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Open Access

Peer-reviewed

Research Article

Geographical and environmental factors in pharmaceuticals and personal care products removal from drinking water plants

  • Sayoni Dutta,

    Roles Data curation, Investigation, Methodology, Writing – original draft

    Affiliation Department of Biological Sciences, Kent State University, Kent, Ohio, United States of America

    ⨯
  • Laura G. Leff,

    Roles Funding acquisition, Investigation, Writing – original draft, Writing – review & editing

    Affiliation Department of Biological Sciences, Kent State University, Kent, Ohio, United States of America

    ⨯
  • Mahinda Gangoda,

    Roles Data curation, Formal analysis, Methodology

    Affiliation Department of Chemistry and Biochemistry, Kent State University, Kent, Ohio, United States of America

    ⨯
  • Megan Zhao,

    Roles Formal analysis, Writing – review & editing

    Affiliation Department of Statistics, The Ohio State University, Columbus Ohio, United States of America

    ⨯
  • Xiaozhen Mou

    Roles Funding acquisition, Investigation, Methodology, Project administration, Supervision, Writing – original draft, Writing – review & editing

    xmou@kent.edu

    Affiliation Department of Biological Sciences, Kent State University, Kent, Ohio, United States of America

    ⨯

Abstract

Pharmaceuticals and personal care products (PPCPs) are unintentionally released into the environment from treated and untreated wastewater, agricultural runoff, and stormwater, and enter drinking water treatment plants (DWTPs). However, very little is known about the occurrence of PPCPs and their removal effectiveness during the drinking water treatment processes (conventional or advanced treatment) in multiple DWTPs, especially within the same geographical area. To address this important knowledge gap, source and finished drinking water samples were collected monthly from May to September in 2018 and 2019 from four DWTPs in Northern Ohio, USA. The total and individual PPCP concentrations varied among DWTPs and sampling dates in both source and finished water. Despite the site and time differences, caffeine and 17-beta-estradiol were detected at a frequency of 100% in all four DWTPs on all dates, while acetaminophen, estrone and nicotine consistently were not detected. Significant correlations were identified between the source water PPCPs and some environmental variables, such as dissolved organic carbon (DOC), temperature and pH. The PPCP removal efficiency from finished water samples varied depending on DWTP (61.2–84.2%), sampling month (28.3–89.7%), and different PPCP compounds (31.2% and 99.3%). Overall, this study demonstrated that PPCPs were consistently present in low concentrations (ng/L) in the freshwater systems; conventional treatment methods could only partially mitigate PPCP contaminants, while advanced techniques, such as UV-peroxide treatment, provided better removal efficiencies. PPCP removal also depended on the season, with summer demonstrating the highest removal in all four treatment plants.

Citation: Dutta S, Leff LG, Gangoda M, Zhao M, Mou X (2024) Geographical and environmental factors in pharmaceuticals and personal care products removal from drinking water plants. PLOS Water 3(10): e0000294. https://doi.org/10.1371/journal.pwat.0000294

Editor: Carl Houtman, USDA Forest Products Laboratory, UNITED STATES OF AMERICA

Received: January 18, 2024; Accepted: August 28, 2024; Published: October 1, 2024

Copyright: © 2024 Dutta et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All data are provided in the manuscript and its Supporting Information files.

Funding: This study was funded by Ohio Sea Grant (NA18OAR4170100-60063785), awarded to XM and LL. SD’s graduate assistantship was partly funded by the above funding.

Competing interests: The authors have declared that no competing interests exist.

1. Introduction

Pharmaceuticals and personal care products (PPCPs) are a broad group of prescribed and over-the-counter medications, such as antibiotics and hormones, and products for personal use, such as cosmetics and pesticides. After use or disposal, PPCPs are primarily collected in industrial and municipal wastewater [1]. Because of the incomplete removal and degradation during wastewater treatment, a proportion of PPCP residuals are released into natural waterways [1, 2]. PPCPs can also enter the natural waters through non-point sources, including agriculture runoff [3], leachates of landfills [4], and septic tanks [1, 5].

