『Abstract
　A considerable amount of colloidally bound Ca has been detected in water samples from Amazonian rivers and the Kalix River, a sub-arctic boreal river. Fractionation experiments using several analytical techniques and processing tools were conducted in order to elucidate the matter. Results show that on average 84％ of the total Ca concentration is present as free Ca. Particulate, colloidal and complexed Ca constitute the remaining 16％, of which the colloidal fraction is significant. Ultrafiltration experiments show that the colloidal fraction in the sampled Amazonian rivers and the Kalix River range between 1％ and 25％.
　In both the Amazonian and the Kalix rivers the technique of cross-flow ultrafiltration was used to isolate particles and colloids. The difference in concentration measured with ICP-AES and a Ca ion-selective electrode in identical samples as used to define the free Ca concentration and thus indirectly the magnitude of the particulate, colloidal and complexed fractions. Results from the Kalix and Amazonian rivers are in excellent agreement. Furthermore, the results show that the colloidal concentrations of Ca can be greatly overestimated (up to 227％) when conventional analysis and calculation of ultrafiltration data is used due to retention of free Ca ions during the ultrafiltration process. Calculation methods for colloidal matter are presented in this work, using complementary data from ISE analysis.
　In the Kalix River temporal changes in the fractionation of Ca were studied before, during and after a spring-flood event. Changes in the size distribution of colloidally associated Ca was studied using FlFFF (Flow Field-Flow Fractionation) coupled on-line to a HR ICP-MS. The FlFFF-HR ICP-MS fractograms clearly show the colloidal component of Ca, supporting the ultrafiltration findings. During winter conditions the size distribution of colloidally associated Ca has a concentration maximum at 〜5 to 10 nm in diameter, shifting to smaller sizes (＜5 nm) during and after the spring flood. This shift in size distribution follows a change in the river during this period from ironoxyhydroxy colloids being the most important colloidal carrier phase to humic substances during and after the spring flood.
　WHAM and NICA-Donnan models were used to calculate the amount of colloidally bound Ca. The results similar for both models, show that on average 16％ of the Ca may be associated to a colloidal phase, which is in broad agreement with the measurements.』

1. Introduction
2. Method
　2.1. Definitions
　　2.1.1. Methods and units
　　2.1.2. Fractions
　　2.1.3. Calculation methods for the colloidal fraction
　2.2. The ‘Kalix 2002’ project
　　2.2.1. Sampling
　　2.2.2. Ultrafiltration
　　2.2.3. FlFFF-ICPMS
　　2.2.4. Ca ISE measurements
　　2.2.5. Modelling of Ca in the Kalix river
　2.3. Purus（後ろのuの頭に´）/Amazonas: 1996
　　2.3.1. Sampling
　　2.3.2. Ultrafiltration
　　2.3.3. Ca ISE measurements
　　2.3.4. Modelling Ca in the Amazonian rivers
3. Results and discussion
　3.1. ICP-AES and ISE concentrations in bulk water from the Kalix and Amazonian rivers
　3.2. A calculation exercise for colloidally associated Ca, and results from the Kalix and Amazonian rivers
　3.3. Kalix river
　　3.3.1. Geochemical variations during the Spring-Flood event
　　3.3.2. Retention of free Ca during ultrafiltration
　　3.3.3. FlFFF results
　　3.3.4. Ca fractionation
　3.4. Modelling
　　3.4.1. Modelling of Ca in the Kalix river
　　3.4.2. Modelling of Ca in the Amazonian rivers
　　3.4.3. Comparison of modelling in the Kalix and Amazonian rivers
4. Conclusions
Acknowledgments
References