Inputs from wastewater treatment plant euent inuence the temporal variability of nutrient uptake in an intermittent stream

Euents from wastewater treatment plants (WWTP) affect water chemistry and in-stream nutrient uptake capacity from receiving freshwaters, thus altering the amount and fate of nutrients exported. In Mediterranean regions, the dilution capacity of receiving streams to buffer the WWTP biogeochemical ngerprint can vary seasonally due to changes in hydrologic conditions. We assessed the temporal patterns and controls on nutrient uptake in an intermittent Mediterranean stream receiving WWTP euent inputs. We compiled data on longitudinal proles of ambient concentrations of dissolved inorganic nitrogen and phosphorus along a 800 m reach on 47 sampling dates between 2001 and 2017 that cover a wide range of hydrological conditions. Data were used to estimate net nutrient uptake in the receiving stream. Ammonium concentration decreased along the reach in 72% of dates, and these decreases were coupled with increases of either nitrite or nitrate. This phenomenon suggests that the stream acted as a hot spot of nitrication. Conversely, concentration of phosphorus did not show any longitudinal pattern in 75% of dates, suggesting that uptake and release processes for this element were commonly counterbalanced. Finally, ammonium net uptake decreased when the stream had a low dilution capacity, suggesting that excess of available nutrients associated with WWTP inputs control de temporal variation of the bioreactive capacity of the receiving streams. Overall, this study suggests that water management should consider the biogeochemical interplay between WWTP operation and the functioning of receiving streams as a strategy to improve stream water quality in urban landscapes.

Freshwater ecosystems have an important capacity to store, transform and retain nutrients during downstream transport because in-stream biota relies on nutrients to develop (Peterson et al. 2001; Mulholland et al. 2008). In-stream biota takes up nutrients from the water column via different assimilatory and dissimilatory biogeochemical uptake pathways (Peterson et  Furthermore, P uptake can also be controlled by sediment adsorption and geochemical processes (Trentman et al. 2020); and N can also undergo different energy-yielding dissimilatory pathways (Kuypers et al. 2018). The most relevant are nitri cation, the aerobic oxidation of ammonium (NH 4 + ) to nitrate (NO 3 − ) (Prosser 1989; Lin et al. 2009), and denitri cation, the reduction of NO 3 − to N 2 gas under sub-oxic conditions during organic matter mineralization (Seitzinger et al. 2006). Together, this in-stream bioreactive capacity may contribute to alleviate the excess of nutrients from WWTP in receiving stream. Previous studies have shown that WWTP e uent inputs diminish the e ciency of receiving streams to take up N and P because excess nutrient loads can cause the saturation of nutrient demand by in-stream biota ( The in uence of WWTP inputs on nutrient uptake in receiving streams can vary seasonally because most processes involved on in-stream nutrient cycling strongly rely, among other factors, on the hydrological conditions and nutrient availability (Peterson et al. 2001;Newbold et al. 2006).
During high ows, the e ciency for stream biota to take up nutrients is low because the interaction between the active streambed and the water column height and the water residence time are also low and therefore, downstream transport might overwhelm in-stream biogeochemical processing (Martí et al. 1997;Algerich et al. 2008 (Earl et al. 2006;Newbold et al. 2006). Therefore, it is also expected that, during extremely low ows, the capacity of stream biota to take up nutrients can be overwhelmed by the high nutrient concentrations. Nevertheless, the extent to which hydrology will interact with chemistry to control temporal variation of nutrient uptake in receiving stream is not fully understood yet. In urban landscapes with water scarcity, such as the Mediterranean region, the temporal patterns of hydrology and dilution capacity in receiving streams as well as the biogeochemical derived effects could be magni ed because the dilution capacity of the receiving stream can largely vary on an annual basis from 0% to > 95% (Martí et al. 2010;Bicknell et al. 2020).
Most existing studies on nutrient uptake in WWTP-receiving streams have focused on quantifying the effects of the WWTP inputs (Gücker et al. 2006;Merseburger et al. 2011;Arnon et al. 2015), but less information is available on assessing its temporal variation and associated controlling factors (Bernal et al. 2020). However, understanding how in-stream nutrient processing varies over time in rivers that receive inputs from WWTP is essential for an integrated management of water resources and their quality in urban landscapes, especially under water scarcity conditions. In this context, we aimed to assess the temporal variability in nutrient uptake and factors controlling it in a Mediterranean stream with an intermittent hydrologic regime that receives the inputs from a WWTP e uent. To this aim, we compiled data on longitudinal pro les of ambient nutrient concentrations collected along a 800 m reach downstream of a WWTP input on different dates (n = 47) between August 2001 and October 2017 that encompassed the four seasons of the Mediterranean climate and covered a wide range of hydrological conditions. From this dataset, we calculated in-stream net nutrient uptake and characterized each sampling date in terms of hydrology and water chemistry to assess the relative in uence of these factors on the temporal variation of net nutrient uptake.

