Behavior of a surface applied radionuclide and a dye tracer in structured and repacked soil monoliths.

There has been increasing evidence in recent years about the impact of soil structure on vadose zone hydrology and the distribution of surface applied chemical substances. We have carried out a combined dye and radionuclide tracer study on two monoliths from the same location, one structured and one repacked, as part of an ongoing study to investigate the link between preferential flow, leaching of surface applied substances and their distribution within the soil.A tracer solution containing 1300 Bq/L (58)Co and 0.31 micromol/L Sulforhodamine B (SB) was added with roughly constant irrigation during a period of three weeks. The dye served as a tracer for water movement within the soil and thus allowed linkage of the radiotracer ((58)Co) with the flow pattern. Both were monitored in the outflow and measured within profile sections after monolith disassembly. Preferential flow in the structured monolith promoted the bypass and transport of both tracers, although transport was impeded at depths greater than 30 cm by compacted soil and reduced hydraulic conductivity. Eighty four percent of radiocobalt and 8% of SB were found in the upper 4 cm of the structured monolith. The homogenized monolith, on the other hand, showed mostly chromatographic infiltration and a more efficient soil filtering capacity with 91% of radiocobalt and 20% SB residing in the upper 4 cm. Furthermore no tracer was found in the outflow of the homogenized monolith during normal to high irrigation or at greater depth within the monolith. We have related flow characteristics and sorption of radiotracers by quantifying dye distributions and radionuclide activities throughout the profiles. Activities within the flow paths are up to 20-times higher than those measured in the soil matrix, and a fraction of radiocobalt follows the dye tracer in spite of cobalt's low mobility. The dye can thus be used to trace radionuclide distribution within the soil block.

Fluorescence imaging of tracer distributions in soil profiles.

To evaluate and parametrize transport models for the vadose (partially water-unsaturated) zone, information about the spatial distributions of solutes is needed. We describe a technique for the simultaneous imaging of several fluorescent tracers in structured field soils. With this technique, we obtain information on local mixing under field conditions. Local dispersion is a decisive process that discriminates different flow regimes. The imaging device consists of a high-power xenon lamp and a sensitive charge coupled device (CCD) camera. The three fluorescent dyes Brilliant sulfaflavine (BF), Sulforhodamine B (SB), and Oxazine 170 (OX) were chosen as solute tracers for their spectroscopic properties and different sorption coefficients. We conducted a field experiment using these tracers and took images of their distribution in a vertical soil profile. The fluorescence images (1242 by 1152 pixels) were corrected for nonuniform lighting, changing surface roughness, and varying optical properties of the soil profile. The resulting two-dimensional relative concentration distributions were similar for BF and SB. The reason might be the fast transport regime, which prevents the establishment of sorption equilibria. According to its higher sorption coefficient, OX was more strongly retarded. In this paper, we show that the fluorescence imaging technique is a powerful tool for the in-situ investigation of transport processes of fluorescent solute tracers in soil profiles. Due to the high spatial resolution of the tracer concentration maps and the ability to detect the flow field characteristics of differently reactive tracers simultaneously under field conditions, this technique provides valuable experimental data for the test and development of theoretical models for heterogeneous solute transport in soils.

First sexual intercourse and contraception: a cross-sectional survey on the sexuality of 16-20-year-olds in Switzerland.

BACKGROUND: The onset of sexual activity represents an important development stage with positive aspects, such as love, discovery, intimate relationships, plans for the future, but also with fears of pregnancy, of sexually transmitted diseases and of AIDS. OBJECTIVE: To study perceptions, attitudes, and behaviour related to sexual life, AIDS and contraception; to explore the onset of first sexual relationships and the process of choice of a contraceptive method by the adolescents, in order to improve prevention programmes for young people. METHOD: Analysis of data from a Swiss national survey on adolescent sexuality using a computerised self-administered questionnaire, involving 2075 girls and 2208 boys between the ages of 16 and 20. The use of computers helps improve confidentiality, response rates and acceptability since survey questions are limited to the subjects' sexual experience only. RESULTS: The young people's responses emphasised the importance of emotion in sexual relationships, girls choosing intimacy and fidelity while boys attached more importance to physical pleasure. Three out of four respondents have had a sexual experience and one out of two have had penetrative sexual intercourse. The percentages of condom or oral contraception use are high: at first sexual intercourse, 86.5% used one or the other, while 7.4% did not declare any contraceptive method. The percentages are lower when age at first intercourse is below 15 years, when a girl had an older partner (age difference 7 years and more) and when the 1st relationship is a casual one. During their first stable relationship 41.1% of girls and 30.9% of boys say they have changed their contraceptive method from condom to contraceptive pill, 2.4% of girls and 2.9% of boys say they have given up any form of contraception. Among girls, condom use at first sexual intercourse with a new partner decreases in favour of oral contraception between first and last steady relationships (75.6 vs 58.0%, p < 0.05), the decrease being insignificant between the first and last casual relationships (73.5 and 62.2%, n.s.). Among boys the rates of condom use are equal at first intercourse with the first and last partner (steady relationship: 74.1 and 77.2%; causal relationship: 78.3 and 76.2% respectively). CONCLUSION: The use of condoms is high among Swiss adolescents, particularly at first sexual intercourse. By integrating the prevention of sexually transmitted diseases and of unwanted pregnancies, preventive programmes would address adolescents' needs more effectively.