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1.
Speciation and solubility relationships of Al, Cu and Fe in solutions associated with sulfuric acid leached mine waste rock 总被引:3,自引:0,他引:3
Solutions from oxidized waste rock originating from an acid-leached waste dump were studied. The dissolution data suggest
that after the majority of the soluble solid phases are removed, remaining solid phases continued to buffer the solutions
in the acidic pH range. Incorporating the solution data into MINTEQA2 identified controls on the solubility of Al, Cu and
Fe at pH values from about 2.5 to slightly over 5. Sulfate appears to play a significant role in the formation of solubility
controlling solid phases for Al and Cu. This is not the case for Fe, and is suggested that Fe and Cu solubility may be controlled
by cupric ferrite at low pH values.
Received: 5 April 1998 · Accepted: 27 July 1998 相似文献
2.
The mineralized district of SW England was one of the world's greatest mining areas, with mining commencing in the Bronze age, peaking in the 1850s to 1890s, but still continuing to the present day. Consequently, it is not surprising that mining has had a major impact on the environmental geochemistry of SW England. In this study, the mineralogical and geochemical signature of mine waste contamination within the Fal Estuary at Tresillian, Cornwall, has been examined. A pulse of mine waste contamination is recognized at approximately 50?cm below present day sediment surface. Sn, As, Cu, Pb, and Zn are all enriched within this contaminated interval with up to 1800 mg?kg–1 Sn, 290 mg?kg–1 As, 508 mg?kg–1 Pb, 2210 mg?kg–1 Zn, and 1380 mg?kg–1 Cu. Within this interval, the dominant minerals present include chalcopyrite, arsenopyrite, pyrite, cassiterite, Fe–Ti oxides (ilmenite and ?rutile), wolframite, sphalerite, baryte, zircon, monazite, tourmaline and xenotime. In addition, man-made slag products commonly occur. The exact timing of the release of mine waste into the estuary is poorly constrained, but probably occurred during or immediately following the peak in mining activity in the nearby Camborne-Redruth district, which was between 1853 and 1893. The mine waste may have entered the estuary either via the Tresillian River and its tributaries or via Calenick Creek and the Truro River and/or the Carnon River which flows into Rostronguet Creek. 相似文献
3.
Forty-three untreated and actively and passively (wetland) treated coal mine drainage sediments and five yellow-red pigments
were characterized using X-ray fluorescence, fusion-inductively coupled plasma atomic emission spectroscopy, X-ray diffraction,
and tristimulus colorimeter. Primary crystalline iron-bearing phases were goethite and lepidocrocite, and iron phases converted
to hematite upon heating. Quartz was nearly ubiquitous except for synthetic pigments. Gypsum, bassinite, calcite, and ettringite
were found in active treatment sediments. Iron concentrations from highest to lowest were synthetic pigment>wetland sediment>natural
pigment>active treatment (untreated sediments varied more widely), and manganese was highest in actively treated sediments.
Loss on ignition was highest for passively treated sediments. No clear trends were observed between quantified color parameters
(L*, a*, b*, and Redness Index) and chemical compositions. Because sediments from passive treatment are similar in chemistry, mineralogy,
and color to natural pigments, the mine drainage sediments may be an untapped resource for pigment.
Received: 29 December 1997 · Accepted: 11 May 1998 相似文献
4.
Toxic metal contamination in the lateral lakes of the Coeur d'Alene River valley, Idaho 总被引:3,自引:0,他引:3
K.F. Sprenke W.C. Rember S.F. Bender M.L. Hoffmann F. Rabbi V.E. Chamberlain 《Environmental Geology》2000,39(6):575-586
The 11 lateral lakes of Coeur d'Alene River valley in northern Idaho have received heavy metal contamination from over a
century of upstream mining. The lateral lakes lie within the flood plain of the Coeur d'Alene River, and in their bottom sediments
is preserved a stratigraphic record of the upstream mining operations. To characterize the contaminated sediments in the lateral
lakes, sampling techniques, including the Livingston piston corer and the Huttenen freeze box, have been developed by Quaternary
geologists to preserve the vertical stratigraphy in the samples. From 26 cm to over 55 cm of undisturbed tailing sediments,
commonly with “varve-like” features, have been found in each of the lateral lakes, with maximum concentrations by weight of
lead at 3.8%, zinc at 3.4%, arsenic at 340 mg/kg, cadmium at 120 mg/kg and mercury at 7 mg/kg. The contamination in the lakes
appears to be restricted to the shallow subsurface and heavy metal concentrations generally drop to background levels within
a meter of depth.
