When soil contamination with heavy metals (including As, although it is technically a metalloid) occurs, whether due to agricultural practices or other human activities, these elements tend to accumulate in the upper soil layers, from where they can leach into groundwater or reach surface water, and can be absorbed by plants (Alloway 2013). In Argentina, soil quality guidelines are established by Regulatory Decree 831/93 of National Law 24,051 on hazardous waste, which defines threshold levels for different land uses (agricultural, residential, and industrial). Our results indicate that the soil used in this study is suitable for agricultural production, with initial concentrations of heavy metals between one and two orders of magnitude below the thresholds established for agricultural soils by current legislation. In our irrigation experiment, the continued use of treated wastewater (13,500 L applied over a 90-day crop cycle) did not significantly increase the concentration of metals in the soil. Final values at harvest were similar to the initial measurements. These results indicate that the contribution of heavy metals by reuse water to the soil is not significant, at least in the volumes of water used during a short crop cycle.

Heavy metals must be bioavailable for plants to be able to incorporate from the soil solution. The bioavailability of heavy metals depends on soil type (e.g., texture) and its physicochemical properties (Reichman 2002; Adamczyk-Szabela and Wolf 2022). Among these factors, soil pH plays a key role, as it affects the solubility and mobility of most metals. With exceptions (Mo, Se, and As), an increase in soil pH generally leads to a decrease in metal bioavailability (Reichman 2002; Silveira et al. 2003; Adamczyk-Szabela and Wolf 2022). In our study, soil pH values were not altered by the application of treated wastewater throughout the experiment. The soil maintained its slightly alkaline character, which is typical of soils in the semiarid Patagonian region (Rostagno et al. 1991; Bouza et al. 2007). Under these pH conditions, it is expected that most metals are present as precipitated forms, such as insoluble hydroxides, carbonates, or inorganic complexes, thus reducing their bioavailability (Alloway 2013; Xu et al. 2022). Electrical conductivity (EC) of the soil was also not significantly affected by the use of reclaimed water, remaining within the non-saline range throughout the crop cycle (Ismayilov et al. 2021).

The physicochemical characteristics of the treated wastewater used in this study are consistent with expected values for urban effluents subjected to secondary treatment processes (U.S. Environmental Protection Agency 2010). Mean pH values above 7, both in water and in soil, could potentially pose limitations to plant growth (Barrow and Hartemink 2023; Hartemink and Barrow 2023). However, no issues related to plant development or symptoms of nutrient deficiency were observed during the crop cycle. This may be explained by the fact that nutrient availability to plants results from a combination of pH effects on soil sorption processes and pH effects on plant uptake mechanisms (Barrow and Hartemink, 2023). The electrical conductivity (EC) of 1,583 μS/cm classifies the water as slightly saline (FAO 1992), which may impose some restrictions depending on the crop’s sensitivity. However, numerous cannabis varieties with salt tolerance have been reported (Huaran et al. 2019; Akram et al. 2021; Zhang et al. 2021).

Dissolved oxygen (DO) levels at the time of refill of the three irrigation tanks were relatively low, likely due to the high organic load of the effluent, as reflected in the elevated BOD₅ and COD values, as well as in the abundance of volatile suspended solids. In lagoon treatment systems, oxygen is supplied by the photosynthetic action of the microalgae growing there and, to a lesser extent, by wind action. In parallel, DO is used for respiration and the degradation of organic matter by microorganisms, corresponded in our study with the recording of a negative correlation between DO and COD concentrations; emphasizing that this has been magnified due to the storage of irrigation water in closed tanks without exposure to sunlight. In oversaturated systems, oxygen consumption may equal or exceed oxygen generation (Faleschini et al. 2012; Ragush et al. 2015).

Regarding nutrients, the water supplied nitrogen in the form of NH₄⁺ (approximately 10 mg/L), as well as NO₃⁻ and NO₂⁻, and contributed with 13 mg/L of PO₄³⁻. Although nutrient concentrations in the soil were not directly measured, this decision was related to the nature of the experimental plot, which was established using locally sourced filling soil rather than a stabilized pedogenetic profile. Under these conditions, soil nutrient levels are highly variable and not necessarily representative of long-term site properties. Nevertheless, the cumulative nutrient input provided by irrigation water throughout the crop cycle was sufficient to support full plant development. In this context, reclaimed water functioned not only as an irrigation source but also as a relevant contributor of nutrients, aligning with the dual-use approach of these effluents in integrated water and nutrient management strategies (Pedrero et al. 2010).

The presence of microorganisms such as total coliforms or E. coli serves as an indicator of potential contamination (Chen et al. 2023). In the case of soil, although there are no regulations in Argentina establishing maximum microbial loads for agricultural soils, sampling at both the beginning and end of the experiment revealed only very low values of total coliforms, with no detection of E. coli, indicating the absence of fecal contamination throughout the crop cycle. Regarding the microbiological quality of the treated water, the results showed a sustained decrease in total coliforms and E. coli over the storage time in tanks or reservoirs. Similar reductions in fecal indicator bacteria during reclaimed water storage and the absence of E. coli in irrigated soils have been reported in studies on other crops, highlighting the effectiveness of storage and management practices in reducing microbiological risks associated with reclaimed water irrigation (Guo et al. 2017; Sunyer-Caldú et al. 2022).The rapid decrease registered may be associated with the high summer temperatures characteristic of the region during those months, which directly promote bacterial inactivation through thermal stress inside the tanks and indirectly, generating a rapid modification of the redox potential, going from widely positive values ​​at the beginning of filling to values ​​below −283 mV. (Nyieku et al. 2020; Wang et al. 2021). Correlations between total coliforms and E. coli, as well as environmental variables such as BOD₅, and total and volatile suspended solids suggest that the availability of organic matter could also be contributing to microbial decline in storage tanks (Kinnaman et al. 2012; Seo et al. 2019).

From a regulatory perspective, E. coli concentrations at tank refilling exceeded the WHO guideline value for the use of treated wastewater in crops with direct contact with irrigation water (≤ 1,000 MPN/100 mL; WHO 2006). However, after storage in the tanks, levels fell below this threshold. In the third refill event, where initial E. coli concentrations were particularly high, an additional sampling was conducted three days after refilling to better capture early dynamics. This sampling confirmed a marked reduction in E. coli concentrations over time. Notably, the rate of reduction observed after the first and second refill events was faster, with values decreasing more rapidly within the first 10 days of storage. It should be noted that, during the third refill event, E. coli concentrations were not assessed at day 10, as sampling efforts were focused on capturing early decay dynamics under high initial contamination conditions. While this approach provided valuable information on short-term microbial reduction, it limited direct comparability with the 10-day assessment performed in the first two events. Future studies should maintain consistent monitoring at this time point to confirm whether a 10-day storage period reliably ensures compliance with recommended microbiological thresholds under varying contamination scenarios. The values recorded after 10 days of storage following the first and second refill events, and after 30 days following the third refill event, were always below the threshold established for industrial crops (WHO 2006). This result highlights the importance of storage as a complementary treatment strategy to improve the microbiological quality of reclaimed water prior to its agricultural application (Jjemba et al. 2014; Sunyer-Caldú et al. 2022). Nevertheless, under the current operational conditions, the WHO guideline value was not met at the time of tank refilling in any of the evaluated events; therefore, compliance at the initiation of irrigation cannot be guaranteed. The variability observed between refill events indicates that bacterial contamination levels are not stable over time and reinforces the need for consistent microbiological monitoring. Additionally, as the present results correspond to a single cultivation cycle, further studies across multiple cycles are required to confirm the robustness of these findings across different operational conditions.