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1.
The Changjiang and the Jiulong Estuaries, located in eastern and southeastern China, respectively, have different geomorphologic
and tidal processes as well as anthropogenic development in their associated watersheds. Sediments in the Changjiang estuary
mostly consist of SiO2, CaO and MgO (mean percentages of 63.9, 4.34 and 2.35%), whereas sediments from the Jiulong estuary mostly consist of Al2O3, Fe2O3 and organic matter (mean percentages 19.2, 6.82 and 4.14%). The Jiulong estuarine sediments contain more than twice the concentrations
of Pb, Zn, Cu, than those from the Changjiang estuary. In the Jiulong estuary, these heavy metals are associated with carbonates
and organic matter, whereas in the Changjiang estuary, they are associated with residual fractions or clay. Sediments from
the Changjiang estuary, mostly sediments with little organic matter, do not efficiently sequester anthropogenic-derived trace
metals. In contrast, sediments from the Jiulong estuary consist of a mixture of fluvial and marine matter which can sequester
heavy metals contributed by larger landscapes with industrial and municipal wastewater. 相似文献
2.
The Tinto and Odiel rivers drain 100 km from the Rio Tinto sulphide mining district, and join at a 20-km long estuary entering the Atlantic Ocean. A reconnaissance study of heavy metal anomalies in channel sand and overbank mud of the river and estuary by semi-quantitative emission dc-arc spectrographic analysis shows the following upstream to downstream ranges in ppm (μg g?1): As 3,000 to <200, Cd 30 to <0.1, Cu 1,500 to 10, Pb 2,000 to <10, Sb 3000 to <150, and Zn 3,000 to <200. Organic-rich (1.3–2.6% total organic carbon, TOC), sandysilty overbank clay has been analyzed to represent suspended load materials. The high content of heavy metals in the overbank clay throughout the river and estuary systems indicates the importance of suspended sediment transport for dispersing heavy metals from natural erosion and anthropogenic mining activities of the sulfide deposit. The organic-poor (0.21–0.37% TOC) river bed sand has been analyzed to represent bedload transport of naturally-occurring sulfide minerals. The sand has high concentrations of metals upstream but these decrease an order of magnitude in the lower estuary. Although heavy metal contamination of estuary mouth beach sand has been diluted to background levels estuary mud exhibits increased contamination apparently related to finer grain size, higher organic carbon content, precipitation of river-borne dissolved solids, and input of anthropogenic heavy metals from industrial sources. The contaminated estuary mud disperses to the inner shelf mud belt and offshore suspended sediment, which exhibit metal anomalies from natural erosion and mining of upstream Rio Tinto sulphide lode sources (Pb, Cu, Zn) and industrial activities within the estuary (Fe, Cr, Ti). Because heavy metal contamination of Tinto-Odiel river sediment reaches or exceeds the highest levels encountered in other river sediments of Spain and Europe, a detailed analysis of metals in water and suspended sediment throughout the system, and epidemiological analysis of heavy metal effects in humans is appropriate. 相似文献
3.
Dissolved trace elements in river water: spatial distribution and the influencing factor, a study for the Pearl River Delta Economic Zone, China 总被引：1，自引：0，他引：1
T. P. Ouyang Z. Y. Zhu Y. Q. Kuang N. S. Huang J. J. Tan G. Z. Guo L. S. Gu B. Sun 《Environmental Geology》2006,49(5):733-742
Twenty-nine water samples were collected from different river channels of the Pearl River Delta Economic Zone, China. An inductively
coupled plasma-mass spectromonitor (ICP-MS) was used to measure concentrations of the trace elements in these samples. The
results suggest that the average concentrations of rare earth elements in river water show an increasing trend from the West
River, the North River, the rivers of the Pearl River Delta, and the Shenzhen River to the East River. Relatively high concentrations
of heavy metals appear in the East River, the rivers of the Pearl River Delta and the Shenzhen River, while the West River
and the North River have relatively low heavy metal concentrations. Trace element concentrations in samples collected near
urban or industrial areas are much higher than those of samples collected from distant areas, away from urban and industrial
areas. After natural conditions, human activities have significant influence on the trace element concentrations in river
water. This trace element concentration’s spatial distribution in the river water from the Pearl River Delta Economic Zone
is actually an integrated effect of natural conditions and human activity. 相似文献
4.
