Spatial distribution and temporal evolution of polycyclic aromatic hydrocarbons in sediment and water in the northern part of Taihu Lake, China

Taihu Lake is the third largest freshwater lake in China, playing an important role for flood control, tourism, shipping and especially as a drinking water source for its neighboring cities, but the lake has been seriously polluted and drinking water supply has been threatened. Polycyclic aromatic hydrocarbons (PAHs) are ubiquitously distributed in the environment with petrogenic and pyrogenic sources, and they are carcinogenic and mutagenic to humans and other organisms. With the development of economy and industries, the rapid increase of fuel and biomass consumption results in significant PAH emission to the environment. In this study, we focus on PAHs in the sediments and water body in the northern part of Taihu Lake where is more polluted than the other part of the lake. We analyzed the concentration patterns of 20 PAHs in 25 surface sediments, 11 sediment cores and 41 water samples which were collected from the northern part of Taihu Lake during 4 times of field campaign (2015-11, 2016-06, 2017-02 and 2017-09). Three of the cores were dated based on 137Cs activity for the deposition age of the sediment. The data on energy consumption and type of vehicles in the lake catchment in the last two decades were collected from regional official websites to explain potential PAH emission histories in the area. The literature data on PAH emissions from their potential sources and on PAH distributions in particle sizes were collected to generalize PAH emission and distribution features. PAH patterns from different emission sources and also from the sediment results of this study were combined to verify the two assumptions for the methods of PAH source track in the environment. The spatial distributions of the PAH concentrations (perylene excluded) show that the inflow rivers into Zhushan bay and Meiliang bay were the main pathways for PAHs and sediments input into the northern part of the lake. This results in substantially higher PAH concentrations (up to 5000 ng/g) and sedimentation rates (higher than the average of 3 – 4 mm/a) in the area close to the river outlets. In addition, the results also show that PAH concentrations in the sediments considerably increased from the late 1950s due to the development of economy and industries, but the relatively decreased or stable concentrations in the upper layers of the sediments could be attributed to the gradual changes in energy structures, emission control in coal combustions since the 1990s and emission control in vehicle exhaust since ca. 2000 in this area. PAHs determined in the sediment cores (perylene excluded) are dominated by 4 light ones (phenanthrene (phen), anthracene (anthra), fluoranthene (fluor), pyrene) and 6 heavy ones (benzo[k]fluoranthene (BkF), benzo[a]pyrene (BaP), benzo[b]fluoranthene (BbF), benzo[e]pyrene (BeP), indeno[1,2,3-c,d]pyrene (INcdP), benzo[g,h,i]perylene (BghiP)) whose concentrations typically reach up to 70% – 80% of that of the 19 PAHs. The concentration fractions of the 6 heavy PAHs are almost double of the fractions of the 4 light ones, and the fractions of the heavy PAHs increase with decreasing depth in the cores, but the fractions of the light PAHs show the opposite trends (except cores GH38 and ZS23 with special features). PAH emission patterns from wood combustion, coal combustion and vehicle exhausts together can, to a great extent, illustrate the distribution patterns of the concentration fractions between the light and the heavy PAHs in the cores. Wood combustion has evidently higher emission fractions of the 4 light PAHs than the fractions of the 6 heavy PAHs compared to the emission patterns from coal and oil combustions, but wood has been gradually phased out as an important energy source since the beginning of industrialization in the 1950s. PAH patterns from coal combustions can somewhat explain the concentration fraction distributions of the two groups of PAHs in the cores through four points: the continuous decrease of residential coal consumption and the significantly increased consumption of emission-controlled coal combustion in this area during the last decades, relatively more ultrafine particles emitted from emission-controlled coal combustion, heavy PAHs bond more in ultrafine particles. Meanwhile, PAH patterns from vehicle exhausts contribute to the distributions of the concentration fractions in the cores through three points: the linear increase of oil consumption in transport, around 24-time increase in the number of light-duty vehicles but only 2-time increase in the number of heavy-duty vehicles in this area during the last two decades, and noticeably higher emission fraction of the heavy PAHs from light-duty vehicle exhausts than that from heavy-duty vehicle exhausts. Several methods are frequently used to interpret PAH source track in the environment, and these methods are based on two assumptions: PAH patterns from different sources are specific and distinguishable from each other; the patterns maintain stable after emission to the environment. The first assumption was mainly verified by the PAH patterns from different emission sources, which shows that PAH patterns from different sources indistinguishably overlap from each other. The second assumption was verified by the spatial and temporal distributions of PAH patterns in the sediment of this study, which shows that PAH patterns in the sediment are ambiguous and variable. Therefore, both of the two assumptions are not valid for PAH source track in the sediment. There were both anthropogenic and biogenic origins of perylene in the lake sediments, which were distinguished based on its spatial distribution patterns and also the concentration proportions of perylene to the sum of the 20 PAHs. In the cores collected close to the river outlets, the concentration proportions of perylene typically range from 0.02 to 0.18 and there are significant positive linear correlations between the concentration of perylene and three anthropogenic PAHs (BaP, BeP, Pyrene), suggesting that perylene was dominated by anthropogenic input. However, the cores collected further away from the river outlets show the concentration proportions between 0.13 and 0.96, and present significant negative correlations or no correlations between perylene and the three PAHs, suggesting that perylene was mainly formed by biogenic activities. Furthermore, the different perylene sources accompanied with the core location distributions imply that anthropogenic activities could inhibit its biogenic formation. In the water samples, naphthalene (naph), 1-methylnaphthalene (1methylnaph) and 2-methylnaphthalene (2methylnaph) have considerably higher concentrations in the samples (2016-06 and 2017-09) taken in warm seasons than the concentrations in the samples (2015-11 and 2017-02) taken in cold seasons, but the concentrations of the other PAHs do not vary with seasonal variations. The distribution patterns of these PAHs might be mainly attributed to ambient temperature effects on the PAH solubility in the water body. It is noteworthy that in the campaign 2017-09, the concentrations of 2methylnaph are particularly around twice as high as the concentrations of naph and reach up to 1350 ng/L. As the emission rates of naph are generally higher than methylnaph from anthropogenic activities, so it is suspected that the relatively higher concentrations of 2methylnaph should be attributed to additional biogenic input in the lake water. Furthermore, the samples collected in cold seasons show that high concentrations are mostly located in the northwestern part of the lake and relatively low concentrations are in the northeastern part of lake, but the concentrations from the warm season samples are homogeneous in the whole sampling area. The reason for the different spatial distributions between the cold and warm seasons is that during the sampling campaigns in cold seasons, there was water recharge from the Yangtze River through the Wangyu River connecting the northeastern part of Taihu Lake, which diluted PAH concentrations in the northeastern part of the lake