3.1. Water simulation system calibration
Figure 3 show results for the pressure and water discharge regression. Calculated minimum Rainfall Intensity (pressure equals to zero) varies between 68 mm/h and 114 mm/h for the grass roof and control roof, respectively. However, empirically, we noticed that minimum pressure to generate water flow in the system considering hydraulic losses varied 2 to 2.5 meters of water gauge (mwg). Under these conditions, we chose a rainfall simulation of 5 minutes using the IDF (Prefeitura do Recife 2016) as reported in the methodology section.
Naturally, the low possibility of choosing the intensity of precipitation duration is a limitation in the designed rainfall simulation system. The elevation of the reservoir above the level of the simulation system could be a solution to this issue, where the water pump would feed the reservoir and no longer the simulation system directly. As we are testing extreme rainfalls (for 5 minutes duration) this would represent the worst-case scenario regarding lixiviate nutrients, as extreme rainfall is associated with leaching more nutrients (Berndtson 2010).
3.2. Water simulation system calibration
All samples showed an increase in pH for both natural and simulated scenarios (Fig. 4). The control roof showed the highest pH value 7.70 ± 0.12 (mean ± standard deviation). This may have been due to the dissolution of alkaline substances related to the cement and the waterproofing material of the slab, especially calcium carbonate. Lima (2012) evaluated the water quality in cisterns also built with cement, she observed that the younger the cistern, the higher the alkalinity levels.
The experimental roofs in this study were built in less than 2 years of the conducted experiment, therefore we can attribute the increase in pH to the roof age. In the studied ecological roofs, the presence of the substrate and vegetation minimized this effect. Among the green roofs the results were quite similar, a pH of 7.38 ± 0.24 for the grass and 7.33 ± 0.24 for the cactus. As for the brown roof, results of 7.50 ± 0.29 were obtained. Carbonates and Bicarbonates results are correlated with pH effect, as they are substances present in cement, especially in the form of calcium bicarbonate (CaCO3), derived from limestone rocks. In all analyzed samples we could not detect presence of carbonate. However, the bicarbonate in control roof (34.2 mg/L ± 4.5 mg/L) and expanded clay (25.5 mg/L ± 4.6 mg/L) showed increase in its concentration. Cactus (13.6 mg/L ± 2.2 mg/L) and Grass green roof (11.2 mg/L ± 1.9 mg/L) showed similar bicarbonate concentrations and overall performed as sink for bicarbonate. In control and expanded clay roofs, the contact of water with the cemented surface of the slab probably promoted the dissolution of these ions. On green roofs, due to the presence of vegetation and substrate layers, this contact was reduced, resulting in lower bicarbonate concentrations. Results for Calcium measures also corroborate with the hypothesis that calcium carbonate may be the cause of the presence of alkalinity in the samples. Control roof showed increase in Calcium, followed by the expanded clay aggregate roof.
Overall, pH neutralization is beneficial for all green roofs. The results agree with studies using different substrates and vegetations present in the literature (Razzaghmanesh et al. 2014; Vijayaraghavan and Joshi 2014; Buffam et al. 2016). In studies in the semi-arid region of Pernambuco, neutral-alkaline pH results were also found (Farias 2012; Lima 2013; Silva 2017). Comparing to the irrigation recommendation levels for the studied variables by Almeida (2010), all roofs met the pH (between 6 and 8.5), bicarbonate (maximum of 10 meq/L, i.e., 610 mg /L) and calcium (maximum of 30 meq/L, i.e., 1063.8 mg /L) thresholds.
It is not possible for the three ions (carbonate, bicarbonate, and hydroxides) to coexist in the same sample, as the bicarbonate reacts with the hydroxide. Whenever a sample has a pH between 4.5 and 8.3 (all samples are in this pH range) the alkalinity is only due to the presence of bicarbonate, explaining the absence of carbonate ion in our results. This effect is due to the balance between carbon dioxide, carbonate, and bicarbonate in the sample.
All roofs revealed to be neither source nor sink of magnesium, except for Coroa-de-Frade which was able to absorb part of magnesium (10,12 mg/L ± 2,86 mg/L to 4,70 mg/L ± 1,14 mg/L). Overall, all roofs recorded magnesium concentration values below the recommended irrigation limit of 486 mg/L by Almeida (2010).
