Risk Assessment Objectives

Risk Assessment Paradigm

What is risk?

the probability that a substance/situation will produce harm under specified conditions (probability that the event occurs AND consequences of event)

Exceedance probability vs risk

What are three uses/objectives for risk assessment?

1. ID hazards

2. Analyze/interpret risk

3. Determine/implement control measures

OR

1. meet regulation

2. meet cleanup criteria

3. meet the expected risk

Environmental risk assessment

process of making a quantitative estimate of human health risks resulting from release or potential release of contaminants to the environment

Problem statement

ask some type of question to lead to a scientific inquiry and hypothesis, can implicitly or explicitly mandate assumptions/methods

System description

qualitative and quantitative info about physical processes in system (timescale, geo and physical configuration), provides key info for risk calculation component

4 steps of environmental risk assessment calculation procedure with description

1. Release assessment (S_dot - contaminant emission rate) - ID of contaminants and quantitative estimation of release probabilities and rates

2. Transport assessment (C) - ID of pathways and estimation of contaminant concentration

3. Exposure assessment (D) - ID exposed populations and exposure routes and calc of rate/duration of exposure

4. Consequence assessment (risk) - adverse aesthetic, ecological, and human effects

Transport pathway vs exposure route

how contaminant moves through environment vs how contaminant moves through body

Aleatory uncertainty

related to chance

Epistemic uncertainty

related to knowledge

Assessment endpoint

valuable ecological or system characteristics you want to protect; all the way down to risk → more uncertainty

Measurement endpoint

measurable biological or physical indicators directly linked to those assessment endpoints that tell you if risk is present/changing; case study, stop and compare concentrations/values to literature

Conceptual model

abstraction of various physical, chemical, and biological processes that affect the behavior of the contaminant in the system

Mathematical model

mathematical representation of conceptual model which permits the calculation of assessment measures

Computational model

math model converted to this, usually identical to math model, calc of assessment involves substituting risk parameters into closed-form analytical expressions using rudimentary computational tool like calculator/spreadsheet

Verification

process of assuring that the math model is accurately translated into the computational model

Calibration

adjustment of risk parameters so that predictions of model match observations

Validation

comparison of predictions of computational model to actual field measurements

What are the appropriate units for radiological and chemical contaminant concentrations in air, soil, food, and water?

mass/time, activity/time

Explain the roles of (a) measurement and (b) process knowledge in release assessment and discuss why one is used over the other

measurement = direct from emissions or indirect (calculate based on known exposure); gives more precise info

Process knowledge = knowledge of various processes responsible for contaminant generation and release at site; good for conceptual model formation, can simplify complex processes or work when general info unknown

Use process knowledge to develop conceptual model of contaminant release

5 classes of contaminants with examples

• organic compounds - Aldrin

• metals - Cadmium

• inorganic gases - Chlorine

• radionuclides - Cesium-134 and Cesium-137

• others - Asbestos

Emission rate

amount of contaminant released per unit time

Specific emission rate

mass emitted per unit volume per time

Given the equation for an actual emission rate calculate total mass released in a time period and max emission rate with time it occurs

Approx for constant emission rate

Sdot_t = Sdot_0

Approx for instantaneous emission rate

Sdot_t = ST * delta_t(t-ta)

Approx for semi-infinite step

Sdot_t = Sdot_0 * h(t-ta)

Approx for finite step

Sdot_t = Sdot_0 [h(t-ta) - h(t-tb)]

Sketches for types of emission rates and approximations of emission rates

Select an appropriate emission rate approx for a given release scenario

What are 2 important considerations when developing emission rate approximation?

total mass released (S_T) and time (Δt)

Calculate contaminant emission rate for a given source using process knowledge and mass balance properties

Fault tree

logic diagram that depicts all possible ways a failure (top event) in a particular system can occur

Event tree

logic diagram that identifies and quantifies possible outcomes of a single initiating event

Difference bt fault and event tree

fault = top down, focus on finding root causes of major event

event = bottom up, predict positive and negative outcomes of some initiating event

For a given system failure condition, construct a simple fault tree

For a given initiating event, construct an event tree

Risk drivers

contaminants and pathways that dominate risk

Contaminant concentration

1) mass per unit volume of medium (m_c/L_medium)

2) mass of contaminant per unit mass of medium (m_c/kg_medium)

Contaminant flux vector

j(r,t); net rate per unit area at which contaminant flows across plane perpendicular to principal direction of flux vector, contaminant flux units are mass/area/time

Advection

transport of a contaminant with the mean flow of the fluid

Dispersion

general term used to describe a variety of processes that cause contaminant movement to deviate from mean flow of fluid, including molecular diffusion, turbulent diffusion, and advective heterogeneities

Contaminant continuity equation

∂C/dt = - ∇•J^A - ∇•J^D

Contaminant transport equation

general equation used for a variety of physical quantities in a flowing fluid, contaminants have to be intimately mixed with fluid through dissolution, entrainment, or suspension, but C contaminants is small enough so fluid flow is not impacted by their presence; first two terms are conservative, other two are not

[time rate of change of C] = [NR of inc in conc due to dispersion] + [NR of inc in conc due to advection] + [generation rate per unit V] - [destruction rate per unit V]

Environmental compartment

useful mathematical construct that is either a distinct component (lake, cow, human body) or distinct subcomponent within a larger component (bottom sediment in a lake, cow’s milk, human liver)

Homogeneous compartment

contaminant concentration does not vary spatially, happens when mixing within compartment is rapid relative to movement into/out of compartment; either advective or non-advective