Sediments have been used to detect sources of contamination in a catchment of the Port Jackson (Sydney Harbour) estuary and
to evaluate the effects of different land-use practices on the fluvial environment. Mean enrichment (mean concentrations over
pre-anthropogenic background) of size-normalized (<62.5 μm) aquatic sediment is 10 × for Cu, 20× for Pb and 90× for Zn adjacent
to industrialized areas and 2×, 7× and 7×, respectively for these metals in highly urbanized subcatchments. Diffuse sources
contribute minor metals to fluvial sediment even in the most underdeveloped subcatchment (2×, 3× and 3× for Cu, Pb and Zn
respectively). Organochlorine pesticide residue concentrations parallel heavy-metal trends due to a common mixed industrial
base. Effects-based sediment criteria suggest that some adverse biological impacts are probably occurring in streams flowing
through the industrial areas. This interpretation is supported by sequential extraction data which show that a moderate proportion
of total heavy metals, especially Zn, is associated with the more bioavailable exchangeable/adsorbed phases in these aquatic
sediments. High total suspended solid loads in water downstream of one of the industrial centers, and high particle-bound
Cu and Pb concentrations, suggest that most contaminants exiting the catchment do so in association with the solid phase.
Received: 29 March 1999 · Accepted: 30 August 1999 相似文献
5.
Dissolved (<1 kDa) and colloidal (1 kDa-0.45 μm) size fractions of sulfate, organic carbon (OC), phosphate and 17 metals/metalloids were investigated in the acidic Vörå River and its estuary in Western Finland. In addition, geochemical modelling was used to predict the formation of free ions and complexes in these waters. The sampling was carried out during high-water flow in autumn and in spring when the abundantly occurring acid sulfate (AS) soils in the catchment area are extensively flushed. Based on the high concentrations of sulfate, acidity and several metals, it is clear that the Vörå River and its estuary is strongly affected by AS soils. The high dissolved form of metals limits also the existence of fish and other organisms in this estuary, and certainly also in other similar shallow brackish estuaries elsewhere in the Gulf of Bothnia. However, generally already <20% saline sea water reduces the concentration for OC and several elements (Al, Cu, Cr, Fe, Pb, PO4 and U) by half and c. 20–30% saline sea water is needed to halve concentrations of Cd, Co, Mn, Ni and Zn. Consequently, these elements as well as organic matters were rapidly precipitated in the estuary, even after mixing with fairly small amounts of the alkaline brackish sea water. Aluminium, Cu, Fe and U most likely precipitate together with organic matter closest to the river mouth. Manganese is relatively persistent in solution and, thus, precipitates further down the estuary as Mn oxides, which concomitantly capture Ba, Cd, Co, Cu, Ni and Zn. In the inner estuary, the high contents of Al is as important than Fe in removing PO4 and, thus, also reducing the risk of algae blooms in near coastal areas influenced by AS soils in the Gulf of Bothnia. Moreover, the dispersion of metals far out in the estuary is dependent on hydrological conditions, i.e. with high flows the plume of metal-rich water will spread further out in the estuary. Furthermore, the extensive drainage of the catchment and subsequent artificial enlargement of the river channel during recent decades has not only enabled oxidation of sulfidic sediments, but strongly increased flow peaks that reach further out in the estuary. 相似文献
6.
V. Yu. Rusakov T. G. Kuzmina M. A. Levitan E. S. Toropchenova A. V. Zhilkina 《Geochemistry International》2017,55(12):1079-1089
This paper addresses the distribution of heavy metals (Co, Ni, Cu, Zn, Cd, Sn, Sb, Pb, and Bi) as well as Si, Al, Fe, and Mn in the surface (0–2 cm) layer of bottom sediments of the Kara Sea. The contents of these elements are determined in each of the previously distinguished facies-genetic types of terrigenous sediments: fluvial, glacial, estuarine, shallow water–marine, “background” marine, and relict sediments. It is shown that these types reflect the modern conditions of accumulation of river discharge material, which forms fans of two greatest Siberian rivers, Ob and Yenisei. The main stages are distinguished in heavy metal accumulation. The first stage is related to the avalanche sedimentation of terrigenous sediments in the estuary and characterized by the elevated contents of Co, Ni, Cu, Zn, Cd, Sb, and Bi. The second stage reflects the mechanical differentiation of sedimentary material by waves and bottom currents in a shallow-water sea part adjacent to the estuarine zone, with accumulation of Pb- and Sn-bearing “heavy” ore minerals. The deepwater background terrigenous–marine sediments accumulate mainly Ni, Zn, and Cd, as well as Mn. The relict sediments differ in the high contents of Si, Mn, and Sn. 相似文献
7.