3.3. N-Ammoniacal, N-Nitrate and Phosphate
Regarding nutrients, for most of the analysis N-ammoniacal was kept below the detection level (0.1 mg/L). 6 samples out of 30 was detected N-ammoniacal, with a maximum value of 0.6, below the Almeida (2010) recommended threshold of mg/L. As inflow water for all tests was below detection level, in these 6 samples roofs acted as sources in a non-clear pattern. The green roof containing the Coroa-de-Frade vegetation acted as source of N-nitrate (Fig. 5a) and Phosphate (Fig. 5b).
Considering N-nitrate, the green roof with Coroa-de-Frade obtained the highest values (6.43 mg/L ± 2.95 mg/L) and in one of the tests (10.30 mg/L) it exceeded the established limit. for nitrate by Almeida (2010) of 10 mg/L (Fig. 5). The green roof with Grass and expanded clay obtained the same means of 1.16 mg/L and deviations of 0.45 mg /L and 0.79 mg/L, respectively. On the control roof, there were no major changes in the concentration of the outflow (0.78 mg/L ± 0.69 mg /L) to the inflow (0.73 mg/L ± 0.85 mg/L). There is a more significant increase in nitrate concentrations for the simulated events compared to natural rainfall, even though the concentrations of N-nitrate in the inflows are similar. This may have occurred due to the intensity of simulated rain compared to natural rain, allowing for higher nitrate leach. In fact, Berndtson (2010) indicate that intense rainfall is responsible for higher concentrations of nitrogen and phosphorus species than less intense rainfall, although other factors may influence these results.
For the analysis of natural rain, the roof with Grass was also found to be above the recommended threshold. On 26th June and 27th June, the concentration on the inflow water were 3.00 mg/L and 2.20 mg/L, respectively, both above the recommended limit. In studies of the semiarid region, the green roofs studied were also sources of phosphorus (Farias, 2012; Lima, 2013; Silva, 2017), with all their effluents above the limit established by Almeida (2010). The reason for the green roof with Coroa-de-Frade leached the highest concentrations of ammoniacal-N, N-nitrate and phosphate was probably because the substrate used was rich in nutrients. The cactus species was not using all the available nutrient resulting in leaching at the observed levels. The use of a substrate in lower quantity of nutrients would be an alternative to reduce the concentrations of nitrogen and phosphorus compounds in the effluents.
Future analyzes are needed to verify whether the amount of leached nutrients has stabilized, since nutrient concentrations are higher for newly constructed roofs and decrease over time (Todorov et al. 2018).
3.4. Turbidity and electric conductivity
For the Turbidity, according to Fig. 6a, we notice that the green roof with Coroa-de-Frade obtained the highest values for the simulated events (277.7 NTU ± 39.2 NTU), followed by the expanded clay roof (17.5 NTU ± 13.5 NTU), by the grass roof (12.7 NTU ± 5.3 NTU) and the control roof (4.2 NTU ± 4.9 NTU). For better visualization of the results, the y axis of Fig. 6a is in logarithmic scale. Turbidity is associated with the type of substrate (Morgan et al. 2011) and the roof age, as young roofs may carry more fine particles. All roofs have the same substrate. Thus, the greater turbidity from the cactus roof can be explained by the higher soil exposure, facilitating the transport of sediments and by the shape of the vegetation's roots, since they are not as ramified as that of the grass ones. The turbidity for the event with natural rain was lower than the average of the simulated events (less expressive in the control roof), probably because the intensity of the simulated rain is greater than the natural rain.
Regarding electric conductivity, in the experiments for the simulated rain, only the green roofs decreased the electric conductivity (EC), and the roof with Coroa-de-Frade had the highest removal efficiency. The inverse effect was noticed for the natural rainfall analyzed. All roofs increased EC, with the greatest increase in the Grass roof (Fig. 6b). Only the EC of the green roofs, when using the average values of the simulated experiments, are within the parameters of Almeida (2010) which has a maximum value of 300 mS/cm. For the experiment using natural rain, all roofs are in accordance with the parameters for irrigation.
3.5. Chloride, sodium, and RAS
All roofs were neither sources nor retained chloride, maintaining all outflow concentrations inside standard deviation of inflow samples. Chloride inflow concentration mean was 60.70, while outflow means varied from 57.40 to 59.44.
