1. U.S. Department of the Interior U.S. Geological Survey Assessment of Shallow Ground-Water Quality in Agricultural and Urban Areas Within the Arid and Semiarid Western United States Angela P. Paul and Ralph L. Seiler USGS, Water Resources Discipline Nevada Water Science Center Carson City, Nevada

2. BACKGROUND NAWQA investigated shallow ground water underlying agricultural and urban areas within the arid/semiarid Western United States: Arizona ( CAZB ) -- Agricultural Utah ( GRSL ) -- Urban Nevada ( NVBR ) -- Agricultural and Urban New Mexico ( RIOG ) -- Agricultural and Urban California ( SACR,SANJ,SOCA ) -- Agricultural and Urban

3. PURPOSE Identify regional factors influencing shallow ground- water quality within agricultural and urban areas in the arid/semiarid Western United States Concern of compromised shallow ground water entering deeper aquifers thereby contaminating possible potable supplies

4. Sampling Sites Study Area Urban Sites Agricultural Sites Explanation GRSL RIOG CAZB NVBR SOCA SANJ SACR

5. BACKGROUND Growing Population From 1990 to 2000 the percent change in population within the study area ranged from 13 to 66 percent. Changing Land Use Agricultural and desert lands are being converted to residential and industrial areas. Increasing Need For High-Quality Water Increasing demand on ground-water supplies Increasing reuse of water

6. SACR SANJ SOCA NVBR GRSL CAZBRIOG Modified from U.S. Census Bureau, Census 2000 Redistricting Data (P.L. 94-171) Summary File.

7. BACKGROUND Growing Population From 1990 to 2000 the percent change in population within the study area ranged from 13 to 66 percent. Changing Land Use Agricultural and desert lands are being converted to residential and industrial areas. Increasing Need For High-Quality Water Increasing demand on ground-water resources Increasing reuse of water

8. APPROACH Use existing NAWQA data available for: Water Quality GIS Datasets Each well was characterized using 500 m buffer zones (0.785 km 2 area). Statistical and geochemical analysis: Nutrients Pesticides VOCs

9. REGIONAL FACTORS Land Use Pesticide Use Population Density Aquifer Characteristics Dissolved Oxygen (redox condition)

10. DATA ANALYSIS Samples were collected between 1986 and 2004 from 179 Urban Wells (11 to 250 feet deep) 272 Agricultural Wells (11 to 390 feet deep) The most recent sample from a given well was used in data analysis

11. Nutrients

12. Detection Frequency is Similar for Agricultural and Urban Areas

13. Nitrate Concentrations 1.Ag and Urban appear similar. 2.Nitrate concen- trations differ greatly among study areas. 3.Many samples exceed MCL (27% Ag; 7% Urban)

14. Bias From Sampling Intensity 1.SANJ comprises 40% of Ag samples 2.Significant difference between SANJ and non- SANJ samples. 3.One study unit can have a disproportionate influence on summary statistics.

15. Pesticides

16. Detection Frequencies of Selected Pesticides In Study Area 0 10 20 30 40 50 Triazines Ureas Carbamates Bentazon Metolachlor Dinoseb Norflurazon PESTICIDE DETECTION FREQUENCY AG, N = 271 URBAN, N = 175

17. Estimated Simazine Use and Detections (Agricultural Wells) 0 20 40 60 80 100 CAZBNVBRRIOGSACRSANJ STUDY UNIT PERCENT APPLICATION 0 20 40 60 80 100 PERCENT DETECTION Estimated Use Detection

18. Triazine Detections and Agricultural Land Use 0 5 10 15 20 25 30 35 40 45 0 - 2526 - 5051 - 7576 - 100 AGRICULTURAL LAND USE (PERCENT) PERCENT DETECTIONS Atrazine Simazine

19. 0 5 10 15 20 25 30 35 40 45 0 - 2526 - 5051 - 7576 - 100 AGRICULTURAL LAND USE (PERCENT) PERCENT DETECTIONS Atrazine Simazine Triazine Detections and Agricultural Land Use 180 Wells 26 Wells 8 Wells

