fate of pollutants in air. downstream and upstream

1. FATE OF POLLUTANTS

2. INTRODUCTION
 A contaminant is a chemical or biological substance in a concentration that can
potentially cause adverse affects on the physical, chemical, or biological properties
of a waterbody.
 It includes pathogens, toxic metals, toxic organic chemicals, and other harmful
substances. Contamination of surface waterbodies poses serious risks to both
aquatic ecosystems and human health.
 Contaminants in a waterbody can be taken up by aquatic organisms in a process
called bioaccumulation.
 Contaminants can be transported through a waterbody by suspended sediment,
deposited to the bottom and/or resuspended from the sediment bed, and
transformed by chemical, biological, and hydrodynamic factors.
 Examples of contaminant sources are sewage treatment plants, urban runoffs,
storm sewers, failing septic systems, industrial discharges, and contaminated
sediments

3. ORIGIN
 The origins of contaminants can be divided into point and
nonpoint sources.
 Point source pollution comes from a specific, identifiable source,
such as a pipe. Nonpoint source pollution ca
 Point sources include wastewater treatment plants, overflows
from combined sanitary and storm sewers, and industry
discharges.
 Nonpoint sources include runoffs from urban, agriculture, and
mining areas not be traced to a specific spot. Atmospheric
deposition is another source of nonpoint pollution. Temporal
variability of nonpoint sources is directly related to watershed
hydrologic variability.

5. Processes Affecting Pathogens/Pollants
 Factors that may influence pathogen concentrations in water include
 (1) hydrodynamic transport, dilution, and settling;
 (2) sunlight;
 (3) temperature;
 (4) salinity;
 (5) predation;
 (6) nutrient levels;
 (7) toxic substances; and
 (8) other environmental factors

7. PATHWAYS
 Toxic substances in a waterbody can exist in two basic forms: the
dissolved and the particulate phase. The former is transported with water
flows, and the latter is often attached to and transported with sediments
(or particulate organic carbon
 Distinct processes that many toxic substances undergo are
bioaccumulation and biomagnification, which do not pertain to many
conventional pollutants, such as nitrogen and phosphorus).
 Bioaccumulation is the process by which some persistent contaminants
concentrate and accumulate as they travel via digestive processes to
higher levels of the food chain.
 Biomagnification is the magnification of contaminant concentrations in
biota at each successive trophic level in a food chain.

8.  In order for a substance to be considered toxic, several factors should be
considered, including the following:
 1. The potential that the environment is exposed to the substance.
 2. The potential that living organisms are exposed to the substance.
 3. The effects that are derived from the exposure.
To assess the effects of a toxic substance, the fate and transport processes of the
substance in the environment should be determined, including the following:
 1. Hydrodynamic processes, such as advection and dispersion of the toxics in the
water column.
 2. Sediment processes, such as sediment transport in the water column; deposition
and resuspension of sorbed toxics due to sediment movement; and sorption and
desorption of the particulate toxics with the sediment.
 3. External sources, such as point sources from wastewater treatment plants,
nonpoint sources from runoffs, and atmospheric depositions.
 4. Decay and transformation processes, such as photolysis, hydrolysis, and
biodegradation

9. Toxic organic chemicals that are frequently cited as causing environmental damage
include
 (1) PCBs,
 (2) PAHs,
 (3) pesticides,and
 (4) dioxins and furans.
Heavy Metals
 Characteristics of heavy metals include
 (1) bioaccumulation and biomagnification,
 (2) long decay time,
 (3) natural occurrences,
 (4) toxicity closely linked to the metal ’ s dissolvability, and
 (5) many chemical forms.
 Frequently cited heavy metals include lead, cadmium, mercury, and others.

10. Processes
Sorption and Desorption
 The sorption – desorption processes influence the concentration of contaminants. Sorption
with solids is a major pathway for the transport of toxic chemicals in natural waters.
 Sorption is the transfer of a substance from the aqueous to the solid phase.
 Desorption is the process by which substances are released from the particles back into water.
key factors that determine the fate and transport of toxic substances, including
(1) inflow and outflow;
(2) settling of particulates in the water column;
(3) sorption and desorption in both the water column and the bed;
(4) exchange between the water column and the bed via deposition/resuspension, diffusion, and
bioturbation;
(5) losses by burial and volatilization; and
(6) bioaccumulation and transformation.