Received: 22 May 1998 · Accepted: 21 September 1998 相似文献
5.
Hydrological and geomorphological criteria to evaluate the dispersion risk of waste sludge generated by the Aznalcollar mine spill (SW Spain) 总被引:2,自引:0,他引:2
G. Benito Alfonso Benito-Calvo Francesc Gallart Juan Pedro Martín-Vide David Regües Ernest Bladé 《Environmental Geology》2001,40(4-5):417-428
Following the Aznalcóllar pyrite mine disaster (Seville, Spain) which caused the spilling of some 4.5?hm3 of acid water, the floodplains of the rivers Agrio and Guadiamar were rapidly cleaned of waste sludge. However, despite the efficiency of cleaning activities, there is still evidence of a fine superficial layer of sludge and some soil contamination, with the consequent risk of remobilisation of the pollutants by water erosion. There is much concern that these contaminated sediments may affect the precious ecosystems of the Doñana National Park and the Guadalquivir marshlands. This report describes the evaluation of the risk of mobilisation of the waste sludge through (1) detailed geomorphological analysis, indicating potential areas of erosion-sedimentation on the floodplains of the rivers Agrio and Guadiamar, and predicted dynamics of the waste sludge, and (2) evaluation of the potential dispersion of the waste sludge provoked by future flood events, including hydraulic calculations to model channel flow and the analysis of the texture of the sludge to obtain critical transport and sedimentation values. Findings suggest that the waste sludge is likely to be transported and deposited within the Doñana National Park during future flood events. 相似文献
6.
Mine waste dumps and heavy metal pollution in abandoned mining district of Boccheggiano (Southern Tuscany, Italy) 总被引:11,自引:1,他引:11
M. Benvenuti I. Mascaro F. Corsini P. Lattanzi P. Parrini G. Tanelli 《Environmental Geology》1997,30(3-4):238-243
Mining activity in the Boccheggiano-Fontalcinaldo area (Southern Tuscany) dates back at least to the 16th century AD and
lasted up to very recent times. Copper-rich hydrothermal veins, massive pyrite deposits, and their gossans were exploited.
Two mine waste dumps (Fontalcinaldo, Fontebona), one flotation tailings impoundment (Gabellino), and one roasting/smelting
waste dump (Merse-Ribudelli) in the study area were selected to ascertain the environmental effects of such protracted mining
activity. Primary waste mineralogy is mainly characterized by pyrite, gypsum, quartz, carbonates, chlorites, and micas. Secondary
oxidation mineralogy includes Fe and Cu sulfates and hydroxy sulfates, Cu carbonates, Fe and Al oxyhydroxides, and other phases
[neogenic cassiterite at Fontalcinaldo; probable calkinsite, (Ce,La)2(CO3)3· 4H2O, at Fontebona]. Mine waste samples show extremely variable contents of toxic elements (Cu, Pb, Zn, Bi, Cd, As), with average
values in the order of hundreds to thousands of parts per million (except for Bi and Cd). In some samples, the abundance of
proper minerals of these metals cannot account for the entire metal load. Conceivably, either solid solution substitutions
or adsorption processes contribute to the intake of released metals into newly formed minerals. Release and transport of pollutants
was affected to variable degrees by acid-neutralization processes. The highest metal and acid concentrations occur close to
the investigated wastes and rapidly decrease moving downstream some hundreds of meters or less, with the partial exception
for Mn and Fe. Other than dilution effects, this phenomenon may be ascribed to metal adsorption and precipitation of solid
phases.
Received: 16 April 1995 · Accepted: 14 December 1995 相似文献
7.