Partition coefficient

describes contaminant partitioning between two compartments as a ratio between those two concentrations

First-order process

time rate of change of contaminant concentration/mass is proportional to concentration/mass present

Dispersion tail

portion of curve to the right of the peak concentration; occurs bc dispersion and advection are acting in concert before peak arrives and in opposition after it leaves

Describe the three mechanisms that contribute to dispersion

molecular diffusion, turbulent diffusion, and advective heterogeneities

Calculate the new rate of flow of a contaminant across a surface, given the flux vector

S_dot = (integral from z=0 to L) (integral from y = 0 to Y) J(y,z,t)*i_x dydz

Write the mass continuity equation for contaminant concentration

Mass change per time = mass in - mass out + mass generated - mass destroyed

Give the empirical expressions relating flux and concentration

J^A = Cv; J^D = -D∇C where D is dispersion coefficient

Combine the empirical expressions with the mass continuity equation for contaminant concentration and obtain the contaminant transport equation

Give the physical significance of each term in the contaminant transport equation

∂C/dt = change in concentration over time

-∇•j^A(r,t) = contaminant flux due to advection

-∇•j^D(r,t) = contaminant flux due to dispersion

gdot(r,t) = generation rate per unit volume

ddot(r,t) = destruction rate per unit volume

Distinguish between conservative and non-conservative processes in the contaminant transport equation

conservative = -∇•j^A(r,t) -∇•j^D(r,t)

non-conservative = gdot(r,t) - ddot(r,t)

Give units of contaminant transport equation

mass/Vt = A/t * mass/V /A - L/t x mass/V/L + mass/Vt - mass/Vt

Write the general procedure for reducing/simplifying the contaminant transport equation

Reduce/simplify the contaminant transport equation given a particular scenario, including the specific generation term g(x,t)

Describe and apply the homogeneous compartmental models for non-advective first-order removal

dC/dt = -kC → C = C0 exp(-kt)

Describe and apply the homogeneous compartmental models for non-advective constant uptake, first-order removal

Ct = g/k (1-exp(-kt))

Describe and apply the homogeneous compartmental models for non-advective instantaneous partitioning

dC/dt = 0

Describe and apply the homogeneous compartmental models for advective conservative

C = C1 (1-exp(-Qt/V))

Describe and apply the homogeneous compartmental models for advective first-order reactions

advective non conservative first order → C = C1Q/V(k+Q/V) (1-e^-(Q/V+k)t)

Provide physical interpretations of C(r,t)

Simplify the contaminant transport equation

Solve the simplified form of the contaminant transport equation for C(r,t) either analytically, with LaPlace transforms, or both

Transport problems 1D advection semi-infinite step emission rate conservative

Transport problems 1D advection semi-infinite step emission rate first-order reactions

Transport problems 1D advection semi-infinite step emission rate variations

Transport problems 1D advection finite-step emission rate conservative

Transport problems 1D advection finite-step emission rate first-order reactions

Transport problems 1D advection finite-step emission rate variations

Transport problems 1D dispersion point, instantaneous emission rate no advection, conservative

Transport problems 1D dispersion point, instantaneous emission rate advection-dispersion, conservative

Transport problems 1D dispersion point, instantaneous emission rate advection-dispersion, first-order reactions

Transport problems 1D dispersion semi-infinite emission rate conservative

Transport problems 1D dispersion semi-infinite emission rate first-order reactions

Transport problems 3D dispersion

Thermocline

thin layer that separates the upper and lower layers that are stratified, regulates mixing and inhabitant life

Stratification

thermal stratification is when there’s a well-mixed upper layer (epilimnion) and a stable pool of cool water at the bottom (hypolimnion)

What are common sources of contamination for surface water?

• discharges from industrial facilities

• runoff from contaminated surface soils

• seeps from contaminated aquifers

• deposition from atmosphere

• spills and leaks from ships

What are common pathways through which humans can be exposed to contaminants in surface water?

• ingestion of drinking water (most important)

• incidental ingestion or skin absorption while swimming or bathing

• inhalation of volatized contaminants

• consumption of food crops irrigated with contaminated surface water or contaminated from atmospheric deposition

• consumption of fish or shellfish

Describe the processes responsible for contaminant transport among the various compartments of surface water

Identify the principal factors that affect contaminant concentration in rivers and streams, lakes, reservoirs, estuaries, and oceans

Quantify the partitioning of a contaminant bt water and suspended sediment using the distribution coefficient

KD = foc*Koc

KD = Cs/Cw

Use the distribution coefficient and suspended solids concentration to evaluate the relative importance of sorption as a process affecting aqueous phase contaminant concentration

Incorporate sorption into simple transport models

Calculate contaminant concentrations in surface water for simple source terms

Calculate contaminant concentrations in surface water for geometries like well-mixed lake

Calculate contaminant concentrations in surface water for geometries like 1D river or stream, instantaneous source (advection/dispersion)

Calculate contaminant concentrations in surface water for geometries like 1D river or stream, constant source (advection only)

Porous media

microscopic spaces bt solid subsurface material

Fractured porous media

macroscopic void spaces in subsurface, transport controlled by velocity, geometry, orientation, rock pores

Fractured non-porous media

macroscopic void spaces in subsurface, transport controlled by pore scale connectivity and Darcy flow

Saturated zone

region below the water table

Vadose (unsaturated) zone

from land surface to the water table

Aquifer

saturated region that can transmit significant quantities of water