Environmental Geochemistry Of Typical Organic Contaminants In The Soil-groundwater System Of Sewage Irrigation Area

In China, agricultural irrigation has been a major sector of water consumption in this country of water shortage. Sewage irrigation has been a common practice in northern China due to subtantial difference in demand and suppy of water resource Sewage irrigation owns certain advantages, such as relieving stress of water supply, increasing fertility, and purifying sewage quality, but because of the low level of sewage treatment, irrigation water contains inorganic contaminants, microorganisms, organic contaminants and other substances. Once released into the soil-groundwater system, the pollutants pose a great threat. Previous studies on the environmental effects of sewage irrigation have been focused on heavy metals and microorganisms, few studies on organic contaminants. The Xiaodian sewage irrigation area in Taiyuan City of Shanxi Province in northern China which has a history of nearly four decades’ history of sewage irrigation was selected for this study. Surface water, groundwater, shallow vadose zone soil and vadose zone sediment samples were collected, and the environmental geochemistry of the occurrence, distribution and migration of organic contaminants in the soil-groundwater system was studied. The results of this sudy provide important data and clue for the management of sewage irrigation in China.The main study contents of this thesis are summarized as follows:1. Hydrochemical characteristicsIn order to characterize the water qualities and identify recharge sources of groundwater, water samples from irrigation canal and drainage canals and groundwater samples were collected for hydrochemical analysis and stable hydrogen and oxygen isotopes analysis. The results can be summarized as follows.(1) The deterioration order of surface water quality in the study area is:river, irrigation canal and drainage canals.(2) Except Se, the concentration levels of other detected components meet China national standards for irrigation water quality. Heavy coal combustion is considered to be the reason causing Se in the aerosols settling into surface water.(3) High concentrations of NO3- in several groundwater samples indicate that irrigation practice has already affected the groundwater quality.(4) For groundwater with lower concentration of Cl, minor halite dissolution is the major source for its salinity; for groundwater samples with high concentration of Cl, evaporation and mixing are the major causes for the salinity.(5) From the relationship between concentrations of Cl and δ18O, the main sources of recharge of groundwater is rainfall, Fenhe River water, sewage and return water after irrigation. Evaporation is not a dominant factor for Cl enrichment in groundwater in winter. Leaching and vertical mixing lead to more salt accumulation in the upper soil and other pollutants from sewage into groundwater and promote halite dissolution.2. Environmental geochemistry of antibiotics(1) All the 25 kinds of antibiotics have been detected in the surface water in this study area. The order of average detection frequency is MLs (98.2%)> FQS (91.3%)> TCs (87.5%)> SMs (75.0%). The sample with the highest concentration of antibiotics (730.19 ng/L) is located at Taiyu Drainage Canal while that with the lowest concentration (245.93 ng/L) at Xiaohe River. The type and concentration of antibiotics in SW-3 and SW-4 were almost the same, indicating the measures to prevent canal banks from seepage were able to retard their transport into the soil and groundwater system via lateral recharge. Along the Beizhang Drainage Canal, the sewage from the swine farm, the cow farm, the irrigation field and domestic usage were all discharged into the canal, so the concentration kept increasing. Along the down gradient direction of Taiyu Drainage Canal, the concentration decreased. Transport of the contaminants via lateral recharge due to lack of seepage prevention measures therefore poses a potential threat for the soil-groundwater system.(2) 15 of 25 kinds of antibiotics were detected in the groundwater. The detection frequency was lower than other study areas in China. The concentrations ranged from 2.75 ng/L to 114.38 ng/L with the average concentration of 27.60 ng/L.