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1.
The distribution coefficient (K d) between the soil solid phase and the liquid phase is a commonly used parameter to evaluate radionuclide behavior in soil. For reliable evaluation, the variability of the K ddue to changes of environmental conditions should be clarified. In the present study, applicability of electrical conductivity (EC) to estimate effects of competing ions on K dwas examined in batch experiments. The K dof five radionuclides (60Co, 54Mn, 85Sr, 137Cs, 65Zn) were found to be in inverse proportion to the supernatant EC. The K dof 60Co and 54Mn were increasing at 7 days and no stable K dcould be determined. For 85Sr, 137Cs, and 63Zn, the regression equations obtained between the K dof the equilibrium state and the supernatant EC were validated in another experiment; it is expected that their K ds can be determined accurately using the EC. Because of its simplicity in measurements, using EC should be very practical for estimating effects of competing ions on K dof selected radionuclides.  相似文献   

2.
Abstract

Both selenium (Se) and antimony (Sb) are major soil and water pollutants. Their sorption behavior in a soil–plant system was studied. Soil–soil solution distribution coefficients (K ds) for Se and Sb were measured, using a radiotracer, as an indicator of their sorption levels. Both Se and Sb behave as oxoanions (SeO2? 4, H2PO? 4 and SO2? 4) in soil; thus, the effects of concentrations of two major oxoanions (SeO2? 4 and SeO2? 3) on Se and Sb sorption were also examined. The K d values for Se for Japanese soils significantly correlated with the K d values for Sb (n = 141). The K ds of both Se and Sb similarly decreased with increasing SbO? 3 concentration. These results indicated that the sorption of Se and Sb was similarly controlled by a ligand-exchange mechanism such as phosphate sorption in soil. However, an increase in the concentration of SeO2? 3 did not decrease the K ds of Se and Sb. Furthermore, the ligand-exchangeable fractions of stable Se and Sb in major Japanese soils were determined by extraction with 0.1 mol L?1 Na2HPO4 solution. For both Se and Sb, the phosphate-extractable fractions were 10-fold higher for Se and fivefold higher for Sb than their water-soluble fractions. Although the total Se and Sb amounts in soils were the same, their ligand-exchangeable fractions were different. Approximately 0.9–12% of total Se and 0.2–1.3% of total Sb were extracted by the phosphate solution. These findings suggested that Se was more likely to be mobilized by the addition of phosphate than Sb. The effect of plant-available phosphate in the soil and the phosphate sorption capacity of soil on Se and Sb availabilities for plants were also examined using a pot experiment with soybean plants. The experimental results suggested that a high content of available phosphate and/or low phosphate sorption capacity of soil increased both Se and Sb availabilities to the plant. However, the results also suggested that the soil Se availability to the plant was higher than that of Sb even though the soil total Se and Sb amounts were the same.  相似文献   

3.
Assessing metal contamination of sediments requires knowledge of the geochemical partitioning of trace metals at the sediment-water interface. Under controlled laboratory conditions, sequential extraction was conducted to determine the associations of metals (Cd, Cr, and Zn) and radiotracers (109Cd, 51Cr, and 65Zn) with various geochemical phases and the different partitioning and mobility of metals for two types of surface sediments collected from the Huanghe and Changjiang Rivers in Eastern China. The residual phase was the major phase for stable metal binding, indicating that these sediments had little subjection to recent anthropogenic influences. Fe–Mn oxides were the next important binding phases for metals. The partitioning of metals in various geochemical phases as a function of the duration of the radiolabeling was also examined. Trace metals transferred among the different geochemical phases over the 30 days radiolabeling period, particularly between the carbonate and Fe–Mn oxides phases. The freshwater-sediment distribution coefficients (K d) of three metals were investigated in batch experiments using the radiotracer technique. The decreasing K d with increasing metal concentration(from 0.5 to 200 μg L-1) may be explained by competitive adsorption. The metal K d in sediments from the Changjiang River was greater than those from the Huanghe River, presumably because of the higher Fe/Mn and organic carbon contents in Changjiang River sediment. The K d decreased with increasing total suspended solid load from 3 to 500 mg L-1, and was Cr > Zn > Cd. For Cd and Zn, increasing the pH from 5 to 8 resulted in an increase in K d due to the reduced H+ competition and increasing sorptionpotential. However, the K d for Cr in the sediments from both rivers showed no relationship with pH, presumably becauseof the complexity of the Cr species and environmental behavior.  相似文献   

