U.S. Department of the Interior U.S. Geological Survey Source, Movement, and Age of Ground Water in a u s Coastal California Aquifer G s
This report is a summary ofisotopic studies EXPLANATION of ground-water source, movement, and age in Santa Clara-Calleguas Ground-water aquifers underlying the Santa Clara- ground-water basin recharge ponds Calleguas basin, Ventura County, California. © Piru It is part of a series summarizing the results of © Saticoy the U.S. Geological Survey's Southern Califor © El Rio nia Regional Aquifer-System Analysis (RASA) study of a southern California coastal ground- water basin. The geologic setting and hydro- logic processes described in this report are similar to those in other coastal basins in southern California.
Introduction
Understanding the contribution of recharge from different sources is important to the management of ground-water supply in coastal aquifers in California especially where water-supply or water-quality problems have developed as a result of ground-water pumping. In areas where water levels have changed greatly as a result of pumping and no longer reflect predevelopment conditions, an analysis of isotopic data can provide informa Figure 1. Santa Clara-Calleguas Hydrologic Unit. tion about the source, movement, and age of ground water that is not readily obtained from a more traditional analysis of ground-water data This information can be used to develop River where ground water discharges at land face water from Piru Creek near Piru and the management strategies that incorporate the surface. In recent years, perennial flow has Santa Clara River near Saticoy and El Rio (fig. availability of natural and artificial recharge to been maintained in some reaches within the 1). In other areas, natural recharge is supple control water-level declines and water-quality Calleguas Creek drainage by the discharge of mented (intentionally and unintentionally) by degradation. treated municipal sewage and irrigation return irrigation return water, discharge of treated water. municipal wastewater, and use of imported water. In this study, the ratios of the stable iso topes of oxygen and hydrogen were used to The Santa Clara-Calleguas Hydrologic determine the source and trace the movement Unit includes the Santa Clara-Calleguas The upper and lower aquifer systems are of ground water in the Santa Clara-Calleguas ground-water basin. The 310-mi2 basin con extensively developed for water supply. In basin, Ventura County, California Tritium and tains a complex system of aquifers that can be many areas, ground-water pumping in excess carbon-14 data were used to estimate the age divided into an upper aquifer system and a of ground-water recharge has resulted in (time since recharge) of ground water. lower aquifer system (see fig. 6, later in this water-level declines greater than 200 feet. As a report). The upper aquifer system consists of result, water supply and water-quality prob alluvial deposits and is generally about 400 lems, such as seawater intrusion and brine Hydrogeology feet thick. The lower aquifer system consists invasion, are important concerns. Because of of alluvial deposits, which grade to marine the large variability in seasonal and annual The 2,010-square-mile (mi2) Santa deposits near the coast and with increasing precipitation, runoff, and streamflow and the Clara-Calleguas Hydrologic Unit, about 60 depth, and is more than 1,000 feet thick in geologic complexity of the aquifer system miles northwest of Los Angeles, has a Medi places. The deposits of the lower aquifer sys there is uncertainty about the sources and terranean climate characterized by warm, dry tem are folded and faulted and crop out in amount of ground-water recharge, and about summers and cool, wet winters. Precipitation some places along the flanks of the mountains the movement of water within the aquifer sys ranges from 14 inches near the coast to more and hills that surround the basin. tems. than 40 inches at higher altitudes in the Topatopa and San Gabriel Mountains. Surface Previous researchers believed that natu drainage is toward the Pacific Ocean through ral recharge to both the upper and lower aqui Source and Movement of Ground Water the Santa Clara River, which drains about fer systems occurred primarily as infiltration 1,600 mi2, and Calleguas Creek, which drains of surface water from larger, perennial streams Oxygen-18 and deuterium are naturally about 400 mi2 (fig. 