Transport and Fate ofContaminants in Surface Water

June 2001

This booklet is part of a series of educational brochures and slide sets that focuses on various aspects ofwater source protection. The series has been prepared jointly by the University of California AgriculturalExtension Service and the California Department of Health Services.

For further information about this and other documents in the series, contact the project team leader (seebelow) or visit the following website:www.dhs.ca.gov/ps/ddwem/dwsap/DWSAPindex.htm

Authors: Rhea Williamson, Department of Civil and Environmental Engineering, San Jose State University, SanJose, Calif., and John Klamut, Sunnyvale, Calif.

Editor: Larry Rollins, Davis, Calif.

Layout crew: Larry Rollins and Pat Suyama

Photo credit (cover): Larry Rollins

Cover photo: Merced River, Yosemite National Park, Calif.

Project Team leader: Thomas Harter, Department of Land, Air, and Water Resources, University of California atDavis

Funding Agency: California Department of Health Services

This document is the result of tax-supported government projects and, therefore, is not copyrighted. Re-printed material, mainly figures, is used with permission, and sources are indicated. Statements, findings,conclusions, and recommendations are solely those of the author(s).

Reasonable efforts have been made to publish reliable information; however, the author and publishingagencies cannot assume responsibility for the validity of information contained herein, nor for the conse-quences of using such information.

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Several mechanisms exist for thetransformation and transport ofcontaminants in water.

Knowledge of a contaminant’s chemicalform and of the chemical’s ability to besorbed, to precipitate, to dissolve, or tobe degraded is necessary if the fate of thechemical is to be determined accurately.

Solids in water exist in two forms: eitheras particulate (filterable) solids or asdissolved (nonfilterable) solids. Withineach group, further differentiation can bemade between organic (volatile) solids andinorganic (nonvolatile, or fixed) solids.

The amount of a contaminant of concern(COC) found at a given location in theaquatic environment depends on thecontaminant’s rate of accumulation. That,in turn, depends on the rates ofcontaminant loading and transport(Tchobanoglous and Schroeder, 1987).Contaminant loading arises from an“input” (with respect to the water body)of allochthonous or autochthonousmaterials.

Allochthonous materials originate fromwithin the watershed, but outside thewater body. A few examples are: erodedsoils and inorganic solids from weatheredrocks, detritus imported from thecatchment basin (e.g., leaves, twigs,grasses), fecal material, and organic andinorganic pollutants from road surfaces.Autochthonous material originates fromwithin the water body. It includesungrazed plant components such as cellsand tissues, undigested plant materialswithin animal feces, and dissolvedsubstances released by phytoplankton,macrophytes, and animals (Cole, 1994). Allochthonousloadings are usually less organic than autochthonousloadings (Chapra, 1997).

Once a material is present in a surface body of water, itcan be transported within the water body, volatilizedor transported into the atmosphere, sorbed into thesoil or solid matter in the water, precipitated or dissolvedinto and out of the water, degraded by photolysis, ortransformed by biodegradation.

Water to Air

Many different contaminant transport processes occurin a water body at the boundary between water and air.

These mechanisms include wind, hydraulic turbulence,and gas transfer.

Wind and hydraulic turbulence represent the transferof airborne contaminants into water by mechanicalmeans. Gas transfer is controlled by the difference inconcentration of a gas in water and air. If theconcentration of a gas in the atmosphere is greater thanthe concentration of that same gas in water, then thegas will tend to be absorbed into the water. The reverseis true, too: if the gas exists in the atmosphere at lowerconcentration than in water, it will tend to come out ofsolution. However, a gas film and water film in betweenthe water body and atmosphere can limit the effect ofthe concentration gradient in gas transfer. Figure 1shows absorption of a gas into a liquid, and desorption

Figure 1: Gas transfer from water to air (and vice versa): (a ) absorptionand (b) desorption (Peavy et al., 1985).

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of a gas out of a liquid, resulting from a concentrationgradient across a gas film and liquid film.

