mapping groundwater quality in a coastal southern ... · modeling software to generate a...
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3D Geologic Framework Model
Major-ion Chemistry
Uncorrected Groundwater Age
Mapping Groundwater Quality in a Coastal Southern California Aquifer SystemLindsey E. White1, 2, Carolyn S. Glockoff1, Donald (D.J.) Martin1, and Robert Anders1
1-USGS San Diego WSC, 4165 Spruance Rd., Ste. 200, San Diego, CA 921012-Naval Facilities Engineering Command, SW, 1220 Pacific Hwy, San Diego, CA 92132
1. Sodium and chloride concentrations are quite high in many samples, showing poor water quality and potentially hiding other trends in the groundwater chemistry. Some of the Stiff diagrams have similar shapes to that of seawater; however, the SDNB wells suggest more than just seawater intrusion is affecting the ion concentration.
2. The stable isotopes of hydrogen and oxygen indicate the groundwater in the Sweetwater and Otay River basins likely originates in the fractured crystalline rock more than 20 km east and upgradient from the coastline.
3. The 87Sr/86Sr ratios in the shallow part of the aquifer system reflect the isotopic composi-tion of Sr in the rocks and minerals that are present. In contrast, 87Sr/86Sr ratios approach the value of seawater at about the time of deposition of the geologic formation in the deeper part of the aquifer system.
4. The age of groundwater in the coastal aquifer system generally correlates with depth. Water in the Quaternary layer appears to be less than 10,000 years old; water in the San Diego formation is generally less than 20,000 years old; and water in the lower formations (Otay and the Eocene layer) is between 25,000 and 45,000 years old.
Stable Isotopes of H and O
87Sr/86Sr Isotopic Ratios
Major-ion composition of groundwater samples collected from 9 multiple-depth monitoring-well sites located in coastal San Diego are presented here using Stiff diagrams. As seen in both the West and East cross sections, much of the groundwater in the coastal southern California aquifer system is of poor quality, having high sodium and chloride concentrations. SDMC and SDNB have particularly poor quality water, requiring scale increases of 50% and 200%, re-spectively, in order to depict the high concentrations of sodium and chloride at these sites.
Strontium isotopes are useful in detecting mixing among waters of different sources and histories, as well as in char-acterizing the effects of water-rock interaction. The 87Sr/86Sr isotopic ratios for coastal San Diego groundwater ranged from about 0.7060 to 0.7090, with the lowest values from groundwater samples collected from SDMC and the highest values from groundwater samples collected from SDNB and SDBP. The greatest variation of 87Sr/86Sr isotopic ratios with depth was found in the groundwater samples collected from SDSW and SDEP.
The stable isotopes of hydrogen and oxygen can be used to identify the different sources of recharge throughout the region. The lighter isotopic values are characteristic of recharge from the mountains in eastern San Diego County, while the heavier isotopic values are characteristic of local precipitation as the source of recharge. The di�erent sources of recharge are distinguishable in general terms, with more locally recharged water being found at shal-lower depths and more distantly recharged water found at greater depths.
Carbon-14 was used to derive the uncorrected groundwater ages from calculations using the percent modern carbon measured in the various samples. The cross sections show a general trend which depicts groundwater in the Quater-nary layer as less than 10,000 years before present, water in the San Diego Formation as generally less than 20,000 years before present, and water in the lower formations (Otay and the Eocene layer) as between 25,000 and 45,000 years before present.
Extensive data, including lithologic information, geophysical logs, and water quality data have been collected by the U.S. Geological Survey in the San Diego, California area to evaluate the suitability of the San Diego Formation and overly-ing alluvial deposits for use as a drinking water supply. The lithologic informa-tion compiled from descriptions of drill cuttings collected during the installation of multiple-depth monitoring-well sites was combined with pre-existing GIS data sets, surface geologic maps, drillings and well logs, and various literature refer-ences to wells and outcroppings to generate a three-dimensional geologic frame-work model of the coastal San Diego area. The analytical protocol included se-lected major- and minor-ions, the stable isotopes of hydrogen, oxygen, and stron-tium, and the radiogenic isotope of carbon-14. Through the use of the water-quality data and cross sections derived from the 3D geologic framework model, it was possible to map the groundwater quality in the coastal southern California aquifer system.
