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42) Minnesota Paitutlon Control Age= r Environmental Outcomes Division Ground Water Monitoring & Assessment Program (ORP) are measured in the field. Iron samples are filtered in the field using a 0.45- micron filter and sent to a lab for analysis. A value of 0.7 part per million for total (unfiltered) iron may be used in place of filtered iron. The criteria shown in the table can be modified if technical staff have a sufficient basis for modifying the values. How does the geochemical sensitivity concept work? We advocate taking ground water samples vertically within the aquifer of concern. For example, consider the figure below. The data for it come from our St. Cloud land use study (MPCA, 1998b, 1999). In the figure, there are three wells sampled at different depths and data for each well are included. The data indicate that the upper portion of the aquifer is sensitive to nitrate contamination but the lower portion is not. A transition zone, where nitrate is denitrified, occurs between the upper and lower zones. 0 • 15 0 5 50 • Sensitive Wells Land surface Not sensitive Well 1 2 3 Oxygen (ppm) 6.3 4.6 1.7 Eh (mV) 308 300 287 Iron (PPm) 0.03 0.07 0.30 3 Nitrate (PPm) 5.60 0.50 0.05 How does this affect aquifer management? In the example above, thesurficial sand and gravel aquifer is mapped as being sensitive to contamination (DNR, 1999). Age dating indicates that water throughout the aquifer is less than 50 years old. Nitrate, however, is denitrified between five and 15 feet. Some important recommendations can be made based on these results: 1. Different land uses are compatible (for example, irrigated agriculture and residential development). 1 The ORP measurement is not the same asEh. ORP measurements must be corrected for temperature and for the particular probe being used. Call the manufacturer of the probe to obtain the correction factor. June199Io 2. Drinking water wells should be completed at depths of more than 30 feet below the top of the aquifer, preferably deeper. 3. Large water supply wells, such as those used for irrigation and municipal supply, should draw from deeper, confined aquifers to preventdrawdown of the surficial system. Drawdown in the surficial system would pull nitrate -rich water deeper into the aquifer, contaminating much of the aquifer. How does one map geochemical sensitivity for nitrate? • Sample several wells at various depths in the aquifer of concern; • measure dissolved oxygen and ORP in the field; • collect nitrate and filtered iron samples for laboratory analysis; • develop a three - dimensional sensitivity map using the .• develop appropriate aquifer management strategies. guidelines stated above; and For more information For more information, contact Mike Trojan at (651) 297- 5219 (e -mail: mike.trojan@pca.state.mn.us). Our Web address is http://www.pca.state.mn.us/water/groundwater/gwmap/in dex.html. References Minnesota Department of Natural Resources. 1999. Geologic Atlas — Stearns County, Minnesota. County Atlas Series. Atlas C -10, Part B. 3 plates. Minnesota Pollution Control Agency. 1998a. Nitrate in Minnesota Ground Water —A GWMAP Perspective. 57 pp. Minnesota Pollution Control Agency. 1998b. Water Quality in the Upper Fifteen Feet of a Shallow Sand Aquifer. 38 pp. Minnesota Pollution Control Agency. 1999. Effects of Land Use on Ground Water Quality —St. Cloud Area, Minnesota. 1998 Results. 46 pp. Estimating Ground Water Sensitivity to Nitrate Contamination Page 2