Equilibrium

Environmental Monitoring and Data Analysis

 

Differences in accuracy (proximity to the true value) and precision (proximity of multiple measurements) are illustrated in these figures. The top figure shows a series of measurements that are neither accurate (close to the bull's eye) or precise (close to each other). The middle figure shows a tightly grouped cluster of measurements (very precise) that are off the bull's eye (in other words, the data appear to be great, but they are erroneous). The lower figure shows data that are both accurate (on the bull's eye) and precise (close together). A fourth figure might show data that was accurate (grouped around the bull's eye), but imprecise (not clustered together). In spite of the statement of a now-retired government official that:

"I don't care if the data are accurate, as long as they are precise"

only the accurate data in the bottom figure are acceptable for scientific research and most other uses.

Box models are commonly used to calculate the input, removal, and residence time of substances within a body. These could include: money in a checking account; salt in the oceans, DDT in a lake, and dioxin in an infant. The models may be as simple or as complex as the data and analytical capabilities allow. This is a more stylized illustration of our first box model of metals in coastal waters in the Southern California Bight. With the evolution of computer capabilities, such models may have an essentially unlimited number of cells with a large number of inputs and outputs. However, the validity of even the most sophisticated computer model depends upon the quality of the data (e.g., gigo or garbage in / garbage out).

An illustration of the problem of erroneous measurements of metal concentration, at an industrial site in Santa Cruz, California. The site has been reported to be highly contaminated, based on analyses that indicate the concentrations of some metals, including cadmium, in groundwater below the site exceed the state and federal MCL (maximum concentration level). However, rigorous (trace metal clean and low flow pumping) measurements of cadmium concentrations in wells at both the "contaminated" and adjacent, "background" sites indicate the cadmium concentrations are below the MCL.

The National Mussel Watch Program is designed to monitor the health of coastal waters through systematic analyses of contaminant concentrations in bivalves (mussels and oysters) and sediments. These figures illustrate the relative amounts of (a) PAH (polyaromatic hydrocarbons) in bivalves

(c) PAH in sediments

(b) PCBs (polychlorinated biphenyls) in bivalves

The global water cycle illustrates the amounts of water in different reservoirs. The limited amount of fresh water in accessible reservoirs relative to the vast amount of salt water available in the oceans accounts for efforts to obtain freshwater by desalinization.

The relative errors caused by the inadvertent (unrecognized) introduction of a contaminant (such as lead) during collecting, processing, and measuring a sample for that contaminant concentration are illustrated in this figure. It shows that the inadvertent contamination (e.g., 3 g/dL), will substantially increase (e.g. quadruple) the reported blood lead concentration in an individual with a relatively low blood lead concentration (e.g., 1 g/dL), but the same amount of inadvertent contamination will not markedly increase (< 10%) the reported blood lead concentration in an individual with a relatively high blood lead concentration (e.g. 50 g/dL). While these changes may seem inconsequential, similar errors in analysis may result in the improper notification of a pregnant woman that the mental health of her fetus may be at risk from high lead exposure from her blood. Similarly, erroneous measurements of contaminant concentrations in the environment may result in actions to reduce contamination that isn't really at a problematic level, while erroneous measurements in toxicity studies may falsely indicate that a contaminant doesn't cause adverse health effects at ambient concentrations.

Box models are commonly used to calculate the input, removal, and residence time of substances within a body. These could include: money in a checking account; salt in the oceans, DDT in a lake, and dioxin in an infant. The models may be as simple or as complex as the data and analytical capabilities allow. For example, (a) illustrates our first box model of metals [Me] in San Francisco Bay

(b) illustrates our subsequent model for those metals, when we segmented the bay into its three principal hydraulic components (North Bay, Central Bay, and South Bay)

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