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3D Geologic Framework Model Major-ion Chemistry Uncorrected Groundwater Age Mapping Groundwater Quality in a Coastal Southern California Aquifer System Lindsey E. White 1, 2 , Carolyn S. Glockoff 1 , Donald (D.J.) Martin 1 , and Robert Anders 1 1-USGS San Diego WSC, 4165 Spruance Rd., Ste. 200, San Diego, CA 92101 2-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 87 Sr/ 86 Sr 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, 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr 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 different 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) SDBP SDHF SDCP SDNB SDMC SDOT SDEP SDSW SDCC SDOR 5 15 8 117 116 116 30’ 33 32 30’ San Dieguito River Drainage Basin San Diego River Drainage Basin Sweetwater River Drainage Basin River Tijuana Drainage Basin Mexico United States CALIFORNIA Otay River Drainage Basin Area of map Sea level 100 –100 –200 –300 –400 –500 –600 DISTANCE ALONG SECTION, IN METERS ELEVATION ABOVE OR BELOW SEA LEVEL, IN METERS SDBP SDNB SDMC SDOT N-S West Carbon-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 Sea level 100 –100 –200 –300 –400 –500 –600 0 1,000 10,000 11,000 12,000 13,000 14,000 15,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 ELEVATION ABOVE OR BELOW SEA LEVEL, IN METERS 9,000 N-S East SDOR SDCC SDSW SDEP SDHF 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 ELEVATION ABOVE OR BELOW SEA LEVEL, IN METERS SDBP SDNB SDMC SDOT N-S West Strontium (87/86) 0.70747 0.70697 0.70704 0.70738 0.70768 0.70857 0.70726 0.70719 0.70720 0.70630 0.70685 0.70898 0.70853 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 15,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 ELEVATION ABOVE OR BELOW SEA LEVEL, IN METERS 9,000 N-S East SDOR SDCC SDSW SDEP SDHF 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 ELEVATION ABOVE OR BELOW SEA LEVEL, IN METERS SDBP SDNB 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 permil delta Oxygen, in permil ( 18 O/ 16 O) Quaternary Pliocene (SD Formation) Oligocene (Otay Formation) Eocene Cretaceous 0 1,000 10,000 11,000 12,000 13,000 14,000 15,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 ELEVATION ABOVE OR BELOW SEA LEVEL, IN METERS 9,000 N-S East SDOR SDCC SDSW SDEP SDHF -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 permil delta Oxygen, in permil ( 18 O/ 16 O) DISTANCE ALONG SECTION, IN METERS ELEVATION ABOVE OR BELOW SEA LEVEL, IN METERS SDBP SDNB SDMC SDOT N-S West Stiff Diagrams milliequivalent per liter 40 40 30 30 20 20 10 10 0 600 200 0 200 600 1,200 1,200 milliequivalent per liter milliequivalent per liter 10 02 2 4 6 8 4 6 810 milliequivalent per liter 140 100 60 20 20 60 100 140 0 30 20 10 0 10 20 30 milliequivalent per liter Mg ++ Ca ++ Na + + K + Cl - HCO 3 - SO 4 -- EXPLANATION Quaternary Pliocene (SD Formation) Oligocene (Otay Formation) Eocene Cretaceous Sea level 100 –100 –200 –300 –400 –500 –600 Sea level 100 –100 –200 –300 –400 –500 –600 0 1,000 10,000 11,000 12,000 13,000 14,000 15,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 ELEVATION ABOVE OR BELOW SEA LEVEL, IN METERS 9,000 N-S East SDOR SDCC SDSW SDEP SDHF 40 30 20 10 0 10 20 30 40 milliequivalent per liter 40 30 20 10 0 10 20 30 40 milliequivalent per liter 40 50 30 20 10 0 10 20 30 40 50 milliequivalent per liter 40 50 30 20 10 0 10 20 30 40 50 milliequivalent per liter EXPLANATION Stiff Diagrams 40 30 20 10 0 10 20 30 40 milliequivalent per liter Mg ++ Ca ++ Na + + K + Cl - HCO 3 - SO 4 -- In cooperation with the Sweetwater Authority
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