PPCP Concentrations in Surface Water and Compositional AlterationA total of 33 PPCPs (∑33w PPCP) were quantified (≥QL) in surface water (ng/L) within the TFPR. The PPCP concentrations showed significant differences spatially (Kruskal–Wallis, pFigure S3A). The lowest ∑33w PPCP median concentrations occurred along the Potomac River mainstem or at the fluvial upsteam reference sites for the tributary embayments, which were located above the head of tide. The five mainstem TFPR sites (1,9,10, 13 and 14) showed significant differences in surface water concentrations (Kruskal–Wallis, p > 0.05), with site 14 being an outlier. However, when site 14 was removed the four remaining TFPR mainstem locations were not significantly different in median PPCP concentrations (Kruskal–Wallis, pTable S2) discharging into Neabsco Creek, indicating that although site 14 was selected at the end of a long pier into the main Potomac River channel located at Leesylvania State Park it was not indicative of the mainstem TFPR. Although the high-capacity WTPs significantly increased surface water concentrations of PPCPs in the immediate vicinity of the outfall zones in the embayment regions, with a downstream influence of ~1 km radius, the WTP effluents did not appear to elevate the concentrations of the PPCPs further downstream in the mainstem Potomac River along the entire length of the TFPR from Chain Bridge to Gunston Cove (site 13). The major WTPs showed a near-field influence in the TFPR overall in terms of ∑PPCP concentrations (Figure S3A).Because caffeine and nicotine were prominent upstream constituents in this geographical region, these two chemicals were removed from ∑PPCP consideration to provide ∑31wPPCP (Figure S3B) as an alternative comparison. The spatial profiles and statistical relationships along the TFPR were similar among the two ∑PPCP groups. However, it is even more clear the ∑31wPPCPs in the discharge zone embayments dispersed to consistent baseline (median) concentrations of 50 to 80 ng/L throughout the mainstem TFPR during the time period sampled at ebb tide. Removal of caffeine and nicotine sharpened the hydrogeochemical perspective of this process.The PPCP concentrations measured in the effluents of WTP 1 and WTP 2 are illustrated in Figures S4 and S5. There were 52 PPCPs quantified in the WTP effluents ranging in concentration from 3.7 ng/L (fentanyl) to 9900 ng/L (fexofenadine) in WPT 1 to 2.8 ng/L (fentanyl) to 10,000 ng/L (fexofenadine) in WTP 2. The PPCP composition and concentrations were similar between the two WTPs. The individual PPCP concentrations (log transformed) were strongly correlated between the two effluents (Spearman’s Rho = 0.861, p

Conversely, when the unmixed WTP effluent concentrations (also log transformed) for WPT 1 and WPT 2 were compared with nearby discharge zone PPCP concentrations (Spearman’s Rho) at sites 3 (Four Mile Run) and 6 (Upper Hunting Creek), respectively, only weak correlations were observed (Rho = 0.25 for site 3, p > 0.05; Rho = 0.38 for site 6, p > 0.05). The weak correlations were not caused simply by dilution of effluent, because some individual PPCPs dissipated or changed in relative abundance relative to their effluent profiles.

