On July 14, 1991, a train derailment resulted in the spill in approximately 13,0

Question

On July 14, 1991, a train derailment resulted in the spill in approximately 13,000 gal of the soil fumigant sodium metham (Vapam) to the Sacramento River, 70 km upstream from Shasta Lake largest reservoir. The spill killed almost all the river. In an aqueous environment, metham (which is highly soluble) hydrolyzes to form methyl isothiocyanate and H_2S as the major products. Rate constants for this hydrolysis reaction are k_a almostequalto 300/(M sec) and K_n almostequalto 1 times 10^-8/sec. Metham also undergoes indirect photolysis to form methyl isothiocyanate, with a pseudo-first order rate constant of approximately 1 times 10^-4/sec. The California Regional Water Quality Control Board urgently needs your advice, both as to the maximum of metham that will be encountered at different points along the Sacramento River, and as to the total load will enter Shasta Lake. a. Calculate the maximum concentration (in moles per liter) of Na^+ behaves a conservative tracer) and metham that will be encountered, both at the town of Dunsmuir (10 km downstream from the spill) and at the point where the Sacramento River reaches Shasta Lake (70 km downstream from the spill) b. How important are the processes of longitudinal dispersion, hydrolysis, and indirect photolysis in attenuating the peak concentration of metham between Dunsmuir and Shasta Lake? Quantify your answer. c What are the half-lives for hydrolysis and indirect photolysis? d. How important are these two processes collectively in attenuating the total of metham between Dunsmuir and Shasta Lake? e. Data are currently not available as to the Henry's law constant for metham. Would you recommend measuring this parameter, or do you feel that it is reasonable to neglect air-water exchange of metham? Indicate your logic f. Estimate the total mass of methyl isothiocyanate (in moles) that will be transported into Shasta Lake, ignoring losses from any process (such as air-water exchange) that might remove this compound during its transport in the Sacramento River. The upper reach of the Sacramento River is a small tributary to Shasta Lake. At the time of the spill, the river had a discharge of 75,000 liter/min, a mean depth of 0.3 m, a pH of 7.8, a mean width of 1.2 m, and a dispersion coefficient of 1.6 times 10^2 m^2/min. The commercial formulation of sodium metham contained 0.33 g sodium salt of metham per milliliter. The molecular weight of sodium salt of metham is 129 g/mol.

Explanation / Answer

29 a) 13000 gal=49210 L Metham contains 423 g/L the active constituent. So total metham=423 g*49210 =20815830 g.

So sodium ion per litre=423/23 moles/L=18.39 moles/L

b) The two processes ie the hydrolisis and indirect photolysis is faster in destroying the agent ie metham.(t1/2)=1*10^-4

c)t1/2=0.693/k=0.693/1*10^-4=6.93*10^-3

Out of the two processes the pseudo1ist order reaction has less halflife ie it's faster as compared to to indirect pht 10^4olysis1*10^-4=6.93*10^-4

d) Both the hydrolysis and indirect photolysis is important as these are taking care of huge metham (around 492910 L where per L metham is giving 4239*10-3

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