On the behavior of approaches to simulate reactive transport

Two families of approaches exist to simulate reactive transport in groundwater: The Direct Substitution Approach (DSA), based on Newton-Raphson and the Picard or Sequential Iteration Approach (SIA). We applied basic versions of both methods to several test cases and compared both computational demands and quality of the solution for varying grid size. Results showed that the behavior of the two approaches is sensitive to both grid size and chemistry. As a general rule, the DSA is more robust than the SIA, in the sense that its convergence is less sensitive to time step size (any approach will converge given a sufficiently small time step). Moreover, the DSA leads to a better simulation of sharp fronts, which can only be reproduced with fine grids after many iterations when the SIA is used. As a consequence, the DSA runs faster than SIA in chemically difficult cases (i.e., highly non-linear and/or very retarded), because the SIA may require very small time steps to converge. On the other hand, the size of the system of equations is much larger for the DSA than for the SIA, so that its CPU time and memory requirements tend to be less favorable with increasing grid size. As a result, the SIA may become faster than the DSA for very large, chemically simple problems. The use of an iterative linear solver for the DSA makes its CPU time less sensitive to grid size.     

A tracer study of headspace ventilation in a collector sewer

A field-scale tracer test was conducted to evaluate in-situ ventilation rates in a major collector sewer. The sewer under study was approximately 11 km long and ranged from 0.61 to 2.1 m in diameter. For the purposes of the tracer testing, the collector was divided into four reaches, each of which was tested individually. The tracer test involved injecting a measured volume of CO gas into a manhole over a short time period. CO concentrations were then measured in the collector headspace at selected manholes along the length of the reach. The technique employed successfully measured average headspace velocities over extended lengths of the collector. In a section that had a relatively stagnant headspace, approximately 1.1 km of sewer could be evaluated, with substantial tracer loss attributed to losses to manholes. In a section of the sewer with elevated headspace velocities, a section approximately 7.0 km long was successfully tested with one injection of tracer gas. The velocities observed in the collector varied substantially with time and location in the collector. The lowest velocities measured were in the upstream sections, with a minimum observed value of 3.8 m/min. The highest velocities were observed in the downstream sections, with a maximum value of 31.5 m/min. The presence of a substantial drop structure appeared to reduce the headspace velocity in the upstream reach. In general, there was an increasing trend in gas-phase flows with distance along the length of the collector. Flows at the discharge end of the collector were almost 2 orders of magnitude greater than those at the beginning.     

Seveso Women's Health Study: does zone of residence predict individual TCDD exposure?

The compound, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), is produced as an unwanted by-product of various chemical reactions and combustion processes, including the manufacture of chlorinated phenols and derivatives. In animals, TCDD exposure is associated with toxic, carcinogenic, developmental, and reproductive effects. In 1976, a chemical plant explosion in Seveso, Italy, exposed the residents in the surrounding community to the highest exposure to TCDD known in humans. Materials from an aerosol cloud of sodium hydroxide, sodium trichlorophenate and TCDD were deposited over an 18.1 km2 area. As evidence of the significant level of TCDD exposure, numerous animals died and 193 cases of chloracne were reported among residents of the area. Initially, the contaminated area was divided into three major exposure Zones (A, B, R) based on the concentration of TCDD in surface soils. To date, the majority of epidemiologic studies conducted in Seveso have used Zone of residence as a proxy measure of exposure. The purpose of the present study is to validate the use of Zone of residence in Seveso as a proxy measure of exposure against individual serum TCDD measurement, and to determine whether questionnaire information can improve the accuracy of the exposure classification. Using data collected from the Seveso Women's Health Study (SWHS), the first comprehensive epidemiologic study of the reproductive health of women in Seveso, we determined that Zone of residence is a good predictor of individual serum TCDD level, explaining 24% of the variance. Using questionnaire information could have improved prediction of individual exposure levels in Seveso, increasing the percent of the variation in serum TCDD levels explained to 42%.     

The modelling of trichloroethene photodegradation in Brij 35 surfactant by two-stage reaction.

Various clean-up technologies have been developed for the removal and/or destruction of trichloroethene (TCE) in the subsurface. Surfactant-aided soil washing followed by photodegradation could be a promising approach to such a task. The modelling of TCE photodegradation by UV in Brij 35 surfactant micelles is therefore investigated. Two stages of TCE degradation are observed in surfactant Brij 35 systems. A lag phase is observed at the commencement of the degradation, but the duration of the lag phase is significantly reduced as the initial pH increases. As the overall decay of TCE is also found to be faster at higher pH levels, it is suggested that the free radical reaction is dominant at high pH levels, and the formation of lag phases is mainly due to the deficiency of free radicals at lower pH levels. Since the period of the lag phase gradually decreases with the increase of initial pH level, and the two pseudo first-order reaction constants (one for the lag phase and one for the subsequent fast decay) for TCE decay in both stages are also pH dependent, a non-steady-state mathematical model is developed for the prediction of TCE photodegradation in Brij 35 solutions, in which the remaining fraction of TCE (C/C0) in the system can be determined at any instant by using a simple parameter of the initial system pH.     