4.
Sampling is conducted during 2006 in Lahontan Reservoir, Nevada to investigate seasonal variation of total mercury (THg) and methylmercury (MeHg) partitioning in different phytoplankton size fractions as a function of point source (fluvial) mercury (Hg) loads, reservoir residence time, and algal growth. Carson River Hg inputs into the reservoir are extremely dynamic with spring loads two orders of magnitude larger than summer loads. Chlorophyll a measurements show two periods of algal growth. A small amount of algal growth occurs March to May. A second more substantial bloom occurs in the late summer, which is dominated by large, filamentous algae. THg concentrations (C b) and partitioning coefficients (K d) in total suspended particulate matter (SPM) are highest when fluvial inputs of Hg-contaminated sediment are large and are not necessarily associated with living biomass. However, MeHg K d in the small size fraction is indirectly related to fluvial loads and more strongly associated with living biomass in the later portion of the summer when algal growth occurs and reservoir residence times are longer. Data suggest size distinction is important to MeHg partitioning in the reservoir. Lumping all sizes into a single SPM sample will bias the analysis toward low MeHg C b and low MeHg K d in late summer when Aphanizomenon flos-aquae dominates the phytoplankton assemblage.  相似文献   

5.
The main objective of this study was to evaluate the contribution of sorption to the removal of two commonly used antibiotics (amoxicillin and ciprofloxacin) from wastewater. These antibiotics are excreted in large quantities with more than 75% of them being unmetabolized and are therefore likely to end up in domestic wastewater in significant quantities. The specific objectives were to determine the sorption behavior in synthetic wastewater (SWW), the effect of pH and contribution of microbial surfaces, to the sorption of these antibiotics. The SWW, adjusted to various pH levels, was used and sorption kinetics conducted at 100 and 250 ??g L?1 concentrations. Adsorption isotherms were determined at different pH levels. The SWW (pH 6.6) was inoculated with Rhodococcus sp. B30 strain to determine the contribution of microbial surfaces to sorption. Generally, both antibiotics revealed a decrease in sorption with pH increase, suggesting that lowering the solution pH of the wastewater may reduce their amounts in wastewater solution. Comparatively, ciprofloxacin exhibited higher sorption than amoxicillin. The sorption distribution coefficient (K d) values for ciprofloxacin ranged from 0.4356 to 0.8902 L?g?1, with pH?=?5.5 exhibiting the highest K d, while that for amoxicillin ranged from 0.1582 to 0.3858 L?g?1 with the highest K d at pH?=?3.5. There was a significant difference (p?<?0.05) in K d values between various pH levels for both antibiotics except between the pH of 5.5 and 6.6. Both antibiotics were not degraded within 48 h by Rhodococcus sp. B30 strain. These results indicate that degradation may not be the major process of removal of compounds from wastewater treatment plants and hence the importance of sorption as an intervention technique.  相似文献   

6.
In this study, distribution coefficients (K d s) of five radionuclides (54Mn, 60Co, 65Zn, 85Sr, 137Cs) were measured by a batch technique for 36 agricultural soils (paddy and upland soils) collected in Japan. Twelve properties of the soils and measured K d ss were examined for their probability distributions. Soil properties showed log-normal type distributions, except pH(H2O), and total calcium and potassium contents for which distributions were normal. The K d s distributions were also lognormal types except for 54Mn-K d s? No significant difference in soil K d s was found between upland and paddy soils. The K d s were analyzed for correlations with each soil property. The combinations showing the highest correlation were: the exchangeable calcium for 54Mn and 60Co; the water content for 65Zn; the cation exchange capacity for 85Sr; and the exchangeable potassium for 137Cs, For 54Mn and 60Co, their K d s also correlated with supernatant pH. Only 85Sr-K d s showed an adequate correlation with the value if the CEC divided by the supernatant EC. For the other four nuclides, some other mechanisms besides ion exchange seemed to be working.  相似文献   

7.
Cadmium distribution coefficients, K d were determined at low Cd concentrations (solute: 0.2 to 3.0 μg Cd dm?3, soil: 0.044 to 1.1 mg Cd kg?1) for 63 Danish agricultural soils. The K d values ranged from 15 to 2450 L kg?1. About 40% of the soils had K d values below 200 L kg?1. The observed K d values correlated very well with soil pH (r 2 = 0.72). Introducing soil organic matter content as a second parameter improved the correlation some (r 2 = 0.79). No further improvements were obtained by introducing traditional soil parameters as clay, silt, fine sand, coarse sand and CEC or ‘reactive’ parameters as oxyhydroxides of Mn, Fe and Al. The identified regression equation for predicting K d values indicates that K d approximately doubles for each 0.5 unit increase in pH or 2% increase (weight basis) in organic matter content.  相似文献   

8.

