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.