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1.
Quality assessment of potable water in the town of Kolasib,Mizoram (India)   总被引：2，自引：1，他引：1  
The potable water for the residents of the town of Kolasib in Mizoram state, India, is supplied by the Public Health Engineering Department (PHED) of the Government of Mizoram without any notable treatment. The source of water is the Tuichhuahen River, flowing from north to south in the area. Water samples were analyzed for their physicochemical and bacteriological characteristics in order to obtain the current quality level of the potable water in the twon. The samples were collected from two different sources, i.e., the supply from the government agency (PHED) and from the naturally occurring springs (tuikhurs). The results suggest that the water supplied by the PHED is better than that from the tuikhurs; however, the quality of water from both sources, which are used for drinking and domestic purposes, were found to be more or less within the tolerance limits.  相似文献   

2.
Hydrogeochemical investigations are carried out in and around Perumal Lake, Cuddalore district, South India in order to assess its suitability in relation to domestic and agricultural uses. The water samples (surface water = 16; groundwater = 12) were analyzed for various physicochemical attributes like pH, electrical conductivity (EC), sodium (Na+), potassium (K+), calcium (Ca2+), magnesium (Mg2+), chloride (Cl−), bicarbonate (HCO3 −), sulfate (SO4 2−), phosphate (PO4), silica (H4SiO4) and total dissolved solids (TDS). Major hydrochemical facies were identified using Piper trilinear diagram. Hydrogeochemical processes controlling the water chemistry are water–rock interaction rather than evaporation and precipitation. Interpretation of isotopic signatures reveals that groundwater samples recharged by meteoric water with few water–rock interactions. A comparison of water quality in relation to drinking water quality standard proves that the surface water samples are suitable for drinking purpose, whereas groundwater in some areas exceeds the permissible limit. Various determinants such as sodium absorption ratio (SAR), percent sodium (Na%), residual sodium carbonate (RSC) and permeability index (PI) revealed that most of the samples are suitable for irrigation.  相似文献   

3.
The overexploitation of groundwater in some parts of the country induces water quality degradation. The untreated industrial effluents discharged on the surface causes severe groundwater pollution in the industrial belt of the country. This poses a problem of supply of hazard free drinking water in the rural parts of the country. There are about 80 tanneries operating in and around Dindigul town in upper Kodaganar river basin, Tamilnadu, India. The untreated effluents from the tanneries have considerably affected the quality of groundwater in this area. To assess the extent of groundwater deterioration, a detailed analysis of groundwater quality data has been carried out. The concentration of cations such as Calcium (Ca2+), Magnesium (Mg2+), Sodium (Na+) and Potassium (K+), and anions such as Bicarbonate (HCO3–), Sulphate (SO42–), Chloride (Cl–) and Nitrate (NO3–) in the groundwater have been studied. Apart from these constituents, pH, electrical conductivity (EC), total dissolved solid and total hardness (TH as CaCO3) were also studied. The correlation of these constituents with the EC has been carried out. The highest correlation is observed between EC and chloride with a correlation coefficient of 0.99. Progressive reduction in correlation coefficients for Mg2+, (Na+ + K+), Ca2+ and SO42– are observed as 0.91, 0.87, 0.86 and 0.56, respectively. It is found that the quality of groundwater in the area under investigation is deteriorated mainly due to extensive use of salt in the leather industries.  相似文献   

4.
A hydrogeochemical study of surface water of the West Bokaro coalfield has been undertaken to assess its quality and suitability for drinking, domestic and irrigation purposes. For this purpose, fourteen samples collected from rivers and ponds of the coalfield were analysed for pH, conductivity, total dissolved solids (TDS), major cations (Ca2+, Mg2+, Na+ and K+), major anions (HCO3-, F-, Cl-, SO42- and NO3-) and trace metals. The pH of the analysed water samples varied from 7.3 to 8.2, indicating slightly alkaline in nature. The electrical conductivity (EC) value varied from 93 μs cm-1 to 906 μs cm-1 while the TDS varied from 76 mg L-1 to 658 mg L-1. HCO3- and SO42- are the dominant anion and Ca2+ and Na+ the cation in the surface water. The concentration of alkaline earth metals (Ca2+ + Mg2+) exceed the alkali metals (Na+ + K+) and HCO3- dominates over SO42- + Cl- concentrations in the majority of the surface water samples. Ca2+ -Mg2+ -HCO3- and Ca2+ -Mg2+ -Cl- are the dominant hydrogeochemical facies in the surface water of the area. The water chemistry is mainly controlled by rock weathering with secondary contribution from anthropogenic sources. For quality assessment, analyzed water parameter values compared with Indian and WHO water quality standard. In majority of the samples, the analyzed parameters are well within the desirable limits and water is potable for drinking purposes. However, concentrations of TDS, TH, Ca2+, Mg2+ and Fe are exceeding the desirable limits in some water samples and needs treatment before its utilization. The calculated parameters such as sodium absorption ration, percent sodium, residual sodium carbonate, permeability index and magnesium hazard revealed good to permissible quality and suitable for irrigation purposes, however, higher salinity, permeability index and Mg-ratio restrict its suitability for irrigation at few sites.  相似文献   