1). In most areas stream- and that in areas where these streams are not occurring stable isotopes of oxygen and flow is intermittent and occurs only for brief present aquifers were readily recharged by hydrogen. Oxygen-18 and deuterium abun periods after storms. However, streamflow is infiltration of precipitation or infiltration of dances are expressed as ratios in delta notation perennial in the larger tributaries to the Santa runoff in smaller, intermittent streams. Natural (8) as per mil (parts per thousand) differences Clara River and in parts of the Santa Clara recharge is supplemented by diversion of sur relative to the standard known as Vienna Stan- dard Mean Ocean Water (VSMOW). By con ing of freshwater with seawater or other saline recharged by direct infiltration of precipitation vention the value of VSMOW is 0 per mil. water is not shown in figure 3.) Water in these or infiltration of runoff in small streams that areas was recharged by infiltration of surface drain the area. Most of the world's precipitation origi water from the Santa Clara River that origi nates from the evaporation of seawater. As a nated largely as runoff from the higher alti No large areas of isotopically heavy result, the oxygen-18 (818O) and deuterium tudes of the Topatopa and San Gabriel water were present in the upper or lower aqui (8D) composition of precipitation throughout Mountains. fer systems underlying the Oxnard Plain. the world are linearly correlated and distrib These data suggest that the amount of ground- uted along a line known as the meteoric water Water from wells in Las Posas Valley, water movement from Las Posas Valley, Pleas line (fig. 2). The 818O and 8D composition of Pleasant Valley, and near the flanks of the ant Valley, and near the flanks of the moun a water sample relative to the meteoric water mountains north of the Santa Clara River had tains to aquifers underlying the Oxnard Plain line and relative to the composition of water 8D values greater than -45 per mil and was is small. from other areas provides a record of the isotopically heavier than water from most source and evaporative history of the water wells in the Santa Clara River valley or Water from some wells in the upper and can be used as a tracer of the movement of Oxnard Plain. Water in these areas was the water. aquifer system near the northeast corner of the Oxnard Plain have an intermediate isotopic UPPER AQUIFER SYSTEM composition (8D values between -45 and -50 The 818O and 8D composition of water ("perched aquifer" omitted) 119° 118°45' per mil) (fig. 3). As a group, water samples from almost 240 wells in the basin ranged ~i- f'JS-rrr from wells in this area were affected by evapo from -8.85 to -1.75 and -62.5 to -9.5, respec T4N tively (Izbicki and others, 1995). The isotopi- ration and plot to the right of the meteoric cally heavier (less negative) waters result from water line. These samples were collected near the mixing of freshwater with seawater or the end of an extended drought when recharge other saline water. The source of these waters from the Santa Clara River was scant. In this has been discussed by Izbicki (1996) and will area, there are no thick clay layers separating the upper aquifer system from the surface; not be discussed further in this report. Many of T2N and, therefore, irrigation return water can infil trate to the water table. Isotopically heavier 0 Native freshwater water is not present in the upper aquifer sys -10 o Freshwater - Imported from N. Calif. tem farther away from the recharge area High-chloride water - Chloride =d'20 concentration >250 mg/L because low-permeability clay deposits under Water recharged by LOWER AQUIFER SYSTEM lying the Oxnard Plain isolate the aquifers s. infiltration of water from 118°45' a:-30 f the Santa Clara River from land surface. ULJ O- and its larger T4N s-40 r tributaries -Water recharged by infiltration °-50 A$ of precipitation or runoff in Isotopically light water (less than -50 per .a^ small streams mil) in the upper aquifer system in parts of Las -60 Posas and Pleasant Valleys near streams (fig. 3) is the result of recharge by treated -70 -9 -8 -7 -6 -5 -4 -3 -2 -1 municipal wastewater. The wastewater origi 8180, IN PER MIL nated as water imported from northern Cali fornia for public supply. This water is Figure 2. Delta deuterium (SD) as a isotopically lighter than water from local pre function of delta oxygen-18 (8f8O) in 34°07'30 cipitation and plots