As a result of widespread use, PPCPs are ubiquitously distributed in aquatic environments, and studies have reported their presence in lakes, rivers, streams, and oceans, with concentrations ranging between ng/L and μg/L [6–9]. Although concentrations of PPCPs in natural environments are typically far below the active dosages they are designed for, some environmental PPCP compounds can still be hazardous to aquatic life [10–12]. For example, environmental PPCPs like analgesics (diclofenac and ibuprofen), antibiotics (ciprofloxacin and sulfamethoxazole), blood lipid-lowering drugs (gemfibrozil and carbamazepine), and hormones (estradiol and estriol) can induce endocrinal and physiological changes in benthic amphipods and midges [13], frogs [14], and fishes [15].

Humans can be exposed to environmental PPCPs from the consumption of drinking water. PPCPs have been repeatedly detected in source water at concentrations up to 1000 ng/L [7, 16] and even in finished water at concentrations up to 150 ng/L [17–19]. Individual PPCPs at such concentrations are unlikely to have detectable effects on human health from short-term exposures, yet combined effects of multiple PPCPs pose a considerable risk of causing adverse and long-term health impacts to humans [6, 20].

Despite the ubiquity of PPCPs and growing concerns about their potential hazardous impacts on human health, federal or state-level regulations do not require assessments of PPCPs in drinking water systems [21]. Moreover, universally adopted conventional treatment procedures for drinking water are not designed to remove PPCPs and have varied efficacy in removing specific PPCP compounds [22–24]. Consequently, the extent of PPCP contamination in drinking water and factors impacting PPCP distribution remain largely unclear. While some studies showed a positive correlation between PPCPs and nutrient loading [25], others did not [26].

In addition, our understanding of the effectiveness of current water treatment methods in removing PPCPs is very limited. Studies that documented the removal of PPCPs in drinking water have primarily focused on single treatment plants [17, 19, 27], and only a handful of studies have examined more than one plant [28]. Of the studies that have worked with more than one plant, only some have compared conventional and advanced treatment methods at multiple plants [28]; none have examined the occurrence and removal of PPCPs at different DWTPs in the same geographic area with different water sources [29].

We hypothesized that the locations that have high nutrient loading would have higher occurrence of PPCPs in the source of drinking water. We also hypothesized that the removal efficiency of PPCPs is impacted by temporal and site differences and as well as the treatment methods. To test these hypotheses, water samples were taken from four drinking water plants with a range of nutrient loading in Northern Ohio, USA, and concentrations of 14 PPCP compounds were determined in both source and finished waters. Water quality variables were also measured in source water and assessed for potential correlations with PPCP occurrence.

2. Materials and methods

2.1 PPCP model compound

A total of 14 PPCP compounds that are commonly detected in freshwater environments [6, 26, 30, 31] were selected for quantification. These included pharmaceuticals like analgesics: acetaminophen (Ace), butalbital (But), naproxen (Nap) and ibuprofen (Ibu); antibiotics: sulfamethoxazole (Sul); anti-seizure and bipolar medication: carbamazepine (Car); lipid regulator: gemfibrozil (Gem); hormones: 17-beta-estradiol (Estdl) and estrone (Est); and stimulants caffeine (Caf), nicotine (Nic) and its metabolites cotinine (Cot); and two personal care products: insect repellent: N, N-Diethyl-meta-toluamide or diethyltoluamide (DEET) and antimicrobial agent: triclosan (Tri). Standards of the PPCP compounds were purchased from Thermo Fisher Scientific (Waltham, Massachusetts, USA).