5.
This study assessed the levels of selected inorganic contaminants in streams and stream sediments in the effluent areas relating to the pyrometallurgical and hydrometallurgical treatment of gold ores in the Obuasi gold mine, Ghana. Water and stream sediment samples were taken from specific locations during the consecutive rainy and dry seasons, and concentrations of phosphate (PO4 3−), nitrate (NO3 −), chloride (Cl−), sulphate (SO4 2−), sodium (Na+), potassium (K+), calcium (Ca2+), magnesium (Mg2+), arsenic (As), copper (Cu), iron (Fe), zinc (Zn) and lead (Pb), were determined. Alkalinity, pH, temperature and specific electrical conductivity were also measured. In the water samples, the average pH range for both the seasons is 6.9–7.4, most anions and metals have relatively higher concentrations in the wet season than in the dry season at both the metallurgical sites. Trace metals concentrations were comparatively low (<0.01–5.00 mg/l), higher in the dry season at the pyrometallurgical sites. Irrespective of seasons, SO4 2− (0.80–949.50 mg/l) and PO4 3− (<0.01–6.30 mg/l) were pronounced at the pyrometallurgical sites, while NO3 − (0.01–98.45 mg/l) and Cl− (1.88-49.05 mg/l) were higher at the hydrometallurgical sites. In water samples, Ca2+ and SO4 2+ were the dominant cation and anion, respectively. In the stream sediments, except pH, NO3 −, Cl−, Na+ and Mg2+, all other parameter values were relatively higher at the hydrometallurgical areas. The average concentrations of Ca2+, Mg2+, As and Fe are remarkably high at both metallurgical sites (3,217–46,026 mg/kg). Overall, the level of parameters in the water samples are pronounced at pyrometallurgical sites, whereas the levels in sediments are higher at the hydrometallurgical sites.  相似文献   

6.
Groundwater samples from three different states of southeastern Nigeria were analyzed for a range of physicochemical parameters and heavy metal constituents in order to determine their potability as drinking water sources. The results indicate that the physicochemical parameters (pH, temperature, TDS, TH, electrical conductivity, Cl?, NO3 ? and SO4 2?) tested for have most of their values fall within the permissible limits given by the Nigerian and World Health Organization standards for drinking water quality. The results also reveal that apart from Cu and Pb the rest of the tested heavy metals (As, Zn, Ni, total Fe, Cd and Mn) recorded values that are significantly above the maximum permissible limits for drinking water purposes and therefore have the potential to cause health impacts for long-term potable use. On the basis of principal component analysis, the sources of the measured chemical constituents are likely to be predominantly geogenic (weathering of soluble minerals in the formations) and partly anthropogenic (industrial effluent, agricultural practices and mining activities). Single-factor ANOVA tests indicated insignificant differences in the datasets within/between states or geological units. Caution or adequate treatment is generally required when utilizing the groundwater from the area for drinking purpose due to the elevated metal concentrations and rating as poor to unsuitable water quality.  相似文献   

7.
Balram Ambade 《Natural Hazards》2014,70(2):1535-1552
In the present work, chemical characterization and sources of fog water contaminants in the most polluted area of central India, Raipur, and its surroundings are described. The fog water (n = 22) was collected during 2010–2011 from six sites. The physical (i.e., pH, fog amount, electrical conductivity and TDS) and chemical (i.e., F?, Cl?, NO3 ?, SO4 2?, NH4 +, Na+, K+, Mg2+, Ca2+, Al, Mn, Fe, Cu, Zn, Pb and Hg) parameters of the fog water were investigated. The effect of meteorology, i.e., temperature, humidity and wind speed, on the precipitation of the fog water contaminants is discussed. The cluster and factor analysis are used to apportion the sources of the contaminants in the fog water.  相似文献   