12. In this case, the solubility of the substance can be measured by its solubility product, which is the
product of the concentrations of the ionic species involved in the dissolution. This product is
considered as a constant under given environmental conditions. For a heavy metal with the following
reaction:

13. FATE AND TRANSPORT PROCESSES
 The fate and transport processes of contaminants are controlled by two factors: their reactivity and
their hydrodynamic transport. Reactivity includes
 (1) chemical processes,
 (2) biological processes, and
 (3) bio uptakes. Hydrodynamic
Transport includes three mass transport processes:
(1) advection of water current,
(2) diffusion and turbulent mixing within the water column, and
(3) deposition and resuspension on the water - sediment bed interface.

14. Mathematical Formulations
 The fate and decay of a contaminant represent the gradual decrease in the amount of a substance
in an environmental system, as the result of various sink processes, including chemical and
biological transformation, or dissipation/ deposition to other environmental systems. The fate and
decay processes are contaminant specific.
 Based on the principle of conservation of mass, the concentration change of a contaminant can be
calculated using mass balance equations. Although reaction kinetics in aquatic systems can be
described in numerous ways, the form for a single reactant is generally expressed as:

17. Processes Affecting Fate and Decay
 The fate and decay of toxic substances can result from
physical, chemical, and/or biological reactions. In addition
to sorption and desorption, processes that can significantly
affect the fate and decay processes include
 (1) mineralization and decomposition,
 (2) hydrolysis,
 (3) photolysis,
 (4) biodegradation,
 (5) bioconcentration, and
 (6) volatilization.

18.  Most decay processes are expressed as first - order
reactions. The first - order decay coefficients for
individual processes are additive and can be linearly
superimposed to form a net decay coefficient:
kd = km + kh + kp + kbd + kbc + kv
(4.4.9)
 where k d = net decay coefficient, k m = mineralization
coefficient, k h = hydrolysis coefficient, k p = photolysis
coefficient, k bd = biodegradation coefficient, k bc =
bioconcentration coefficient, and k v = volatilization
coefficient

19. Mineralization and Decomposition
 Bacteria decompose organic material to obtain energy for growth. Plant residue is broken down
into glucose that is then converted to energy:
Hydrolysis. Hydrolysis is the reaction of a chemical with water, in which splitting of a molecular bond
occurs in the chemical and there is a formation of a new bond with either the hydrogen (H + ) component or
the hydroxyl (OH − ) component of a water molecule.
Photolysis.
Photolysis (photodegradation) is the transformation of a compound that results directly from the adsorption
of light energy. Compounds that absorb sunlight may gain sufficient energy to initiate a chemical reaction.
Some of these photochemical reactions result in the decomposition or transformation of a substance.

20.  Biodegradation. Biodegradation (biolysis) is the breakdown of a compound by enzyme - mediated
transformation, primarily due to bacteria, and to a lesser extent, fungi

21. Volatilization.
 Volatilization represents a chemical substance entering the atmosphere by evaporation from water.
 Volatilization is often treated as an irreversible decay process, because of its mathematical
similarities to these decay processes.

22. p H
 The rate of chemical reactions can be significantly altered by changing the pH.
 The solubility and bioavailability of many chemicals are also dependent on pH.
 The pH value is defined as the exponent to the base 10 of the hydrogen ion concentration:
 The pH scale is used to determine the acidic or alkaline nature of water. It ranges from 0 to 14 pH
units, with pH 0 being the most acidic, pH 7 being neutral, and pH 14 being the most alkaline
(basic).

23. CONTAMINANT MODELING
 The fate and transport of contaminants are complicated processes that include physical transport
and chemical and biological kinetics.
 A toxicant is usually composed of two forms: dissolved and particulate. A typical toxic model should
include the following:
 1. A hydrodynamic and sediment model that provides the transport and settling information.
 2. Sorption – desorption interaction between dissolved and particulate toxics.
 3. Interactions and exchanges between the sediment bed and the overlying water column.
 4. Transport, fate, and decay of the toxics in the water column and the sediment bed.
 5. External loadings to the system.

25. In addition to the hydrodynamic and sediment parameters, toxic modeling often requires adjustment of
the following parameters:
 1. Partition coefficient (mostly adjusted in toxic metal and TOC modeling).
 2. Decay rate (mostly adjusted in pathogen and TOC modeling).
Data used in a toxic model include the following:
 1. Concentrations of the contaminants (pathogens, metals, or TOCs) in the water column. For metals
and TOCs, the concentrations should be separated into dissolved and particulate phases.
 2. Concentrations of the contaminants in the sediment bed.
 3. External toxic loadings to the waterbody.