2.2 Sample collection and processing

Water samples (in triplicates) were taken from four municipal drinking water treatment plants (DWTPs) in Northern Ohio, USA, including plants AK, AL, RA, and SA (Table 1). All four of the DWTPs use conventional drinking water treatment processes that consist of coagulation, flocculation, sedimentation, and gravity-fed sand filtration. AL and SA use powdered activated carbon (PAC) during the flocculation and granulated activated carbon (GAC) during filtration, whereas AK and RA only use PAC during the coagulation-flocculation process. AL also uses UV- H2O2 advanced oxidation process (AOP) post filtration (Table 1).

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Table 1. DWTP sites and their major treatment steps.

Treatment steps are included in selected drinking water plants. ’+’ means the site has that treatment method and ‘-’ means that it does not. PAC, powdered activated carbon; GAC, granular activated carbon; C-F, coagulation and flocculation; AOP, advanced oxidation process. * Chlorinated tap water is used to backflush the sand filters regularly.

https://doi.org/10.1371/journal.pwat.0000294.t001

Source and finished drinking water samples (4L each) were collected monthly using acid-washed, high-density polyethylene bottles at each DWTP from May to September in 2018 and 2019. Source water was collected from the reservoir pumping stations, whereas finished water samples were collected from faucets inside the plants before water distribution to households. During the sampling, the temperature (T) and pH of source and finished water samples were obtained from the water treatment plants. After collection, water samples were put on ice and transported back to the laboratory within 1.5 hr. All DWTP samples were taken with the assistance of the local drinking water plants without interference with any of the drinking water plant’s operation. No permits were required for taking and analyzing the obtained DWTP samples.

Once arrived at the laboratory, source and finished water samples were immediately filtered using 0.45 μm membrane filters (Thermo Fisher Scientific, Waltham, MA). The resulting filtrates for source water samples were split into two parts, one for PPCP analysis (3L) and the other for nutrient measurement (1L), and both were stored at 4°C until further analysis. The resulting filtrates for finished water samples were not split and stored at 4°C until further analysis only for PPCP measurements.

2.3 PPCP extraction and preparation

Filtered source water samples were mixed with hydrochloric acid (HCl) in tetrasodium ethylenediamine-tetraacetate hydrate (Na4 EDTA·4H2O, final pH = 2). Acidified samples were then loaded into pre-washed hydrophilic-lipophilic (HLB) cartridges (Waters Corporation, Milford, Massachusetts, USA) for solid-phase extraction (SPE) of PPCPs following the manufacture’s instruction and the EPA method 1694 [32] and EPA method 539 [33]. Filtered finished water samples were prepared following the same EPA methods for PPCP extraction and preparation [32, 33], except that prior to the SPE, they were treated with sodium thiosulfate (80 mg/L in water) to remove residual chlorine. SPE cartridges were then air-dried, washed with HPLC grade water, before PPCPs (except for triclosan) were eluted by gravity filtration using 12 mL methanol. For measurements of triclosan, 6 mL each of acetone and methanol were used for elution.

2.4 HPLC-MS measurement of PPCPs

Extracted PPCPs were analyzed using high-performance liquid chromatography (HPLC) coupled with an electrospray ionization tandem mass spectrometry (MS; Agilent, Santa Clara, California, USA) using a Waters (Milford, Massachusetts, USA) C18 HPLC column (10 cm, 2.1 mm id, 3.5 μm particle size). Twelve PPCP compounds (Ace, Sul, But, Ibu, Nap, Car, Gem, Caf, Cot, Nic, DEET and Tri) were analyzed following the EPA method 1694, [32] whereas Est and Estdl were analyzed following the EPA method 539 [33].

PPCP standards were used to generate the standard curves. At least five concentrations of standards were prepared to generate a calibration curve for determining PPCP concentrations in the samples. Limit of detection (LOD) was defined as concentrations corresponding to 3 times of signal to noise (S/N) ratios [34] During statistical analysis, concentrations of PPCPs that were below LOD were substituted with half of the LOD values.