8.
The River Ganges being the most sacred river and lifeline to millions of Indians in serving their water requirements is facing excessive threat of pollution. Under various river management and conservation strategies for its protection, the assessment of water quality of its main tributary Ramganga River is lacking. This study focuses on assessment of physicochemical and heavy metal pollution of the Ramganga River by application of multivariate statistical techniques. Sampling of Ramganga River at sixteen sampling sites was carried out in three seasons (summer, monsoon and winter) of 2014. The collected water samples were analyzed for physicochemical parameters and heavy metals. Results from cluster analysis (CA) of the data divided the whole stretch of the river into three clusters as elevation from 1304 to 259 m as less polluted, from 207 to 154 m as moderately polluted and from elevation 154 to 139 m as high-polluted stretches with anthropogenic as main sources of pollution in high-polluted stretch. Principal component analysis of the seasonal dataset resulted in three significant principal components (PC) in each season explaining 72–8% of total variance with strong loadings (>0.75) of PC1 on fluoride (F?), chloride (Cl?), sodium (Na+), calcium (Ca2+), magnesium (Mg2+), bicarbonate (HCO3 ?), total dissolved solids and electrical conductivity. Temporal variation by one-way ANOVA (Analysis of Variance) showed significant seasonal variation was in the pH, chemical oxygen demand, biochemical oxygen demand, turbidity, HCO3 ?, F?, Zn, cadmium (Cd) and Mn (p < 0.05). Turbidity showed approximately a twofold increase in monsoon season due to rainfall in the catchment area and subsequent flow of runoff into the river. Concentration of HCO3 ?, F? and pH also showed similar increase in monsoon. The concentration of Zn, Cd and Mn showed an increasing trend in summers compared to monsoon and winter season due to dilution effect in the monsoon season and its lasting effect in winters.  相似文献   

9.
Hydrochemical studies were carried out in parts of Oban Massif and Mamfe Embayment, Southeastern Nigeria to examine the contributions of barite mining activities on water quality. Pond and stream water samples were collected from six abandoned barite mines and adjoining streams areas during dry and wet seasons. These samples were analysed for physicochemical parameters using standard techniques. The results showed that the quality of water samples in the vicinity of barite mine sites was characterized by low pH, low mineralization, high suspended solids and deep colour. Sodium (Na+) and calcium (Ca2+) are the dominant cations and bicarbonate (HCO3 ?) and sulphate (SO4 2?) the dominant anions. The low concentration of dissolved silica, relatively high concentrations of Na+, HCO3 ? and SO4 2? suggest a combined influence of silicate, carbonate and weathering of mine spoils. The total dissolved solids, total suspended solids, electrical conductivity and major ions (except Na+ and K+) are higher for water draining barite mines from Cretaceous sediments, while Na+, K+, Ba2+ are higher in basement rock areas. Correlation and factor analyses suggest that the main components of the water samples are related to anthropogenic, geogenic, mineralization and environmental controls, while Gibbs diagram plots indicate weathering as the main process controlling the chemistry of water. Computed values of hardness and some irrigational parameters showed that the pond and stream water samples are generally soft and can be used for irrigation. Generally, the water quality satisfied the WHO (2008) and NIS (2007) standards for domestic, agriculture and industrial. However, there is the need to assess the impact of the pollution on the ecosystem and human health while remediation measures are being considered.  相似文献   

