In 1993, the Orange County Water District(OCWD) was planning a comprehensivewater quality study of Santa Ana River

(SAR) water prior to and following rechargein the aquifers that comprise the 350-squaremile Orange County groundwater basin innorthern Orange County, California.Considering that more than 60 percent of theapproximately 200,000 acre-feet/year (af/yr)of SAR water that replenishes thegroundwater basin is composed of tertiary-treated wastewater from upstreamcommunities, OCWD wanted to evaluate theoverall quality of this important water source.In addition to SAR water, during the last tenyears OCWD has recharged an average of60,000 af/yr of imported water from theColorado River and northern California tomeet groundwater pumping demands.

Understanding the subsurface flowdynamics of the recharged SAR water wascritical to assessing changes in waterquality with distance from the rechargebasins. Of particular interest was thedelineation of the 1-year travel timeboundary of recharge water, as this criterionis being considered by California’sDepartment of Health Services in its draftrecycled water recharge regulations. Whilehistorical groundwater level measurementsand contour maps provided a reasonableunderstanding of general gradients inOCWD’s recharge area, determination ofdetailed groundwater flow paths andvelocities required more precisemeasurements. OCWD staff consideredusing existing geochemical data butconcluded that they were not sufficientlydistinctive to be useful in delineatinggroundwater flow paths and travel times. In1993, staff of OCWD and LawrenceLivermore National Laboratory (LLNL)met at a water conference and exchangedideas that resulted in a phased project thatused isotopes to characterize subsurface flowconditions in the vicinity of OCWD’s

recharge basins. This article summarizestheir methods and experiences, which aredescribed in more detail by Davisson et al.(1996, 1998, 1999) and Fujita et al. (1998).

Isotope Feasibility StudyAn isotope feasibility study was performedin 1995 by LLNL under contract to OCWD.The purposes of the feasibility study were to:

• Assess whether sufficient oxygen andhydrogen isotopic distinctions existed inthe recharge waters to serve as tracers.

• Develop a baseline groundwater agedistribution to identify potential "fast"recharge flow paths that should beprioritized for frequent monitoring duringthe subsequent tracer testing.

The scope of work consisted of sampling thesurface recharge water (SAR and importedwater) and the existing network of about 30wells for stable isotopes of oxygen (18O/16O)and hydrogen (2H) as well as age-dating the

groundwater samples using the tritium-helium-3 dating method.

Stable Oxygen Isotopic Tracers FingerprintColorado River Water

The 18O/16O ratio in Colorado River rechargewater can be distinguished from SAR waterand ambient groundwater because of thedepletion of 18O in Colorado River waterrelative to local waters (Williams, 1997).This depletion is caused by selectiveremoval or "rain out" of 18O relative to 16O asair masses containing ocean-derivedmoisture move inland. Because OCWD hadnot recharged Colorado River water inalmost two years, the groundwater samplescollected from wells close to the rechargebasins had an 18O/16O signature representativeof local water; however, groundwatersamples collected from a well approximatelyone mile downgradient of the rechargefacilities contained an 18O/16O signature thatwas clearly influenced by the last ColoradoRiver water recharge event. Based on the

Roy L. Herndon, R.G. and Greg D. Woodside, R.G. – Orange County Water District, and M. Lee Davisson and G. Bryant Hudson,Ph.D. – Lawrence Livermore National Laboratory

Figure 1. Groundwater ages (years) from 3H/3He age-dating (300-500 feet depth interval)

distinct oxygen isotopic character ofColorado River water, OCWD and LLNLstaff concluded that a tracer study usingColorado River water to determinegroundwater flowpaths would be feasibleand that this water could be reliably detectedto dilutions as low as 10 percent.

Tritium/Helium Method Provides Groundwater AgesPrior to initiating the tracer test usingColorado River water, groundwater ages atselected wells were estimated by measuringtritium (3H) and helium-3 (3He) concentrations(Schlosser et al., 1988). Because of its 12.4-year radioactive half-life, 3H is a goodchronometer for groundwater that wasrecharged within the last 40 years. Theproblem with attempting to age-dategroundwater using 3H measurements alone isthat atmospheric 3H concentrations havesteadily decreased over approximately thepast 40 years due to radioactive decay,making the estimation of the initial 3Hconcentration in the water at the time ofrecharge imprecise. This uncertainty wasaddressed by quantifying the amount of 3He,3H’s decay product, in each sample. Bysimultaneously measuring 3H and the daugherproduct 3He, known as tritiogenic helium, thetime, T, in years since the groundwater waslast in contact with the atmosphere wascalculated as follows:

T = 17.8 x ln(1 + 3He/3H)

Groundwater mixing within the screenedintervals of the monitoring and productionwells was found to be a major influence onthe age-dating results. Mixing ofgroundwaters that span the 40 years ofatmospheric tritium dispersal creates 3H/3Heages that represent a composite of the mixed"modern" waters. On the other hand, mixingof older, "pre-bomb" groundwaters devoid of3H and 3He with modern groundwaters doesnot affect the age estimate of the moderncomponent of groundwater, even if themodern component constitutes a smallpercentage of the total flow into a well. Formonitoring wells with screened intervals ofless than 20 feet in which mixing wasconsidered insignificant, the estimated 3H/3Hegroundwater ages were assigned anuncertainty of ±2 years. The resultantgroundwater ages were plotted and contouredwithin the study area (Figure 1). The

configuration of these contours indicatedpreferential flow paths of recharge water andhelped to establish a sampling plan for thesubsequent tracer tests.