Study site
This study was conducted in the catchment of La Tordera river, which is located near Barcelona (NE of Catalonia, Spain). Within this catchment, we selected a third-order stream reach immediately located downstream of the input from the WWTP e uent of the municipalities of Sant Esteve and Santa Maria de Palautordera (lat. 41º41'3.47''N, long. 2º27'33.19''W). The reach is 800 m long with no major lateral inputs (e.g., tributaries). Channel morphology is characterized by a low sinuosity, a run-ri e sequence with a few shallow pools, and a slope close to 1%. Streambed substrata is dominated by cobbles (34%), pebbles (22%), and boulders (22%) (Merseburger et al. 2005). The reach is anked by a dense canopy of riparian trees, with some areas of sparse vegetation (Bernal et al. 2020). At the study location, the mean annual stream discharge is 267 ± 115 L/s, but it varies several orders of magnitude within and among hydrological years due to the intrinsic characteristics of Mediterranean climate of the region (Merseburger et al. 2005). In summer, the stream commonly dries out upstream of the WWTP input.
The WWTP treats 17,900 population equivalents, where 1 population equivalent is the biodegradable organic-matter load corresponding to a biological O 2 demand (BOD5) of 60 g O 2 /d. This WWTP lacks tertiary treatment; and, despite it ful ls de legislation requirements concerning Here, we compiled data on longitudinal pro les of concentrations of NH 4 + , nitrite (NO 2 − ), NO 3 − , and SRP collected along this reach during different studies (n = 47 sampling dates) encompassing an overall period from August 2001 until October 2017. We used this data set to estimate net nutrient uptake for the different DIN and P forms to examine its temporal variability in the receiving stream. Despite the sampling periodicity is irregular, the data set covers a wide range of hydrological conditions and it is representative of the four climatic seasons of this Mediterranean region (winter n = 7; spring n = 16; summer n = 13; and autumn n = 11).

Field sampling and laboratory analysis
We de ned eight sampling sites along the reach, which were evenly distributed every 100 m. In addition, we selected a sampling site upstream of the WWTP that served as control to assess the relative contribution of the WWTP input to the receiving stream. On each sampling date, we took surface water samples from each sampling site (3 replicates) using 100 mL acid-washed plastic syringes. Water samples were immediately ltered through a GF/F glass ber lters (Whatman ®) with a pore of 0.7 µm and kept refrigerated on ice in the eld. Once in the laboratory, we Concentration of NO 2 − was not included in DIN estimates because values were only available for half of the dates; and in average, it represented < 1% of DIN concentration. We also calculated the molar ratios between DIN and SRP and between NO 3 − : NH 4 + concentrations as proxies of potential N or P limitation and relative proportion of oxidized and reduced DIN forms, respectively. Samples were analyzed at the Labqa Service of the Centre for Advanced Studies of Blanes (CEAB-CSIC).

Parameter calculations
On each sampling date, we estimated the contribution of the WWTP e uent to stream discharge (in %) by using conductivity as a conservative tracer as follows: Contribution WWTP (%) = 100 * (EC 1 -EC UP /EC WWTP -EC UP ) Eq. 1 where EC 1 is the electrical conductivity at the rst sampling site of the stream reach located at 100 m downstream of the WWTP. Empirical measurements in 2001 indicated that this distance from the WWTP input is enough to ensure a complete mixing of the e uent water with stream water under different hydrological conditions. EC up is the electrical conductivity at the sampling site located upstream of WWTP; and EC WWTP is the electrical conductivity of the WWTP e uent. Then, we estimated the hydrologic dilution factor of the receiving stream (DF, in %) as the inverse of the WWTP contribution to discharge (i.e. DF = 100 -Contribution WWTP). Thus, low DF values indicate that the stream ow is low relative to WWTP inputs (Gupta 2008 We used data from the longitudinal pro les in nutrient concentrations along the reach to estimate net nutrient uptake following (von Schiller et al.

