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Mapping Saline Groundwater in a Coastal Southern California Aquifer System Robert Anders1, Danny A

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Mapping Saline Groundwater in a Coastal Southern California Aquifer System Robert Anders1, Danny A Mapping Saline Groundwater in a Coastal Southern California Aquifer System Robert Anders1, Danny A. Cronquist1, 2, Lindsey E. White1, 3, and Wesley R. Danskin1 1–U.S. Geological Survey, CaWSC, 4165 Spruance Road, Ste. 200, San Diego, CA, 92101 H53B-1524 2–San Diego State University, 5500 Campanile Drive, San Diego, CA 92182 3–Naval Facilities Engineering Command, SW, 1220 Pacific Hwy, San Diego, CA 92132 Major-ion Chemistry Stable Isotopes of H and O Extensive data, including lithologic information, geophysical logs, and water- Major-ion composition of groundwater samples collected from 9 multiple-well monitoring sites located in coastal San The stable isotopes of hydrogen and oxygen 0 Seawater quality data have been collected in the San Diego, California area since 2001 to Diego County are presented here using Stiff diagrams. As seen in both the West and East cross sections, much of the can be used to identify the different sources of groundwater in the coastal southern California aquifer system is of poor quality, having high sodium and chloride concentrations. recharge throughout the region. These different Global Meteoric Water Line evaluate the suitability of the San Diego Formation and overlying alluvial depos- –10 SDMC and SDNB have particularly poor quality water, requiring scale increases of 50% and 200%, respectively, in order to depict sources are distinguishable by: (1) isotopic values its for use as a drinking-water supply. Lithologic information was compiled from the high concentrations of sodium and chloride at these sites. comprised of a mixture of groundwater and sea- 100 descriptions of drill cuttings collected during the installation of 9 multiple-well water: (2) lighter (more negative) isotopic values –20 Group 1 SDEP SDCC SDOR that are characteristic of recharge originating in monitoring sites to depths of more than 700 meters and was combined with avail- SDHF SDSW Sea the fractured crystalline rock more than 20 km east Seawater Mixing Line able geospatial data sets, surface geologic maps, and various literature references level and upgradient from the coastline; and (3) interme- –30 to wells and outcroppings to generate a three-dimensional geologic framework diate isotopic values that are characteristic of pre- Group 3 cipitation originating in the coastal plain as the model of the coastal San Diego area. Natural gamma-ray and electromagnetic in- –100 –40 source of recharge. duction (EM) logs were obtained prior to the installation of the monitoring wells DEUTERIUM, IN PER MIL δ Group 2 and during subsequent site visits. EM logs are sensitive to changes in lithology and –200 –50 water quality; because the lithology remains constant, repeated EM logs were used to show changes in water-quality due to natural recharge processes, seawater in- –300 –60 trusion, or other processes. Water samples were collected and analyzed for selected –8 –6 –4 –2 0 δOXYGEN-18, IN PERMIL major- and minor-ions, the stable isotopes of hydrogen, oxygen, and strontium, 405030 20 10 0 10 20 30 40 50 –400 milliequivalent per liter and the radiogenic isotope of carbon-14. Through the use of the geophysical logs, 40 30 20 10 0 10 20 30 40 milliequivalent per liter Na/Ca Molar Ratios water-chemistry data, and cross sections derived from the 3D geologic framework ELEVATION ABOVE OR BELOW SEA LEVEL, IN METERS 40 30 20 10 0 10 20 30 40 –500 milliequivalent per liter N-S East 40 30 20 10 0 10 20 30 40 model, it was possible to map the groundwater quality in the coastal aquifer Stiff Diagrams milliequivalent per liter EXPLANATION The relationship between Na/Ca molar ratios Na+ + K+ Cl− 200 ++ − Ca HCO3 system. In addition, by using sodium-to-calcium molar ratios, in combination Quaternary Oligocene Cretaceous ++ −− Mg SO4 (Otay Formation) and Cl concentrations can be used to distin- Pliocene –600 (SD Formation) Eocene with the isotopic data, the occurrence of saline groundwater as a result of seawa- 405030 20 10 0 10 20 30 40 50 guish between the processes responsible for milliequivalent per liter the occurrence of saline groundwater in a ter intrusion was distinguishable from groundwater in a previously-saline aquifer 100 150 which has been “freshened” by continental recharge. These ndings indicate mul- SDBP coastal aquifer system. For the San Diego area, SDNB the composition of fresh groundwater is repre- tiple groundwater ow paths exist in the coastal aquifer system. SDMC SDOT “Freshening” of a Sea sented by low Na/Ca molar ratios with low to inter- previously-saline level mediate Cl concentrations. In contrast, increasing aquifer Na/Ca molar ratios without a corresponding in- 100 Background –100 