1. Chemicals in the Environment
Hazardous Chemicals
Ignitability-Materials that pose a fire hazard during routine handling
Corrosivity-Materials that require special containers because they corrode standard containers.
Reactivity-Materials that react spontaneously with air or water, are unstable to shock or heat, generate toxic gases, or explode during routine handling.
Toxicity-Materials that release toxicants in quantities that pose a threat to human or environmental health when improperly handled.

2. Effects Acute effects- immediate damage
Chronic effects-long term damage; many years until effect is noticed.

3. Transport Mechanism
Principal mechanism of transport through the environment is from the movement of fluids in which the materials are suspended
Water and air through atmospheric, surface and ground systems.
Emphasis in the course is water quality, but air is equally and possibly more important.
Live 3 days without water, but only 6 minutes without air.

4. Exposure Sources
Biological cycles-Uptake and decay of animal and plant life, excretion of materials, etc.
Domestic waste-Discharges of raw and treated wastewater.
Industrial waste-Discharges of raw and treated wastewater, discharges of raw and treated off-gases.
Nonpoint source-Landfill leachate, stormwater runoff.
As a rule of thumb, a nonpoint source is any source that you cannot “point” to. (Although humorous, this definition is quite practical)

5. Exposure and Risk
Fundamental question is what are the risks associated with assimilation of a certain compound at a certain concentration over short (acute) and long (chronic) term?
Risk Assessment
Toxic response analysis
Exposure Concentration
Cost-benefit analysis
Revealed and expressed preference analysis
Economics

6. Exposure Concentration
Source of compound, production rates, and release rates to environment.
Characteristics of compound relevant to its ability to travel and react in the natural environment
Data to estimate the population at risk
Occupation
Medical surveillance
Socioeconomic use habits
The source and compound characteristics can be incorporated into models. The risk analysis is a second step.

7. Dose Response
For response to a contaminant the material must be toxic and the receptor must be exposed.
A highly toxic material with no exposure is not a hazard.
A mildly toxic material with high exposure could be very hazardous.
Environmental toxicology typically assumes:
For dilute pollutants, toxicity is proportional to concentration.
The longer the contact time, the greater the probability of toxic effects.
Amount of toxicant initially absorbed is gradually decreased by metabolic activity and excretion with other bodily wastes.

8. Retention Dose
Time integral of the retention curve is called the retention dose.
The lifetime retention dose is called the dose commitment.
A typical formula for estimating retention dose is:
Toxicant Retained
Time

9. Threshold
In drug therapy there exist threshold doses where response to the drug changes.
Typically two thresholds in drugs exist, a lower bound where no therapeutic effect is observed, and an upper threshold where damage (usually death) occurs.
Similarly toxicants are thought to also have thresholds.
A practice used is that one-percent of the threshold dose for animals is acceptable for humans (normalized by body weight)

10. Latency
In support of the threshold hypothesis it has been observed that the period between exposure and response (tumors) for carcinogen increases as dose decreases.
Generally it is accepted that the product of dose and latent time raised to a power is a constant.

11. Estimating Fate of Chemicals
The fundamental tool used to predict concentrations in the environment is the mathematical model, supported by data, laboratory experiments, and judgement.
Models are used in many disciplines
Economics: predict market activity, occurrence or recessions or periods of productivity.
Meteorology: short-term weather conditions, long-term climatological conditions.
Engineering: predict performance of engineered structures. Predict transport and fate of pollutants.

12. Modeling A model is used to answer important questions about a system.
What changes can be expected in the aquifer water levels in the Houston-Galveston region by 2020?
What is the capture area for a well that supplies drinking water to San Angelo, Texas?
What fundamental processes govern behavior of a contaminant in a leaching experiment?
What level of contaminant can be introduced into a reservoir with minimal health effects?

13. What is a Model?
A model is a device that represents an approximation to a real situation.
Physical Models (Sandbox)
Analog Models (Viscous Flow)
Mathematical Models
Numerical Solutions (MODFLOW)
Analytical Solutions (Theis)
Numerical Models
Solution Algorithm (Code)
Experiments and scaling laws

14. Applied Modeling Governing principles are well tested
The appropriate computer codes exist
The Modeling Exercise becomes:
Purpose (What questions will be asked)
Conceptualization (simplifying assumptions)
Code Selection
Model Design
Grid design, boundaries, sources/sinks
Calibration,sensitivity analysis
Prediction
Presentation and interpretation of results

15. Investigative Modeling
Governing principles are not understood
Computer codes may not exist to do the job.
The Modeling Exercise becomes:
Purpose
Conceptualization
Code Development
Model Design
Calibration,sensitivity analysis
Prediction
Presentation and interpretation of results

16. Types of Modeling Predictive (Applied Modeling)
Used to predict the future
Requires calibration
Interpretive (Applied and Investigative)
Used as a framework for studying system dynamics.
Used to organize data
Calibration is not always needed
Generic (Applied and Investigative)
Used to analyze hypothetical situations.
Helpful to frame regulatory guidelines

17. Establishing the Purpose
Is the model to be constructed for prediction, system interpretation, or generic modeling?
What do you need to learn from the model?
What questions do you want the model to answer?
Is modeling the best way to answer the question(s)?
Will an analytical model suffice?