W. C. Rember T. W. Erdman M. L. Hoffmann V. E. Chamberlain K. F. Sprenke 《Environmental Geology》1993,22(3):242-245
For over a century Medicine Lake in northern Idaho has received heavy-metal-laden tailings from the Coeur d'Alene mining district. Establishing the depositional chronology of the lake bottom sediments provides information on the source and rate of deposition of the tailings. Cesium-137, an isotope produced in the atmosphere by nuclear bomb tests, was virtually absent in the environment prior to 1951, but reached its apex in 1964. Our analysis of cesium-137 in the sediments of Medicine Lake revealed that 14 cm of fine-grained tailings were deposited in the lake from 1951 to 1964 and tailing deposition downstream was greatly reduced by the installation of tailings dams in the district in 1968. Cesium-137 analysis is accomplished by a fairly simple gamma-ray counting technique and should be a valuable tool for analyzing sedimentation in any lacustrine environment that was active during the 1950s and 1960s. 相似文献
8.
Role of carbonates in mitigation of metal release from mining waste. Evidence from humidity cells tests 总被引:7,自引:0,他引:7
Leaching of two contrasting types of sulphidic tailings in humidity cells has been performed. The release of heavy metals
and the oxidation rate have been studied. Tailings from the Laver mine contain a few percent sulphides and lack carbonates,
whereas tailings from the Stekenjokk mine are both sulphide- and carbonate-rich. The results showed that in the leachates
from the Laver samples, the metal concentrations increased and pH decreased with time, indicating an increased oxidation rate.
In the Stekenjokk samples, pH remained high during the experiment, thereby keeping the metal concentrations low in the leachates.
The oxidation rate also decreased with time, probably due to Fe-hydroxide coatings on sulphide surfaces. The results show
that addition of carbonates and the maintenance of a high pH not only reduce the solubility of heavy metals, but also decrease
the oxidation rate of sulphides.
Received: 20 January 1998 · Accepted: 2 April 1998 相似文献
9.
10.
The San Antonio-El Triunfo mining district, located at a mountainous region 45 km south-east of La Paz, Baja California,
has been worked since the late 1700s. Mine waste material produced during 200 years of mineral extraction area poses a risk
of local groundwater pollution and eventually, regional pollution to the Carrizal (west basin) and the Los Planes (east basin)
aquifers. There are different types of deposits in the mining area. These are dominated by epithermal veins, in which arsenopyrite
is an important component. Carrillo and Drever (1998a) concluded that, even though the amount of mine waste is relatively
small in comparison to the large scale area, significant As in groundwater derived from the mine waste piles is found locally
in the groundwater. This paper shows the results of geochemical analyses of groundwater samples from the San Antonio-El Triunfo
area and the Carrizal and Los Planes aquifers during several years of monitoring (1993–1997). The highest values of total
dissolved solids (TDS) and As are in the mineralized area where the mining operations occurred (∼1500 ppm TDS and 0.41 ppm
As). The lowest concentrations of TDS and As are, in general, away from the mineralized area (∼500 ppm TDS and 0.01 ppm As).
Sulfate and bicarbonate (alkalinity) are, in general, high near the mineralized area and low away from it. The arsenic concentrations
vary seasonally, especially after the heavy summer thunderstorms. Geochemical modeling (MINTEQA2 and NETPATH) and analysis
of the regional geochemical evolution of the groundwater from the mining area towards the aquifer of Los Planes shows that
the most likely hydrochemical processes include: dilution, precipitation of calcite, and adsorption of As onto surfaces of
iron oxyhydroxides (ferrihydrite). These processes act as natural controls to the extent and amount of As pollution in the
Carrizal and Los Planes aquifers.
Received: 4 May 1999 · Accepted: 22 February 2000 相似文献
11.
The mineralogy of arsenic in uranium mine tailings at the Rabbit Lake In-pit Facility, northern Saskatchewan, Canada 总被引:3,自引:2,他引:3
A detailed investigation of the mineralogy of As in the tailings of the Rabbit Lake uranium ore processing facility was conducted.
The milling/ore extraction process was sampled at three different locations to obtain information about when, where and under
what condition secondary As phases form. These samples were compared with four samples of varying As content from the Rabbit
Lake in-pit tailings management facility (TMF). Up to 20% As in the tailings are present in primary minerals that reach the
tailings directly because they are not dissolved during the uranium extraction. The remaining 80% constitute As that was dissolved
during ore extraction and then re-precipitated before being discharged into the tailings pond. It was not possible to conclusively
identify any individual re-precipitated (secondary) As minerals in the Rabbit Lake TMF. Indirect evidence from sequential
extraction analyses suggests the presence of an amorphous Ca-As phase and a possible, but unlikely, minor amount of an amorphous
Fe-As phase. However, the close association between hydrous ferric oxide (HFO) and As could be clearly demonstrated. HFO was
identified to be 2-line ferrihydrite and its XRD pattern geometry indicates a substantial amount of adsorbed As. This is in
good agreement with SEM, TEM and sequential extraction analyses that all showed the close association of HFO and As.
