an ecological risk assessment for chlorpyrifos in an agriculturally dominated tributary of the san...

An Ecological Risk Assessment for Chlorpyrifosin an Agriculturally Dominated Tributaryof the San Joaquin River

Nicholas N. Poletika,1* Kent B. Woodburn,2 and Kevin S. Henry2

Single-species toxicity testing of ambient water samples and national-scale probabilistic riskassessment have implicated the organophosphorous (OP) insecticide chlorpyrifos (O,O-diethyl O-(3,5,6-trichloro-2-pyridyl)-phosphorothioate) as a potential chemical stressor ofaquatic organisms residing in the lower San Joaquin River basin. This site-specific aquaticecological risk assessment was conducted to determine the probability of adverse effectsoccurring from exposure to chlorpyrifos in an agriculturally dominated tributary of the SanJoaquin River and to assess the ecological significance of such effects. Assessment endpointswere fish population persistence and invertebrate community productivity. Daily chemicalmeasurements collected over a period of one year were analyzed temporally for frequency,duration, and spacing between events for acute and chronic exposure episodes. Effectsthresholds for fish and freshwater lotic invertebrates were determined from single-specieslaboratory toxicity tests. Potential risk was characterized by the degree of overlap ofdistributions of exposure events and effects, with consideration given to additive toxicity ofother OP insecticides, recovery periods, and duration of chronic exposure ( 21 d).Ecological significance was determined by analysis of fish assemblage dietary andreproductive habits in relation to the surrogate invertebrate taxa judged at risk. Results ofanalysis indicated no direct effects on fish, and indirect effects on fish through elimination ofinvertebrate food items were considered unlikely. Biological survey information will benecessary to address uncertainty in this risk conclusion, especially as it relates to the benthicinvertebrate community. Results of this site-specific risk analysis suggest that fish populationpersistence and invertebrate community productivity were not adversely affected bymeasured chlorpyrifos residues during a year-long monitoring period.

KEY WORDS: Chlorpyrifos; diazinon; aquatic risk assessment; ecological significance; agriculturallydominated tributary

1. INTRODUCTION

1.1. Background

Chlorpyrifos is a widely used agricultural OPinsecticide. Although only one freshwater aquatic

incident report has been compiled by the U.S.Environmental Protection Agency (U.S. EPA) inthe 30 years this product has been marketed foragricultural use,(1) toxicity testing in California(2) andpreliminary risk assessments for U.S. EPA pesticidereregistration(3) have suggested the potential foradverse ecological effects of chlorpyrifos in aquaticsystems. A recent probabilistic risk assessment forchlorpyrifos in North American aquatic ecosystems

1 Dow AgroSciences, Indianapolis, IN.2 The Dow Chemical Company.* Address correspondence to Nicholas N. Poletika, Dow Agro-

Sciences LLC, 9330 Zionsville Road, Indianapolis, IN 46268.

Risk Analysis, Vol. 22, No. 2, 2002

291 0272-4332/02/0400-0291$22.00/1 2002 Society for Risk Analysis

has addressed this potential problem on a nationalscale.(3) The conclusions of the assessment were that,overall, chemical monitoring data in freshwatersystems do not suggest ecologically significant risks,except in a few locations. The authors recommended,however, conducting site-specific risk assessments forthese few locations to refine the risk characterizationand to reduce the uncertainty associated with thenational assessment. One area identified as possiblyexperiencing risk was a region in the San JoaquinValley of California with heavy chlorpyrifos use thatcontributes agricultural drainage to the perennialreach of the San Joaquin River. The tributaries inthis region thus appeared to be good candidates for asite-specific ecological risk assessment for the stres-sor chlorpyrifos.

1.2. Objectives

The primary objectives of this risk assessmentwere (1) to estimate the probability of chlorpyrifosuse to cause adverse effects in various taxa ofaquatic organisms in an agriculturally dominatedtributary of the San Joaquin River, and (2) todetermine the ecological relevance of any predictedrisk in the local aquatic community. Ideally, detailedinformation on exposure and effects would becoupled with site-specific biological and habitatsurvey information in performing this type ofassessment. Intensive chemical monitoring data areavailable for a site where significant chlorpyrifos useoccurs within the San Joaquin Valley,(4) and thenational-scale assessment provides an excellent setof effect endpoints, derived from laboratory toxicitytests and microcosm/mesocosm studies, which aregenerally applicable to theoretical assemblages ofvertebrate and invertebrate freshwater organisms.(3)

Unfortunately, little biological survey work has beenconducted in the agricultural regions of California,(5)

nor has biocriteria development progressed to thepoint where established indices of biological integ-rity are available to interpret the effects of varioustypes of human activity on California surfacewaters.(6)

2. PROBLEM FORMULATION

2.1. Site Description

Detailed chemical monitoring, stream flow, andweather data are available for the lower reach ofOrestimba Creek,(4) an agriculturally dominated

natural drainage in Stanislaus County, California(Fig. 1). Orestimba Creek originates in the CoastRange of mountains in western Stanislaus County,passes through irrigated farmland in the San JoaquinValley at elevations of 66 to 20 m above sea level,and terminates at its confluence with the SanJoaquin River. The most important crops in thestudy area receiving insecticide applications arealfalfa, walnuts, almonds, and dry beans.

