Mike Trojan and Mark Ferrey Minnesota Pollution Control Agency

1. MEASURING THE CAPACITY FOR CONTAMINANT BIODEGRADATION IN GROUND WATER: REGIONAL ASSESSMENTS OF AQUIFER SENSITIVITY Mike Trojan and Mark Ferrey Minnesota Pollution Control Agency

2. ACKNOWLEDGEMENTS Erin Eid Dan Helwig Jennifer Maloney Jim Stockinger Kent Verill Moira Campion

3. OUTLINE • Definitions and Concepts • Studies • Application

4. Nitrogen, Volatile Organic Compounds, many metals and trace inorganics, pesticides, and other organic contaminants are affected by biological processes within ground water

5. In spite of this, we often ignore these processes in our work • Using time of travel for pollution sensitivity and wellhead protection doesn’t incorporate likelihood a chemical will be attenuated • Pooling all data to conduct water quality analyses may mix data from different geochemical groups • Remediation decisions based on chemical concentrations or changes in concentration over space or time may ignore reasons for observed changes or differences

6. Within an aquifer, geochemistry is a critical factor affecting chemical concentrations Feet below water table Redox condition 0 Pesticides Nitrate Nitrate-reducing Cr Pb Iron reducing Halogenated VOCs BTEX 50 Sulfate reducing Mn As Methanogenic 100

7. We should therefore not treat an aquifer as a homogenous unit when managing it Land surface 0 Pesticides Nitrate Cr Nitrate-reducing Pb Halogenated VOCs Iron reducing BTEX 50 Sulfate reducing Mn As Methanogenic t = 7 yrs 100

8. # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # STUDY 1: Between 1992 and 1996 we sampled 954 wells statewide

9. 3.4 % of all sampled wells exceeded the MCL of 10 ppm for nitrate < 1 ppm 1 to 10 ppm > 10 ppm

10. What is a nitrate-vulnerable aquifer? • An aquifer in which nitrate will not be denitrified. We use the following criteria: • Eh > 250 mV • Dissolved oxygen > 1 ppm • Total iron < 0.70 ppm

11. Considering only the 121 wells considered to be vulnerable to nitrate contamination, 18% of samples exceeded the MCL < 1 ppm 1 to 10 ppm > 10 ppm

12. # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # In nitrate-vulnerable aquifers from agricultural areas, nearly 27 percent of the samples exceeded the MCL < 1 ppm 1 to 10 ppm > 10 ppm

13. Geochemical stability was the best indicator of nitrate in ground water Presence of Nitrate > 1 ppm

14. Nitrate Sensitive Aquifers Glacial Bedrock Hydrogeology based Geochemical based

15. Other baseline results • The median oxygen concentration in wells with detectable halogenated VOCs was 2.04 ppm compared to 0.82 in wells with detectable non-halogenated VOCs • In wells with arsenic concentrations exceeding 10 ppm, the median Eh was 146 mV, compared to 223 mV in wells with arsenic concentrations less than 1 ppm • and so on (Cr, V, Se, Sb, Cu, Pb, SO4, …..)

16. Study 2 : St. Cloud Land Use Study 6 miles Sewered Irrigated/dryland agriculture Commercial Unsewered 0 Feet Till Sand 100 Well Ground water flow Geoprobe

17. Nitrate concentrations decrease with depth, presumably due to denitrification

18. Consider a well and nitrate source: • horizontally at t=13, nitrate decreases from 15 to 3 ppm • vertically at t=16, nitrate decreases from 15 to < 0.2 ppm Irrigated field : nitrate = 15 ppm t = 13 t = 16

19. SENSITIVITY- NITRATE Elevation (feet) 0 -50 Geochemical based 0 Vulnerability V. high -50 Wellhead based High 0 Low -50 Hydrologic based

20. SENSITIVITY-FUEL OILS/GASOLINE Elevation (feet) 0 -50 Geochemical based Vulnerability 0 V. high -50 Wellhead based High Low Varies but generally increases with depth Hydrologic based

21. SENSITIVITY-CHLORINATED SOLVENTS Elevation (feet) 0 -50 Geochemical based 0 Vulnerability -50 V. high Wellhead based High 0 Low -50 Hydrologic based

22. Geochemical conditions in bedrock aquifers appear to differ from sand and gravel aquifers. For example, we often see less evidence for denitrification in bedrock aquifers

23. Uses of geochemical sensitivity maps • locating wells • determining if land use or aquifer management is more important • identifying importance of riparian buffers