Received: 14 February 2000 · Accepted: 9 May 2000 相似文献
12.
Characterisation of mine waste rock with respect to acid generation potential is a necessary part of routine mine operations,
so that environmentally benign waste rock stacks can be constructed for permanent storage. Standard static characterisation
techniques, such as acid neutralisation capacity (ANC), maximum potential acidity, and associated acid–base accounting, require
laboratory tests that can be difficult to obtain rapidly at remote mine sites. We show that a combination of paste pH and
a simple portable carbonate dissolution test, both techniques that can be done in the field in a 15 min time-frame, is useful
for distinguishing rocks that are potentially acid-forming from those that are acid-neutralising. Use of these techniques
could allow characterisation of mine wastes at the metre scale during mine excavation operations. Our application of these
techniques to pyrite-bearing (total S = 1–4 wt%) but variably calcareous coal mine overburden shows that there is a strong
correlation between the portable carbonate dissolution technique and laboratory-determined ANC measurements (range of 0–10 wt%
calcite equivalent). Paste pH measurements on the same rocks are bimodal, with high-sulphur, low-calcite rocks yielding pH
near 3 after 10 min, whereas high-ANC rocks yield paste pH of 7–8. In our coal mine example, the field tests were most effective
when used in conjunction with stratigraphy. However, the same field tests have potential for routine use in any mine in which
distinction of acid-generating rocks from acid-neutralising rocks is required. Calibration of field-based acid–base accounting
characteristics of the rocks with laboratory-based static and/or kinetic tests is still necessary. 相似文献
13.
Time-of-flight laser-ionization mass spectrometry was applied to study the chemical composition of mineral particle surfaces
in a sulphide-rich mine-tailings impoundment. This surface-sensitive technique provides chemical information from surfaces
of irregularly shaped mineral particles (both conductive and insulators) less than 100 μm in diameter, which are considered
to be representative of particle surface coatings in the tailings pile (after drying). In addition, depth profiles in the
mineral particles were obtained. The combination of speed of analysis (1 min), small beam-diameter (2–4 μm), surface sensitivity
(2–10 nm), trace-element sensitivity, and capability to analyze rough surfaces makes this method useful as a complement to
studies of pore-water geochemistry and tailings mineralogy. As an example, the behavior of Pb and As in the Kidd Creek tailings
dam near Timmins, Ontario, Canada, was studied, using a combination of surface analyses, and pore-water geochemical data.
Received: 22 February 1995 / Accepted: 6 January 1996 相似文献
14.
Field and laboratory studies of the sulphide-bearing tailings at Laver, northern Sweden, show that the present release of
metals from the tailings is low, especially with regard to Cu. A large part of the Cu released by sulphide oxidation is enriched
in a distinct zone just below the oxidation front. The enrichment zone occurs almost all over the tailings area except in
areas with a shallow groundwater table. The Cu enrichment is caused by formation of covellite and adsorption onto mineral
surfaces. The transport of Zn, Co, Cd, Ni and S seems to be controlled mainly by adsorption. No secondary zone or secondary
minerals containing these metals have been found. Just below the groundwater table, metals are released into solution when
the enrichment zone reaches the groundwater due to the low pH. An increased release of metals, especially Cu, can be expected
in the future, since the enrichment zone is moving towards the groundwater table.
Received: 4 December 1997 · Accepted: 17 December 1998 相似文献
15.