2011)
Speci cally, for each nutrient, we used the following rst-order equation to estimate the net uptake coe cient per unit length of the reach (k, in 1/m) : where C 1 is the nutrient concentration at the rst sampling site of the reach below the WWTP e uent (in mgN/L or mgP/L); C x is the nutrient concentration at sampling site located x meters from the WWTP e uent input; and EC x is the electrical conductivity at that sampling site. We estimate k as the slope of the regression between the natural logarithm of nutrient concentration (corrected by the conductivity) and the distance from the WWTP e uent input of each sampling site. Longitudinal pro les of ambient nutrient concentrations represent the net balance between in-stream uptake and release processes; and thus, k values can be positive (uptake > release), negative (uptake < release), or ≈ 0 (uptake ~ release) (Merseburger et al. 2005).
We used k to estimate the net uptake velocity (V f , in mm/min), that describes the velocity at which a nutrient molecule is removed from the water column and is a proxy of in-stream nutrient demand (Bernhardt et al. 2002). Values of V f will be positive, negative, or ≈ 0 depending on the value of k; and are calculated as follows (Bernal et al. 2020):

Data analysis
We inferred potential seasonal differences in stream physical and chemical characteristics based on the variables measured on each sampling date (see Table 1) by using a non-parametric Kruskal-Wallis test (season as a factor) because data sets were not normally distributed and showed heteroscedasticity. Table 1 Physical and chemical characteristics of the study stream, which receives inputs from a wastewater treatment plant e uent, measured on different seasons over the study period. For each variable, we report the range of values (minimum and maximum) and the number of data available (n) corresponding to all sampling dates. We also show the average ± Standard Error values of each variable considering data from each season separately. For each season, n indicates the number of sampling dates with available data. We also examined how measured physical and chemicals variables contributed to the observed temporal variability among sampling dates by using a principal component analyses (PCA). The weight of a variable on a PCA component was considered signi cant when its loading was > 0.7. Unfortunately, some of the variables were not measured in all sampling dates. Therefore, to avoid a signi cant reduction of available sampling dates to be considered in the PCA analysis, we excluded from the analysis those variables with a high number of missing values (i.e., concentrations of DO, NO 2 − , and SRP; and the DIN:SRP molar ratio; Table 1). In addition, we used non-parametric Kruskal-Wallis test on the scores of PC1 and PC2 to examine differences among seasons.
For each nutrient, we estimated the frequency of cases in which V f < 0 (release > uptake), V f > 0 (release < uptake) and V f ≈ 0 (release ≈ uptake) considering all data together to compare in-stream responses among nutrients, and separated by seasons to identify temporal patterns in dominant pathways of nutrient cycling. We also used a non-parametric Kruskal-Wallis test to test for seasonal differences on V f for each particular nutrient. For this analysis, we only selected those dates when longitudinal pro les showed either a signi cant decline (i.e., uptake > release; V f positive or net uptake) or increase (i.e., uptake < release;V f negative or net release). In the case of V f for DIN, we could not consider winter in the analysis because none of the longitudinal pro les of this season showed signi cant longitudinal trends.
Further, we used simple regression analysis between V f for NH 4 + and V f for NO 3 −  We ran the PCA analysis with the R software (version 3.2.2; R project for Statistical Computing, Vienna, Austria). The rest of statistical analyses were performed with the SPSS 11.0 statistical package. We considered statistical results to be signi cant at a p-value < 0.05