The removal efficiency of each PPCP was calculated using the following formula:

100% × ([influent concentration]–[effluent concentration]/ [influent concentration])

2.5 Nutrient analysis

Nutrients in the source water samples were measured according to standard EPA procedures. Briefly, raw water samples were filtered using 0.2 μm filters and filters were stored at 4°C in the dark for subsequent analysis of chlorophyll-a concentration using a spectrophotometer following the method used by Knefelkamp et al., 2007 [35]. Concentrations of ammonium, nitrate, and soluble reactive phosphorus (SRP) were spectrophotometrically (Beckman Counter DL730 UV, California, USA) determined using the indophenol blue or EPA 349 method [36], azo dye or EPA 353.2 method [37], and antimony-phospho-molybdate complex or EPA 365.3 method [38], respectively. Dissolved organic carbon (DOC) was determined with a TOC/TN analyzer (Shimadzu Corp., Tokyo, Japan) after samples were acidified overnight using 2 M HCl to remove inorganic carbon using EPA method 351.2 [39].

2.6 Statistical analyses

All statistical analyses were performed using the Vegan package in R [40]. PPCP concentrations and PPCP removal efficiency of different sampling sites, sampling locations and dates (months) were compared with a three-way analysis of variance (ANOVA) using water plant, month, and year variables for source and finished water. Significant results derived from ANOVA analysis were further evaluated by a posthoc Tukey’s test. P values < 0.05 were regarded as significant. Stepwise multiple linear regression (MLR) analysis was performed with backward elimination to examine potential effects of environmental variables (Chla, ammonium, SRP, DOC, nitrate, pH, and T) on individual PPCP concentrations. Prior to the MLR analysis, Pearson correlation analysis was performed to eliminate highly correlated environmental variables that had correlation coefficient r ≥0.8. The overall relationship between PPCPs and the sites and months were examined using non-metric multidimensional scaling (NMDS) based on the Bray-Curtis similarity matrix.

3. Results

3.1 Occurrence of PPCPs

In source water, 11 out of the 14 PPCPs were detected at least once in the DWTPs during our two-year sampling period (May-September 2018 and 2019; Table 2 and Fig 1). Caf and Estdl were found in every source water sample, i.e., with a frequency of detection (FD) of 100%. Nap (FD, 40%), But, Cot, and Sul (FD, 35%) were measured in over a third of the samples. With a decreasing FD, Car, Ibu, Gem, and Tri were found in 30%-15% samples. Ace, Est, and Nic were not detected in any source water samples (FD, 0%).

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Fig 1. Distribution and average concentration of PPCPs in the source (S, solid bars) and finished (F, stripped bars) water of four DWTPs (AK, AL, RA, SA) over two years of sampling period.

https://doi.org/10.1371/journal.pwat.0000294.g001

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Table 2. PPCP compounds and their limit of detection (LOD) concentration, average and standard deviation Avg (SD) concentrations (ng/L level; n = 3), and frequency of detection (FD%) in the source and finished water samples from four DWTPs.

The standard deviation of each measurement is provided in the parentheses for each PPCP compound. "-" indicates a PPCP concentration below the LOD; it is assigned as half of the LOD when used in statistical analyses. Compound names are shown in abbreviation, and their respective full names are provided in the method.

https://doi.org/10.1371/journal.pwat.0000294.t002

In finished water, 7 out of 14 PPCPs were present at least once in the DWTPs (Table 2 and Fig 1). Consistently, as observed for source water, Caf and Estdl showed 100% FD. Cot had the second-highest FD (20%) in the finished water, followed by Sul (FD,15%), Ibu (FD, 15%), Car (FD,10%), and Gem (FD, 5%). DEET, Nap, and Tri were not detected in any finished water samples (FD 0%), contrasting their FD values (15%-30%) in the source water. Like the source water samples, Ace, Est, and Nic were not detected in any finished water samples.

