# Presentation: 504949

## 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

**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

## 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**** **
**	**

## Sampling Sites

**GRSL**

**RIOG**

**CAZB**

**NVBR**

**SOCA**

**SANJ**

**SACR**

## 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**

**Notes:**
(U.S. Census Bureau, Population Division, Interim State Population Projections, 2005)

## SACR

**SACR**

**SANJ**

**SOCA**

**NVBR**

**GRSL**

**CAZB**

**RIOG**

**Modified from U.S. Census Bureau, Census 2000 Redistricting Data (P.L. 94-171) Summary File.**

## 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**

**Notes:**
(U.S. Census Bureau, Population Division, Interim State Population Projections, 2005)

## 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****	****	****	**

## REGIONAL FACTORS

**Land Use**
**Pesticide Use**
**Population Density**
**Aquifer Characteristics**
**Dissolved Oxygen (redox condition)**
**	**

## 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**

## Nutrients

## Detection Frequency is Similar for Agricultural and Urban Areas

**Detection Frequency is Similar for Agricultural and Urban Areas**

## Nitrate Concentrations

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

## 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.

## Pesticides

## Detection Frequencies of Selected Pesticides In Study Area

**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**

## Estimated Simazine Use and Detections(Agricultural Wells)

**0**

**20**

**40**

**60**

**80**

**100**

**CAZB**

**NVBR**

**RIOG**

**SACR**

**SANJ**

**STUDY UNIT**

**PERCENT APPLICATION**

**0**

**20**

**40**

**60**

**80**

**100**

**PERCENT DETECTION**

**Estimated Use**

**Detection**

## Triazine Detections and Agricultural Land Use

## Triazine Detections and Agricultural Land Use

**Triazine Detections and Agricultural Land Use**

**180 Wells**

**26 Wells**

**26 Wells**

**8 Wells**

## Triazine Detections and Agricultural Land Use

**Triazine Detections and Agricultural Land Use**

**180 Wells**

**26 Wells**

**26 Wells**

**8 Wells**

**Dominated by the California Central Valley Agricultural Area (SANJ)**

## Triazine Detections and Agricultural Land Use(Without SANJ Data)

## Triazine Detections and Agricultural Land Use(Without SANJ Data)

**Triazine Detections and Agricultural Land Use****(Without SANJ Data)**

**8 Wells**

**26 Wells**

**23 Wells**

**75 Wells**

## VOCs

## DETECTION FREQUNCY OF SELECTED VOCS

**0**

**5**

**10**

**15**

**20**

**25**

**30**

**35**

**THM**

**Solvent**

**Oxygenate**

**CFC**

**BTEX**

**Fumigants**

**VOC GROUP**

**DETECTION FREQUENCY**

**AG, N = 238**

**URBAN, N = 174**

## Selected VOC Detections With Land Use(Urban Wells)

## Selected VOC Detections With Land Use(Urban Wells)

**Selected VOC Detections With Land Use****(Urban Wells)**

**19 Wells**

**24 Wells**

**37 Wells**

**98 Wells**

## POPULATION DENSITY AND VOCs(Urban Wells)

## POPULATION DENSITY AND VOCs(Urban Wells)

**53 Wells**

**63 Wells**

**43 Wells**

**19 Wells**

## Chloroform, Solvent, Oxygenate Detections

**Chloroform, Solvent, Oxygenate Detections**

**15 Wells**

**51 Wells**

**7 Wells**

**11 Wells**

**30 Wells**

**46 Wells**

## Chloroform, Solvent, Oxygenate Detections

**Chloroform, Solvent, Oxygenate Detections**

**Chloroform occurs more frequently under aerobic conditons**

## 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**

## 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**

## 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**** **

## CONCLUSIONS

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