Over the past decade the USGS has been conducting a regional assessment of the groundwa-ter resources in the San Diego area. An integral part of the regional assessment has been the installation of multiple-depth monitoring-well sites, each with as many as 6 monitoring wells to depths of more than 600 meters. Lithologic information was compiled from descriptions of drill cuttings collected at each borehole and from observations recorded during drilling. These observations provided direct, reliable data for the geologic boundaries of the subsur-face. Additional data from wells, borings and seismic shot points provided age and depth in-formation. These values were coupled with pre-existing GIS data sets, surface geologic maps, drillings and well logs, the aforementioned direct observations of lithology, and various lit-erature references to wells and outcroppings to provide the input data used by RockWorks modeling software to generate a three-dimensional geologic framework model. Two north-south cross-sections were derived from the 3D geologic framework model, one located along the San Diego Bay (West) and the other one located along the coastal plain away from the bay (East). The East and West cross sections were used to map the groundwater quality in the coastal southern California aquifer system.
Additionally, the multiple-depth monitoring-well sites have been equipped with real-time, water-level recording equipment and the data is available via the project website. (For more information see—http://ca.water.usgs.gov/sandiego)
SDBPSDHF
SDCP
SDNB
SDMC
SDOT
SDEP
SDSW
SDCC
SDOR
5
15
8
117 116116 30’
33
3230’
San Dieguito River Drainage Basin
San Diego River Drainage Basin
Sweetwater River Drainage Basin
RiverTijuana
Drainage Basin
MexicoUnited States
CALIFORNIA
Otay RiverDrainage Basin
Area of map
Sealevel
100
–100
–200
–300
–400
–500
–600DISTANCE ALONG SECTION, IN METERS
ELEV
ATIO
N A
BOVE
OR
BELO
W S
EA L
EVEL
, IN
MET
ERS
SDBPSDNB SDMC SDOT
N-S WestCarbon-14 age,
in years before present
1,656
1,024
12,272
30,363
22,960
14,390
19,202
29,154
2,172
14,333
9,095
20,828
4,852
13,375
16,306
27,643
30,202
Quaternary
Pliocene (SD Formation)
Oligocene(Otay Formation)
Eocene
Cretaceous
Sealevel
100
–100
–200
–300
–400
–500
–600
0 1,000 10,000 11,000 12,000 13,000 14,000 15,0002,000 3,000 4,000 5,000 6,000 7,000 8,000
ELEV
ATIO
N A
BOVE
OR
BELO
W S
EA L
EVEL
, IN
MET
ERS
9,000
N-S East
SDORSDCCSDSW
SDEPSDHF
3,847
8,695
17,548
32,055
6,168
21,120
22,520
24,346
26,385
28,824
1,942
13
14,062
18,475
36,701
44,856
9,125
8,188
17,276
12,528
36,029
9,983
13,616
25,102
27,231
29,528
Carbon-14 age,in years before present
DISTANCE ALONG SECTION, IN METERS
ELEV
ATIO
N A
BOVE
OR
BELO