江苏典型地区河流沉积物重金属元素分布特征及其污染来源 总被引：3，自引：0，他引：3
通过对河流沉积物等地表沉积物中重金属元素分布等环境地球化学调查数据的统计分析,总结了江苏典型地区河流沉积物中重金属元素的分布特点与规律:江苏局部地区河流沉积物中,Cd、Pb、Zn、Cr、Cu、Ni、Hg等相对富集趋势明显,Cd最大富集倍数 100;不同河流沉积物的重金属元素分布存在差异,与其污染来源密切相关;工业排放是导致河流重金属污染的主要原因,使用含Cd的颜料作为调色剂导致部分河流出现严重Cd污染;与土壤等其他地表沉积物相比,河流沉积物中重金属元素的分布更不均衡。查明河流沉积物的重金属元素分布可为相关农产品安全性预测、确定重金属污染来源、追踪涉重产业的发展历程、防治局部耕地重金属污染等提供科学依据。 相似文献
8.
Thirty-eight sediment samples from 15 primary rivers on Taiwan were retrieved to characterize the rare earth element (REE) signature of fluvial fine sediment sources. Compared to the three large rivers on the Chinese mainland, distinct differences were observed in the REE contents, upper continental crust normalized patterns and fractionation factors of the sediment samples. The average REE concentrations of the Taiwanese river sediments are higher than those of the Changjiang and Huanghe, but lower than the Zhujiang. Light rare earth elements (LREEs) are enriched relative to heavy rare earth elements (HREEs) with ratios from 7.48 to 13.03. We found that the variations in (La/Lu)UCC–(Gd/Lu)UCC and (La/Yb)UCC–(Gd/Yb)UCC are good proxies for tracing the source sediments of Taiwanese and Chinese rivers due to their distinguishable values. Our analyses indicate that the REE compositions of Taiwanese river sediments were primarily determined by the properties of the bedrock, and the intensity of chemical weathering in the drainage areas. The relatively high relief and heavy rainfall also have caused the REEs in the fluvial sediments from Taiwan to be transported to the estuaries down rivers from the mountains, and in turn delivered nearly coincidently to the adjacent seas by currents and waves. Our studies suggest that the REE patterns of the river sediments from Taiwan are distinguishable from those from the other sources of sediments transported into the adjacent seas, and therefore are useful proxies for tracing the provenances and dispersal patterns of sediments, as well as paleoenvironmental changes in the marginal seas. 相似文献
9.
On their way from the Rhine estuary into the North Sea and Dutch Wadden Sea, Rhine sediments “lose” large portions of their
original heavy metal concentrations. Until now these losses were explained by a mobilization process, solubilization — the
decomposition products of organic matter form soluble organometallic complexes with the metals of the sediment.
Our investigations of the sediments of the Elbe clearly indicate that a mixing process, whereby highly polluted Elbe sediments
mix with relatively non-polluted North Sea sediments, rather than solubilization, is the cause of the dilution of heavy metals
in the sediments of the Elbe estuary. Because of the similarity of the Elbe data with those from the Rhine River, we propose
that a mixing process is also effective in the Rhine estuary and adjacent North Sea areas.
The mechanism by which heavy metals are “diluted” is important to the marine ecosystem. In the mixing process proposed in
this paper, the heavy metals fixed to the suspected material are trapped in bottom sediments of the marine environment, whereas
solubilization would increase the concentration of heavy metals in the sea water and thus they would be more available for
uptake by aquatic organisms. 相似文献
10.