Water to Water

Within the water body itself, the possible modes oftransport include advection and diffusion (Figure 2).Advection occurs when a COC moves as a result offluid motion but the contaminant’s concentrationremains unchanged. Chapra (1997) defines advectionas a flow that is unidirectional and does not change theidentity of the substance being transported. Basicexamples of advection are the downstream transportof a contaminant (1) in a river or (2) through a conduit.

Diffusion occurs when a COC is transported along aconcentration gradient from a region of highconcentration toward a region of lower concentration(Tchobanoglous and Schroeder, 1987). Diffusivetransport occurs because of random mixing or motionsin the water. Diffusion can be modeled by using Fick’sFirst Law:

Jx = –D(dc/dx)

where:

Jx= mass flux in the x direction

D = the diffusion coefficient

dc/dx= the concentration gradient

Fick’s First Law shows that the mass flux is proportionalto the rate of change of the concentration of thecontaminant. The diffusion coefficient is used toquantify the rate of the diffusive process. Therefore, iflittle or no mixing or turbulence occurs in a water body,the diffusion coefficient would be low. On the otherhand, if mechanical mixers are present in a water body,then the diffusion coefficient would be high (Chapra,1997).

Dispersion is a third possible mode oftransport within a water body.Dispersion is related to diffusion in thatit will mix a contaminant in a waterbody. However, whereas diffusionmixes contaminants in water due torandom motions in the water over time(i.e., wind, waves, turbulance),dispersion mixes contaminants in waterdue to velocity differences in a givenspace (Chapra, 1997).

Figure 3 shows an example ofdispersion in a pipe. The velocity nearthe walls of the pipe are less than thevelocities in the center of the pipe. Thedifference in these velocities results ina gradual mixing of the water along

the axis of the pipe. The figure also demonstratesdiffusive mixing, which is completely random in nature.

Most mixing that occurs in narrow conduits of rapidlyflowing water, such as a river or pipe, is a result ofdispersion. Most long-term mixing that occurs due towind action, such as in a lake or bay, is a result ofdiffusion (Chapra, 1997).

Water to Soil

Water-to-sediment transport mechanisms includesorption (adsorption and desorption), precipitation, anddissolution. Sorption describes a contaminant’sattachment to and movement within the soil. The termis often used to refer to the transfer of organics in surfacewaters to a solid surface (Chapra, 1997; Sawyer, et al.,1994). Sorption can occur either by the compoundadsorbing chemically onto the surface of the solid orby the compound absorbing (penetrating into) thesolid’s surface (Chapra, 1997).

Several factors affect a chemical’s potential to adsorbto soil. These include: (1) the chemical and physicalcharacteristics of the soil, (2) whether or not adsorptionwill be chemical or physical, (3) the type of bondingthat will occur between the COC and the soil, and (4)such characteristics of the environment as pH andtemperature (Tchobanoglous and Schroeder, 1987).

Adsorption is evident when there is an accumulationof material at the water-soil interface. Desorption canoccur when the conditions resulting in adsorptionreverse, causing a release of the contaminant from thesoil back to the water column (Tchobanoglous andSchroeder, 1987).

Precipitation and Dissolution

Precipitation is the removal of a chemical species fromwater by formation of solid mineral(s). This can occur

Figure 2: Transport of a contaminant in water: (a) advection and (b) diffusion(Chapra, 1997).

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when the concentrations of the mineral-forming chemical species exceed thesolubilities of the minerals they form. Insuch situations, the solution is said to besupersaturated with respect to thoseminerals.

Dissolution—essentially the reverse ofprecipitation—typically occurs when thewater is undersaturated. The net result,assuming reaction kinetics are favorable, isthe diffusion of ionic species away from thesolid(s). The rate of dissolution is affectedby several factors: (1) the saturationconcentration, (2) the actual concentrationof the limiting ion, (3) the rate constantfor the reaction, and (4) the surface areaavailable for precipitation (Snoeyink andJenkins, 1980).

The reaction that occurs when a precipitatedissolves in water to form its constituentions (AzBy(s) = zAy+ + yBz-) is described bythe equilibrium constant as follows:

Kso = [Ay+]z [Bz-]y

where:

Kso = the solubility product

A = the cation

B = the anion

This relationship can be used to determinewhether a solution will precipitate ions,forming a solid, or result in the dissolutionof ions from a solid.

