Introduction toNon-Point Source Pollution Models.ppt

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1、Introduction toNon-Point Source Pollution Models,Pollution from diffuse land sources and not from a point source discharge. 50-70% NPS pollutants loads to surface waters 75 % of which are agricultural related. Sediments are very costly soil loss, chemical transport reduce reservoir capacity Characte

2、ristics: diffuse pathways, spatially and temporally heterogeneous intermittent transport is important- extensive land areas hard to monitor at the source, no standards can be set, BMPs are used.,Sediments,5 billion from erosion per year in US 2.7 billion tons reach small streams 60% from sheet & ril

3、l Ag erosion 40% from bank, gully, road, construction 10% total erosion enters ocean 45% deposited before streams 45% deposited flood plains, ponds, reservoirs, channels, wetlands Cost: US Army Corps dredging $300 million in1980, $450 million now disposal cost may average 100% of dredging, total cos

4、t $1.5 billion Reservoirs in 30 years 20% of nation small reservoirs will be 50% full. Carrier of P and pesticides, crop loss, aquatic habitat degradation,Nutrient,N and P, accelerated eutrophication of lakes and estuaries P: 2,000,000 tons/year 71% Ag sources, 84% NPS P sediment adsorbed, sediment

5、control P control N dissolved nitrate NO3- Pesticides 2179 tons / year, 95 % Ag sources Toxicity to human and wild life BOD 33.5 million tons/year, 91% NPS, 81% Ag sources Oxygen depletion in rivers and streams Pathogen Bacterial and viral from feed lots, manure fields, pasture, range land.,Why Mode

6、l NPS,Diffuse, Intermittent Process Stream monitoring may not be sufficient. will not pinpoint source will not predict land use changes will not represent high load events unless continuos Land use Planning Assess impact of land use changes on NPS loads, example BMP Resource Inventory and Assessment

7、- Stream Assessment Regional assessment of overall problems Identifying potential problem areas Scientific understanding Models and process integrator Sensitivity analysis Error analysis Validation,Kind of Models,Process description Empirical vs. Physically based Variables presented Runoff volume, P

8、eak discharge, Erosion, Sediment yield, Nutrients, Pesticides, Temporal scale Single event, continuos simulation, Average annual Spatial Scale Hillslope, Field, Basin Dynamic vs. Steady State Deterministic vs. stochastic,Transport Processes,Advection: Transport by the water current Dispersion: Mixin

9、g within the water body Molecular diffusion induced by concentration gradient, very slow Turbulent diffusion eddies and mixing due to microscale turbulence moderately slow Dispersion due to velocity gradient Transport of sediment particles, erosion, tillage, suspension and deposition Chemical proces

10、ses Sorption Ion exchange Crystallization Hydrolysis Oxidation-Reduction Phytochemical and Biochemical,Nitrogen Ballance,NAL = Nfertilizer + Nrain + Nresidual + Nnitrification N leaching below the root zone is:NAL(kg/ha) (1-e(-K*WAL)/nN is porosity in cm K is leaching coeficient 1.2 WAL is water ava

11、ilable for leaching (cm) = water inflow losses AWHC (cm) actual water in soil (cm),Contamination Problem,Uncontrolled spill of chemical; subsequently distribute in the subsurface, and create a health hazard.,Water-NAPL air distribution on an initially wet scratched glass,Removal Processes,The two la

12、yer theory,Multiphase Transport Model,C : is the solute concentration (M/L3) D :is the hydrodynamic dispersion coefficient (L2/T) v : is the specific velocity (L/T). r : represents the interphase transfer to or from phase : represents the loss or gain due to chemical or biochemical reactions.,J : is

13、 the flux from the non-aqueous phase (M/L3.T-1) kf : is the first order mass transfer coefficient (T-1) C : is the contaminant concentration (M/L3) Keq : represents the thermodynamic equilibrium constant as Henrys law constant KH (dimensionless) , : denotes the phases involved,Random conductivity fi

14、eld,Hypothetical case study,The general layout for the hypothetical case study Hydraulic conductivity = 3.0 m/day Porosity =0.30 Longitudinal dispersivity = 2.0 meter,Sample result,The contaminant plume after 50 days of heterogeneous hydraulic conductivity field of variances of (a) 0.45 and (b) 1.2.

15、,a,b,Preliminary Results,The contaminant plume in Bekaa basin after 300 days of instantaneous spill of 100 ppm contaminant at a point of local coordinate of X=15000, and Y=7500 m. The transmissivities were varied by +15%,Preliminary Results,The contaminant plume in Bekaa basin after 700 days of instantaneous spill of 100 ppm contaminant at a point of local coordinate of X=15000, and Y=7500 m. The transmissivities were varied by +15%,