Selenomethionine (SeM) is an organic toxicant that is present in seleniferous environments. No kinetic data is yet available
regarding SeM reactions in coal mine environments, where selenium (Se) toxicity is a potential concern. A kinetic study was
conducted on two reclaimed coal mine soils (Typic Torriorthents) from Wyoming having sandy and clayey textures. Four levels
of SeM treatments (0, 50, 100 μM, and plant amendment from the mine vegetation) were reacted with the soils for 4, 7, 14,
28, 42, 56, and 84 days to characterize the kinetic behavior of overall SeM disappearance from soil solutions. Detection of
SeM in soil solutions at the control level (0 μM SeM) indicated occurrence of indigenous SeM in the soils. In the plant-amended
soil solutions, much greater concentrations of SeM were observed as compared with the soil-only systems. This indicated the
plant material was a more potential source of SeM than the mine soils. A time-dependent loss in solution SeM concentrations
was observed for both soils under 0, 50, 100 μM SeM treatments. For the soil-plant mixtures, the solution SeM concentration
increased initially, reached a maximum after 14 days, and then decreased thereafter. In the plant-amended soil solutions,
SeM concentrations at all time intervals were higher for the sandy as compared to the clayey soil. At 50 and 100 μM SeM treatments,
the solution pH was linearly related to the percentages of SeM disappeared from the solutions; greater percentage of SeM was
removed from solutions at comparatively lower pH levels, which was ≥90% at pH 7.7 for both soils. Solution SeM concentrations
decreased exponentially with time following first-order kinetic reactions. Under all applications (except for the control),
C
0 (SeM concentration at t=0) values for the sandy soil were greater than those determined for the clayey soil, indicating higher solution SeM availability
for the former and more SeM retention by the latter at t=0. Comparison of C
0 in controls (0 μM SeM addition) suggested greater indigenous SeM in the clayey soil. For both soils, C
0 values under different treatments followed the order, (soil+100 μM)>(soil+50 μM)>(soil+0 μM). The specific reaction rate
constants (K
r) of SeM for both soils were similar (0.031 and 0.029 day–1 for sandy and clayey soils, respectively); low K
r values indicated that SeM loss from our reclaimed coal mine soil solutions would follow rather slow kinetics. The half-life
(t
0.5) of SeM varied from 15 to 55 days depending on treatment level. The knowledge obtained from this study should contribute
in developing time-based Se reclamation strategies in coal mine environments.
Received: 18 September 1995 · Accepted: 28 December 1995 相似文献
16.
Mobilization and attenuation of heavy metals within a nickel mine tailings impoundment near Sudbury, Ontario, Canada 总被引:1,自引:0,他引:1
R. G. McGregor D. W. Blowes J. L. Jambor W. D. Robertson 《Environmental Geology》1998,36(3-4):305-319
The oxidation and the subsequent dissolution of sulfide minerals within the Copper Cliff tailings area have led to the release
of heavy metals such as Fe, Ni, and Co to the tailings pore water. Dissolved concentrations in excess of 10 g/l Fe and 2.2
g/l Ni have been detected within the shallow pore water of the tailings, with increasing depth these concentrations decrease
to or near analytical detection limits. Geochemical modelling of the pore-water chemistry suggests that pH-buffering reactions
are occurring within the shallow oxidized zones, and that secondary phases are precipitating at or near the underlying hardpan
and transition zones. Mineralogical study of the tailings confirmed the presence of goethite, jarosite, gypsum, native sulfur,
and a vermiculite-type clay mineral. Goethite, jarosite, and native sulfur form alteration rims and pseudo-morphs of the sulfide
minerals. Interstitial cements, composed of goethite, jarosite, and gypsum, locally bind the tailings particles, forming hardpan
layers. Microprobe analyses of the goethite indicate that it contains up to 0.6 weight % Ni, suggesting that the goethite
is a repository for Ni. Other sinks detected for heavy metals include jarosite and a vemiculite-type clay mineral which locally
contains up to 1.6 weight % Ni. To estimate the mass and distribution of heavy metals associated with the secondary phases
within the shallow tailings, a series of chemical extractions was completed. The experimental design permitted four fractions
of the tailings to be evaluated independently. These four fractions consisted of a water-soluble, an acid-leachable, and a
reducible fraction, as well as the whole-rock total. Twenty-five percent of the total mass of heavy metals was removed in
the acid-leaching experiments, and 100% of the same components were removed in the reduction experiments. The data suggest
that precipitation/coprecipitation reactions are providing an effective sink for most of the heavy metals released by sulfide
mineral oxidation. In light of these results, potential decommissioning strategies should be evaluated with the recognition
that changing the geochemical conditions may alter the stability of the secondary phases within the shallow tailings.
Received: 9 April 1997 · Accepted: 21 July 1997 相似文献
17.