Purpose

Understanding the fate and behavior of polycyclic aromatic hydrocarbon (PAH) sources in aquatic systems is important for the efficiency of control policies. In this work, a new approach??organic carbon-normalized sediment?Cpore water partition coefficients of PAH source contributions (logK??Osource)??was developed to study the sediment?Cpore water partition of PAH source contributions. The focus of this study was the Yellow River, which is the second largest river in China and one of the largest rivers in the world.

Materials and methods

Sixteen priority US Environmental Protection Agency PAHs were analyzed in 14 surface sediments and 11 pore water samples. Principal component analysis?Cmultiple linear regression (PCA-MLR) and Unmix models were employed to estimate the source contributions of PAHs in sediments and pore water samples. Finally, logK??Osource values were calculated according to the modeled source contributions of PAHs.

Results and discussion

??PAHs (sum of the 16 PAH concentrations) in 14 sediment samples and 11 pore water samples from the Yellow River were 1,415?±?726?ng?g?1 dry weight (dw) and 123?±?57.4???g?l?1, respectively. The source apportionment results indicate the following: (1) for sediment samples, the contributions to ??PAHs from vehicular emissions, coal combustion, and petrogenic sources were 41.07?C61.05, 38.83?C45.56, and 11.18?C14.92?%, respectively, and (2) for pore water samples, vehicular emissions were the most significant contributor (45.51?C69.39?%), followed by petrogenic sources (29.80?C34.22?%) and coal combustion (7.35?C21.59?%). Coal combustion had the highest logK??Osource values (4.15?C4.26) among the three categories, followed by vehicular emissions (3.51?C3.57) and petrogenic sources (3.30?C3.43).

Conclusions

The possible categories of PAH sources identified by hierarchical cluster analysis, PCA-MLR, and Unmix models were consistent, indicating that vehicular emissions, coal combustion, and petrogenic sources were three important categories. The logK??Osource values indicate that contributions from coal combustion had a higher partition for the sediment phase compared with the other two source categories.  相似文献   

9.
The retention of antimony (a potential toxin) in polluted soils or waterway sediments can involve interaction with several component phases. One of these, humic acid, has been found to adsorb antimony (III) from solutions of Sb(OH)3 or potassium antimonyl tartrate (C8H4K2Sb2O12) in accordance with Langmuir type isotherms. Using Sb(OH)3 solutions (initial Sb levels < 10 μM) the bonding constant value (at pH 4) was 6 × 105, with a calculated saturation capacity of 23 μmol g?1. In the antimonyl tartrate systems (initial Sb levels 0.5 to 75 μM) the bonding constant value for the sorbed species was 1.6 × 105 and the saturation capacity 53 μmol g?1. Addition of small amounts of HCl or NaOH (to vary the pH between 3.1 and 5.4) had little effect on the amount sorbed from KSbT solutions but with Sb(OH)3 solutions uptake was reduced (by about 15%). In the presence of NaCl (0.5 or 0.05M) Sb uptake increased (by about 15%). Antimony (V) (introduced as KSb (OH)6) was not sorbed from solutions < 10 μM in this salt. Using more concentrated solutions, uptake gradually increased, reaching a plateau value of around 8 μmol g?1 with solutions initially 50 or 75 μM.  相似文献   