Colorado River Water Tracer TestIn 1996, a tracer test was performed byrecharging 6,000 af of Colorado River waterat OCWD’s Anaheim Lake recharge basin.While the 18O/16O isotopes of the ColoradoRiver water were the primary "intrinsic"

tracer, LLNL utilized an additional tracer,xenon-124 (124Xe), to investigate the use ofthis stable noble gas as a conservative tracer.The 124Xe-spiked Colorado River water wasrecharged over a period of approximately 50days and then monitored for arrival at a seriesof downgradient wells. The test was repeatedin 1998 using a xenon-129 (129Xe) tracer andprovided results that were generally consistentwith the 1996 124Xe tracer test.

-1000

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02,0004,0006,000

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Figure 2. Tracers introduced in Anaheim Lake demonstrate that recharge water moves deep as itmoves horizontally

See Groundwater Ages, page 36

Tracer arrivals at shallow wells within 100feet of Anaheim Lake occurred withinapproximately two weeks, while arrivals atwells 3,000 feet downgradient occurred atabout four months, indicating averagegroundwater velocities of about 25 feet/day.The detections of Xe tracers and 18O/16O in therecharge water showed consistent arrivaltimes, demonstrating that the Xe isotopesacted as conservative tracers. Tracer arrivalsat depths of 800 feet indicated that recharge

flow paths have a significant verticalcomponent (Figure 2).

Summary of FindingsIsotopic measurements allowed OCWD todelineate groundwater flow paths andvelocities laterally and verticallydowngradient from its recharge basins in acomplex alluvial environment. These methodsprovided a level of detail of flow dynamicsbeyond that of conventional hydraulicgradient and geochemical interpretative

methods. The researchers’ findings aresummarized as follows:

• Stable oxygen isotopic signatures weredistinctive between imported ColoradoRiver water and local recharge water andgroundwater, making Colorado River watera viable tracer.

• Tritium/helium age-dating proved valuablein establishing general groundwater flowcharacteristics and ages prior to conductingthe tracer tests.

• Tritium/helium-derived ages wereestimated to within ±2 years when mixingwas minimal and represented compositeages when mixing of different "modern"waters occurred in long-screened wells.

• Noble gas isotopes such as xenon-124 canbe practically mixed with recharge watersand used as groundwater tracers; however,the manufacture and analyses of thesenoble gas isotopes can be performed byonly a few research laboratories, includingLLNL, making the common application ofnoble gas tracers less practical than thelower-cost oxygen isotopes.

Contact Roy Herndon at RHerndon@ocwd.com

ReferencesDavisson, M.L., G.B. Hudson, R.L. Herndon, S.

Niemeyer, and J. Beiriger, 1996. Report on theFeasibility of Using Isotopes to Source and Age-Date Groundwater in Orange County WaterDistrict’s Forebay Region, Orange County,California. Lawrence Livermore NationalLaboratory Isotope Sciences Division, May 1996(UCRL-ID-123953).

Davisson, M.L., G.B. Hudson, J.E. Moran, S.Niemeyer, and R.L. Herndon, 1998. IsotopeTracer Approaches for Characterizing ArtificialRecharge and Demonstrating RegulatoryCompliance. Annual UC Water Reuse ResearchConference, Monterey, California, June 1998.

Davisson, M.L., G.B. Hudson, R. Herndon, and G.Woodside, 1999. Report on Isotope TracerInvestigations in the Forebay of the OrangeCounty Groundwater Basin: Fiscal Years 1996and 1997. Lawrence Livermore NationalLaboratory, March 1999 (UCRL-ID-133531).

Fujita, Y., J. Zhou, E. Orwin, M. Reinhard, M.L.Davisson, and G.B. Hudson, 1998. Tracking theMovement of Recharge Water After Infiltration.Lawrence Livermore National Laboratory,March 1998 (UCRL-ID-130194).

Schlosser, P. Stute, M. Dorr, H. Sonntag, C.Munnich,O., 1988, Tritium/3He dating of shallowgroundwater. Earth, Planet. Sci. Lett., 89, 353-362.

Williams, A.E., 1997, Stable isotope tracers: naturaland anthropogenic recharge, Orange County,California. Journal of Hydrology, 201 230-248.

Groundwater Ages, continued from page 25