A field survey and modelling of the oxidation and carbonate buffering reactions inside the alum-shale-containing waste rock
dumps located in Maardu, Estonia, was accomplished. In the slope areas, the shale has been altered at high temperatures due
to the spontaneous combustion and the pyritic acidity has been eliminated through migration of SOx gases out from the dump. In the central parts of the waste rock plateaus, low temperature pyrite oxidation fronts develop
towards the dump depth and towards the centres of individual shale lumps. The main secondary phases precipitating in the weathering
profile are gypsum, ferric oxyhydroxide, K-jarosite and smectite. The respective field data made it possible to calibrate
the two-stage oxygen diffusion model and the characteristic pyrite oxidation rate 0.06–0.08 mol of pyrite reacted per kg of
available water (pyrox/H2O value) was estimated to describe the first tens of years of dump performance. The model is capable to compare different
shale disposal strategies that are illustrated with two case scenarios. The buffering of sulphuric acid by Mg-calcite appears
to be an incongruent reaction with gypsum precipitating that leads to the build-up of the high Mg/Ca ratio in the leachate.
Application of the Mg/Ca method estimates the pyrox/H2O value in the range of 0.05–0.14 mol/kg.
Received: 26 January 1999 · Accepted: 23. February 1999 相似文献
18.
Application of statistical methods to study seasonal variation in the mine contaminants in soil and groundwater of Goa, India 总被引:1,自引:0,他引:1
Soil and groundwater samples were collected during two seasons, premonsoon (April 1990) and postmonsoon (December 1990),
and analyzed for major elements (Na, Mg, Al, Si, P, K, Ca, Ti, Mn and Fe), trace elements (Ni, Pb, Co, Cr and Zn) and water
parameters (pH, conductivity, acidity, alkalinity, hardness, Cl and SO4). All the data were subjected to linear discriminant analysis and partial correlation analysis in order to understand the
seasonal variation in the data. It was observed from the Mahalanobis generalized distance that in both soil and groundwater
samples there was a large difference in the concentration level of premonsoon and postmonsoon data. Linear discriminant functions
were calculated to distinguish between premonsoon and postmonsoon samples. From the partial correlation coefficient analysis
of soil samples, dominance of chemical weathering and precipitation of atmospheric fallout during monsoon were inferred. In
the case of the water samples, high conductivity and high hardness in the postmonsoon samples as well as atmospheric fallout
of Pb and Ni during the premonsoon period was suggested from partial correlation of water samples.
Received: 19 September 1995 · Accepted: 12 December 1995 相似文献
19.
The assessment of the aluminosilicate buffering potential during acid weathering of the Estonian alum shale is provided.
It is found that the stoichiometric interaction between dissolved pyrite oxidation products and illite of the shale best describe
the buffering process and are consistent with earlier field studies. The scheme includes incongruent dissolution of illite
with smectite and K-jarosite precipitating. This complex mechanism involves buffering of 8% of the acidity by K+ and temporary precipitation of 25% of the acidity as K-jarosite. Dissolution proceeds at a low pH (1.5–3) until all pyrite
in the shale particle is oxidised. Hence, if the total amount of illite present is larger than needed for stoichiometric interactions,
only part of it is involved in a buffering process, neutralising a certain percentage of acidity. The next stage in shale
weathering is the incongruent dissolution of K-jarosite with the release of the precipitated acidity and the formation of
ferric oxyhydroxide.
Received: 3 August 1998 · Revised paper: 26 January 1999 · Accepted: 23 February 1999 相似文献
20.
Integrated use of hydrochemistry and resistivity methods in groundwater contamination caused by a recently closed solid waste site 总被引:6,自引:0,他引:6
In order to investigate the groundwater contamination by solid waste disposal using both hydrochemical and geophysical methods,
the Halkalı (I˙stanbul) solid waste disposal site which was closed in 1994 was investigated. The disposal site lies on a ridge
between two valleys filled with alluvium. A total of six boreholes were drilled on two lines across the Menekşe valley adjacent
to the Halkalı site. Groundwater samples collected from these boreholes were analyzed for various contaminant parameters.
The results indicate that TDS and chloride concentrations decrease horizontally away from the waste site whereas they increase
with depth. Electrical soundings carried out at 12 locations yielded high resistivity values at the upstream part of Menekşe
valley while lower values were obtained from the locations near the leachate seepage points.
Received: 11 November 1997 · Accepted: 23 February 1998 相似文献