As evidenced by catastrophic cadmium and mercury poisonings in japan, heavy metals belong to the most toxic environmental pollutants. Through the investigation of sediments, the extent, distribution and provenance of heavy metal contamination in rivers and lakes can be determined and traced. Eight heavy metals from the clay fraction of sediments from major rivers within the Federal Republic of Germany were determined by means of atomic adsorption spectrometry. Heavy metals especially known for their high toxicity are enriched most: mercury, lead and zinc by a factor of 10; cadmium by a factor of 50, as compared with the natural background of these elements. A mobilisation of heavy metals from the suspended load and from the sediments, as to be observed in rivers approaching the marine enviromment, could endanger marine organisms, thus negatively influencing the acquatic food chain. With a further increase of heavy metal pollution, a threat to the drinking water supplied by rivers and lakes cannot be excluded. 相似文献
11.
12.
Tillamook Bay, Oregon, is a drowned river estuary that receives freshwater input from 5 rivers and exchanges ocean water through
a single channel. Similar to other western United States estuaries, the bay exhibits a strong seasonal change in river discharge
in which there is a pronounced winter maximum and summer minimum in precipitation and runoff. The behavior of major inorganic
nutrients (phosphorus, nitrogen, and silica) within the watershed is examined over seasonal cycles and under a range of river
discharge conditions for October 1997–December 1999. Monthly and seasonal sampling stations include transects extending from
the mouth of each river to the mouth of the estuary as well as 6–10 sites upstream along each of the 5 major rivers. Few studies
have examined nutrient cycling in Pacific Northwest estuaries. This study evaluates the distributions of inorganic nutrients
to understand the net processes occurring within this estuary. Based upon this approach, we hypothesize that nutrient behavior
in the Tillamook Bay estuary can be explained by two dominant factors: freshwater flushing time and biological uptake and
regeneration. Superimposed on these two processes is seasonal variability in nutrient concentrations of coastal waters via
upwelling. Freshwater flushing time determines the amount of time for the uptake of nutrients by phytoplankton, for exchange
with suspended particles, and for interaction with the sediments. Seasonal coastal upwelling controls the timing and extent
of oceanic delivery of nutrients to the estuary. We suggest that benthic regeneration of nutrients is also an important process
within the estuary occurring seasonally according to the flushing characteristics of the estuary. Silicic acid, nitrate, and
NH4
+ supply to the bay appears to be dominated by riverine input. PO4
−3 supply is dominated by river input during periods of high river flow (winter months) with oceanic input via upwelling and
tidal exchange important during other times (spring, summer, and fall months). Departures from conservative mixing indicate
that internal estuarine sources of dissolved inorganic phosphorus and nitrogen are also significant over an annual cycle. 相似文献
13.
Jorge Marcovecchio Sandra Botté Melisa Fernández Severini Federico Delucchi 《Aquatic Geochemistry》2010,16(2):251-266
Chromium and lead concentrations and distribution have been fully studied within Bahía Blanca estuary inner area, which is
strongly influenced by urban and industrial stress. Not only metals dissolved in estuarine water but also those included in
sediments and suspended particulate matters (SPM) were measured. In all cases, internationally standardized protocols were
applied to metal measurements, including analytical quality check test through analysis of certified reference materials.
Total metal contents from surface sediments and SPM were compared with those from a historical database of the area, as well
as with values representing the natural geochemical baseline within the system. Results showed that heavy metal pollution
is mainly localized in the areas close to both industrial effluents discharge system and urban sewage outfall discharge. Data
from sequential extractions indicate that metals from anthropogenic sources (i.e., Pb) are potentially more mobile than those
inherited from geological parent material (i.e., Cr). The influences of other potential sources of metals (i.e., streams,
runoff) were also considered. SPM was clearly identified as the main carrier of the studied heavy metals within the system,
and its significance to metals input into sediments and/or biota was verified. Finally, the normalization of measured metal
concentrations against background reference elements (i.e., Al or Fe) has allowed to identify that most of the measured Cr
was lithogenic, while a significant percentage of Pb was from anthropogenic origin within Bahía Blanca estuary. 相似文献
14.