10.
The present study assesses the impact of coal mining on surface and groundwater resources of Korba Coalfield, Central India. Accordingly, water samples collected from various sources are analyzed for major ions, trace elements, and other mine effluent parameters. Results show that the groundwater samples are slightly acidic, whereas river water and mine water samples are mildly alkaline. Elevated concentrations of Ca2+, Na+, HCO3 ?, and SO4 2? alongside the molar ratios (Ca2++Mg2+)/(SO4 2?+HCO3 ?) <1 and Na+/Cl? >1 suggest that silicate weathering (water-rock interaction) coupled with ion exchange are dominant solute acquisition processes controlling the chemistry of groundwater in the study area. The overall hydrogeochemistry of the area is dominated by two major hydrogeochemical facies (i.e., Ca–Cl–SO4 and Ca–HCO3). Analysis of groundwater and river water quality index (GRWQI) elucidates that majority (82%) of samples are of “excellent” to “good” category, and the remaining 12% are of “poor” quality. Similarly, the effluent water quality index (EWQI) indicates that 6 out of 8 samples belong to excellent quality. Concentration of trace element constituents such as As, Zn, Cu, Cr, and Cd is found to be well within the stipulated limits for potable use, except for Fe, Mn, and Pb. Suitability of water samples for irrigation purpose, established using standard tools like Wilcox and USSL diagrams, reveal “excellent to permissible” category for majority of the samples. The present study also substantiates the effectiveness of the measures implemented for the treatment of mine effluent water.  相似文献   

11.
This study was carried out to analyze groundwater quality in selected villages of Nalbari district, Assam, India, where groundwater is the main source of drinking water. 40 groundwater samples collected from hand pumps and analyzed for pH, EC, TDS, Ca2+, Mg2+, Na+, K+, HCO3 −, SO4 2−, Cl− and F−. Chemical analysis of the groundwater showed that mean concentration of cations in (mg/L) is in the order Ca2+ > Mg2+ > Na+ > K+ while for anions it is HCO3 − > Cl− > SO4 2− > F−. Fluoride concentration was recorded in the range of 0.02–1.56 mg/L. As per the desirable and maximum permissible limits for fluoride in drinking water recommended by WHO and by Bureau of Indian Standards (BIS), which is 1.5 mg/L, the groundwater of about 97% of the samples were found to be suitable for drinking purpose. The suitability of the groundwater for irrigation purpose was investigated by some determining factors such as sodium adsorption ratio, soluble sodium percentage, Kelly’s ratio and electrical conductivity. The value of the sodium absorption ratio and electrical conductivity of the groundwater samples were plotted in the US Salinity laboratory diagram for irrigation water. Most of the groundwater samples fall in the field of C2S1 and C3S1 indicating medium to high salinity and low sodium water, which can be used for irrigation on almost all types of soil with little doubt of exchangeable sodium. The hydrochemical facies shows that the groundwater is Ca-HCO3 type.  相似文献   

12.
Hydrogeochemical investigation of groundwater has been carried out in the coastal aquifers of southern Tamil Nadu, India. Seventy-nine dug well samples were collected and analyzed for various physicochemical parameters. The result of the geochemical analysis indicates the groundwater in the study area is slightly alkaline with moderate saline water. The cation and anion concentrations confirm most of the groundwater samples belong to the order of Na+ > Mg2+ > Ca2+ > K+ and Cl? > SO4 2? > HCO3 ?. Thereby three major hydrochemical facies (Ca–Cl, mixed Ca–Mg–Cl and Na–Cl) were identified. Based on the US Salinity diagram, majority of the samples fall under medium to very high salinity with low to high sodium hazard. The cross plot of Ca2+ + Mg2+ versus chloride shows 61 % of the samples fall under saline water category. Higher EC, TDS and Cl concentrations were observed from Tiruchendur to Koodankulam coastal zone. It indicates that these regions are significantly affected by saltwater contamination due to seawater intrusion, saltpan deposits, and beach placer mining activities.  相似文献   

13.
This paper deals with the importance of integrated hydrogeological, geological and geophysical applications in identifying suitable site for treatment, storage and disposal facility (TDSF) for industrial waste management. The pollution control authorities (Civic body) have a mandate to provide proper TSDF for the industrial zones. One such zone of 2800 acres, established by State Industries Promotion Corporation of Tamil Nadu (SIPCOT) at Perundurai in southern India, is selected to identify suitable TSDF site. The zone is envisaged to cater 22500 MTA (metric tones per annum) of industrial waste generated by 565 units. Hydrogeological and geological setup forms an important criterion in the process of environmental impact assessment (EIA) to establish proposed activity. Hydrogeological, geophysical and socio-economic/ecological studies were carried out to identify suitable TSDF site. Vertical electrical sounding (VES), multi-electrode resistivity imaging (MERI), ground penetrating radar (GPR) scanning, and infiltration tests were performed within the proposed TSDF site of 50 acres land. An inventory of 54 key wells includes depth to water level (varies from 9.7 to 15 m, below ground level) and water chemistry (pH, TDS, Ca2+, Mg2+, Na+, K+, Cl?, HCO 3 ? , SO 4 2? , NO 3 ? and F?) helped in identifying physical environment around (5 km radius) proposed TSDF site. Geophysical and hydrogeological investigations verified with drillings (at 5 locations) within the proposed TSDF site resulted, shallow hard rock at 4.5 m depth, the absence of groundwater, and low infiltration rate (4.3 cm/hour) of soil in the western side (45 acres) of proposed 50 acres land reveals suitability of TSDF site.  相似文献   