The Cameron Run/Hunting Creek watershed yielded the best trend of PPCP compositional alterations occurring in the downstream direction because of the extended transect at that location. The PPCP composition profiles demonstrated the typical upstream fluvial signature of caffeine and nicotine (Figure 2) as the predominant PPCPs (45 and 15 ng/L, respectively) and much lower concentrations of carbamazepine and tramadol (Tables S2 and S5). Caffeine was a reliable WTP marker and illustrated the expected 2-fold dilution between WTP effluent and surface water in both Four Mile Run and Cameron Run (at sites 3 and 6). The individual PPCP concentrations quantitated in >50% of the surface water samples are shown in Figure S6 for sites 3 and 6. Furthermore, there was a considerable reduction in desvenlafaxine between Upper Hunting Creek and the mainstem TFPR through the Lower Hunting Creek embayment. The alteration in PPCP composition continued in the downstream direction through Lower Hunting Creek to the mainstem Potomac River along the site 6 to 9 transect (Figure 2). Caffeine and nicotine were minor constituents in WTP effluent, albeit at much higher concentrations than in upstream surface water. Caffeine and nicotine were, however, abundant in the mainstem TFPR. The contribution of both caffeine and nicotine from the Cameron Run upland watershed and WTP 2 was approximately equal on a first-order approximation. When caffeine and nicotine were removed from the compositional comparison for the two TFPR mainstem sites at this transect the remaining pharmaceutical composition at sites 9 (downstream of WTP 2) and 10 (upstream of WTP 2) were nearly identical (Figure 2). Thus, this illustrated the consistency of the PPCP composition (without caffeine and nicotine) following the tributary embayment input of a major WTP in the TFPR.PPCP concentrations in water were converted to mole fraction-PPCP concentrations and analyzed by principal component analysis (PCA), revealing a continuum of changing composition from the upland streams to the mainstem TFPR. A spread of eigenvalues is apparent across a compositional arc beginning with the WTP effluents and culminating in the mainstem TFPR and upland locations. The PCA can be divided into 4 compositional segments (Figure 3), including (i) the WTP effluent from WTPs 1 and 2 (upper left quadrant in Figure 3), (ii) WTP discharge zone, (iii) transition mixture where the embayments flow into the mainstem TFPR and (iv) the mainstem TFPR itself in combination with the upland watersheds (lower right quadrant). The first two PCAs accounted for 87% of the compositional variability. The TFPR sites nearest the WTP outfalls showed a composition on the PCA in closest proximity to the effluents (e.g., sites 3, 4, 6–9, 12 and 14), but trending down and to the right. The opposite end of the PCA in the lower right quadrant included the sites directly upstream (above the head of tide) from the WTP outfalls (e.g., sites 2, 5, and 11) and some of the mainstem TFPR sites (e.g., sites 1, 10 and 13). The mixed zone included several sites that were primarily mainstem sites (e.g., sites 9, 10, and 13) that trended upward toward the discharge zone in the PCA. The PCA was very useful in proving that site 14 was impacted by WTP 4 effluent discharged near Leesynvania SP (site 14) because of its proximity to WTP effluents 1 and 2 in the loadings plot.There were 39 PPCPs quantified in TFPR sediments (∑39s PPCP), for which several PPCPs were unique to sediment. The ∑39s PPCP concentrations (ng/g dwt) in sediment also varied spatially, with the greatest concentrations observed within a 1 km radius of WTP discharge (Figure S7). However, the distinct spatial feature of sediment PPCPs showed concentrations maximized in the embayments and not directly adjacent to the discharge points as was the case with surface water. The sediment spatial presence is again best exemplified by Hunting Creek (sites 6–8), whereby the maximum concentrations were found in the deposition zone of the Lower Hunting Creek (sites 7, 8). The deposition zone occurs where Lower Hunting Creek empties into its shoal and forms a bayhead delta. This sediment deposition zone clearly traps PPCPs emerging from WTP discharge undergoing downstream transport. Such a sedimentary process creates greater ecotoxicological risk in the benthic shoal community in the TFPR from PPCPs entering through the tributaries. However, the presence of relatively large ∑39s PPCP concentrations in the Lower Hunting Creek shoal was a localized phenomenon because the mainstem TFPR sediments were much lower in concentration relative to the embayments.The change in chemical composition of the PPCPs between effluent and nearby receiving waters shows that dispersal forces beyond dilution and reactivity acted on PPCPs rapidly following discharge. The most likely physical process acting on PPCPs immediately upon discharge was sorption in geosolids. PPCPs for the most part have low Henry’s law constants, such that air-water exchange is likely negligible. However, some PPCPs are highly particle-sorptive, and it has been shown