Reviews of water-quality data have identified some of the pitfalls of extending data interpretations to evaluate trends using data that were collected for other purposes. For example, data reviews by the U.S. Geological Survey, Minnesota Pollution Control Agency, and the Minnesota Department of Agriculture have shown apparent increasing trends in nitrate concentrations over time in Minnesota ground water. However, samples to date are not sufficient to quantify trends in space or time.

Surficial aquifers in Minnesota are among the most susceptible to contamination from land-surface activities such as urbanization, agriculture, or accidental spills. An early warning approach to ground-water monitoring may be desirable for these surficial aquifers to identify causes of water-quality degradation and to effect remediation before contamination spreads. Recent research tools, such as age dating of ground water using chloroflourocarbons, have shown that ground-water residence time can vary from a few years to decades within a vertical distance of 30 feet in some surficial sand and gravel aquifers. Sampling of shallow wells, which are completed near the water table of surficial aquifers, could be important as an early warning method. Unfortunately, existing water wells rarely meet this criterion. Ground-water monitoring can be costly, but technically valid implementation of monitoring to achieve early warning objectives can promote wise water management and reduce costs of remediation.