Physical and chemical stream characterization and seasonal variation
Most of the physical and chemical variables in the receiving stream varied among seasons (Table 1). Average stream water temperature was relatively warm, with lowest values in winter and highest in summer (Table 1). Stream Q varied up to 3 orders of magnitude among seasons, being the lowest in summer and the highest in spring (Table 1). Similarly, the DF ranged from 0-100%, with higher values in winter and spring than in summer and autumn, when Q upstream of the WWTP was low or even nil. Under these seasonal conditions, average DF values were ≤ 40%. Water EC was moderate and varied seasonally following the inverse pattern of Q ( The two rst components of the PCA, considering the physical and chemical variables measured in the study reach, accounted for 66.8% of the variance over sampling dates (Fig. 1). The rst component of the PCA (PC1) explained 47.6% of the total variance, with a positive loading of EC, temperature and NH 4 + concentration, and a negative loading of the DF and Q (Fig. 1). Thus, this axis indicated a gradient of the relative physical and chemical in uence of the WWTP e uent inputs over the temporal changes in stream hydrology (i.e., its dilution capacity). The second component of the PCA (PC2) explained 19.2% of the total variance, with a positive loading of NO 3 concentration and the NO 3 -:NH 4 + ratio (Fig. 1).
Given that DIN in the WWTP e uent is relatively enriched in NH 4 + compared to upstream (Bernal et al. 2020), this axis can be related to the in uence of upstream vs. point-source inputs onto stream water chemistry.
Finally, a Kruskal-Wallis test indicated that there were signi cant differences among seasons for the score values of the PCA components. In relation to PC1 scores, summer cases showed signi cantly higher values than those in spring and winter (Kruskal-Wallis test, p-value < 0.05). In relation to PC2 scores, there were signi cant differences between autumn and spring (Kruskal-Wallis test, p-value < 0.05).

Temporal variation of stream net nutrient uptake
Both the magnitude and the temporal variation of net uptake rates differed among nutrients ( Fig. 2 and Table 2). We found that NH 4 + concentration signi cantly decreased along the reach (i.e., net uptake) on most sampling dates (i.e., 72% of all cases); yet, this proportion slightly varied among seasons ( Fig. 2A). In particular, the seasons with the highest and lowest proportion of dates with net uptake were summer (85%) and spring (56%), respectively ( Fig. 2A). In this sense, average V f for NH 4 + was consistently > 0 in all seasons (   2 Net nutrient uptake velocity (Vf) measured in the study stream, which receives inputs from a wastewater treatment plant e uent, on different seasons over the study period. Values of Vf were estimated from pro les of ambient nutrient concentrations that showed signi cant trends along the stream reach. For each nutrient, we report the range of Vf values (minimum and maximum) and the total number of data available (n), which in all cases is a fraction of all sampling dates ( There was a negative relationship between V f for NH 4 + and the scores of PC1 and a positive relationship between V f for NO 2 − and the scores of PC1 (Fig. 4). The remaining nutrients showed no relationship between V f and the scores from PC1. For any of the studied nutrients, V f showed no relationship with the scores of PC2

