Chemicals in the Environment

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 • Calibration is not always needed

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?