Sodium and RAS values were found to be much lower the limits established by Almeida (2010) of 919.6 mg /L and 15 , respectively, for all roofs studied. Silva (2010) found similar results for green roofs, from 19.1 mg/L to 20.9 mg/L of sodium and 1.4 to 2.3 . This is indicative of the absence of sodicity in the soil, a factor that could decrease hydraulic conductivity and affect vegetation growth and water retention.
Regarding potassium (Fig. 7a), the effluents from the brown roof with expanded clay (1.30 mg/L ± 0.75 mg/L), the control effluent (1.22 mg/L ± 0.53 mg/L) and the Grass roof (1.37 mg/L ± 0.88 mg/L) were below the limit recommended by Almeida (2010) of 2 mg/L. The effluents from the green roof with Coroa-de-Frade obtained an average of potassium samples approximately 5 times higher than the recommended limit for irrigation (10.94 ± 1.49). Silva (2017) found values between 18.0 mg/L and 18.8 mg/L for effluents from the green roofs of both cactus species, Babosa and Coroa-de-Frade, in the semi-arid region of Pernambuco State.
The well water used for the simulated rainfall analysis contained high levels of sulfate compared to natural rainwater. The presence of these sulfate concentrations in well water may be due to the decomposition of soils and rocks present in the aquifer such as gypsum (CaSO4) and magnesium sulfate (MgSO4). Sulfate analysis of natural rainwater resulted in values below the detection level (0.1 mg/L). The green roof with Frade's Crown was a source of sulfate (Fig. 7b) both for the simulated experiments and for the one with natural rain, the others did not show such significant differences in relation to the initial concentrations. The green roof with Coroa-de-Frade was a source of sulfate for all experiments analyzed. According to Almeida (2010), the maximum recommended limit for sulfate is 960.6 mg/L, so all samples were within the recommended values for irrigation.
Sulfate (which also obtained values below the detection level of 0.1 mg/L) and nitrate are substances that contribute to acid rain, as they can produce sulfuric (H2SO4) and nitric acids in an aqueous medium. (HNO3). The low concentration of these ions contributed to a more neutral pH of the rain of 6.4. According to the studies analyzed by Hashemi et al. (2015) normally the rain pH is between 5 and 6. Studies in the dry period are necessary to evaluate the effect of these variables.
No boron levels (Fig. 7c) above the lower limit of quantification (0.1 mg/L) were detected in natural rainfall, the brown roof with expanded clay and the control roof. As in the experiments using simulated rain. In agreement with Ferrans et al. (2018) the green roofs were sources of Boron, being more pronounced in the one that contains Coroa-de-Frade as vegetation. The presence of Boron is associated with the composition of the substrate and in Coroa-de-Frade to the low degree of assimilation by the vegetation. The higher concentration of boron in the simulated experiments was possibly due to the presence of boron-containing aquifer rocks, as groundwater was used for simulated. Silva (2017) found values far above Sulfates in his analyzes (2685 mg/L to 3050 mg/L) and values below Boron (0.6 mg/L to 0.9 mg/L) than those found in the presented study.
3.6. Temporal discretization of a single event
Figure 7 shows the main results of the temporal discretization sampling in a single event experiment. The cactus green roof was the one that showed the greatest changes with the collection of samples at different times, especially there was a more expressive decrease in the variables: N-ammoniacal, calcium and potassium. Variables that exceeded the recommended parameters for irrigation (Almeida 2010) using natural rainfall were phosphate and potassium. This experiment by sampling at different times of the same precipitation event indicates that a device for the disposal of first waters would be an alternative to mitigate the leaching effects of potassium but not phosphate. The first water disposal device is a sanitary barrier that diverts the first waters of each rain to discard the waters that wash the atmosphere and the catchment surface. This type of device has proven to be very efficient (Carvalho et al. 2018).
Silva (2017) developed a filter containing sand and activated carbon that was able to decrease the phosphate levels of effluents from green roofs containing the two types of cactuses: Coroa-de-Frade and Babosa. Thus, a treatment system containing a first water diverter and a sand and activated carbon filter could keep all parameters in the recommendation levels of Almeida (2010) and should be further investigated.