Recent assessments of fish communities in theNational Water Quality Assessment (NAWQA)Program San Joaquin-Tulare Basin study unit wereperformed by the U.S. Geological Survey (U.S. GS)in conjunction with habitat surveys and waterchemistry determinations.(5) The San Joaquin MainStem group was characterized by high specificconductance (high salinity), decreased fish cover,and high percentage of agricultural land. Largepercentages of introduced fish species tolerant ofaltered environmental conditions were found atthese sites: fathead minnow (Cyprinidae: Pimiphalespromelas), red shiner (Cyprinidae: Cyprinellalutrensis), threadfin shad (Clupeidae: Dorosoma

N

0 10 Kilometers

0 100 Kilometers

StanislausCounty

SanJoaquinR

.

San J

oaqu

inR.

Orestimba C

r.

AgriculturallyDominated Reach

Fig. 1. Location of study area.

292 Poletika, Woodburn, and Henry

petenense), and inland silverside (Atherinidae: Men-idia beryllina). Minor components of the SanJoaquin Main Stem group include, in order ofdecreasing importance, other introduced species,largemouth bass and sunfish, catfish, native species,and smallmouth bass. Incidence of external abnor-malities (parasites and lesions) in San Joaquin MainStem resident fish individuals averaged 17%, indi-cating impaired conditions may exist.(5)

The benthic invertebrate species assemblagespecific to Orestimba Creek is unknown. However, arecent paper by Brown and May(7) examined benthicinvertebrates in lotic systems in the lower Sacra-mento and San Joaquin river drainage basins. Theyfound that tributaries have statistically greater spe-cies richness than the main stem rivers and drainagewatersheds (Orestimba Creek and others, N 7), adifference likely due to land-use practices. Theauthors report that the predominate benthic inverte-brate taxa in the drainage watersheds are Odonata,Oligochaeta, Ephemeroptera, and Diptera, and thatthe drainage watersheds have substrates of a finerparticle size.

2.2. Conceptual Model

In view of the site description given above, wedecided to evaluate the risk of chlorpyrifos use inthe agriculturally dominated reach of OrestimbaCreek (Fig. 1) for ecologically significant adverseeffects on the principal components of the SanJoaquin Main Stem fish assemblage. It would bepreferable to focus the assessment on native speciesof concern across the entire Sacramento-San Joa-quin River system rather than on these introducedspecies tolerant of altered environmental condi-tions. However, there is no site-specific informationavailable to determine whether particular nativespecies inhabited the lower reach of OrestimbaCreek before commercial agriculture was intro-duced or whether the current habitat would supportnative fish of concern should restoration beattempted.

Important invertebrate components of fishdiets were identified from the literature andassumed to be present in the creek for riskcharacterization using surrogate species in thetoxicity database. Thus, both direct effects on fishand invertebrates and indirect effects on fishthrough impact on their diet were evaluated. Basedon the toxicity profile of chlorpyrifos(8) and thereview by Giesy et al.,(3) effects on aquatic plants

and microorganisms were judged unlikely to occur.The toxicity data also indicate that, as a taxonomicgroup, aquatic invertebrates are more sensitive tothe broad-spectrum insecticide chlorpyrifos thanare fish; therefore, for a given exposure concentra-tion, more severe direct effects were expected withinvertebrate organisms.

2.3. Assessment Endpoints and Measures of Effect

The ecological entities of value in this assess-ment were fish and benthic invertebrates (plank-tonic invertebrates were assumed to be minorcomponents of this lotic system). Ecologically rele-vant characteristics of these valued entities requir-ing protection were fish population persistence andinvertebrate community productivity. Indicators ofchlorpyrifos effects included the concentration caus-ing death in 50% of a population of organisms(LC50), and, for chronic effects, the no-observed-effect concentration (NOEC). The effect endpointsdetermined in Giesy et al.,(3) as discussed below inSection 4.3, were used as measures of adverseimpact.

2.4. Analysis Plan

The analysis plan for the assessment consistedof the following elements: stressor characteriza-tion, exposure analysis, effects analysis, riskcharacterization, and analysis of uncertainty andvariability. Stressor characterization includeddescriptions of chlorpyrifos physicochemical prop-erties, environmental fate, and mechanism ofaction. Reviews of chlorpyrifos properties andfate(9) and mechanism of toxicity(3) were brieflysummarized.

Exposure analysis focused principally on chlor-pyrifos use in the watershed and the temporalpatterns of chemical pulses (frequency, duration,and intervals between pulses). Previously reportedOP insecticide dissolved concentrations present indaily time-proportional composite samples collectedfor the period of May 1996 through April 1997(4)

were examined with the data analysis computerprogram RADAR (Risk Assessment Tool to Evalu-ate Duration and Recovery) (Waterborne Environ-mental, Inc., Leesburg, VA). Using the dailycomposite concentration data, exposure events wereidentified by RADAR in terms of exceedance of athreshold concentration value appropriate for acuteor chronic effects.

Risk Assessment for Chlorpyrifos 293

Effects analysis evaluated effects on individualspecies, populations, and communities (fish andinvertebrate). Several measures of effects associatedwith acute and chronic exposure were compared tothe pattern of exposure events identified byRADAR to determine which endpoints wereappropriate.

Risk characterization related the previous ana-lysis to the ecologically relevant assessment end-points of fish population persistence andinvertebrate community productivity. Where appro-priate, additive toxicity from co-occurring OP insec-ticides was also considered. Surrogate speciespresent in the laboratory toxicity database wereselected to represent fish species identified byNAWQA as components of the San Joaquin MainStem group fish assemblage. Invertebrate food itemsconsumed by fish were identified from publisheddescriptions of fish feeding habits. Invertebratesdocumented to be important in fish diets wererelated to rank-order distributions of chemicalsensitivity to determine whether a tested taxon (orclosely related surrogate) was likely to be affected atthe observed exposure levels.