3.2 PPCP concentrations in source water

In source water, the total PPCP concentration (i.e., sum of 11 detected individual PPCPs) ranged between 22.8 ng/L and 175.1 ng/L (Fig 1). A three-way ANOVA analysis showed that the total PPCP concentrations in source water were significantly different among sampling dates and sites (P<0.05). The total PPCP concentrations were higher in 2019 (46.9–181.6 ng/L) compared to 2018 (44.3–133.2 ng/L; P<0.05). For monthly variations, July (48.0–167.6 ng/L), and August (73.5–181.6 ng/L) had the highest total concentrations, whereas May (44.3–68.4 ng/L) and September (46.9–50.9 ng/L) was found to have the lowest total PPCP concentrations (P<0.05).

Concentrations of detected individual PPCPs in source water ranged from 9.8 ng/L (DEET) to 36.6 ng/L (Cot) (Fig 1). The stimulant Caf and the hormone Estdl were detected repeatedly across the sampling sites and months within a range of 20.8–34.5 ng/L and (23.3–32.5 ng/L, respectively, and counted for > 25% of total PPCPs in the water samples. The NMDS ordination illustrated PPCP grouping patterns based on site and sampling month for source water (Fig 2A). Source water PPCP measurements of May and September clustered tightly and generally away from June, July, and August samples. Caf and Estdl had consistent clustering patterns for all four sites and five sampling months. Cot, Car and But showed a separate distinct clustering for July and August for the sites AK and RA. Gem, Ibu and Sul formed a distinct group for the sites AL and SA due to their higher occurrence in those sites.

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Fig 2.

NMDS plots using the Bray Curtis dissimilarity plot illustrating the temporal and spatial PPCP distribution across five months (May, June, July, Aug, Sep) and four sites for source (a) and finished (b).

https://doi.org/10.1371/journal.pwat.0000294.g002

3.3 Correlations between source water PPCP and environmental variables

To examine the potential relationship between PPCP concentrations and environmental variables in the source water multiple linear regression analysis was performed (Table 3 and S1 Table). Concentrations of PPCPs were best predicted by different sets of environmental variables. For example, But was best predicted by DOC concentration and T, while Nap was bested predicted by concentrations of ammonium, SRP and pH. MLR analysis also showed that specific environmental factors might have opposite correlation trends with different PPCPs. Out of 11 PPCPs, DOC had significant correlations with 8 of them. DOC concentration was positively correlated with But, Caf, Car, Cot, DEET and Estdl, but negatively correlated with Gem, Ibu and Sul. Meanwhile, DOC concentration had no significant correlations with DEET, Nap and Tri. pH and T had significant correlations with 5 and 6 PPCPs, respectively. pH had significant positive correlations with Nap, and negative correlations with Caf, Gem, Ibu and Tri. T had significant positive correlations with Estdl, Gem, Ibu, Nap and Sul.

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Table 3. Stepwise multiple linear regression coefficient and P values for individual PPCP compounds and environmental variables in the source water (n = 120).

*p <0.05; **p <0.01; *** p <0.001.

https://doi.org/10.1371/journal.pwat.0000294.t003

3.4 Concentrations of PPCPs in the finished water

Concentrations of total PPCPs in finished water samples were about half of those in source water (P < 0.05) and ranged between 20.7 and 75.6 ng/L (Fig 1). Concentrations of detected individual PPCPs in the finished water ranged from 8.1 ng/L (Car) to 22.9 ng/L (Cot), (Table 2 and Fig 1). A three-way ANOVA analysis showed that the total PPCP concentrations in finished water were significantly different among sampling dates and sites (P<0.05). The total PPCP concentrations in fished water were higher in 2019 (21.8–82.8 ng/L) compared to 2018 (20.5–43.6 ng/L), similar as for the source water. Temporal difference of PPCP concentration in finished water was observed and July (20.5–82.8 ng/L) and August (25.9–62.1 ng/L) samples had the highest values of total PPCP concentrations for both 2018 and 2019, whereas May (20.7–28.1 ng/L), June (22.0–43.6 ng/L) and September (20.9–27.0 ng/L) had lower values in finished water.