W S
EA L
EVEL
, IN
MET
ERS
SDBPSDNB SDMC SDOT
N-S WestStrontium (87/86)
0.70747
0.70697
0.70704
0.70738
0.70768
0.70857
0.70726
0.70719
0.70720
0.70630
0.70685
0.708980.70853
0.70829
0.70782
0.70770
0.70859
0.70887
0.70893
0.70853
Quaternary
Pliocene (SD Formation)
Oligocene(Otay Formation)
Eocene
Cretaceous
0 1,000 10,000 11,000 12,000 13,000 14,000 15,0002,000 3,000 4,000 5,000 6,000 7,000 8,000
ELEV
ATIO
N A
BOVE
OR
BELO
W S
EA L
EVEL
, IN
MET
ERS
9,000
N-S East
SDORSDCCSDSW
SDEPSDHF
0.70790
0.70790
0.70810
0.70770
0.70824
0.70834
0.70830
0.70853
0.70804
0.70817
0.70804
0.70828
0.70748
0.70650
0.70701
0.70848
0.70697
0.70713
0.70703
0.70678
0.70820
Strontium (87/86)
DISTANCE ALONG SECTION, IN METERS
ELEV
ATIO
N A
BOVE
OR
BELO
W S
EA L
EVEL
, IN
MET
ERS
SDBPSDNB SDMC SDOT
N-S West
−35.8−5.39
−39.8−6.08
−42−6.18
−40.7−6.14
−34.5−4.94−12.8−1.18−16.1−2.28
−36.2−5.58
−39.5−5.66
−32.5−5.13
−35.5−5.54
−32.2−4.72
−37.3−5.48
−45.4−6.6
−42.2−6.48
−35.2−5.51
−34.3−5.25
−24.8−3.76
−40.2−5.99
delta Deuterium, in permildelta Oxygen, in permil
(18O/16O)
Quaternary
Pliocene (SD Formation)
Oligocene(Otay Formation)
Eocene
Cretaceous
0 1,000 10,000 11,000 12,000 13,000 14,000 15,0002,000 3,000 4,000 5,000 6,000 7,000 8,000
ELEV
ATIO
N A
BOVE
OR
BELO
W S
EA L
EVEL
, IN
MET
ERS
9,000
N-S East
SDORSDCCSDSW
SDEPSDHF
−42.4−6.3
−44.2−6.56
−35.8−5.42
−34.3−5.4
−35.3−5.43
−35.2−5.37−37.5−5.52
−36.4−5.59
−44.5−6.67
−46−6.85
−40.7−6.03
−43.8−6.51
−44.2−6.51
−35.7−5.39
−40.1−5.78
−43.4−6.44
−41.2−6.34
−45.8−6.82
delta Deuterium, in permildelta Oxygen, in permil
(18O/16O)
DISTANCE ALONG SECTION, IN METERS
ELEV
ATIO
N A
BOVE
OR
BELO
W S
EA L
EVEL
, IN
MET
ERS
SDBPSDNB SDMC SDOT
N-S WestStiff Diagrams
milliequivalent per liter40 4030 3020 2010 100
600 200 0 200 6001,200 1,200milliequivalent per liter
milliequivalent per liter10 0 22468 4 6 810
milliequivalent per liter140 100 60 20 20 60 100 1400 30 20 10 0 10 20 30
milliequivalent per liter
Mg++ Ca++
Na+ + K+ Cl−
HCO3−
SO4−−
EXPLANATION
Quaternary
Pliocene (SD Formation)
Oligocene(Otay Formation)
Eocene
Cretaceous
Sealevel
100
–100
–200
–300
–400
–500
–600
Sealevel
100
–100
–200
–300
–400
–500
–600
0 1,000 10,000 11,000 12,000 13,000 14,000 15,0002,000 3,000 4,000 5,000 6,000 7,000 8,000
ELEV
ATIO
N A
BOVE
OR
BELO
W S
EA L
EVEL
, IN
MET
ERS
9,000
N-S East
SDORSDCCSDSW
SDEPSDHF
40 30 20 10 0 10 20 30 40milliequivalent per liter
40 30 20 10 0 10 20 30 40milliequivalent per liter
4050 30 20 10 0 10 20 30 40 50milliequivalent per liter
4050 30 20 10 0 10 20 30 40 50milliequivalent per liter
EXPLANATIONStiff Diagrams
40 30 20 10 0 10 20 30 40milliequivalent per liter
Mg++ Ca++
Na+ + K+ Cl−
HCO3−
SO4−−
In cooperation with theSweetwater Authority