10.
Antimony (Sb) and Copper (Cu) are two metals of major concern in sewage sludge. Antimony because its use in society is increasing and this might lead to increased Sb concentrations in sludge. Copper because its total volume in use in society is large and because of corrosion from water pipes it is most difficult to reduce the Cu concentrations in sludge. Fresh digested sewage sludge was spiked with Cu or Sb and the sludge was cultivated with oat (Avena sativa), lettuce (Lactuca sativa) or radish (Raphanus sativus). Elutriates from the cultivated sludge were tested for toxicity with Lemna minor (7-d growth) and Daphnia magna (48 h immobility). Before cultivation the elutriates were toxic to Lemna and Daphnia due to high concentrations of ammonia (NH3) and nitrite (NO2 -). Cultivation decreased the concentrations of both NH3 and NO2 -, thereby reducing the impact of these compounds in the toxicity tests. Cultivation also decreased the metal concentrations and pH. Daphnia magna was the most sensitive test organism in this study with a 48 h EC50 of 1130 mg Cu kg-1 dry wt and 5 mg Sb kg-1 dry wt in elutriates from sludge cultivated with oat. In sludge cultivated with radish the 48 h EC50 was 1700 mg Cu kg-1 dry wt and 22 mg Sb kg-1 dry wt. The effect of Cu could be predicted by pH and Cu concentrationin the elutriate, but the effect of Sb could not solely be explained by its concentration in the elutriate.  相似文献   

11.
Varying amounts of humic acid (Na-salt; 0 to 10 ppm) were first equilibrated with a dilute suspension of H+ -Kaolin and then with spiked (high activity radioactive 64Cu) water. After filtration, through a 0.45 µm Millipore membrane, the amount of radioactivity on the filter and the filtrate was measured separately using a Gamma ray well counter. Adsorption of Cu (expressed as distribution coefficient — K d ) was plotted against humic acid concentration. K d values decreased sharply upto 1 ppm humic acid concentration and thereafter remained constant up to 10 ppm.  相似文献   

12.
Varying amounts of CaCl2 or MgCl2 were first equilibrated with a dilute suspension of H+-kaolin and then with spiked (high activity radioactive 64Cu) water. After filtration, through 0.45 μm Millipore membrane the amount of radioactivity on the filter and the filtrate was measured separately using a Gamma ray well counter. Adsorption of Cu (expressed as distribution coefficient ?K d ) was plotted against Ca or Mg concentration. K d values decreased sharply at Ca or Mg concentration from 0 to 10 mg l?1 and changed only slightly from 10 to 200 mg l?1. Increase in the concentration of Ca or Mg reduces the Cu binding capacity of the clay.  相似文献   

13.
In hydroponic plant nutritional research, nutrient solutions can be considered as aqueous solutions of inorganic ions. In this aqueous solution, the ions are submitted to the laws of aquatic inorganic chemistry. This means that the ions are involved in the dynamic equilibria between complexation, dissociation, and precipitation reactions. These chemical reactions seriously impact elemental speciation and bioavailability. As a result, plant roots experience a different nutritional composition. Ions withdrawn from the‐nutrient solution due to precipitation reactions, change the nutritional composition and are not available for uptake by plant roots. Like complexes, precipitates can buffer a nutrient solution, exchanging nutrients as these decrease by plant uptake. This research looks into the precipitation reactions that occur in hydroponic nutrient solutions. In the concentration range of nutrient solutions, no precipitates are formed involving potassium (K+), nitrate (NO3 ‐), ammonium (NH4 +), or sulphate (SO4 2‐), while calcium (Ca2+) and magnesium (Mg2+) form mainly at a higher pH precipitates with hydrogen phosphate (HPO4 2‐). Preparing nutrient solutions with tap water, calcium carbonate (CaCO3) is likely to precipitate. A good knowledge of the chemical reactions occurring in nutrient solutions is the first prerequisite in hydroponic plant nutritional research.  相似文献   