3. ANALYSIS

3.1. Stressor Characteristics

3.1.1. Physicochemical Properties andEnvironmental Fate

Chlorpyrifos is moderately lipophilic, whichlimits its water solubility (1.4 mg L1) and explainsits tendency to partition (log Kow of 4.75.3) intosoil, sediment, and organic material.(9) The dissipa-tion of chlorpyrifos in water is relatively rapid, withhalf-lives on the order of days, while dissipation ratesin soil and sediment are only moderate (half-lives ofweeks). When used at agricultural application rates,residues of chlorpyrifos do not accumulate over timein soil/sediment systems. The principal degradate ofchlorpyrifos is 3,5,6-trichloro-2-pyridinol (TCP).TCP is considerably more polar than the parentchlorpyrifos molecule and is therefore more hydro-philic.

3.1.2. Mechanism of Action

Toxicity of chlorpyrifos results from metabolicactivation to form the chlorpyrifos oxon, whichinactivates the neurotransmitter acteylcholinest-erase at neural junctions. Inactivation occurs

through reversible phosphorylation of the enzymeactive site. Enzyme inactivation exerts toxicity byoverstimulation of the peripheral nervous system.Acute toxicity of OP insecticides co-occurring in thewater column appears to be adequately explained bya simple additive model.(10) The TCP metabolitedoes not contain the phosphorous-dialkyl ethermoiety and cannot phosphorylate the target enzymeactive site. Consequently, TCP does not exhibit OPinsecticide activity, and TCP was not considered achemical stressor in this risk assessment. Thisjudgment is supported by the toxicity data availablefor TCP, where the lowest reported LC50/EC50 valueis 1,800,000 ng L1.

3.2. Exposure Data

3.2.1. Insecticide Use

Previous monitoring(2,11,12) in the perennialreach of the San Joaquin River identified three OPinsecticides as important contributors of residues tosurface water, especially during the dormant tree app-lication period (DecemberFebruary): chlorpyrifos,diazinon (O,O-diethyl-O-(2-isopropyl-4-methyl-6-pyrimidinyl) phosphorothioate), and methidathion(O,O-dimethylphosphorodithioate, S-ester with4-(mercaptomethyl)-2-methoxyD2-1,3,4-thiadiazolin-5-one). Monitoring conducted by Poletika et al.(4) fromMay 1, 1996 to April 30, 1997 generated daily data setsfor each of these compounds, although a staggeredmethod validation process limited analysis for resi-dues of diazinon and methidathion to days21364 and 56364, respectively.

3.2.2. Daily Chemical Monitoring in Water:May 1996April 1997

ISCO Model 2700 autosamplers (ISCO, Inc.,Lincoln, NE) were used to collect hourly samplescomposited over 24 hours. Data used in the riskassessment came from a sampling location at RiverRoad, near the confluence with the San JoaquinRiver. Methods of sample handling, extraction,analysis, and quality control were previouslyreported.(4) Method detection limits were 10, 10,and 24 ng L1, for chlorpyrifos, diazinon, andmethidathion, respectively. Individual chemical datawere combined and plotted on the same graph toshow the overall detection patterns for the threemonitored OP insecticides (Fig. 2). The patterns arecharacterized by numerous, moderate chlorpyrifos


peaks, fewer, larger diazinon peaks, and few, smallmethidathion peaks.

3.2.3. Exposure in Sediment

No sediment data were collected during thisstudy period. However, bed sediment samples col-lected near River Road in October 1992 andanalyzed by the U.S. GS contained

The 10th centile single-point estimate of spe-cies acute sensitivity was selected as a benchmarkto evaluate exposure profiles that could impact themost sensitive organisms in the tail of the distribu-tion and direct further analysis of indirect effectson taxa higher in the food chain. Previous prob-abilistic aquatic ecological risk assessments haveused the 10th centile benchmark in a similarmanner.(3,15)

Bluegill sunfish (Lepomis macrochirus) was themost sensitive fish species, with a 48-h LC50 value of4800 ng L1. The 48-hour period was viewed as atypical maximum exposure window, similar to thepattern found in midwestern U.S. stream chemicalmonitoring data.(3) We assumed initially that the48-hour exposure timeframe would also be applic-able to Orestimba Creek and later confirmed thevalidity of the assumption through temporal analysisof exposure data.

The freshwater organism acute toxicity databaseused in this site-specific risk assessment was subdi-vided in the following manner. First, due to therelative insensitivity of fish, and to a greater extent,plants, only aquatic invertebrates were considered.Second, each taxon in the database was evaluatedfor habitat requirements with respect to lentic andlotic environments. Grouping was based on descrip-tions in the literature(1618) and professional judg-ment. Tables I and II list the freshwater invertebratespecies sensitivity distributions resulting from thissubdivision. Note that where an animal inhabits bothstill and moving water, the species appears in bothtables. Table II is referenced as a listing of surrogatespecies for invertebrates likely to be found inOrestimba Creek.

Fig. 3 presents the rank-ordered (i1 . . . in)cumulative freshwater invertebrate species sensitiv-ity distributions for the lentic and lotic groups,plotted using the formula i/n +1 100.(19) Also givenin Fig. 3 are the best-fit linear regressions and 10thcentile sensitivity estimates calculated from theregression equations. Compared to the all-species(vertebrates and invertebrates) 10th centile sensitiv-ity of 102 ng L1, the equivalent 10th centile valuewas lower for the lentic group of invertebrates, 35 ngL1, and higher for the lotic invertebrate group, 177ng L1. As shown in Table I, the sensitivity of thelentic group was greatly influenced by the inclusionof mosquito larvae and cladocerans, taxa unlikely tobe found in Orestimba Creek. The most sensitivelotic species include amphipods, a mysid, midges,and mayflies (Table II).