14.
In India, the quantity and quality of water available for irrigation is variable from place to place. Assessment of water quality has been carried out to determine the sources of dissolved ions in groundwater. Quality of groundwater in a 398 km2 Peddavanka watershed of a semi-arid region of south India is evaluated for its suitability for drinking and irrigation purposes. The middle Proterozoic Cuddapah Supergroup and Kurnool Group of rocks underlie most of the watershed. The main lithologic units consist chiefly of quartzite, limestone, and shale. Seventy-six water samples were collected from open-wells and bore-holes. Water samples were collected representative of the post-monsoon (winter) and pre-monsoon (summer). The quality assessment is made through the estimation of Ca2+, Mg2+, Na+, K+, Cl−, SO42−, CO32−, HCO3−, total hardness as CaCO3, TDS, EC, and pH. Based on these analyses, parameters like sodium adsorption ratio, % sodium, residual sodium carbonate, non-carbonate hardness, potential salinity, Kelley’s ratio, magnesium ratio, index of base exchange and permeability index were calculated. According to Gibbs‘ ratio samples in both seasons fall in the rock dominance field. The overall quality of waters in the study area in post-monsoon season is high for all constituents ruling out pollution from extraneous sources.  相似文献   

15.
This study was carried out to simulate the forest cover changes in India using Land Change Modeler. Classified multi-temporal long-term forest cover data was used to generate the forest covers of 1880 and 2025. The spatial data were overlaid with variables such as the proximity to roads, settlements, water bodies, elevation and slope to determine the relationship between forest cover change and explanatory variables. The predicted forest cover in 1880 indicates an area of 10,42,008 km2, which represents 31.7% of the geographical area of India. About 40% of the forest cover in India was lost during the time interval of 1880–2013. Ownership of majority of forest lands by non-governmental agencies and large scale shifting cultivation are responsible for higher deforestation rates in the Northeastern states. The six states of the Northeast (Assam, Manipur, Meghalaya, Mizoram, Nagaland, Tripura) and one union territory (Andaman & Nicobar Islands) had shown an annual gross rate of deforestation of >0.3 from 2005 to 2013 and has been considered in the present study for the prediction of future forest cover in 2025. The modelling results predicted widespread deforestation in Northeast India and in Andaman & Nicobar Islands and hence is likely to affect the remaining forests significantly before 2025. The multi-layer perceptron neural network has predicted the forest cover for the period of 1880 and 2025 with a Kappa statistic of >0.70. The model predicted a further decrease of 2305 km2 of forest area in the Northeast and Andaman & Nicobar Islands by 2025. The majority of the protected areas are successful in the protection of the forest cover in the Northeast due to management practices, with the exception of Manas, Sonai-Rupai, Nameri and Marat Longri. The predicted forest cover scenario for the year 2025 would provide useful inputs for effective resource management and help in biodiversity conservation and for mitigating climate change.  相似文献   

16.
The tsunami sediments deposited after the December 2004 tsunami were sampled immediately in the coastal environment of Tamil Nadu State on the southeast coast of India. Fifty-four sediment samples were collected and 14 representative samples were selected to identify the level of metal contamination in tsunami sediments. The results indicate that the sediments are mainly of fine to medium-grained sand and contain significantly high contents of dissolved salts in sediments (Na+, K+, Ca+2, Mg+2, Cl−) in water-soluble fraction due to seawater deposition and evaporation. Correlation of acid leachable trace metals (Cr, Cu, Ni, Co, Pb, Zn) indicate that Fe-Mn oxyhydroxides might play an important role in controlling their association between them. Enrichment of trace metals is observed in all the locations with reference to the background samples. High values of trace metals in the southern part of the study area are due to the large-scale industries along the coast, and they are probably anthropogenic in nature and of marine origin, which could cause serious environmental problems.  相似文献   