14.
《Geoderma》2005,124(3-4):335-348
In order to gain understanding of the movement of pollutant metals in soil, the chemical mechanisms involved in the transport of zinc were studied. The displacement of zinc through mixtures of sand and cation exchange resin was measured to validate the methods used for soil. With cation exchange capacities of 2.5 and 5.0 cmolc kg−1, 5.6 and 8.4 pore volumes of 10 mM CaCl2, respectively, were required to displace a pulse of ZnCl2 . A simple Burns-type model (Wineglass) using an adsorption coefficient (Kd) determined by fitting a straight line relationship to an adsorption isotherm gave a good fit to the data (Kd=0.73 and 1.29 ml g−1, respectively).Surface and subsurface samples of an acidic sandy loam (organic matter 4.7 and 1.0%, cation exchange capacity (CEC) 11.8 and 6.1 cmolc kg−1, respectively) were leached with 10 mM calcium chloride, nitrate and perchlorate. With chloride, the zinc pulse was displaced after 25 and 5 pore volumes, respectively. The Kd values were 6.1 and 2.0 ml g−1, but are based on linear relationships fitted to isotherms which are both curved and show hysteresis. Thus, a simple model has limited value although it does give a general indication of rate of displacement. Leaching with chloride and perchlorate gave similar displacement and Kd values, but slower movement occurred with nitrate in both soil samples (35 and 7 pore volumes, respectively) which reflected higher Kd values when the isotherms were measured using this anion (7.7 and 2.8 ml g−1, respectively). Although pH values were a little higher with nitrate in the leachates, the differences were insufficient to suggest that this increased the CEC enough to cause the delay. No increases in pH occurred with nitrate in the isotherm experiments. Geochem was used to calculate the proportions of Zn complexed with the three anions and with fulvic acid determined from measurements of dissolved organic matter. In all cases, more than 91% of the Zn was present as Zn2+ and there were only minor differences between the anions. Thus, there is an unexplained factor associated with the greater adsorption of Zn in the presence of nitrate.Because as little as five pore volumes of solution displaced Zn through the subsurface soil, contamination of ground waters may be a hazard where Zn is entering a light-textured soil, particularly where soil salinity is increased. Reductions in organic matter content due to cultivation will increase the hazard.  相似文献   

15.
The saturated hydraulic conductivities (Ksat) of 22 spodic horizons with and without a thin iron pan, were measured in situ with a new technique, using large, carved-out columns, encased in gypsum. Measured infiltration rates and pressure heads above and below the spodic horizons allowed calculation of Ksat, which averaged 8 cm d?1. Flow rates averaged 32 cm d?1, however, due to a hydraulic head gradient across the spodic horizon of 4 cm cm?1. Occurrence of a thin iron pan in the spodic horizon did not affect its Ksat-value. The measured high flow rates exclude the occurrence of perched watertables. Lateral flow of water, forming surface ponds in local depressions, was due to surface runoff, rather than to lateral movement of perched water: surface ponding of water occurred also in soils in which the spodic horizon had been mixed by tillage.  相似文献   

16.
The potential for surface and groundwater contamination of soil applied herbicides is partly dependent on soil properties. Sorption and desorption of diuron and norflurazon were studied in seven soils representative of the southern citrus-belt of Florida using the batch-equilibrium technique. Sorption of herbicides was influenced by soil properties. Sorption coefficients (K d) ranged from 0.84 to 3.26 mL g?1 for diuron and 0.63 to 2.20 mL g?1 for norflurazon indicating weak to moderate binding of herbicides to soil. For norflurazon, K dwas significantly related to organic C content, soil pH, and cation exchange capacity. For diuron, absence of a significant relationship between K dand selected soil properties suggests that the soil properties other than those studied may play a role in determining sorption on these soils. Desorption studies showed that higher amounts of diuron and norflurazon was desorbed by water than by 0.5 M CaCl2. An inverse relationship was apparent between herbicides sorbed and that which was desorbed among the soils studied. The soil which exhibited higher sorption had lower desorption and the soil which exhibited lower sorption had higher desorption.  相似文献   

17.
Chestnut agro-industrial companies consume a high volume of water for washing and processing fruit, generating a large volume of wastewater. This work studied the biodegradation of chestnut processing wastewater through aerobic assays, varying substrate, and biomass concentrations. In general, this wastewater presents a good biodegradability, especially in experiments with relatively low chemical oxygen demand (COD) (0.4 and 0.6?g O2 L?1) allowing a COD removal of 85?C90?%. The best results were obtained in the reactor initially loaded with 2?g?L?1 of biomass and 0.4 or 0.6?g O2 L?1 of COD. These experiments also showed high COD removal rates: 4.25 and 3.88?g COD g?1 volatile suspended solids (VSS) h?1, respectively. The sedimentation rate, evaluated for different initial values of biomass (1, 2, and 3?g?L?1), always presented higher values in the experiments with 2 and 3?g?L?1 of biomass, regardless of the initial COD value used. After comparing different kinetic models (Monod, Contois, and Haldane), it was observed that the Haldane inhibition model satisfactorily describes the COD biodegradation. AQUASIM software allowed calculating the kinetic constant ranges: K s, 1.59?C6.99?g COD L?1; ?? max, 25?C40?g COD g?1 VSS day?1; and K i values, 0.07?C0.11. These kinetic constants corresponds to maximum rates (??*) between 1.48 and 4.25?g COD g?1 VSS day?1 for substrate concentrations (S*) from 0.38 to 0.88?g COD L?1.  相似文献   