Giesy et al.(3) used reported sediment acutetoxicity data for a midge (Chironomus tentans) andan amphipod (Hyalella azteca) and equilibriumpartitioning methods to determine whole-sedimentchlorpyrifos concentration ranges having potentialfor adverse effects. The analysis suggests that effectsare not probable at dry weight sediment concentra-tions 500 lg kg1.Applying the local estimated Kd value of 379lg mL1 to these whole-sediment concentrationranges for potential adverse effects, the equivalentpore-water concentrations are 1; 320 ng L1, respectively.

Fewer data are available to describe chlorpy-rifos chronic toxicity endpoints. Chronic toxicity ininvertebrates has been observed as reproductiveinhibition in two sensitive species, the freshwaterdaphnid, Daphnia magna, and the saltwater mysid,Mysidopsis bahia.(8) Life-cycle studies (exposureperiods of 21 to 35 d) report ranges of lowest-observed effect concentration (LOEC) of 100 to300 ng L1 for D. magna and 4 to 10 ng L1 forM. bahia. Reported LOECs for saltwater andfreshwater fish evaluated in early life stage studies(flow-through exposures of 28 to 32 d) range from480 ng L1 with the Atlantic silverside, Menidiamenidia, to 3,200 ng L1 with the fatheadminnow, Pimephales promelas.(8) These findings,combined with results from two full life-cyclestudies of Pimephales promelas and other chronictoxicity tests on various saltwater species, indicatethat growth generally is the most sensitive meas-ure of chronic toxicity for vertebrates exposed tochlorpyrifos.(8) However, survival may occasionallybe as important or a more sensitive chronicendpoint.

3.3.2. Microcosm Studies

Microcosm and mesocosm studies providevaluable effects information from controlled experi-mental ecosystems containing several trophic levels.Both direct and indirect effects can be observed, andif a suitable dosing regime is employed, generationof chronic effects is also possible. Giesy et al.(3)

summarized several microcosm studies conducted inthe United States and the Netherlands and conclud-ed that consistent findings from these studiessupported a no-observable-adverse-effect concen-tration (NOAEC) of 100 ng L1. Recovery ofaffected sensitive invertebrate populations usually


Table I. Acute Toxicity Values Estimated for 48-Hour Exposure to Chlorpyrifos for Invertebrates Requiring Lentic Habitat

Organism Taxonomic Name TaxonHabitat

Requirement Rank48-h Acute

Value (ng L1)

Mosquito various species I Lentic 1 0.7Mosquito Aedes aegypti I Lentic (temporary pools

and ponds)2 4.8

Mosquito Culex pipiens I Lentic 3 28Cladoceran Ceriodaphnia dubia C Lentic 4 113Cladoceran Ceriodaphnia sp. C Lentic 5 148Midge Chironomus tentans I Lentic 6 157Mysid Neomysis mercedis I Estuarine, brackish lakes,

freshwater lakes and streams(lotic and lentic)

7 198

Cladoceran Daphnia pulex C Lentic 8 210Mosquito Culex quinquefasciatus I Lentic 9 212Cladoceran Daphnia sp. C Lentic 10 255Amphipod Gammarus lacustris C Lentic and lotic 11 289Midge (various species) I Lentic, lotic, estuarine, some

marine12 297

Mosquito Culicoides pipiensquinquefasciatus

I Probably lentic 13 354

Cladoceran Daphnia longispina C Lentic 14 424Mosquito Aedes vexans I Lentic 15 424Fairy Shrimp [Anostraca] C Lentic 16 438Amphipod Gammarus fasciatus C Lentic, lotic, estuarine 17 453Cladoceran Simocephalus vetulus C Lentic 18 707Mosquito Culicoides variipennis I Lentic 19 771Pygmy Backswimmer Neoplea striola I Lentic 20 849Caddisfly Leptoceridae sp. I Lotic and lentic 21 900Mosquito Aedes cantans I Lentic (temporary pools

and ponds)22 900

Amphipod Hyalella sp. C Primarily lentic, occasionallylotic

23 919

Cladoceran (not specified) C Lentic 24 1,000Midge Tanypus grodhaus I Lentic 25 1,061Diptera Paratanytarsus sp. I Lentic and lotic 26 1,131Crawling water beetle Peltodytes sp. I Lentic 27 1,131Diving Beetle Laccophilus fasciatus I Lentic and lotic 28 1,485Diptera Chaoborus sp. I Primarily lentic, occasionally

lotic29 1,543

Cladoceran Daphnia magna C Lentic 30 1,700Pygmy Backswimmer Plea sp. I Lentic 31 2,400Midge Chricotopus sp. I Unknown distribution,

collected from S. Californiadrainage ditches

32 2,475

Tadpole shrimp Triops longicaudatus C Lentic 33 2,828Water Boatman

(not Water Strider)Corixa punctata I Uncertain, generally lentic 34 2,828

Planaria Dugesia dorotocephala O Lentic and lotic 35 3,742Diptera Chaborus punctipennis I Lentic 36 3,818Dragonfly Crocothemis erthryaea I Uncertain, collected from

Sudanese irrigation canals37 4,101

Mayfly Caenis horaria I Lotic and lentic 38 4,243Ostracod (not specified) C Lentic and lotic 39 6,300Damselfly Enallagma/Ishnura spp. I Primarily lentic, also lotic

depositional40 8,061

Crayfish Orconectes immunis C Lentic and lotic 41 8,485Diptera Chaoborus obscuripes I Lentic 42 9,334Cladoceran Daphnia sp. C Lentic 43 11,314Ostracod Cyprinotus incongruens O Lentic and lotic 44 14,142


occurred at concentrations < 500 ng L1. Becausemost experimental ecosystems of this type tend to belentic systems, the 100 ng L1 NOAEC in partreflects the more sensitive species associated withlentic habitats (Fig. 3, Table I) and is thereforeprobably too conservative for moving water bodies.The lotic species sensitivity distribution (Fig. 3,Table II) suggests that a higher value such as the10th centile concentration of 177 ng L1 is moreappropriate for characterizing acute effects in Ores-timba Creek.