along a line parallel to but water from wells. R22W below the meteoric water line (fig. 2). North ern California water also was present in parts 0 510 KILOMETERS of the Santa Clara River valley, but its pres ence is less obvious because native water in the remaining differences in the isotopic com EXPLANATION position of ground water are the result of dif the Santa Clara River valley is lighter than UNCONSOLIDATED DEPOSITS- water in other parts of the basin. ferences in the temperature of condensation Santa Clara-Calleguas (dew point) of precipitation in different parts ground-water basin The lightest water (less than -60 per mil) of the basin. Water recharged from precipita UNCONSOLIDATED DEPOSITS- tion (or resulting runoff and streamflow) that Outside Santa Clara-Calleguas in the basin is from deep wells at the end of condensed at higher altitudes and lower tem ground-water basin long flow paths in aquifers underlying the peratures is isotopically lighter (more nega CONSOLIDATED ROCKS Oxnard Plain. The isotopic composition of this tive) than water recharged from precipitation water reflects a cooler temperature of conden 5 D COMPOSITION- sation and cooler climatic conditions than that condensed at lower altitudes and higher Water from unconsolidated deposits temperatures. Most other observed differences those found in the basin today. It is possible in the isotopic composition of sampled waters > -45 per mil that these waters are very old and were are the result of evaporation, the use of -45 to -50 per mil recharged many thousands of years ago. imported water, or (for water recharged many < -50 per mil thousands of years ago) long-term changes in HYDROLOGIC UNIT BOUNDARY climate. STREAM Age of Ground Water WELL Water from most wells in the Santa Clara MULTIPLE-WELL SITE Naturally occurring radioactive isotopes River valley and the upper and lower aquifer of hydrogen (tritium) and carbon (carbon-14) systems underlying the Oxnard Plain had 8D were used to determine the age (time since values less than -50 per mil (fig. 3). (The 8D Figure 3. Delta deuterium (8D) recharge) of ground water. Tritium was used to composition of water resulting from the mix composition of water from selected wells. identify recently recharged (post 1952) ground water and carbon-14 was used to estimate the upper and lower aquifer systems are raised by Tritium was not present in water from age of older ground water. hydraulic pressure from this recharge water. most wells in Las Posas Valley and Pleasant Valley, in deep wells at multiple-well sites in the Santa Clara River valley, and in deep wells Recently Recharged Water Tritium was present in the upper aquifer system near streams in Las Posas Valley. On in most parts of the Oxnard Plain. These data show that water in these areas was recharged Tritium (3H) has a half-life about 12.4 the basis of the 818O and 8D isotopic composi tion, this water was identified as water prior to 1952. Carbon-14 data were used to years and is measured in tritium units (TU) determine the age of this older water. each tritium unit equals one tritium atom in imported from northern California for public 1018 atoms of hydrogen. Prior to 1952, the tri supply and discharged as treated municipal tium concentration of precipitation in coastal wastewater. Small amounts of tritium also Older Water southern California was about 2 TU. Begin were present in water from wells in the upper ning in 1952, tritium was released to the atmo aquifer system near the coast as a result of sea- Carbon-14 (14C) has a half-life of about sphere as a result of the atmospheric testing of water intrusion in those areas (Michel and oth 5,730 years and is measured as percent mod nuclear weapons, and the tritium concentration ers, in press). ern carbon by comparing carbon-14 activities of precipitation increased (fig. 4). Because tri to the radioactivity of National Bureau of tium is part of the water molecule, tritium con 119° 118°45' Standards oxalic acid prepared in 1950 12.88 disintegrations per minute per gram of centrations are not affected significantly by T4N reactions other than radioactive decay. Tritium carbon equals 100 percent modern carbon. is an excellent tracer of the movement of water Carbon-14 is a tracer of the movement and rel on time scales ranging from 10 to less than 34°22130" ative age of ground water on time scales rang 100 years before present. In this report, ground ing from recent to more than 