Assessing Contamination Susceptibility at Public Water Supply Wells in Northern California
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Assessing Contamination Susceptibility at Public Water Supply Wells in Northern California. This project is sponsored by the State Water Resources Control Board (SWRCB) Under the Ambient Groundwater Monitoring and Assessment (GAMA) Program

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Assessing Contamination Susceptibility at Public Water Supply Wells in Northern California

  • This project is sponsored by the State Water Resources Control Board (SWRCB)

  • Under the Ambient Groundwater Monitoring and Assessment (GAMA) Program

  • Called the California Aquifer Susceptibility (CAS) project

  • Carried out in collaboration with the U.S. Geological Survey

http://www.swrcb.ca.gov/cwphome/land/gama/webpages/gamahome.htm


California Groundwater Vulnerability Supply Wells in Northern California

Number of Leaking Underground Fuel

Tank (LUFT) Sites Within 1/2 Mile of a water supply well

CAS Project

Takes a probablistic approach in assessing relative contamination susceptibility

4-9 LUFT sites within 1/2 mile


The question
The Question Supply Wells in Northern California

  • Will contaminants present at and near the surface reach drinking water supply wells?

The Tools

  • Oxygen isotopes (water source)

  • Ultra low level Volatile Organic Compound analysis (part per trillion detection limits)

  • Groundwater Age (Tritium-Helium method)


The ratio of Supply Wells in Northern California18O to 16O in precipitation varies because lighter 16O is favored during evaporation while 18O is favored during condensation. The map shows the value of 18O/16O relative to seawater in parts per thousand


Stable isotopes of the water Supply Wells in Northern California

molecule are good indicators

of water source in California

The isotopic signatures of the

large, perennial Rivers with

Sierran headwaters are distinct

from locally-sourced water in the Central Valley


D 18 o ranges and end members
d Supply Wells in Northern California18O Ranges and End Members

  • Northern Sacramento Valley Precipitation: -8 (volume weighted, time-integrated)

  • High Elevation Sierra Precipitation: -14

  • Range in measured rain/snow samples: -4.4 to -22.8

  • Surface Water:

    • Sacramento River above Shasta -13.3 (3), below Shasta -10.8 (12), Feather River -11.0 (2)

    • Wild Goose Canal: -10.6

    • McCloud River: -12.4 (3), Pit River: -13.0 (3), Fall River: -13.5 (2), Susan River: -13.1 (2), Burney Falls: -13.0 (3)

    • Lake Davis: -5.7

  • Range in GAMA northern CA study groundwaters: -14.7 (Weed) to -6.0 (Durham Esquon shallow monitoring well)


Results are from long screened production wells

Advantages Supply Wells in Northern California

You get a good, clean sample (both dissolved gases and low level VOCs)

Characteristics are usually known

It’s the water people are using and drinking

Disadvantages

Vertical information is smeared

‘Signal’ is diluted

Flow field is altered

It’s the water people are drinking

Results are from Long-screened Production Wells


Assessing groundwater vulnerability
Assessing Groundwater Vulnerability Supply Wells in Northern California

Type(s) of VOCs detected, and age analysis including mixing, give

additional information about transport.


Sample statistics and fun facts
Sample Statistics and Fun Facts Supply Wells in Northern California

  • 168 samples

    • 121 public supply, 39 monitoring wells, 6 private wells, 2 springs

  • 74 (44%) with < 3 pCi/L tritium

    • 47 (28%) with < 1 pCi/L

  • 18 with clear mantle He component

  • 51/123 (41%) MtBE hits (30 in Chico); 4 > 51 ppt (2 Portola, 1 Alturas, 1 Chico)

  • 3 BTEX hits (Dorris, Chico, Weed)

  • 55/123 (45%) PCE hits (47 in Chico)

  • 23 TCE hits (19 in Chico)

  • 5 1,1,1TCA hits (Chico, Orland, Portola)

  • 2 DBCP hits (Chico and Gridley)


Nested monitor Supply Wells in Northern California


The ‘river recharge’ Supply Wells in Northern California

phenomenon occurs

all along the Central

Valley


N = 606 Supply Wells in Northern California

Analyte

< Reporting Limit

> Reporting Limit

> 50 ppt

>500 ppt

MtBE

450

156

28

3

BTEX

599

71

22

13

PCE

405

201

80

23

TCE

510

96

34

9

CHCl3

249

357

140

26

DBCP

582

24

15

3

All VOCs4

180

426

1 5 samples had all four BTEX compounds, an additional 2 samples had toluene, ethylbenzene and o-xylene or m,p-xylene detections without benzene

2At least one of the BTEX analytes was detected at a concentration > 50 ppt

3One toluene detection of 750 ppt

4Analytes listed in Table 1; sample had <RL for all compounds or >RL for at least one compound


Coastal Basins Supply Wells in Northern California1 N=194

Central Valley Basins2 N=338

No VOCs3

105 (54%)

48 (14%)

MtBE

29 (15%)

87 (25%)

PCE

18 (9%)

166 (49%)

CHCl3

68 (35%)

252 (74%)

11 Includes Santa Clara Valley, Basins of San Mateo County (San Mateo Plain, Westside Basin and Coastside Basin), Livermore-Amador Basin, and Niles Cone (Fremont, CA)

22 Includes Bakersfield, Chico and surrounding northern Sacramento Valley, Sacramento, and San Joaquin County urban areas (Stockton, Lodi, and Manteca); wells from areas outside of alluvial basins are not included on this table

3           3 Samples had <RL for all compounds listed in Table 1


Voc statistics
VOC Statistics Supply Wells in Northern California

  • 51/123 (41%) MtBE hits (30 in Chico); 4 > 51 ppt (2 Portola, 1 Alturas, 1 Chico)

  • 3 BTEX hits (Dorris, Chico, Weed)

  • 55/123 (45%) PCE hits (47 in Chico)

  • 23 TCE hits (19 in Chico)

  • 5 1,1,1TCA hits (Chico, Orland, Portola)

  • 2 DBCP hits (Chico and Gridley)


MtBE Supply Wells in Northern California


Conclusions
Conclusions Supply Wells in Northern California

  • River water is an important source of recharge for a large portion of the study area

  • A large volume of ‘old’ (pre-modern) groundwater is produced at drinking water wells

  • Low-level contamination is widespread in Chico, and reaches deep, old groundwater

  • Paleowater is present in deep aquifers on the western side of the valley


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