3.3.3. Effects Thresholds

3.3.3.1. Chlorpyrifos Alone. For temporal eventanalysis, the acute effect threshold used was 177 ngL1, the 10th centile sensitivity of the lotic speciesgroup. Applying a mean acute-to-chronic ratio of 8(3)

to the acute value produced an estimated chroniceffect threshold of 22 ng L1. Note that thesethresholds are most accurate for event periods of 2 dfor acute events and 21 d or longer for chronicevents. The acute duration is based on the normal-ization of acute toxicity tests to a constant exposureperiod of 48 hours. A 21-d minimum for chroniceffects is associated with the exposure periodrequired in laboratory toxicity studies to observetypical chronic endpoints for both invertebrate andvertebrate aquatic species.

3.3.3.2. Chlorpyrifos and Diazinon. The expo-sure data in Fig. 2 indicate that diazinon may havecontributed additional toxicity to increase theseverity of adverse effects in Orestimba Creek.Methidathions contribution appears to be negli-gible. Accordingly, we utilized the diazinon com-bined acute toxicity database(20) to form a subgroup-ing of invertebrates requiring a lotic habitat,employing the same approach as that used forchlorpyrifos. These diazinon data are presented inTable III. We then estimated the 10th centile speciessensitivity value for diazinon by linear regression onthe cumulative rank-order distribution of acuteEC50/LC50 concentrations (r

2 0.878). The resulting10th centile value for lotic invertebrates was 1,142ng L1. Diazinon acute values in the distributionwere not normalized to 48 hours as was done forchlorpyrifos. Instead, Giddings et al.(20) combined alldata for exposure periods of 4896 hours.

Assuming that the additive effect modelexplains acute toxicity,(10) we assessed exposure forthe combined residues of chlorpyrifos and diazinonby summing the daily measured water concentra-tions of chlorpyrifos and the toxic equivalent con-centrations of diazinon to estimate a totalchlorpyrifos equivalent concentration. Total chlor-pyrifos equivalent concentrations were computed asfollows. First, each daily diazinon concentration wasconverted to a chlorpyrifos equivalent by taking the

Table I. Continued



Value (ng L1)

Backswimmer Notonecta undulata I Lentic and lotic 45 24,890Crayfish Procambarus clarki C Lentic and lotic 46 41,713Oligochaete Limnodrilus hoffmeisteri O Lentic and lotic 47 50,912Diving Beetle Hydrophylus spp. I Lentic and lotic 48 70,711Ostracod Chlamydotheca arcuata C Lentic 49 141,421Snail Anius vortex O Lentic 50 210,190Snail Lymnaea stagnalis O Lentic 51 210,190Leech Nephelopsis obscura O Lentic 52 586,570Midge Chironomus decorus I Primarily lentic, also lotic

depositional53 1,039,447

Snail Aplexa hypnorum O Lentic 54 1,139,856Snail Lanistes carinatus O Unknown, probably lentic,

African genus55 1,916,259

Snail Bromphalaria alexandra O Unknown, probably lenticEgyptian genus

56 2,070,391

Snail Helisoma trivolvis O Lentic and lotic 57 2,449,490Rotifer Brachionus calyciflorus O Lentic 58 8,449,928

Note: Crustacea (C), Insect (I), or Other (O).


product of the diazinon concentration and the ratioof the chlorpyrifos and diazinon 10th centile acutelotic species sensitivities, 177/1,142. Second, each ofthe resulting daily chlorpyrifos equivalent concen-trations was summed with the corresponding repor-ted daily chlorpyrifos concentration.

Chronic toxicity from the presence of diazinonresidues does not appear to be important, based on a

comparison of chronic endpoints derived in speciestested for both chlorpyrifos and diazinon toxi-city.(8,20) Typically, the no-observed-effect levels fordiazinon chronic effects were two to three orders ofmagnitude higher than those determined for chlor-pyrifos.

Table II. Acute Toxicity Values Estimated for 48-Hour Exposure to Chlorpyrifos for Invertebrates Requiring Lotic Habitat



Value (ng L1)

Amphipod Gammarus pulex C Generally lotic, occasionallylentic

1 99

Mysid Neomysis mercedis I Estuarine, brackish lakes,freshwater lakes and streams(lotic and lentic)

2 198

Amphipod Gammarus pseudolimnaeus C Lotic 3 248Amphipod Gammarus fasciatus C Lentic, lotic, estuarine 4 285Amphipod Gammarus lacustris C Lentic and lotic 5 289Midge (various species) I Lentic, lotic, estuarine, some

marine6 297

Mayfly Cloeon dipterum I Lotic 7 360Mayfly Ephemerella sp. I Lotic 8 383Stonefly Pteronarcella badia I Lotic 9 537Caddisfly Leptoceridae sp I Lotic and lentic 10 922Diptera Paratanytarsus sp. I Lentic and lotic 11 1,131Diving Beetle Laccophilus fasciatus I Lentic and lotic 12 1,485Stonefly Claassenia sabulosa I Lotic 13 2,162Midge Chricotopus sp. I Unknown distribution,

collected fromS. California drainage ditches

14 2,475

Planaria Dugesia dorotocephala O Lentic and lotic 15 3,742Isopod Asellus aquaticus O Lotic 16 3,818Dragonfly Crocothemis erthryaea I Uncertain, collected from