20,000 years water having tritium concentrations less than before present. the detection limit of 0.3 TU is interpreted as water recharged prior to 1952. Ground water Carbon-14 activities for water from 34 having measurable tritium is interpreted as wells in the study area ranged from 89.4 to less water recharged after 1952. than the detection limit of 1 percent modern carbon (Izbicki and others, 1995). Carbon-14 10 MILES Tritium concentrations in water from R22W activities were higher in areas where ground almost 150 wells sampled as part of this study 10 KILOMETERS water contained tritium and lower near the ranged from 9.4 TU to less than the detection downgradient ends of long flow paths through EXPLANATION the lower aquifer system. limit of 0.3 TU (Izbicki and others, 1995). (See fig.3 for base map explanation) Ground water containing tritium was present TRITIUM (<0.3TU) - Carbon-14 is not part of the water mole throughout large areas of the upper aquifer Upper aquifer system system underlying the Santa Clara River val cule, and in addition to radioactive decay, ley and parts of the Oxnard Plain near the Lower aquifer system carbon-14 activities are affected by chemical Santa Clara River (fig. 5). Between 1952 and J*' LINE OF SECTION reactions that occur between dissolved constit 1993 about 1.8 million acre-feet of water was uents and the material that composes the aqui WELL fer. For example, in the Santa Clara-Calleguas recharged in ponds at Saticoy and El Rio. By MULTIPLE WELL-SITE 1991, this water was present throughout much basin, oxidation of organic material (that does of the upper aquifer system underlying the not contain carbon-14) within the aquifer Oxnard Plain and had moved more than 6 Figure 5. Tritium in water from selected deposits during the reduction of sulfate to miles downgradient from the recharge area but wells. hydrogen sulfide can reduce the carbon-14 had not yet reached the coast. Ground water activity of water from wells. Izbicki and others containing tritium was present only in small (1992) and Izbicki and Martin (written areas of the lower aquifer system near Saticoy A' commun., 1996) evaluated the mass transfer of El Rip Saticoy carbon, sulfur, and other elements between the and El Rio (fig. 6). These data suggest that spreading spreading infiltration of surface water in ponds at these grounds grounds solid and dissolved phases and the subse locations is more effective at recharging the quent changes in carbon-13 (813C), sulfur-34 upper aquifer system than the lower aquifer (834S), and carbon-14 composition of ground system. However, water levels throughout the water using the computer program NET- PATH (Plummer, and others, 1994). Inter preted carbon-14 ages ranged from recent (less than 50 years before present) for water from wells near sources of ground-water recharge along the Santa Clara River to more than VERTICAL SCALE GREATLY EXAGGERATED 20,000 years before present (fig. 7). 0 4 MILES 4 KILOMETERS Changes in ground-water ages with EXPLANATION depth (figs. 6 and 7) were evaluated on the CONSOLIDATED J WELL- Number is basis of samples collected from 19 wells at 8 ROCKS interpreted 14 Cage, in thousands of years sites that were screened in individual aquifers 1950 1960 1970 1980 1990 | | UPPER SYSTEM <7-5 i before present (Densmore, 1996). Interpreted carbon-14 ages YEAR LOWER SYSTEM
Isotopic data show that recent recharge Densmore, J.N., 1996, Lithologic and ground- occurs primarily as infiltration of surface water data for monitoring wells in the water from large streams. Water from these Santa Clara-Calleguas ground-water basin, Ventura County, California, 1989- sources does not effectively recharge the lower 95, U.S. Geological Survey Open-File aquifer system or aquifers in parts of the basin Report 96-120,179 p. that are distant from these large streams. In Densmore, J.N., Middleton, G.K., and Izbicki, some areas recharge occurs as infiltration of J.A., 1992, Surface-water releases for precipitation or as infiltration of runoff in ground-water recharge, Santa Clara smaller, intermittent streams (especially in River, Ventura County, California, in areas where flow in these streams has been Hermann, Raymond, ed., Managing increased by discharge of treated municipal Water Resources During Global Change: American Water Resources Association, wastewater). However, in most areas where p. 407-416. aquifers are distant from large streams ground Izbicki, J.A., Michel, R.L., and