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Coni Minnesota Pollution Control Agency v.� Percent exceeding secondary drinking water standards • Data should be reanalyzed at approximately 10 -year intervals. What is the difference between ambient and baseline data? Baseline provides a snapshot of water quality at a particular point in time. It is used as a point of reference and can therefore be considered to represent a background condition. This concept works well for naturally- occurring chemicals, because concentrations of these chemicals in ground water should change slowly, if at all. For aquifers affected by human activity, a different approach is needed because water quality may change in response to human activity. An ambient program also provides a snapshot of water quality at a particular point in time, but when measured over several different times, trends in water quality can be assessed. An ambient program therefore measures several "baselines" to determine if they are equal. If they are not, then ground water quality is changing. Another difference between baseline and ambient is that only chemicals which may be expected to change in response to human activity are sampled in an ambient design. Examples include nitrate and VOCs. If water quality is changing, the following questions need to be answered. • Is water quality getting better or worse? • What is the lateral and vertical extent of change within the aquifer? • What will the water quality be when change ends? • What factors are contributing to change? • What human activities can be implemented to maintain or improve water quality at a sustainable level for human consumption? The following aquifers are potentially sensitive to huma. activity. • Surficial drift aquifers. • Bedrock aquifers with thin cover of glacial materials. • Karst bedrock. • Fractured bedrock near the land surface. • Deeper aquifers which are extensively pumped, thus inducing flow of ground water from more sensitive aquifers. Ambient monitoring networks should be established in areas mapped as being hydrologically sensitive. The principal components of such networks are listed below. • Wells should be completed at the water table and at receptor points. The shallow wells are designed to identify impacts in the most sensitive portions of the aquifer; the receptor wells identify the risk to humans. • Monitoring points should be located so that spatial analysis of the data can be conducted. Separation distances between wells will vary with the sampling location and may require some preliminary sampling from temporary and existing wells. • Conduct quarterly sampling for at least four years or until seasonal variations can be quantified. Samplir may then be reduced to once or twice a year. • Sample parameters include the chemicals of concern for the aquifer being sampled and field parameters. Most monitoring programs will also include sampling for major cations and anions and the redox parameters (field Eh and dissolved oxygen, reduced iron and manganese, total and dissolved organic carbon, sulfate, and nitrate). • Field sampling, laboratory QA/QC, data storage, and data analysis procedures must be documented • Completion of an annual report is required. Where can 1 get more information or data? Additional reports, information, and presentations will be prepared during the ensuing months to reach all potential audiences.For further information, contact Tom Clark (project coordinator, 612 - 296 -8580) or Mike Trojan (technical analyst, 612 - 297 - 5219). GWMAP reports and data can be mailed electronically or found on the MPCA web site at <http: / /www.pca.state.mn.us >. Page 4 Baseline Water Quality of Minnesota's Principal Aquifers