Sudanese irrigation canals17 4,101

Mayfly Caenis horaria I Lotic and lentic 18 4,243Midge Dicrotendipes californicus I Lotic 19 4,950Ostracod (not specified) C Lentic and lotic 20 6,300Crayfish Orconectes immunis C Lentic and lotic 21 8,485Ostracod Cyprinotus incongruens O Lentic and lotic 22 14,142Blackfly Simulium vitattum I Lotic 23 19,092Mayfly [Heptageniidae] I Primarily lotic, occasionally

lentic24 20,506

Caddisfly Hydropschy/Cheumatopsyche sp. I Lotic 25 21,637Stonefly Pteronarcys californica I Lotic 26 22,361Stonefly Claassenin sp. I Lotic 27 24,495Backswimmer Notonecta undulata I Lentic and lotic 28 24,890Isopod Proasellus coxalis O Probably lotic 29 28,284Crayfish Procambarus clarki C Lentic and lotic 30 41,497Oligochaete Limnodrilus hoffmeisteri O Lentic and lotic 31 50,912Water Scavenging

BeetleHydrophylus spp. I Lentic and lotic 32 70,711

Snail Bithynia tentaculata O Primarily lotic, also lentic 33 210,190Snail Helisoma trivolvis O Lentic and lotic 34 2,449,490



4. RISK CHARACTERIZATION

4.1. Temporal Analysis of Chlorpyrifos WaterExposure Patterns

Fig. 4 presents the RADAR acute event analy-sis for chlorpyrifos alone and chlorpyrifos + diazinonfor events at or above the chlorpyrifos lotic thresh-old value of 177 ng L1. There were eight acutechlorpyrifos events with an average duration ofthree days, and the percent of time above thethreshold was 7%. Addition of the diazinon chlor-pyrifos-equivalent concentrations to the data analy-sis increased the number of acute events to 11, with

an average duration of two days. The percent of timeabove the threshold increased slightly to 9%. Theseacute exposure patterns were characterized by along recovery duration of 175 days, occurring in theperiod between the end of summer applications andbefore initiation of winter treatments.

Chlorpyrifos exposure events at or above thechronic effect threshold of 22 ng L1 are depicted inFig. 5. A total of 25 exposure periods were found toexceed the chronic effect level, averaging five daysin duration (range of one to 32 days). Only one ofthese episodes, the one beginning on Day 315 (3/10/97), persisted beyond the 21-day exposure periodnecessary to elicit chronic effects in laboratory

Fig. 3. Distribution of 48-hour normalizedchlorpyrifos EC50/LC50s for all freshwaterinvertebrate species tested. Data pointswith error bars are geometric means andgeometric standard errors. The linearregression line is plotted with the 95thcentile prediction interval. Table inset: 10thcentile species sensitivity concentrationwith upper and lower concentration boundsof regression probability point estimate.NS Normal Score. Upper: species asso-ciated with lentic habitat. Lower: speciesassociated with lotic habitat.


studies with freshwater invertebrates. The amount oftime the exposure concentrations were above thelotic chronic threshold value was 36%. The long fall/winter recovery period observed in the acute eventanalysis also appeared in the pattern of chronicevents (155 d). As Fig. 5 shows, interpretation of thechronic exposure pattern is complicated by thepresence of acute events for sensitive invertebrates.

4.2. Probability of Exposures Exceeding EffectsThresholds and Taxa at Risk

4.2.1. Acute Effects

The 11 chlorpyrifos + diazinon acute exposureevents (Fig. 4) can be rank-ordered by arithmeticmean concentration and plotted as a cumulativeprobability distribution (Fig. 6). Using linear regres-sion, estimates of the typical (50th centile) andtypical worst-case (90th centile) acute event weregenerated: 356 and 766 ng L1, respectively. Com-parison of these values to the lotic species chlorpy-rifos sensitivity ranking in Table II suggests thatsome species of amphipods, mysids, and midges maybe impacted by the typical event, and some mayfliesand stoneflies could be affected by the typical worst-case event. Regression on a similar distribution forchlorpyrifos-alone acute-event concentrations (r2 0.912, data not shown) produced 50th and 89thcentiles of 330 and 624 ng L1, respectively (N 8,so the 90th centile cannot be computed). The

significance of this analysis is that diazinon contri-buted to the upper end of the event distributionmore than chlorpyrifos did.

The 50th and 89th centile chlorpyrifos-aloneexposure events can be related to reference water-column concentrations predicting risk in bed sedi-ment habitat. (No sediment acute toxicity data orsite-specific Kd information are available for diazi-non, so the analysis is restricted to chlorpyrifosonly.) In this approach, we utilized the single-valueestimate of Kd to predict bioavailable, toxic concen-trations in bed sediment pore water, assumingequilibrium conditions exist in each time period ofinterest in the water column (water + suspendedsediment) and in the sediment bed (pore water +sediment). Thus, dissolved concentrations in bothcompartments are assumed to be equal or nearly so.The 50th centile event (330 ng L1 ) falls at thelower end of the range in possible effect concentra-tions in bed sediment pore water (264 to 1,320 ngL1 ), while the 89th centile event (624 ng L1 ) fallsin the middle of the range. Effects on sediment-exposed life stages are slightly possible to possiblebut not probable.