Concentrations of nutrients (NO3 ?, NO2 ?, NH4 +, PO4 3?, and dissolved SiO2) were examined in three North China estuaries—the Luanhe, Shuangtaizihe, and Yalujiang. These riverine-estuarine systems provide distinct geographic and hydrodynamic conditions, that is, a shallow water zone embraced by shoals and sandbars (Luanhe), the confluence of two streams in the upper estuary with different water and sediment loads, and a turbidity maximum in the upper estuarine mixing zone (Yalujiang). Nutrient element concentrations in these rivers are high in comparison with large, less disturbed systems but similar to those from polluted and/or eutrophic European and North American rivers. This is attributed to intensive weathering and erosion and extensive use of chemical fertilizers. In the fresh-marine waters mixing zone, nutrient species can behave either conservatively or nonconservatively, or both. Wherever nonconservative behaviours of nutrient elements are observed, remobilization from solid phases is probably the predominant mechanism. The extrapolation of dilution curves to the fresh water end-members gives estimated riverine concentrations, which can be between two and ten times higher than those from field observations. Taking into account the high N:P ratios (102–104) from North China rivers and very low concentrations of nitrogen species in the Northwest Pacific coastal oceans (e.g., Yellow Sea), the estuaries in this study may act as regions in which production is limited by phosphorus to regions in which production is limited by nitrogen. 相似文献
15.
笔者应用因子克里格分析方法,研究了珠江口西部海域388个表层沉积物中7种重金属元素Cd, Ni,Cu,Zn,Pb,Cr和As的空间结构特征,识别并分离了重金属元素不同尺度的空间主成分及其分布特征,探讨了不同空间尺度重金属的物源及控制因素。结果显示,研究区7种重金属元素在空间上存在块金尺度、局部尺度(变程为60 km的球状结构)和区域尺度(变程为160 km的球状结构)3种尺度的空间变化。以迭代算法模拟了研究区重金属元素线性协同区域化模型。根据不同尺度上区域化因子的主成分得分分布特征可知:局部尺度上,F1因子(Zn,Cr,Ni,Cu)和F2因子(As)的高值区表现为"牛眼"状局部特征,并分布在陆地沿海的河口或者港湾区,指示了可能受人为污染成分影响的重金属区域。其中,雷州半岛东部沿海是最可能的重金属污染区,其空间分布主要受控于局部的地形、海流等因素。F2因子不同于F1的空间分布,主要在于As不同于Zn,Cr,Ni,Cu等的地球化学行为。区域尺度上,F1(Zn,Cr,Ni)和F2(As)因子主要反映了不同陆源母岩物质的影响。Zn,Cr,Ni等主要源于华南大陆陆源母岩物质,而As主要受到海南岛陆源母岩物质的控制。F1和F2因子得分高值区整体上表现为NNE向自陆地向较深海域延伸的"片状"分布特征,推测其主要受到海平面变化及NNE向区域性海洋环流的控制。 相似文献
16.
长江口滨岸潮滩重金属源汇通量估算 总被引：8，自引：3，他引：5
根据2002～2003年长江口滨岸潮滩16个站位1年的沉积物重金属含量、粒度和密度等季节性测试数据,结合其他文献的沉积物累积速率资料,初步估算了长江口滨岸潮滩重金属的源汇通量。结果表明,2002年徐六泾以下的河口地区,沉积物中Cu、Pb、Fe、Mn、Zn、Cr的年滞留量分别为3425t/a、2751t/a、3451599t/a、69505t/a、14045t/a和3176t/a,石洞口排污口和黄浦江仍然是这几种重金属的主要污染输入来源之一。潮滩能有效滞留河口水体中的重金属,在长江口0m线以上的潮滩沉积物中,Cu、Pb、Fe、Mn、Zn、Cr的年沉降量分别占到整个河口区重金属沉降量的24.3%、27.6%、32.3%、32.4%、20.5%和23.2%。从潮滩重金属的沉降通量来看,南汇边滩的沉降通量最大,长江口南岸潮滩明显高于长江口北支潮滩,说明位于排污口下游的滩地能有效削减入海重金属污染负荷。 相似文献
17.
金矿区河水和底泥中重金属含量分布与耦合关系 总被引：3，自引：1，他引：2
河流中河水与底泥中重金属含量之间的关系对于河流污染防治意义重大。对某金矿区3种典型河流的河水与底泥中的重金属含量进行分析,发现4条河流均有超过国家标准限值的情况,河水中7种重金属元素均出现超标,Cr、As元素超标不严重;底泥中Hg、Pb、Cr、Cu、Zn超标,其中Hg超标最严重,河水和底泥中Hg最大超标倍数分别达到3099倍和244倍。河流中重金属主要赋存形态为沉积态,底泥的吸附解吸作用是河流底泥和河水中重金属沿程变化的主控因素;金矿区区域上河水和底泥中重金属很好地服从Langmuir等温吸附模式。矿业活动、地层岩性均会影响底泥对重金属的平均最大吸附容量,流径黄土区的双桥河平均最大吸附容量最大。研究结果为矿山河流污染防治与预警提供了参考依据。 相似文献
18.