crease in Cl concentrations indicate "freshening" of a previously-saline aquifer by continental recharge. Na/Ca ratio Over the past decade, the USGS has been conducting a regional assessment of the groundwa- Finally, Na/Ca ratios approaching seawater with in- Na/Ca MOLAR RATIO for seawater ter resources in the San Diego area. The regional assessment of groundwater resources in the San –200 creasing chloride concentrations indicate the 50 10 0 22468 4 6 810 Diego area was designed as an integrated set of five drainage-basin investigations to evaluate the milliequivalent per liter source of saline groundwater to these wells is the suitability of the San Diego Formation and overlying alluvial deposits for use as a drinking water result of seawater intrusion. Seawater intrusion supply. The San Diego Formation is composed of thinly bedded sandstone and conglomerate, which –300 originated as marine and non-marine sediment during the late Pliocene and early Pleistocene, ranges 10 in thickness from about 30 meters to more than 250 meters, and is overlain by unconsolidated Quater- 10 100 1,000 10,000 100,000 –400 CHLORIDE, IN MILLIGRAMS PER LITER nary alluvium of the Linda Vista and Bay Point Formations. An integral part of the regional assess- 40 30 2010 100 20 30 40 ment has been the installation of 9 multiple-well monitoring sites, each with as many as 6 monitoring milliequivalent per liter 1,200 600 200 0 200 600 1,200 140 100 60 200 20 60 100 140 30 20 10 0 10 20 30 wells, to depths of more than 700 meters. ELEVATION ABOVE OR BELOW SEA LEVEL, IN METERS N-S West milliequivalent per liter milliequivalent per liter milliequivalent per liter –500 Stiff Diagrams Changes in Geophysical Logs and Cl Concentrations EXPLANATION Na+ + K+ Cl− ++ − Ca HCO3 Additionally, the multiple-depth monitoring-well sites have been equipped with real-time, water-level ++ −− Mg SO4 recording equipment and the data is available via the project website. (For more information see- –600 Gamma and electromagnetic induction (EM) logs were obtained prior to the installation of the monitoring wells, and during http://ca.water.usgs.gov/sandiego) 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000 11,000 12,000 13,000 14,000 15,000 subsequent site visits. A natural gamma tool is designed to measure the total intensity of gamma-ray emissions from the for- DISTANCE ALONG SECTION, IN METERS mation. EM logs are sensitive to changes in lithology and water quality; because the lithology remains constant repeated EM logs show changes in water-quality due to natural recharge processes, seawater intrusion, or other processes. Recent in- Uncorrected Groundwater Age creases in EM conductivity and Cl concentrations for two multiple-well monitoring sites (SDNB and SDMC) located along the 3D Geologic Framework Model coast indicate seawater intrusion is occurring within these zones. In contrast, decreases in EM conductivity and Cl concen- Carbon-14 was used to derive the uncorrected groundwater ages from calculations using the percent modern carbon trations indicate zones of “freshening” of a previously-saline aquifer by recently recharged water. Lithologic information was compiled from descriptions of drill cuttings collected at each measured in the various samples. The cross sections show a general trend which depicts groundwater in the Quaternary allu- CHANGE IN ELECTROMAGNETIC CONDUCTIVITY, IN MILLIMHOS PER METER borehole and from observations recorded during drilling. These observations provided direct, vium as less than 10,000 years before present, groundwater in the San Diego Formation as generally less than 20,000 years −200 0 200 400 600 −50 0 50 100 150 SDNB 0 0 SDMC before present, and groundwater in the Otay Formation and the Eocene layer as between 25,000 and 45,000 years before present. 320 reliable data for the geologic boundaries of the subsurface. Additional data from wells, borings and 60 100 100 seismic shot points provided age and depth information. These values were coupled with pre- The presence of modern groundwater in only the shallowest monitoring wells indicates most of the groundwater in the coastal −600 700 aquifer system was recharged many thousands of years ago. 200 200 existing GIS data sets, surface geologic maps, drilling and well logs, the aforementioned direct ob- Chloride 86 servations of lithology, and various literature references to wells and outcroppings to provide the 100 300 300 Seawater input data used by RockWorks modeling software to generate a three-dimensional geologic frame- SDCC intrusion SDHF SDEP SDOR 400 400 work model. Two north-south cross-sections were derived from the 3D geologic framework model, SDSW Sea 500 500 one located along the San Diego Bay (West) and the other one located along the coastal plain away level 9,125 13 6,168 3,847 600 600 from the bay (East). The East and West cross sections were used to map the groundwater
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