4.1. PFASs in Surface Waters
In this work, we studied the concentrations of 23 PFASs in 11 riverine waters during four seasons. The average concentration of ∑PFAS (7.9 ± 11 ng L−1
) was on the same level as concentrations in Swedish rivers (9.9 ± 15 ng L−
1) presented by Nguyen et al. [17
]. However, the mean concentrations of PFBS, PFHxS and PFDA were 42, 23 and 15 times higher in Sweden than in Finland, respectively. Also, PFASs with carbon chains longer than 9 carbons were detected in higher frequencies in Swedish rivers. One reason for this may be Sweden’s higher population and thus higher loading of PFASs. Similarly, higher concentrations of short-chained PFSAs and PFCAs in the rivers of southern Finland compared to the rivers of northern Finland are most likely a consequence of denser population and industrial activities, which leads to larger usage of products containing PFASs.
In a previous study, PFASs were detected in the effluents of Finnish WWTPs [18
]. In W2 and W3, the concentrations of PFASs were remarkably high in June and July during the low flow period. The proportion of municipal wastewater, which is not dependent on weather conditions—in contrast to, for example, PFAS load via surface runoff from contaminated land areas—is at its highest in river water during low flow conditions. These results give preliminary indications that WWTPs are major sources of PFASs to these rivers.
The AA concentrations of PFOS exceeded the AA-EQS (0.65 ng L−1) in W3 (11 ng L−1), W2 (1.3 ng L−1) and W6 (0.80 ng L−1). The AA concentrations were close to AA-EQS in W5 (0.64 ng L−1) and W7 (0.61 ng L−1).
A few cities use the studied rivers as a source for drinking water production. For example, the city of Turku and its nearby municipalities use the water of W6 to make artificial ground water. Water utilities of the cities of Oulu and Vaasa use the Oulujoki (W9) and W7, respectively, as their raw water sources. However, the concentrations of PFOS (0.10–0.49 ng L−1) or PFOA (0.26–1.3 ng L−1) in these rivers did not exceed the WHO’s proposed limit values (4.0 µg L−1 and 40 µg L−1, respectively) for drinking water. Even so, since PFASs are present in the rivers, the quality of raw water supply should be monitored.
Absolute and relative concentrations of several PFASs were almost identical in the background sites, W13 and the outlet of W12 (Figure 2
), although these sites are 750 km apart. PFASs detected in the background sites were also present in relatively similar concentrations in the other rivers, excluding W3 and W2. This implies that atmospheric deposition is an important route for these compounds (e.g., PFPeA, PFHxA, PFOS, PFOA) or their precursors to the environment. On the other hand, W3 and W2 appear to also have specific sources for PFPeA, PFHxA, PFOA, PFOS and 6:2 FTSA based on their distinguishable concentrations in these rivers.
PFPeA and PFHxA, which are degradation products of 6:2 FTSA and of other fluorotelomer-based precursors [19
], were the only short-chained PFASs detected in background sites. According to Ahrens et al. [5
], 6:2 FTSA, along with PFOS and PFHxS, dominates in a biotic environment near firefighting sites. PFHxS is also one substituent for long-chained PFASs in firefighting foams [22
]. Increasing use of 6:2 FTSA may thus be reflected in higher environmental concentrations of its degradation products.
The PFAS precursors are typically volatile [7
], unlike their degradation products. Woldegiorgis et al. [24
] documented the presence of 6:2 FTSA in air particles and deposition samples collected from a monitoring site near W13. Further, Filipovic et al. [25
] showed that PFHxA was the dominant PFCA compound in wet deposition samples collected from a pristine Nordic study area. Along with these studies, this work provides evidence that the increased use of 6:2 FTSA may result in higher PFHxA concentrations in the environment in the future, even in very remote areas.
Some of the most commonly found PFASs were not detected in our background sites. PFHxS, which has been noted to exhibit positive correlation in concentrations with PFOS [26
], was undetectable in background sites and in sites in northern RBDs 5 and 6, but was detected in all of the rivers in RBDs 2–4. Similarly, PFBA and PFBS, whose elevated concentrations in the environment are assumed to be the outcome of their uses as substitutes of PFOS and PFOA [13
], were detected in all of the rivers, but not in the background sites. These findings strongly suggest that the occurrence of these compounds in surface water is predominantly linked to industrial wastewaters and to the usage of firefighting foams, rather than to long-range atmospheric transport.