Discussion
The contribution of the WWTP e uent to the ow of the receiving stream was highly variable (from < 1 to 100%) depending on the period of the Bicknell et al. 2020), because their hydrologic regime is characterized by extreme events (i.e., oods and droughts). This nding explains the strong temporal shifts in the impact that WWTP e uent inputs have on the receiving streams of these regions as shown in our study. During high ow conditions in winter and spring, landscape features and upstream conditions have a major in uence on stream physical and chemical characteristics, whereas these characteristics become increasingly subjected to WWTP e uent inputs as the dilution capacity of the receiving stream decreases during summer low ow (Keller et al. 2014). In the study stream, the impact of the WWTP e uent was dramatic for both physical and chemical variables, especially during summer and autumn when the dilution factor was well below the 40% threshold. In particular, the stream commonly dried out upstream of the WWTP input for some weeks in summer; and thus, the dilution capacity of the stream was nil.
Under these conditions, water temperature and electrical conductivity were higher and oxygen concentration was relatively lower than on dates when the stream dilution factor was > 40%. Low dissolved oxygen concentration in the receiving stream could be explained by the increases in ecosystem respiration that are usually observed downstream of WWTP inputs (Gücker et al. 2006;Bernal et al. 2020). In addition, mean SRP concentration increased between 4-and 8-fold during summer, and there was a shift towards N limitation as indicated by the low DIN:SRP molar ratios. These physicochemical changes suggest that in-stream net nutrient uptake could also show marked seasonal patterns because temperature, oxygen availability, and nutrient availability can strongly in uence the metabolic activity, the demand of nutrients and the preferential biogeochemical pathways of microbial in-stream communities ( The longitudinal pro les in nutrient concentrations allowed examining the temporal variability in the N and P uptake and transformation capacity of receiving stream while allowing the evaluation of the in uence of the WWTP contribution to the temporal dynamics. Results indicate higher instream bioreactivity for DIN forms than for SRP because longitudinal pro les exhibited more signi cant trends (i.e., indicative of net uptake or release) for the former element than for the latter. This nding is relatively common among WWTP-receiving streams (Martí et al. 2010).
Concentration of SRP did not show any signi cant trend along the stream on most of the sampling dates, which suggests that uptake and release processes are counterbalanced. Only in very few dates we found net uptake of SRP, although the values of Vf for SRP were in general lower than those for DIN. Low or nil net uptake of SRP could be explained by the excess of SRP from the WWTP input that could saturate the demand by microbial assemblages as well as the buffering capacity of streambed sediments (House and Denison 1998;Haggard et al. 2005).
In contrast, despite DIN concentration did not show any signi cant longitudinal trends on most sampling dates, each particular DIN form (NH 4 + NO 2 − and NO 3 − ) did. Only on very few days we found a signi cant net uptake of DIN, but these dates did not follow any seasonal pattern. This nding suggests that the receiving stream acts more as a transformer than as a sink of N. Our results showed signi cant longitudinal decreases in the concentration of NH 4 + in the study reach, an indication that this nutrient was highly processed along the stream and that uptake processes prevailed over release processes. Regardless of the season, declines in NH 4 + concentration were accompanied by increases in NO 2 − and NO 3 − concentrations; supporting the idea that nitri cation is a prevailing process in the receiving stream over time (Merseburger et al. 2005;Bernal et al. 2020). This pattern is consistent with previous results showing that nitri cation can represent up to 90% of the uptake of NH 4 + in the study stream (Bernal et al., 2017). Similar patterns have also been described in urban streams worldwide (Cébron et al. 2003;Marti et al. 2004;Gammons et al. 2011), suggesting that high in-stream nitri cation rates might be a common phenomenon downstream of WWTP e uent inputs in urban streams.
These high nitri cation rates can be explained by the high inputs of both NH 4 + and nitrifying bacteria from active sludge discharged into the receiving streams from the e uents of the WWTPs (Merbt et al. 2015). We also found evidence for the occurrence of other biogeochemical processes associated with N cycling within the stream. The slope between V f for NO 3 − and V f for NH 4 + was clearly below 1 (Fig. 3), indicating that the biogeochemical demand of NH 4 + uptake was higher than expected solely from nitri cation. This result suggests that assimilatory NH 4 + uptake by photoautotrophs and heterotrophs additionally contribute to the observed declines in NH 4 + concentration downstream of the WWTP input. In contrast, net uptake of NO 3 − occurred on few dates mostly during spring. This suggests, that except for these dates, denitri cation and assimilation of NO 3 − were overwhelmed NH 4 + nitri cation. The fact that uptake e ciency for NO 3 − tends to be lower than that for NH 4 + could explain this nding   (Fig. 4). This result could be explained by the large impact of the WWTP e uent on stream physicochemistry during low ows, which lead to decreased dissolved oxygen concentrations that likely inhibited the activity of nitri ers, despite of increases in stream water temperature. In this sense, previous studies have shown that bacterial assemblages and associated microbial activity can experience dramatic shifts when dilution factors in WWTP impacted streams are < 50% (Romero et al. 2019). Moreover, high nutrient concentrations likely had a saturation effect on in-stream NH 4 + demand during these low ow periods when the receiving stream had a small dilution capacity. A similar saturation effect has been reported for NO 3 − uptake in urban arid land streams, despite these streams tends to be strongly N limited under pristine conditions (Grimm et al. 2005). In this study, results from the relationship between V f for NH 4 + and PC1 supports the saturation idea, because V f tends to decrease in these sampling case when the dilution factor is low and the effect of WWTP input on increases in NH 4 + concentration is high. The concomitant reduction in NO 2 − net releases (i.e.,less negative values of Vf for NO 2 − ) under these conditions further suggests a clear impact on the in-stream nitri cation capacity. The characterization of NH 4 + uptake kinetics of epilithic and episammic bio lms from this receiving stream additionally suggests that stream ambient NH 4 + concentration can be above limitation (i.e., > than half saturation values) for NH 4 + uptake and nitri cation rates by microbial assemblages (Bernal et al. 2018). We further examined how NH 4 + uptake demand varies with NH 4 + concentration considering a wider set of pristine and human impacted streams, using the data set generated by Marcé at al. (2018) to verify if the saturation explanation holds. We found that variability in V f for NH 4 + from the study stream was relatively constrained and values were in the lower range of all the data set ( Fig. 5) This comparison needs to be done with caution because the data set includes both gross and net uptake rates and there was no signi cant regression between V f for NH 4 + and NH 4 + concentration (Fig. 5). However, the funnel-type pattern observed suggests that with increasing ambient NH 4 + concentration, the biogeochemical reactivity of stream ecosystems, at least for NH 4 + , is seriously threatened, even under hydrological conditions favoring the interaction between nutrients and biota.
The hydrologic regime and the biogeochemical reactivity are essential factors to understand the variability of nutrient uptake in streams and understand the role of these ecosystems in the regulation of nutrient cycling and exports along the river networks (Battin et al. 2008;Acuña et al. 2019). Our results are concordant with the idea that biogeochemical reactivity can be equal or even more important that hydrological opportunity to drive nutrient cycling in stream ecosystems (Marcé et al. 2018), especially in highly perturbed streams receiving chronic nutrient inputs. In particular, this study shows the relative importance of bioreactive controls (i.e., nutrient saturation) over hydrologic controls (i.e., high water residence time) on the temporal variation of nutrient uptake in WWTP-receiving streams. Moreover, the study suggests that this is in uenced by the interplay between the hydrologic regime and the WWTP in uence (i.e the dilution capacity), especially in Mediterranean regions. Overall, this study contributes to emphasize the distinct biogeochemical heartbeat of streams under human pressure (Grimm et al. 2005). Therefore, a better understanding of the temporal variability in nutrient uptake capacity of the receiving streams, and especially of the biogeochemical processes prevailing during low ow conditions, is important for improving the management of urban streams impacted by point-sources. In this context, our study conveys with the perspective of current studies suggesting management strategies for WWTP-receiving streams (i.e., Bicknell et al. 2020). In particular, our study suggests that it is critical to design WWTP operation procedures taking into account both the dilution and the bioreactive capacity of receiving streams for an integrated management of water resources and their quality in urban landscapes, especially under water scarcity conditions. from the Spanish Ministry of Science, Innovation, and Universities.