Returning to the situation in the water column,data from Table III indicate that there are nosensitive taxa in the group of lotic invertebratestested for diazinon acute effects. However, amphi-pods are sensitive to relatively low concentrations ofchlorpyrifos (Table II). Stoneflies appear to be

Table III. Acute Toxicity Values for Exposure to Diazinon for Invertebrates Requiring Lotic Habitat

Organism Taxonomic Name Taxon Habitat Requirement RankAcute

Value (ng L1 )

Amphipod Gammarus pseudolimnaeus C Lotic 1 2,000Mysid Neomysis mercedis I Estuarine, brackish lakes, freshwater

lakes and streams (lotic and lentic)2 4,150

Mayfly Cloeon dipterum I Lotic 3 7,800Crayfish Orconectes propinquus C Lentic and lotic, primarily Laurentian

Great Lakes and drainages4 15,000

Stonefly Acroneuria ruralis I Lotic and lentic 5 16,000Amphipod Asellus communis C Probably lotic 6 21,000Amphipod Hyallela azteca C Primarily lentic, occasionally lotic 7 22,000Mayfly Baetis intermedius I Lotic and lentic 8 24,000Stonefly Pteronarcys californica I Lotic 9 25,000Mayfly Paraleptophlebia pallipes I Lotic 10 44,000Snail Helisoma trivolvis O Lentic and lotic 11 528,000Asian leech Hirudo nipponia O Uncertain, probably lakes and/or slow

streams12 1,900,000

Cyclops Cyclops sp. O Primarily lentic, occasionally lotic 13 2,510,000Oligochaete Tubifex sp. O Lentic and lotic 14 3,160,000



tolerant to diazinon, and most tested stonefly generaare also tolerant to chlorpyrifos. Stoneflies, there-fore, can be removed from the group that could beaffected by the typical worst-case event dominatedby diazinon residues. Equal numbers of mayflygenera were sensitive and tolerant to chlorpyrifos,while all tested genera were tolerant to diazinon.More of the chlorpyrifos-tested midge genera weretolerant of the observed acute events than weresensitive, but there is no information on midgesensitivity to diazinon. Only the tested amphipodgenera were uniformly sensitive to the observedtypical and typical worst-case acute events. Tosummarize: (1) amphipods and mysids appear to

be at risk, primarily from the presence of chlorpy-rifos, (2) mayflies and stoneflies are less likely to beaffected by either compound due to a range ofgenera sensitivities, and (3) for the same reason,midges are also less likely to suffer acute effectsfrom chlorpyrifos alone (no data for diazinon).

Table IV lists the spawning and dietary habitatsof the fish known to dominate the main stem regionof the San Joaquin River watershed. When thisinformation is related to the groups of potential preyspecies at risk from typical and typical worst-caseacute effects, it is possible to determine whetherindirect effects on fish through dietary impacts arelikely to have occurred.

Fig. 4. Temporal analysis of acute toxicevents occurring above a threshold of177 ng L1 (horizontal line). Upper:chlorpyrifos alone. Lower: chlorpyrifosequivalent concentrations ([chlorpyrifos]+ 177/1,142[diazinon]).


During the spring to fall period, flow of waterinto the lower reach of Orestimba Creek came fromirrigation tailwater and spillwater. Flow volumesvaried daily,(4) and only during winter flooding in themain stem river was the water flow consistentlyslow-moving in the lower reach of the creek. Habitatrequirements for spawning and for juveniles suggestthat few immature individuals would inhabit thecreek (Table IV). Adult fathead minnow, red shiner,and inland silverside are more dependent on aqua-tic insects and other small macroinvertebrates,and therefore would be more likely to enter thelower reach of Orestimba Creek from downstream

breeding areas in search of food. Adult threadfinshad would be less attracted to the creek, where onlysmall populations of microscopic food sources wouldbe available for filter feeding.

In summary, the potential impacts on amphi-pods and mysids during acute chlorpyrifos + diazi-non events would appear to be relativelyunimportant to the feeding requirements of adultfish (Table IV), the life stage most likely to inhabitthe lower reach of the creek. However, if there werealso reductions in populations of mayflies, stoneflies,and midges, due to the presence of sensitive species,indirect impacts on fish could have occurred.

Fig. 5. Temporal analysis of chronic toxicevents occurring above a threshold of22 ng L1 (lower horizontal line; upperhorizontal line represents the acutethreshold of 177 ng L1).

Fig. 6. Chlorpyrifos + diazinon acute toxicevents from the lower panel of Fig. 4plotted as a cumulative probability distri-bution of event arithmetic average con-centrations. NS Normal Score.


4.2.2. Chronic Effects

The arithmetic average concentration of the32-day chlorpyrifos chronic event was 155 ng L1,considerably lower than the reported freshwater fish(fathead minnow) LOEC of 3,200 ng L1 and nearthe midpoint of the 100300 ng L1 range ofDaphnia magna LOECs. Assuming that lenticcladocerans such as Daphnia magna, which areacutely sensitive to chlorpyrifos, possess more sen-sitive chronic endpoints than do animals that areless acutely sensitive, then chronic effects on loticinvertebrates are unexpected from this 32-dayevent.

