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Angela P. Paul and Ralph L. Seiler USGS, Water Resources Discipline Nevada Water Science Center

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. BACKGROUND.

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Angela P. Paul and Ralph L. Seiler USGS, Water Resources Discipline Nevada Water Science Center

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

  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 NVBR GRSL SANJ SOCA CAZB RIOG 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 km2 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 • Ag and Urban appear similar. • Nitrate concen-trations differ greatly among study areas. • Many samples exceed MCL (27% Ag; 7% Urban)

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

  15. Pesticides

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

  17. Estimated Simazine Use and Detections(Agricultural Wells) 100 100 Estimated Use Detection 80 80 60 60 PERCENT APPLICATION PERCENT DETECTION 40 40 20 20 0 0 CAZB NVBR RIOG SACR SANJ STUDY UNIT

  18. 45 40 Atrazine 35 Simazine 30 PERCENT DETECTIONS 25 20 15 10 5 0 0 - 25 26 - 50 51 - 75 76 - 100 AGRICULTURAL LAND USE (PERCENT) Triazine Detections and Agricultural Land Use

  19. 45 40 Atrazine 35 Simazine 30 PERCENT DETECTIONS 25 20 15 10 5 0 0 - 25 26 - 50 51 - 75 76 - 100 AGRICULTURAL LAND USE (PERCENT) Triazine Detections and Agricultural Land Use 180 Wells 8 Wells 26 Wells 26 Wells

  20. 45 40 Atrazine 35 Simazine 30 PERCENT DETECTIONS 25 20 15 10 5 0 0 - 25 26 - 50 51 - 75 76 - 100 AGRICULTURAL LAND USE (PERCENT) Triazine Detections and Agricultural Land Use Dominated by the California Central Valley Agricultural Area (SANJ) 180 Wells 8 Wells 26 Wells 26 Wells

  21. 30 25 Atrazine Simazine 20 PERCENT DETECTIONS 15 10 5 0 0 - 25 26 - 50 51 - 75 76 - 100 AGRICULTURAL LAND USE (PERCENT) Triazine Detections and Agricultural Land Use(Without SANJ Data)

  22. 30 25 Atrazine Simazine 20 PERCENT DETECTIONS 15 10 5 0 0 - 25 26 - 50 51 - 75 76 - 100 AGRICULTURAL LAND USE (PERCENT) Triazine Detections and Agricultural Land Use(Without SANJ Data) 75 Wells 8 Wells 26 Wells 23 Wells

  23. VOCs

  24. DETECTION FREQUNCY OF SELECTED VOCS 35 30 AG, N = 238 25 URBAN, N = 174 20 DETECTION FREQUENCY 15 10 5 0 THM Solvent Oxygenate CFC BTEX Fumigants VOC GROUP

  25. 40 35 Chloroform PCE 30 TCE 25 MTBE PERCENT DETECTIONS 20 15 10 5 0 0 - 25 26 - 50 51 - 75 76 - 100 PERCENT URBANIZATION Selected VOC Detections With Land Use(Urban Wells)

  26. 40 35 Chloroform PCE 30 TCE 25 MTBE PERCENT DETECTIONS 20 15 10 5 0 0 - 25 26 - 50 51 - 75 76 - 100 PERCENT URBANIZATION Selected VOC Detections With Land Use(Urban Wells) 98 Wells 37 Wells 24 Wells 19 Wells

  27. 50 45 Chloroform 40 PCE 35 TCE MTBE 30 PERCENT DETECTIONS 25 20 15 10 5 0 0 - 1000 1001 - 2000 2001 - 3000 >3000 2 POPULATION DENSITY (People/km ) POPULATION DENSITY AND VOCs(Urban Wells)

  28. 50 45 Chloroform 40 PCE 35 TCE MTBE 30 PERCENT DETECTIONS 25 20 15 10 5 0 0 - 1000 1001 - 2000 2001 - 3000 >3000 2 POPULATION DENSITY (People/km ) POPULATION DENSITY AND VOCs(Urban Wells) 53 Wells 43 Wells 19 Wells 63 Wells

  29. 60 Chloroform 50 PCE TCE 40 MTBE PERCENT DETECTION 30 20 10 0 0 - 0.5 0.6 - 1.0 1.1 - 1.5 1.6 - 2.0 2.1 - 4.0 >4.0 DISSOLVED OXYGEN (mg/L) Chloroform, Solvent, Oxygenate Detections 30 Wells 11 Wells 46 Wells 15 Wells 51 Wells 7 Wells

  30. 60 Chloroform 50 PCE TCE 40 MTBE PERCENT DETECTION 30 20 10 0 0 - 0.5 0.6 - 1.0 1.1 - 1.5 1.6 - 2.0 2.1 - 4.0 >4.0 DISSOLVED OXYGEN (mg/L) 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

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