Martin, Peter, water does not contain tritium and, on the 1992, 3H and 14C as tracers of ground basis of carbon-14 data, is very old. water recharge, in Engman, Ted, Saving 0 5 10 KILOMETERS a threatened resource In search of solu Results of a recently completed U.S. tions: American Society of Civil Engi EXPLANATION neers, IR Div/ASCE, p. 122-127. (See fig. 3 for base map information) Geological Survey simulation-optimization study of alternative management strategies Izbicki, J.A., Martin, Peter, Densmore, J.N., Existing well Clark, D.A., 1995, Water-quality data for o Multiple-well site - (Reichard, 1995) showed that ground-water the Santa Clara-Calleguas Hydrologic resources in the Santa Clara-Calleguas basin Unit, Ventura County, California, Octo Perforated interval Sea level could be used more efficiently by increasing No data ber 1989 through December 1993: U.S. artificial recharge programs and reducing Geological Survey Open-File Report 95- Number is inter <7.S preted carbon-14 Increments of 500, pumping from the lower aquifer system. Local 315,125 p. age of water in -1,000 in feet below Izbicki, J.A., 1996, in press, Seawater intru thousands of years sea level agencies responsible for ground water man before present 23'4 sion in a coastal California aquifer: U.S. 1,500 agement are increasing recharge to the upper aquifer system by releasing water for ground- Geological Survey Fact Sheet FS-125- 96. Figure 7. Age of water from selected water recharge to the Santa Clara River from wells. Michel R.L., Busenberg, E., Plummer, L.N., Lake Piru (Densmore and others, 1992), Izbicki, J.A., Martin, Peter, and Dens increasing the capacity of facilities used to more, J.N., in press, Use of tritium and supplement natural recharge through the diver chlorofluorocarbons to determine the depth confirm that the deeper aquifers of the sion of surface water, and injecting water in rate of seawater intrusion in a coastal lower aquifer system are largely isolated from the lower aquifer system and other parts of the aquifer: Proceedings of the 16th Seawa overlying aquifers and surface sources of basin where present day recharge is scant ter Intrusion Meeting, Caligari, Italy. ground-water recharge. (Ventura County Public Works Agency, 1995). Plummer, L.N., Prestemon, E.C., and Parkhurst, D.L., 1994, An interactive Local agencies also are proposing to reduce code (NETPATH) for modeling NET Water from most wells sampled in the pumping from the lower aquifer system geochemical reactions along a flow Pleasant Valley and Las Posas areas also is rel through a combination of water conservation PATH, version 2.0: U.S. Geological Sur atively old, reflecting the smaller quantities of measures, a shift in pumping to the upper aqui vey Water-Resources Investigations recharge available to aquifers underlying these fer system, and delivery of surface water for Report 94-4169 130 p. areas in comparison with aquifers underlying agricultural supply to replace ground-water Reichard, E.G., 1995, Groundwater-surface the Santa Clara River valley or the Oxnard pumping (Ventura County Public Works water management with stochastic sur Plain. However, water from the wells sampled Agency, 1995). Information obtained from the face water supplies: A simulation opti U.S. Geological Survey RASA study of south mization approach: Water Resources in Pleasant Valley is not as old (and not as iso Research, v. 31, no. 11, p. 2845-2865. lated from sources of recharge) as water from ern California coastal basins will aid in the Ventura County Public Works Agency, 1995, wells sampled in the western part of Las Posas development of those projects and in the effec Annual Report Fox Canyon Groundwa- Valley or from some of the deeper wells sam tive management of the ground-water ter Management Agency, Ventura Cali pled in the Oxnard Plain. resources of the Santa Clara-Calleguas basin. fornia, 32 p.
For additional information on this For information on the southern For information on other RASA report contact the author: California RASA study contact the studies contact the program director: John A. Izbicki project chief: Hayes Grubb 5735 Kearny Villa Road, Suite O Peter Martin 8011 CameronRoad San Diego, CA 92123 5735 Kearny Villa Road, Suite O Austin, TX 78754 (619)637-6831 San Diego, CA 92123 (512)873-3000 e-mail: [email protected] (619)637-6827 e-mail: [email protected] e-mail: [email protected]
July Fact Sheet 126-96 GPO 786-714/59153