	0.60 aA *	0.11 aA **	0.13 aA ns	2.38 aA ns	3.71 aB **	6.09 aB **	3.84 aB **	57 aB **	5.00 aA **

Means followed by the same lowercase letter for soil layers and uppercase letter for periods do not differ according to the Tukey test at the 5% significance level. Means followed by ^ns, *, and **: non-significant and significant at 5% and 1% probability, respectively, concerning the reference treatment (initial condition) according to the Dunnett test for each period and soil layer. WS_0.5: water supply at 0.5 L plant/week; TE_0.5: treated effluent at 0.5 L plant/week; TE₁: treated effluent at 1 L plant/week.

Table 3

Nutrient input applied using two types of water and different irrigation depths over two years (2013–2015).

Treatment	Irrigation Depth	Ca	Mg	Na	K	P
	Mm	---------------------- kg ha−1 ----------------------
WS0.5	53	5.91	3.38	4.80	2.85	0.9
TE0.5	53	12.94	5.65	11.77	14.57	7.39
TE1	76	18.62	8.13	16.95	20.98	10.64

Table 4

Total contents of major elements (%) in the sand, silt, and clay fractions of the soil in the Initial Condition (IC) after two years of irrigation with treated effluent (with effluent) and after two years without irrigation with treated effluent (no effluent).

Period	SiO2	Al2O3	Fe2O3	Cl	SO3	K2O	CaO	MnO	MgO
	--------------------------------------%---------------------------------------
	Sand
IC	70.99	18.93	3.21	2.81	1.73	1.48	0.54	-	-
With Effluent	72.33	17.60	3.67	2.53	1.32	1.45	0.52	-	-
No Effluent	71.99	18.51	1.81	3.17	2.18	1.47	0.47	-	-
	Silt
IC	65.96	16.55	6.59	1.10	0.89	2.54	1.13	0.11	3.10
With Effluent	29.17	17.56	5.11	2.90	3.97	5.47	4.40	0.18	20.00
No Effluent	53.07	17.14	6.38	2.14	1.92	3.99	2.30	0.12	6.00
	Clay
IC	42.46	32.54	8.76	0.08	0.11	1.63	0.13	0.06	7.40
With Effluent	42.72	29.86	7.44	0.09	0.11	1.90	0.16	0.08	7.00
No Effluent	47.81	18.37	20.06	0.33	0.13	6.39	0.68	0.24	2.60

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