20. 0 5 10 15 20 25 30 35 40 45 0 - 2526 - 5051 - 7576 - 100 AGRICULTURAL LAND USE (PERCENT) PERCENT DETECTIONS Atrazine Simazine Triazine Detections and Agricultural Land Use 180 Wells 26 Wells 8 Wells Dominated by the California Central Valley Agricultural Area (SANJ)

21. Triazine Detections and Agricultural Land Use (Without SANJ Data) 0 5 10 15 20 25 30 0 - 2526 - 5051 - 7576 - 100 AGRICULTURAL LAND USE (PERCENT) PERCENT DETECTIONS Atrazine Simazine

22. 0 5 10 15 20 25 30 0 - 2526 - 5051 - 7576 - 100 AGRICULTURAL LAND USE (PERCENT) PERCENT DETECTIONS Atrazine Simazine Triazine Detections and Agricultural Land Use (Without SANJ Data) 8 Wells 26 Wells 23 Wells 75 Wells

23. VOCs

24. DETECTION FREQUNCY OF SELECTED VOCS 0 5 10 15 20 25 30 35 THMSolventOxygenateCFCBTEXFumigants VOC GROUP DETECTION FREQUENCY AG, N = 238 URBAN, N = 174

25. Selected VOC Detections With Land Use (Urban Wells) 0 5 10 15 20 25 30 35 40 0 - 2526 - 5051 - 7576 - 100 PERCENT URBANIZATION PERCENT DETECTIONS Chloroform PCE TCE MTBE

26. 0 5 10 15 20 25 30 35 40 0 - 2526 - 5051 - 7576 - 100 PERCENT URBANIZATION PERCENT DETECTIONS Chloroform PCE TCE MTBE Selected VOC Detections With Land Use (Urban Wells) 19 Wells 24 Wells 37 Wells 98 Wells

27. POPULATION DENSITY AND VOCs (Urban Wells) 0 5 10 15 20 25 30 35 40 45 50 0 - 10001001 - 20002001 - 3000>3000 POPULATION DENSITY (People/km 2 ) PERCENT DETECTIONS Chloroform PCE TCE MTBE

28. POPULATION DENSITY AND VOCs (Urban Wells) 0 5 10 15 20 25 30 35 40 45 50 0 - 10001001 - 20002001 - 3000>3000 POPULATION DENSITY (People/km 2 ) PERCENT DETECTIONS Chloroform PCE TCE MTBE 53 Wells 63 Wells 43 Wells 19 Wells

29. 0 10 20 30 40 50 60 0 - 0.50.6 - 1.01.1 - 1.51.6 - 2.02.1 - 4.0>4.0 DISSOLVED OXYGEN (mg/L) PERCENT DETECTION Chloroform PCE TCE MTBE Chloroform, Solvent, Oxygenate Detections 15 Wells 51 Wells 7 Wells 11 Wells 30 Wells 46 Wells

30. 0 10 20 30 40 50 60 0 - 0.50.6 - 1.01.1 - 1.51.6 - 2.02.1 - 4.0>4.0 DISSOLVED OXYGEN (mg/L) PERCENT DETECTION Chloroform PCE TCE MTBE Chloroform, Solvent, Oxygenate Detections Chloroform occurs more frequently under aerobic conditons

31. Possible Sources of VOCs Chloroform is likely due to chlorination of water PCE and TCE detections are possibly from solvent plumes within the area of influence for wells showing detections (local or upgradient) Additional data of solvent plumes

32. Possible Sources of VOCs MTBE may be from exposed surface water used in artificial recharge of aquifers, leaking underground storage tanks, or accidental gas spills

33. CONCLUSIONS Nitrate MCL (10 mg/L) was exceeded in 27 and 7 percent of the agricultural and urban wells sampled, respectively Pesticide use data needs to be refined Pesticide concentrations were below EPA Criteria

34. CONCLUSIONS Two TCE concentrations exceeded the EPA Criterion of 5 µg/L The MTBE health advisory (20 – 40 µg/L) was exceeded in two samples Further our understanding of shallow to deep ground-water interactions