The NMDS analysis ordinated finished water PPCP concentrations based on site and month (Fig 2B). Like the source water, Caf and Estdl showed clusters for all four sites and five months. Sul and Ibu measurements of June, July, and August for SA were closely clustered, whereas Cot and Car measurements of July and August for the sites AK and RA were closely clustered.

3.5 Removal efficiency of PPCPs

PPCP removal efficiency varied among sites, with the average highest removal efficiency (84.2%) at AL (P<0.05). The plant with the second highest removal efficiency was AK (68.1%), then SA (66.5%) and finally RA (61.2%) (P<0.05). The PPCP removal was highest in August (89.7%) and the lowest in September (28.3%). The overall removal efficiency of individual PPCPs ranged between 31.2–99.3%, with an average of 55.2% (Fig 3). Out of 11 detected individual PPCPs in the source water, 4 compounds, i.e., But, DEET, Nap, and Tri, were completely removed after water treatment (>99% removal) from all four sites. Gem, Ibu and Sul were completely removed from AL (>99%) but only partially (39.1%, 34.5%, and 53.3%, respectively) removed from SA. The compound with the lowest removal rate was Cot, with an average removal rate of 36.6%. The two compounds that showed no significant differences in removal among sites were Caf and Estdl, which had 45.2–66.8% removal efficiency. These two compounds were consistently found in the source and finished water of all four water plants for all sampling months.

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Fig 3. Average removal efficiency (%) of detected PPCPs from the drinking water in each of the four drinking water treatment plants.

The error bars represent the standard deviations of means. The empty spaces represent compounds that were below the limit of quantitation in both source and finished water samples.

https://doi.org/10.1371/journal.pwat.0000294.g003

4. Discussion

The increase in the global human population and continuous usage of PPCPs have resulted in pseudo-persistence of PPCPs in natural water; in turn this causes adverse effects on environmental health [41]. Our studies have identified PPCPs from all tested source and finished drinking waters, suggesting PPCPs can directly reach humans. Measured PPCPs were at levels of several to hundreds of ng/L. Although PPCP at such levels do not pose immediate risk to human health, the long-term additive effects of PPCP mixtures on humans are unknown and concerning [42]. Among the 14 targeted PPCPs compounds, three (Ace, Est, and Nic) were always below the detection limits. These three PPCP compounds have been identified in many other natural waters [6, 7, 26], but their absence in water samples have also been frequently reported [26, 43–46], especially in areas with low anthropogenic impacts. The presence of any PPCP is the net result of the input rate being higher than the removal rate. Generally, sites of drinking water sources are in protected areas with reduced public access and thus potentially have low inputs of PPCPs. It is also possible that microbial, photochemical and other PPCP transformation processes outcompeted the input rate of these PPCPs in the sampling sites. Ace can be degraded by various aquatic microorganisms in environments, and the transformation process appear to be enhanced by solar radiation [47, 48]. Similar findings have also been reported for Est [49, 50] and Nic [51, 52].

Despite their different geographical locations and water sources (Upper and middle Cuyahoga River, Lake Erie, and Mahoning River basin), Caf and Estdl were consistently found in the source and finished water of all four plants during our two years of sampling. One probable reason for this persistent occurrence of Caf and Estdl could be a consistent high input of these compounds in the source water sites (due to high, consistent human use), and partial removal of these two compounds on sites and by all four DWTPs. Consistent presence of Estdl is particularly concerning since it can be bioaccumulated (hundreds of folds) by aquatic organisms [53]; and at μg/L levels it can cause animal reproductive and developmental issues [54].