Precipitation chemistry in the coast of the Metropolitan Region of Rio de Janeiro, Brazil

Precipitation chemistry was studied in the Metropolitan Region of Rio de Janeiro (MRRJ). This study reveals that rainwater in the MRRJ is affected by emissions of air pollutants and provides essential data for future estimates of regional biogeochemical cycles and the impacts of acid deposition on tropical ecosystems. The volume-weighted mean (VWM) pH was 4.77, varying from 3.50 to 6.85. Sea-salt aerosols were the dominant sources of the Na+, Cl- and Mg2+. Excess SO4(2-), Ca2+ and K+ comprised 82, 91, and 87% of their total VWM concentrations, respectively. There were very strong correlations (r > 0.75, P > 0.01) for NO3- and H+, NO3- and excess(exc-)SO4(2-), NH4+ and exc-K+, and exc-SO4(2-) and exc-Ca2+, suggesting causal relationships between these ion pairs. The VWM concentrations of all major ions, except H+, were higher in the dry season, with dry to wet VWM concentration ratios varying from 1.1 (NH4+) to 4.7 (for total K+).     

Characteristics of organic precursors and their relationship with disinfection by-products

The molecular weight distribution and chemical composition of precursors and their relationship with disinfection by-products (DBPs) were investigated. Most of the organic matter responsible for the major DBP precursors in the Pan-Hsin water are small compounds with a molecular weight less than 1 kDa. The hydrophobic acids display the greatest ability to produce DBP. Therefore, effective removal of small molecules or hydrophobic acidic organics prior to disinfection process will significantly reduce the DBP concentration in the finished water. Although the coagulation process is effective in removing large organic precursors and the removal efficiencies of CHCl3 formation potential and organic carbon increase proportionally to the molecular weight of the precursors, the conventional treatment methods have limited efficiency in eliminating small precursors, which have high DBP formation potential.     

Modeling the quantum yields of herbicide 2,4-D decay in UV/H2O2 process

The photodecay of herbicide 2,4-D in a hydrogen peroxide-aided photolysis process was studied and modeled. The decay rate of 2,4-D was known to be low in the natural environment, but rate improvement was achieved in an H2O2/UV system. The 2,4-D decay quantum yields under ultraviolet (UV) light at 253.7 nm increased from 4.86 x 10(-6) to 1.30 x 10(-4) as the ratio of [H2O2]/[2,4-D] increased from 0.05 to 12.5. Apparently, in the presence of UV light, the decay rate of 2,4-D could be greatly improved as the concentration of hydrogen peroxide increased. However, the efficiency of 2,4-D photodecay was retarded if the concentration of H2O2 was overdosed, because the excess hydrogen peroxide consumes the hydroxyl radicals (HO*) in the solution, resulting in a much weaker oxidant HO2*. The decay of 2,4-D was also pH dependent. A ranking of acid (highest), base (middle) and neutral (lowest) was observed owing to the property change of reactants and the shifting of dominant mechanisms among photolysis, photohydrolysis and chemical oxidation. Two mathematical models were proposed to predict the quantum yield for various [H2O2]/[2,4-D] ratios and initial pH levels, in which very good correlation was found for the ranges of regular application.     

The rate improvement and modeling of trichloroethene photodegradation by acetone sensitizer in surfactant solution

The photodecay of trichloroethene (TCE) in surfactant solution by the help of photosensitizer (acetone, ACE) was investigated and modeled. Apart from the direct photodegradation, photosensitization is presumed to be one of the major mechanisms contributing to overall decay. Quantum yields of TCE photodecay in solution with surfactant Brij 35 and optimal ACE dosage are about 25 times higher than in Brij 35 alone. However, with an excess ACE dosage, ACE will act as a light barrier and attenuate the light intensity available for TCE photodegradation. TCE photodegradation follows a two-stage kinetics, in which a lag phase is followed by a fast decay. Mathematical models were developed for the prediction of the two-stage photodegradation, in which the remaining fraction of TCE (C/C0) in the system becomes predictable.     

Synthesis of polychlorinated biphenyls (PCBs) using the Suzuki-coupling

An improved synthesis of polychlorinated biphenyls (PCBs) utilizing a palladium-catalyzed cross-coupling reaction (Suzuki-coupling) is described. The coupling of (chlorinated) aryl boronic acids 1-3 with bromochlorobenzenes 4 using the standard conditions of the Suzuki-coupling gave the desired PCB congeners 5-7 in good to excellent yields. The self-coupling product of the aryl boronic acids is the major impurity of this reaction. 3,4,5-trichlorophenyl derivatives such as 10 can be synthesized by coupling of an aryl boronic acid with the corresponding bromochloroaniline 8. The approach offers the advantage of high selectivity and good yields compared to conventional methods such as the Cadogan reaction and allows the use of less toxic starting materials.