Physical Transport in Surface Waters

Transcription

1 Physical Transpor in Surface Waers odule : Surface Waers, ecure 1 Chemical Fae and Transpor in he Environmen, nd ediion. H.F. Hemond and E.J. Fechner-evy. Academic Press. ondon Naure of Surface Waers Rivers and sreams are relaively long, shallow and narrow, and have obvious horizonal movemen in he downsream direcion. akes end o be deeper and wider and are ofen sraified ino disinc layers ha impede verical mixing. Esuaries are he inerface beween rivers and he ocean and are also commonly sraified due o saliniy differences (he heavier sal waer sinking and he ligher fresh waer rising o he surface see he aached paper in odule 1 where his feaure was used o rap conaminaed CSOs for pumpback reamen a he local wasewaer reamen plan.).1. Sources of Polluan Chemicals o Surface Waers Poin sources of waer polluans are discree, localized, discharges, such as from indusrial or municipal reamen operaions They are regulaed by he NPDES (Naional Polluan Discharge Eliminaion Sysem) program adminisraed by mos saes) for he EPA.

2 Nonpoin sources of polluans are more difficul o measure because hey cover a wide area or are composies of numerous smaller poin sources...1 Physical Transpor in Rivers Graviy-driven advecion (waer flows downsream by graviy). The anning s equaion is commonly used o predic his movemen: 1.49R V n / 3 S 1/ V is he waer velociy [/T] R is he hydraulic radius A/P [] S is he slope of he energy gradien n is he anning s roughness coefficien Table -1. anning s Roughness Coefficiens Travel ime along a river, afer he release of a chemical, if given by: τ V Where is he reach lengh and V is he average velociy. When he waer velociy is no uniform, he ravel ime mus be expressed as an inegral: τ x x 1 dx 1 V ( x) Hemond and Fechner-evy 000 Dye releases are ofen used o direcly measure ravel ime.

3 The mass of he chemical ranspored by a river pas a given poin per uni ime is: J o Q C Where J o is he oal flux of he chemical and C is he average chemical concenraion Hemond and Fechner-evy 000 Fickian ixing Processes A mass of chemical released in a river will spread ou as i moves downsream. This dispersion is caused by he velociy shear wihin he river, and urbulen diffusion. Waer moves more rapidly down he cener of he channel, near he surface, ransporing chemicals faser, and elongaing he plume. Plo of concenraion vs. disance has he shape of a Gaussian (normal) curve: φ( x) σ 1 π e x / σ For a pulse injecion, here is a close relaionship beween a Fickian mixing, or ranspor, coefficien D n a given direcion and he sandard deviaion of he chemical disribuion in ha direcion. D can be calculaed from: Dσ / where σ is he spaial variance (he square of he sandard deviaion) and is he ime since he injecion. The concenraion of a conservaion racer (C) a any ime ( afer injecion and any disance (x) downsream is: C( x, e ( x V ) /(4D Where D is he longiudinal Fickian mixing coefficien [ /T]

4 If he chemical undergoes a firs-order decay, hen he following predics downsream concenraions: Hemond and Fechner-evy 000 ( x V /( 4D ) k C( x, e e A any given ime, he maximum concenraion of he chemical (C max ) is found using: C max e k If he chemical is no insananeously mixed across he river, a mixing zone is creaed. The chemical mus ravel a cerain disance before he chemical is uniform across he channel. The laeral sandard deviaion of he chemical s concenraion disribuion can be esimaed when his value is approximaely equally o he river widh (w): σ D w Subsiuing he earlier expression for ravel ime,, resuls in he following equaion ha can be used o predic he lengh of he ransverse mixing zone: w V Thomann and ueller 1987 D

5 Hemond and Fechner-evy 000