18.
Denitrification capacity (Dcap) was measured in 595 samples from the saturated zone of 18 hydromorphic soils by anaerobic incubation of K15NO3 amended slurries in the laboratory and analyzing 15N2+15N2O in the sample headspace. Furthermore, texture, pH, C (total, dissolved, hot-water soluble organic and inorganic C), total N and sulfide were measured in order to model Dcap by multiple regression analysis. Dcap was significantly correlated with most properties, but exhibited closest relationships to electron donors (Corg, sulfide). Mean Dcap of the soil material groups sand, loam and peat was 0.13, 1.34 and 26.6 mg N kg−1 d−1. Regression models for predicting Dcap were varied in the selection of independent variables and in the differentiation of the source data-set. The models were found to be suitable to estimate mean Dcap of sites with medium or high activity. Best fits were obtained with the most complex models, but prediction for individual samples and for means of sites with low activity was not satisfactory with any of the models. The model based on organic C and total N and differentiating by type of soil material and position above or below the groundwater level was validated using an independent data-set of 46 samples from 10 sites of fluvial, glaciofluvial or glacial till sediments. The fluvial sediments exhibited satisfactory agreement between measured and predicted Dcap, whereas the other sites were overestimated because Dcap was too low and possibly because glacial till was not part of the source data-set. Our study provides for the first time statistical models of denitrification in shallow groundwater which are transferable to other sites with certain restrictions and which can thus be used for environmental modeling and soil mapping.  相似文献   

19.
A pot trial was conducted to clarify the effects of molybdenum (Mo) on photosynthesis and ionic homeostasis of Chinese cabbage under salinity stress. Mo was applied at three levels (0, 0.15, 0.3 mg kg?1). Ten days after sowing, 500 ml of 0.8% of NaCl solution was added to half of the plants for each treatment every 10th day for three consecutive times. The results revealed that fresh weight was significantly increased by application of Mo under salt stress; contents of chlorophyll a, chlorophyll b, carotene, and total chlorophyll were all raised by application of Mo; photosynthesis rate was enhanced by nonstomatal factors by application of Mo; and the ratios of potassium/sodium ions (K+/Na+), calcium/sodium ions (Ca2+/Na+), and magnesium/sodium ions (Mg2+/Na+) were all increased by application of Mo under salt stress. The study suggests that the application of Mo enhances salinity stress tolerance in Chinese cabbage by increasing the photosynthesis rate and the ionic homeostasis adjustment.  相似文献   

20.
Nitrogen isotope abundance (?? 15N) of paddy rice (Oryza sativa L.) grown for 110?days after transplanting (DAT) under field conditions with ammonium sulfate (AS with ?0.4?? as a synthetic fertilizer), pig manure compost (PMC with 15.3?? as a livestock manure compost), and hairy vetch (HV with ?0.5?? as a green manure) was investigated to test the possible use of ?? 15N technique in discriminating organically grown from conventionally grown rice. At 15 DAT, the ?? 15N of whole rice decreased (P?<?0.05) in the order of 10.5?? for PMC > 5.5?? for control (without N input) > 4.0?? for HV > 1.8?? for AS. This difference seemed to reflect primarily the ?? 15N signal of N sources. Although differences in ?? 15N of rice grown with isotopically distinct N inputs (i.e. PMC vs. AS and PMC vs. HV) became smaller over time, the difference (2.8 and 3.0?? difference at harvest on 110 DAT, respectively) was still significant (P?<?0.05). However, there was no distinguishable difference between AS and HV treatment after 42 DAT. Such effect of N inputs on ?? 15N of whole rice was also observed for root, shoot, and grain at harvest. Therefore, our study suggests that it is possible to distinguish rice grown with manure composts from that grown with synthetic fertilizers. However, if green manure of preceding N2-fixing plants is used as the N source, ?? 15N of rice may not be a good surrogate of N sources.  相似文献   

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