17.
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18.
Fifty groundwater samples were collected from Al-Hasa to analyze the pH, electrical conductivity (EC, dS m?1), total dissolved solids (TDS), major anions (HCO3?, CO32?, Cl?, SO42?, and NO3?), major cations (Ca2+, Mg2+, Na+, and K+), and total hardness. The analyzed data plotted in the Piper, Gibbs, and Durov diagrams, and water quality index (WQI) were calculated to evaluate the groundwater geochemistry and its water quality. The results reveal that most of the investigated samples are Ca2+, Mg2+, SO42?, Cl? and Na+, and HCO3? water types using the Piper diagram. Na+?>?Ca2+?>?Mg2+ are the dominant cations, while Cl??>?HCO3??>?SO42??>?CO32? are the dominant anions. Sodium adsorption ratio (SAR) values varied from 0.79 to 10; however, the Kelly ratio (KR) ranged between 0.1 and 2.2. The permeability index (PI) showed that well water is suitable for irrigation purposes with 75% or more of maximum permeability. The US salinity diagram revealed that the water quality classes of studied waters were CIII-SI, CIII-SII, and CIV-SII, representing height hazards of salinity and medium- to low-sodium hazard. The water quality index (WQI) results indicated that total dissolved solids are out of the drinking water standard limits in Saudi Arabia. The WQI revealed that 38% of the studied wells were considered as poor water (class III), 52% are found as very poor water class (IV), and 10% are unsuitable water for drinking class (V).  相似文献   

19.
Study of precipitation chemistry over an industrial city   总被引：2，自引：2，他引：0  
The growing trend of industrialization has marked impact on the atmospheric chemistry around the globe. Such impacts are also predicted for developing countries like India. Acid precipitation due to secondary pollutants is one of them. The present investigation was conducted with an aim of studying the precipitation chemistry over Asansol city, India during the monsoon season. The rain water samples, collected on event basis during the period of June to September 2006, were analyzed for HCO3 ?, Cl?, SO4 2?, NO3 ?, NH4 +, K+, Na+, Mg2+, Ca2+ and pH using standard procedures. The study revealed that the precipitation was alkaline in nature with mean pH value of 6.08. Dominant anion in rain water samples was HCO3 ? (36 %), whereas Ca2+ was highest among the cations (53 %). The observed acidic events, based on pH value, were only 17 %, indicating the alkaline components effectively neutralize rain water over the area. This was also supported by the total alkaline to acid ion ration of 1.05. The ratio of (NO3-?+Cl?) and SO4 2? was >1, indicating that acidic properties were influenced by HNO3 and HCl. In the ratio between (Ca2++NH4 +) and (NO3 ?+SO4 2?), values >1 were obtained, indicating significant alkaline influence, which effectively prevented acid depositions in the area.  相似文献   

20.
In this study, it is determined physicochemical properties of the groundwater and the spatial variability of physicochemical properties of the groundwater in the Amik plain and as well as assess its suitability for drinking and irrigation. A total 92 groundwater samples were collected from drilled well in June 2012. In addition, the 42 of 92 drilled wells were also sampled in September 2012 to evaluate the changes of groundwater properties. According to t test values, the mean Ca2+, SO4 2?, Mg2+ and TH values in the June period were significantly lower than those of the September period. On the other hand, the mean (CO 3 2?  + HCO3 ?) and RSC and MR values in the June period were significantly higher than those of the September period. The order of relative abundance of major cations in the groundwater (in mg L?1) is in order Na+ > Mg2+ > Ca2+ > K+. The order of the anions abundance (mg L?1) is SO4 2? > Cl? > HCO3 ? > CO3 2? in this research. Log and square-root transformation were carried out for the most of the water properties before the calculation of semivariance. The nugget ratio showed all the variables were strongly spatial dependent except for K+ and Mg2+ and (CO3 2? + HCO3 ?) which showed moderate spatial dependence (ratio ranging from 28 to 49.9 %). Spatial distribution analysis of groundwater quality indicated that The EC, TDS values and Na and Cl? contents which is important water quality parameters increase from northern towards southern. The total hardness values increase also from northwestern towards southwestern.  相似文献   

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