Biodegradation

Biodegradation processes typically resultfrom microorganism activity that causesthe transformation or breakdown oforganic materials. Chemical conversionsassociated with biodegradation include:(1) organics to inorganics; (2) toxiccompounds to innocuous compounds; and(3) non-toxic compounds to toxiccompounds (Chapra, 1997).

Biodegradation of organic substances canbe carried out by aerobic or anaerobicorganisms. Aerobic organisms utilizeoxygen to metabolize organics. Anaerobicorganisms do not use oxygen to metabolize organics.Figure 4 outlines the aerobic and anaerobic processesthat degrade organics composed of nitrogen, sulfur, andcarbon.

As mentioned above, aerobic organisms require oxygento function. Therefore, when an organic waste is

discharged into an aquatic system, a biological oxygendemand (BOD) is created. BOD is a measure of theoxygen required to break down organic compounds(Tchobanoglous and Schroeder, 1987). High BODlevels significantly deplete the amount of dissolvedoxygen (DO) in a surface water. Consequently, high

Figure 3: Contrast between (a) diffusion and (b) dispersion. Diffusion iscaused by random motions over time; dispersion is caused by differentialmovement of water over a space (Chapra, 1997).

Figure 4: Nitrogen, carbon, and sulfur cycles: (a) aerobic and (b) anaerobic( Peavy et al., 1985).

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CsQs + CwQw = QmCm

where:

C is the concentration in mass per unitvolume,

Q is the flow rate in volume per unit time,

s refers to the stream conditions,

w refers to the waste conditions, and

m refers to the mixed conditions.

This equation can be used to determinewhether compliance with regulatorystandards, guidelines, and goals is beingachieved.

Photodegradation

Chemical degradation due to the radiantenergy of light is referred to asphotodegradation or photolysis. It typicallyoccurs when a compound absorbs light.This type of degradation depends on theamount of sunlight available, the physicalwater characteristics that allow the sunlightto penetrate the water, the degree to whichthe chemical will absorb sunlight, and manyother factors.

COCs near the water surface are particularly vulnerableto solar radiation and resulting photodegradation(Chapra, 1997). Figure 5 shows a schematic of thephotodegradation process. The figure shows that solarradiation is absorbed by a toxicant, which thenundergoes a chemical reaction that results in decay ofthe toxin.

BOD levels can have a detrimental effect on the healthof aquatic species that require elevated DO levels, suchas fish. This happens, for example, when algae reproducerapidly within a reservoir in response to input ofphosphorus or other contaminants that happen to benutrients for algae. The algae biodegrade the nutrients,at the same time decreasing the amount of availableoxygen. Thus, the increased amount of algae creates ahigh BOD, which depletes the DO, possibly resultingin the death of fish in the reservoir.

Dilution

Contaminants of concern in the aquatic environmentcan be reduced by dilution, a natural self-cleaningmechanism in many surface waters. In the past, dilutionwas considered a viable method for disposal ofwastewater. However, steady increase in the world’shuman population, paired with an increase in the per-capita volume of wastewater to be discharged, hasprecluded dilution as a solution in many water bodies.The net result is increased constraints on disposal intomany surface waters.

According to Peavy et al. (1985), the capacity of astream to receive a waste discharge with a knowncontaminant concentration can be calculated using thefollowing equation:

References

• Chapra, S.C. 1997. Surface Water QualityModeling. McGraw-Hill, Inc., New York, NewYork.

• Cole, G.A. 1994. Textbook of Limnology.Waveland Press, Inc., Illinois, 412 p.

• Peavy, H.S., D.R. Rowe, G. Tchobanoglous.1985. Environmental Engineering. McGraw-Hill, Inc., New York, New York.

• Snoeyink, V.L. and D. Jenkins. 1980. WaterChemistry. Wiley and Sons, New York, NewYork.

• Tchobanoglous, G. and E.D. Shroeder, 1985.Water Quality - Characteristics, Modeling,Modification. Addison-Wesley PublishingCompany, Reading, Massachusetts.

Figure 5: A simplified schematic of photodegradation in natural waters(Chapra, 1997).