Lan Wang Yaping Wang Wenzhao Zhang Chunxue Xu Ziyi An 《Environmental Earth Sciences》2014,71(3):1183-1193
The concentrations of heavy metals (Cr, Co, Ni, Cu, Zn, Pb, Cd, As, Hg, and Fe) in sediments of the Yangtze River, China, were investigated to evaluate levels of contamination and their potential sources. The lowest heavy metal concentrations were found in the source regions of the river basin. Relatively high concentrations of metals, except Cr, were found in the Sichuan Basin, and the highest concentrations were in the Xiangjiang and Shun’anhe rivers. All concentrations, except Ni, were higher than global averages. Principal component analysis and hierarchical cluster analysis showed that Zn, Pb, As, Hg, and Cd were derived mainly from the exploitation of various multi-metal minerals, industrial wastewater, and domestic sewage. Cu, Co, and Fe were derived mainly from natural weathering (erosion). Cr and Ni were derived mainly from agricultural activities, municipal and industrial wastewater. Sediment pollution was assessed using the geoaccumulation index (I geo) and enrichment factor (EF). Among the ten heavy metals assessed, Cd and Pb had the highest I geo values, followed by Cu, As, Zn, and Hg. The I geo values of Fe, Cr, Co, and Ni were <0 in all sediments. EF provided similar information to I geo: no enrichment was found for Cr, Co, and Ni. Cu, Zn, As, and Hg were relatively enriched at some sites while Cd and Pb showed significant enrichment. 相似文献
19.
F. Ruiz M. L. González-Regalado J. M. Muñoz M. Abad A. Toscano M. I. Prudencio M. I. Dias 《International Journal of Environmental Science and Technology》2014,11(5):1249-1258
The spatial distribution and geoaccumulation indices of four heavy metals were investigated in very shallow marine sediments of southwestern Spain. Surface sediments were collected from 43 sites with water depth ranging from 3 to 20 m. High to very high pollution levels (I geo > 4 for zinc, lead and copper) were detected near the end of the Huelva bank, whereas chromium shows a more hazardous distribution in the southwestern Spanish littoral. Low to moderate heavy metal contents (mainly zinc and lead) were also observed in other two areas at different water depths (Isla Cristina-Piedras River: 10–18 m water depth; Mazagón–Matalascañas: <10 m water depth), whereas unpolluted to moderately polluted sediments were detected in the very shallow zones (<8 m water depth) located between the mouths of the Guadiana and the Piedras Rivers. A regional scenario indicates a strong pollution of the adjacent marine areas by polluted inputs derived from the Tinto–Odiel rivers, with a partial transport of heavy metals by W–E littoral currents even 40 km eastward. The Guadiana River is an additional source of zinc–lead contamination near the Spanish–Portuguese border, mainly at water depths up to 10 m. All these rivers are affected by acid mine drainage processes, derived from millennial mining activities. This pollution affects the sediment quality even 40 km eastward. 相似文献
20.
典型岩溶区地下水系统重金属元素分布特征浅析——以广西坡月地下河为例 总被引：1，自引：0，他引：1
地下河为岩溶地下水系统的重要组成部分,为查清典型岩溶区地下水系统中重金属指标的含量变化及空间分布特征,文章以广西坡月地下河系统为例,通过不同断面两次采样分析,重点剖析地下河系统在水文地球化学作用下,地下水中以“五毒”元素及Zn、Al和Mn为代表的重金属元素含量。分析结果表明:坡月地下河水体pH值在7.34～8.10 之间,水体中“五毒”元素、Zn、 Al和Mn的检出含量极低或未检出,质量浓度均远小于国家地下水环境质量标准中的Ⅲ类水质标准,适合各种用途。检出含量较高的点主要为地下河明流段或补给区地表水,含量最高分布在坡心地下河出口水体,其原因是坡心地下河系统区内工矿企业和其它经济活动干扰相对较强,特别是工矿企业的矿石和尾矿渣的露天堆放（如金矿）是引起地下水中重金属元素的检出或超标的重要因素。 相似文献