(3) MLs and FQs were the main antibiotics detected in the shallow vadose zone. The contents of antibiotics in the profile GS-1-GS-4 were 2-3 times as those in groundwater irrigation area, indicating that sewage irrigation has a significant impact on the contents of antibiotics in the soil. The contents of antibiotics in both GS-1-GS-4 and GS-5 exceeded MCL of 100 ng/g. The elevation of their contents in the wet season could be related to that of water level. The highest content of antibiotics occurring at the top left corner is at the same elevation as the sewage in the Beizhang Drainage Canal at wet season. The trend of decrease in antibiotics contents in the profile is as follows:from the top left corner to bottom right corner, the content reduces unequally. The distribution of antibiotics was influenced by the irrigation flow direction. Moreover, the possibility of introducing antibiotics into the shallow vadose soil via the lateral recharge of sewage in the Beizhang Drainage Canal could not be excluded. The contour of FQs is quite similar to that of the total antibiotics, with the content of FQs reducing all the way from the top left corner to the bottom right corner. Approximately 50% and 70% of the FQs was reduced at the upper 50 cm and 80 cm depth, respectively. TCs may have experienced significant lateral migration. MLs and SMs has a similar distribution on the profile. The decrease of their contents occurred in the upper 20 cm.(4) 22 and 20 kinds of 25 antibiotics were detected respectively in the sediment core of sewage irrigation area and groundwater irrigation area. The order of contents of 4 categories of antibiotics in both cores was:fluoroquinolones> tetracyclines> sulfonamides> macrolides. The sewage irrigation influenced the distribution of antibiotics in the sediment core. FQs was the predominant contaminant with the highest average content (264.29 ng/g). The sewage which was used for irrigation both enhanced the amount of antibiotics and took a role of medium in the antibiotics migration.There was moderate correlation (r=0.591, p<0.05) between FQs and TOC, strong correlation (r=0.712, p<0.01) between SMs and TOC, and less strong correlation (r=0.621, p<0.05) between total antibiotics and TOC. Therefore, TOC was the main factor which influencing the residues of antibiotics in soil. The contents of antibiotics slightly decreased with the increase of depth and the antibiotics showed a higher transport in sewage irrigation area than groundwater irrigation area. Groundwater ubiquity score showed FQs present the highest migration capacity, followed by TCs, MLs and SMs.3.Environmental geochemistry of PAHs(1) The data showed the concentrations of PAHs in surface water in winter is higher than autumn, this is related to increase in coal combustion in winter for heating, leading to the increase of PAHs release into the environment.2+3 rings PAHs are the primary content of the total PAHs in surface water. The order of average concentration of surface water was East Main Canal> Beizhang Drainage Canal> Taiyu Drainage Canal> Fenhe River> Xiaohe River.(2) The pollution of PAHs in groundwater in winter was worse than in autumn, which was consistent with the situation of surface water. It can also be attributed to large scale coal combustion in winter.2+3 rings were the primary contaminants in autumn while 4 rings increase to 57.23% of PAHs in winter, indicating the concentration of PAHs in groundwater was strongly affected by coal combustion.(3) The contents of PAHs in the profile GS-1-GS-4 were 2-9 times higher than those of of GS-5, indicating that the pollution of PAHs in sewage irrigation area was much higher than the groundwater irrigation area. The pollution of PAHs in topsoil of Xiaodian irrigation area was higher than Hunpu irrigation area while lighter than Shenfu irrigation area. The reducing of PAHs occurred mainly at the upper 30 cm layer and approximately 95% of total PAHs was reduced at this layer. LMW migrate more easily than MMW and HMW in the soil. As compared to antibiotics, PAHs present higher residual levels, due to the less water solubility and higher koc of PAHs. The highest content occurred at the top left corner for all the PAHs, implying that the lateral recharge of sewage contribute to the residues of PAHs in the shallow vadose zone.(4) All the 16 kinds of PAHs were detected in the topsoil samples of both boreholes. The highest content of PAHs of F4 occurred at the topsoil which was one order of magnitude higher than the highest content of F15. It can be indicated that sewage irrigation introduced higher content of PAHs to the sediment. The content of PAHs presented a similar trend with that of TOC, indicating that TOC was the main factor controlling the retention of PAHs. The soil in the groundwater irrigation area could be considered to be slightly contaminated. However, the soil in the sewage irrigation area was still heavily contaminated, to which great attention should be paid.The novel point of this thesis:combined the hydrochemical characteristics of groundwater and surface water, systematically studied the environmental geochemistry behavior (occurrence, distribution and migration) of typical organic contaminant such as antibiotics and PAHs.