There was no significant difference in concentrations of long-chained PFCAs between the river water samples. An exception to this rule was W3, which exhibited high concentrations of long-chained PFCAs, in particular PFOA and PFNA. The similar concentrations of long-chained PFCAs between the sites indicate that atmospheric deposition is an important route for long-chained PFCAs to the aquatic environment of Finland. However, despite being below LOQ in the river waters, the concentrations of poorly water-soluble PFUnDA, PFDA and PFTrDA in perch caught from point source-polluted sites imply that these long-chained PFCAs or their precursors end up or have ended up in the environment also from point sources.
PFASs are known to be predominant compounds in contaminated land areas such as firefighting training areas and airports [5
]. High concentrations of PFASs have also been detected in landfill leachates [18
]. However, significance of contaminated soils as sources for PFASs to Finnish waterbodies is unknown, and it is recommended that this be studied in the future.
4.2. PFASs in Fish
Higher concentrations of PFOS in perch sampled from point source-polluted sites compared to perch from diffuse source-polluted sites indicate that PFOS originates from point rather than diffuse sources. The high PFOS concentration in perch in P1 (18 µg kg−1) is most likely a consequence of the high PFOS loading from W3, which discharges into the bay.
Geometric mean PFOS concentrations exceeded the EQSbiota
in three sampling sites of perch: P1, Porvoonjoki (P5) and Tuusulanjärvi (P6), but in none of the herrings. Overall, fewer compounds were detected in herrings than in perch. It is recommended in the WFD guidance document [31
] that the trophic level of the sampled biota is determined for the adjustment of monitoring data for comparison with the relevant EQSbiota
. Trophic level was not determined in our study, but fish size was selected in terms of WFD recommendations. Based on the size of the fish (see the Table S1
), we assume that they represent trophic levels 3–4, which, according to the WFD guidance document, is considered to be sufficient for estimations of both chemical status of water bodies and safety for human fish consumption.
PFOS concentrations in herrings were generally lower than the concentrations reported earlier from the Estonian coast of the Gulf of Finland [32
]. The concentration of PFOS in limnic perch in Sweden was between 0.1 and 4 µg kg−1
f.w., and in marine perch, was between 1.2 and 30 µg kg−1
], which is quite well in line with the results presented here. Markedly higher PFOS concentrations in various fish species have been measured in Central Europe, e.g., in Germany (39 µg kg−1
] and in the Czech Republic (61 µg kg−1
PFOS has adverse effects on human health, and the tolerable weekly intake of PFOS is estimated to be 13 ng kg−1
body weight, which is exceeded in a considerable proportion of the population [36
]. The high PFOS concentrations in perch from the most severely polluted sites, e.g., P1, should be taken into account when making recommendations on eating fish.
PFOA seemed to be assimilated more easily into the tissue of herrings compared to perch, although herrings tend to live in open sea, where there are no direct sources of PFOA. These results are in accordance with previous studies [15
], in which PFOA was analysed from several Baltic fish species and the highest concentrations were detected in herrings. This indicates that species-specific mechanisms are responsible for a higher accumulation of PFOA in herrings. PFOA is harmful to humans [36
], but, according to the results of this study, the concentrations of PFOA in fish in Finland are low and therefore pose no risk to humans from eating fish.
4.3. PFAS Load to the Baltic Sea
For the first time in Finland, the annual riverine PFAS load to the Baltic Sea was calculated using actual measured AA concentrations of PFASs. As a result of more accurate data, the load proved to be lower than in previous estimations [37
]. This study also covered a wider geographical area than the previous study. One reason for the lower load of PFASs could also be the substitution of PFOS with other unknown substances not analysed in our study.
The load of PFBA, which is a substituent of longer-chained PFASs [11
], from the studied rivers leading to the Baltic Sea was remarkably high: 17 kg yr−1
in total. Since PFBA and other short-chained PFASs have replaced longer-chained PFASs in applications, and while PFOS load is expected to decrease owing to the restrictions, the load of short-chained PFASs is likely to increase in the future. Concerning the PFASs that are still used, wastewaters remain a significant source of PFASs to the receiving water bodies.
Despite the regulations, PFOS loading from Finnish rivers to the Baltic Sea remains to be 10 kg yr−1
. The load of PFOS from WWTPs to surface waters in Finland was estimated to be 12 kg yr−1
in 2013 [38
]. Since the use of PFOS in industry has for the most part ceased, we assume that products in use are currently the main source of PFOS to WWTPs and from them to receiving water bodies. Moreover, as PFOS is no longer industrially used in Finland, the emissions from WWTPs will decrease in the future when products containing PFOS are replaced.
Contaminated land areas, such as areas where firefighting foams have been used or WWTP sludge has been used as a soil amendment, are possibly major contributors to riverine PFAS loading. Unfortunately, the extent and locations of these activities are not well known in Finland, and thus the role of storm waters and utilisation of WWTP sludge in PFAS loading should be investigated.