Con icts of interest/Competing interests
The authors declare that the research was conducted in the absence of any commercial or nancial relationships that could be construed as a potential con ict of interest.

Availability of data and material
After manuscript acceptance, all data will be archived and available at Zenodo repository.
Code availability Not applicable.
Authors' contributions EM, FS, SB, MR participated in the design of the study. All co-authors contributed to the eldwork on different sampling dates. SC, SB and EM conducted the compilation of the data set, the calculation of nutrient uptake metrics, and the data analyses. SC, EM, SB and MR wrote the manuscript with contributions from the rest of the co-authors.
Additional declarations for articles in life science journals that report the results of studies involving humans and/or animals Not applicable.

Ethics approval
No studies involving animal or human subjects are presented in this manuscript.

Consent to participate
All authors participating in this study can withdraw their consent and stop participating at any time without prejudice.   Frequency of the different trends (i.e., signi cant decreases or increases, and no signi cant changes) in the longitudinal pro les of ambient nutrient concentrations of NH4+ (A), NO2-(B) and NO3-(C) along the study reach of the WWTP receiving stream. The longitudinal trends, when signi cant, were used to estimate net nutrient uptake (estimated as uptake velocity, Vf, which could be either positive, negative or nil). Different colors in the column indicate the frequency of the three types of trends. A longitudinal decrease in ambient concentration indicates net nutrient uptake and it is labeled in black; a longitudinal increase indicates net nutrient release and it is labeled in gray; no signi cant trend indicates balance between nutrient uptake and release processes and it is labeled in white. For each dissolved inorganic nitrogen form, the frequency of the different longitudinal trends is reported for all data together and for data collected in each season separately.

Figure 3
Linear relationship between net uptake velocities (Vf) for NH4+ and for NO3-measured in the WWTP-receiving stream over the study period (n = 23 sampling dates). The equation for the relationship and the statistic results are shown. Different symbols distinguish data by seasons, with crosses = winter; triangles = spring; squares = summer; and circles = autumn.

Figure 4
Relationships between net uptake velocity (Vf) for NH4+ (A) and for NO2-(B) and the scores from the principal component 1 (PC1) of the PCA conducted with physical and chemical characteristics of the receiving stream. The arrow above the graphs indicates the signi cant variables