Other detected PPCPs were differentially distributed in the source and finished drinking waters among sites. For example, Nap and Tri were only detected in the source water of the SA plant. This could be due to higher levels of urbanization, agriculture, and industrialization in the drainage basin of Lake Erie, where the SA plant draws its source water [55, 56], than the other sites. Nap is a non-steroidal anti-inflammatory drug that has been found to cause acute and chronic toxicity to algae and rotifers [57]. Tri on the other hand is a synthetic antimicrobial compound that has been used in toothpaste, hand soaps and detergents for more than thirty years [58]. Tri, at higher concentrations (high μg/L- mg/L), affects the behavior and cause mortality in several organisms including algae, arthropods, mollusks, amphibians, and fishes [13, 59, 60]. The widespread usage of Tri has also been blamed for the increased antibacterial resistance in numerous bacteria [61, 62]. The Food and Drug Administration (FDA) has banned the use of Tri in consumer soap products in September 2016. However, it is still widely used in other consumer products such as mouthwash, toothpaste, hand sanitizer, trash bags, socks and bedding which could result in the continuous presence and steady concentrations of Tri in the environment [63]. The presence of these two and other PPCPs in SA highlights the potential ecological threats of PPCPs to the Laurentian Great Lakes, the largest freshwater system in the world.

High nutrient loading is often a consequence of increased human activities, we therefore hypothesized its association with PPCPs occurrence [64, 65]. Indeed, DOC were found as an important predictor for 8 out of 11 detected PPCPs; and the other 3 PPCPs were significantly correlated with either N, P or S content (Table 3). Moreover, consistent with other studies, 8 of the 11 tested PPCP were significantly correlated with T and pH changes [44, 66, 67]. These two environmental variables also interact with photochemical and microbial activities and may synergistically impact PPCP degradations [44, 68]. In addition, differences in PPCP chemical structures have also been found to play a role in the distribution of PPCPs in aquatic environments [27, 69].

Consistent with our hypothesis, PPCP removal data showed site difference and the plant (AL) that equipped with advanced oxidation processes (AOPs) had the highest efficiency in removing PPCPs from the finished water than plants of conventional methods [46, 47]. Car, Ibu and Gem were completely removed (>99%) in the AL water plant but not the other plants, and these results were consistent with previous studies that showed >99% removal of these PPCPs from drinking water plants with UV disinfection [22, 24]. However, even in AL, Caf and Estdl were still not completely removed, indicating that persistence of PPCPs is a challenge to current DWTP treatment methods and requires further research and development of more effective removal techniques. Among the three traditional DWTPs, trends of PPCP removal appeared to be site specific, which is likely due to the operational difference among DWTPs (Table 1) [70].

5. Conclusion

In conclusion, PPCPs were detected in both source and finished water of Northern Ohio in low concentrations (ng/L). Contrary to our hypothesis, nutrient loading cannot serve as a general predictor for PPCP occurrence. The correlation between PPCP distribution in source drinking water and environmental variables (nutrient loading, T and pH) were compound-specific. The removal activity of PPCPs varied temporally and among sites and had the highest rate in the DWTP with AOP. These findings support our hypothesis that the removal efficiency of PPCPs is impacted by temporal and site differences and as well as the treatment methods. Further studies are needed to investigate the health impact of chronic exposure to low levels of PPCP mixtures on wildlife and humans and to develop more effective PPCP removal methods.

Supporting information

S1 Table. Environmental variables measured in the source of the drinking water from 4 drinking water treatment plants.

https://doi.org/10.1371/journal.pwat.0000294.s001

(DOCX)

Acknowledgments

We would like to thank the superintendents, managers, and staff members of participating water treatment plants, particularly Amy Elliot from AL, Jessica Glowczewski, Charles Lacy, Amy Wilson from AK, Tim Stephens from RA, and Orin McMonigle from SA, for helping with sample collection. We would also like to thank Dr. Kai Wang for helping with sampling.

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