05/31/2000 Env Bd Packet - Laserfiche WebLink

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•Minnesota Pollution Control Agency Environmental Outcomes Division Ground Water Monitoring & Assessment Program Estimating Ground Water Sensitivity to Nitrate Contamination Water resource managers often view ground water pollution sensitivity maps as an important tool for ground water protection. Methods for mapping sensitivity consider factors such as soil permeability, aquifer conductivity, depth to water, and recharge. Sensitivity ratings are based on the time it takes for water to travel from a point of origin (for example, a septic system or a farm field) to the water table or an aquifer. Sensitivity methods based strictly on time of travel provide little information about the potential fate of contaminants in ground water. For example, nitrate can be transformed in ground water through a process called "denitrification." Denitrification reduces potential negative effects of nitrate on drinking water or surface water receptors. The Ground Water Monitoring and Assessment Program (GWMAP) of the Minnesota Pollution Control Agency (MPCA) has collected data that suggest denitrification is an important process in ground water. Denitrification occurs within well- defined conditions in ground water. Assessing these conditions is an important tool for water managers, and is more useful in aquifer management than simple estimates of hydrologic sensitivity. `Geochemical sensitivity" is the term we June 1999 use to describe sensitivity of ground water to nitrate contamination. Under what conditions is an aquifer sensitive to nitrate contamination? In Minnesota, surficial aquifers are recharged annually. This means the time it takes water to move from a point of origin to the water table is less than one year. By traditional mapping techniques, these surficial aquifers are considered sensitive to nitrate contamination. Resource managers, in turn, may use these sensitivity estimates for water planning, even though deeper portions of these aquifers may not be sensitive because nitrate will be denitrified. Recharge and infiltration certainly affect the fate of nitrate in the environment, but we emphasize the importance of characterizing the aquifer chemistry to predict the fate of nitrate. These concepts are explained in greater detail in a report (MPCA, 1998a). The following table can be used as a rough guide to identify ground water that is sensitive to nitrate contamination. When using this table, all three criteria must apply for a ground water sample to be classified as sensitive or not sensitive. This is important, because we have observed mixed results for many samples. Dissolved oxygen and oxidation - reduction potential Characteristic Sensitive Not sensitive Filtered iron concentration Less than 0.1 part per million More than 1 part per million Eh More than 250 millivolts Less than 225 millivolts Dissolved oxygen More than 1 part per million Less than 1 part per million Minnesota Pollution Control Agency, 520 Lafayette Rd. N., Saint Paul, MN 55155 -4194 (651) 296 -6300, toll -free (800) 657 -3864, TTY (651) 282 -5332 or (800) 657 -3864 This material can be made available in alternative formats for people with disabilities. Printed on rervnled naner nnntaininn at least 20 nement fihers from nanar recvc:Ied by consumers.