previously that substantial PPCP fluxes occur into bed sediments within the discharge zone of Hunting Creek [30]. Degradative pathways such as biotransformation, hydrolysis and photolysis are also likely to alter the PPCP compositions in surface waters. For example, photolysis of hydrochlorothiazide, which was a predominant PPCP in our study, occurs rapidly in water with a half-life of 0.43 h [31]. As stated above, diphenhydramine, fexofenadine, sertraline, and escitalopram were observed in sediments in the TFPR, where affinity for sediment has also been reported by other studies [24,32,33,34]. Atenolol, metoprolol, caffeine and carbamazepine can be rapidly degraded by residual chlorine alone or in combination with UV-light [35]. Effluent has moderate residual chlorine concentrations, which is used as a disinfectant in tertiary treatment at WTPs. Ranitidine is rapidly transformed into a nitrosamine by-product in the presence of chlorine and UV-light [36]. Bupropion undergoes rapid hydrolytic degradation in aqueous solution at pH >5 to its most prominent degradation pathway that involves a hydroxide-catalyzed catalysis of the neutral base form [37]. The pH of receiving waters reported for the TFPR estuary have ranged from 6.8 to 7.8, depending on the season [38], promoting hydrolysis. All these examples above show how geochemical partitioning along with degradative forces act on PPCPs discharged into surface waters, contributing in many cases to rapid and extensive alterations in chemical composition.There were 11 PPCPs detected at concentrations above the quantitation limit in sediments at ≥50% detection frequency. These included all the PPCPs shown in Figure 4 for site 8. All TFPR sediment sites showed PPCPs that were composed of subsets of these 11 constituents, except for Pohick Bay where triclocarban was quantified at >50% frequency in sediments. Further, the PPCPs detected in sediments showed no significant correlation (Spearman’s Rho = 0.15, p > 0.05) between log Dow (n-octanol/water distribution constant at pH 7.4) and the measured conditional distribution constant, Kd-cond (Kd-cond = Cs/Cw for PPCP concentrations quantified in sediments (Cs) and surface water (Cw) estimated in our study). The dynamic interaction of PPCPs with sediments is only partially explained by log Dow because sorption to sediment occurs through electrostatic complexation mechanisms in addition to organic matter partitioning [39,40,41,42]. The Kd-cond estimates were often much larger than expected based upon the magnitude of log Dow, especially for PPCPs predicted to be positively charged (i.e., protonated) at ambient pH. Another possible reason for lack of correlation with log Dow is because of rapid transformation that may be taking place in the environment (yielding low water concentrations). Furthermore, the sediment concentrations were not normalized to organic carbon levels because there was no observed correlation between Kd-cond and %TOC (Spearman’s Rho 0.10, p > 0.05). The %TOC in sediments is shown in Figure S8. It is generally assumed that organic micropollutants partition primarily into natural organic matter based on polarity and the (increasing) magnitude of Dow. Interactions of PPCPs between water and geosolids is a mixed complexation process, and the role neutral organic carbon plays in geochemical fate is likely not a dominant factor for many ionized PPCPs at ambient pH.The PPCPs that contributed most of the variability in composition ranked according to the largest PCA eigenvalues are shown in Table 1. The list is divided between likely sorption dominant and reaction dominant processes. The sorption dominant factors are based upon those PPCPs found to be most enriched in TFPR sediment (Figure 4). Upon WTP discharge into the TFPR it is expected these PPCPs undergo rapid sorption to geosolids followed by deposition and incorporation into benthic sediment. As shown in Table 1, sorption accounted for the highest eigenvalue ranks and largest influence in altering PPCP concentrations in surface water transport. All five sorption-dominant PPCPs listed in Table 1 are positively charged conjugate acids of weak bases at ambient pH (~7.5), which form chemical complexes with negatively charged aluminosilicate geosolids [41]. The remaining PPCPs that were not observed to be particularly enriched in sediment are expected to undergo reactions primarily through photolysis (including hydrolysis) or biodegradation leading to a depletion of concentration in the TFPR. The most obvious case is hydrochlorothiazide with a reported photolysis half-life of 0.43 hr. Hydrochlorothiazide was a prominent constituent in WTP effluent but virtually non-existent in surface water. Alternatively, metoprolol showed an increase in abundance in surface water relative to effluent reflecting a lack of reactivity with only a minor degree of sorption. Venlafaxine is known to be demethylated to form desvenlafaxine as a major metabolite, which occurs primarily during wastewater treatment. Caffeine and nicotine are enriched in the TFPR predominantly from upstream sources and not WTP as described above.