5. ECOLOGICAL RELEVANCE

5.1. Population and Community Effects

Interpretation of available data discussedabove suggests that populations of sensitive inver-tebrates could be impacted, if present. Affectedpopulations, however, do not appear to be critic-ally important components of fish diets. Alternat-

ive food sources such as algae, other plantmaterial, other insects and small invertebrates,insensitive crustaceans, and mollusks would beunaffected by exposure to chlorpyrifos + diazinon.Food of terrestrial origin, including live inverte-brates and invertebrate carcasses, commonlymakes up a significant percentage of energy flowin low-order streams with adequate riparian hab-itat, so this would also mitigate against an adversechemical effect on fish diets (good riparian habitatis reported for Orestimba Creek(5)). Recovery oftemporarily affected invertebrate populationswould take place via reproduction by unaffectedindividuals and those protected by refugia, repop-ulation from downstream drift, or immigration byterrestrial life stages. If recovery did not occur,functional replacement by more tolerant inverteb-rate species would likely fill the vacated ecologicalniches.(21,22) In view of all these considerations, weconclude that no significant alterations in ecosys-tem function are expected from the occurrence ofthese chemical stressors in the temporal patternobserved during the year-long chemical monitor-ing.

Table IV. Habitat, Spawning Periods, and Diet and for Fish Inhabiting the Main Stem Region of the San Joaquin Watershed

Fish Spawning Habitat Spawning Period Foraging Habitat Diet References

Fathead minnow Shallow waterwith vegetation

As early as MarchLate spring tomid-summer

Juveniles:shallow weedy areas

Young: filamentousalgae, diatoms,detritus, smallinvertebrates Algae,other plant material,aquatic insects

38, 39, 40, 41, 42

Red shiner Aquatic vegetation,gravel, sand, mud,mostly in calm water

More than onceper season Late springto early fall,mid-summer peak

Shallow waters Juveniles: smallcrustaceans, aquaticinsects, larvae, algae,plant leaves, detritusInsects, other smallinvertebrates

43, 44, 45

Threadfin shad Submergedvegetation inshallow sluggishwater

Spring, when waterwarms to 21 C, maycontinue atintervals throughoutthe warmer monthsof the year

Shallow and openwater

Filter feeder(young and adults):microscopic plants andanimals suspended inwater column

44, 46, 47, 48

Inland silverside Shallow weedywaters

April throughSeptember, withpeaks in May andAugust

Shallow sluggishweedy areas;ditches, reservoirs,irrigation systems;avoid fast currents

Juveniles:zooplankton, includinglarge cladocerans,larvae of chironomidand phantom midgesSmall crustaceans,mollusks, insects

38, 49


5.2. Ecological Significance of AgriculturallyDominated Streams

Presence of the San Joaquin Main Stem groupof fish species indicates the existence of alteredenvironmental conditions, many of which resultdirectly from human activities in the lower Ores-timba Creek watershed. In addition to the occur-rence of pesticide residues in surface water,additional habitat alterations resulting from agri-culture include increased salinity and nutrients,eroded soil contributions to suspended and bedsediment, channel dredging, flow inconsistency,and, possibly, presence of animal waste.(5) Thebasic stream type has changed from intermittent toperennial. Ecological effects from the presence ofOP insecticides over time must be interpreted inthe context of the type of habitat and aquaticcommunities present in a creek dominated byhuman activities. Cumulative ecological risk assess-ment, a process that considers aggregate riskcaused by multiple stressors,(23) could be useful togain better understanding of this situation andpoint to actions necessary to implement any desiredhabitat restoration.

6. UNCERTAINITY AND VARIABILITYANALYSIS

6.1. Exposure Characterization

The exposure data are limited to a one-yearsampling period. Farming practices were typical forthe year, with the exception of fewer winter insec-ticide applications to dormant trees. Increaseddormant applications could result in shortening ofthe long recovery period observed in the monitoringdata set. Cholinesterase-inhibiting insecticides otherthan chorpyrifos, diazinon, and methidathion werenot monitored. Historically, use of other products ofthis type has been minimal.(24)

Sources of variation in the exposure datainclude sampling error and analytical error. Theuse of autosamplers, calibrated daily, to collecthourly water samples composited each 24 hoursfor analysis provided samples representative of thetemporal changes in concentration from day today. Analysis of additional samples collected usingthe U.S. GS Equal Width Increment protocol(25)

indicated that the autosampler single-point intakesalso were representative of the spatial concentra-tion profile across the stream channel in this

well-mixed stream.(26) Sampling error appears tobe minimal.

Variability in the analytical data set was quan-tified by quality control samples. Samples fortified ata level of 25 ng L1 yielded a mean standarddeviation of 23 2.9 ng L1 for chlorpyrifos (N 218) and 24 1.9 ng L1 for diazinon (N 188).(26)Compared to the variation in effects data discussedin the following section, analytical variability wasnegligible in the key threshold concentrations usedin the RADAR analysis (22177 ng L1). Conse-quently, only the reported point estimates of con-centration were utilized in the assessment.

The sediment concentration estimates should beconsidered more a screening level exposure assess-ment than a quantitative analysis due to uncertaintyin the cross-compartment equilibrium assumption.Paired sampling of the water column and bedsediment, along with abundance measurements ofbenthos, is necessary to test the assumption andgeneral approach.

6.2. Effects Data

The use of the constructed surrogate assem-blage for the invertebrate community inhabitingOrestimba Creek generates uncertainty related tothe presence or absence of entire groups (example:amphipods and caddisflies). Another uncertaintyarises from the relative sensitivities of genera withina group. A good example is mayfly sensitivity tochlorpyrifos: two genera were tolerant and two weresusceptible (Table II). It is not known whethermayflies inhabit Orestimba Creek, or which generaor species are represented.

There are unequal numbers of species tested forchlorpyrifos and diazinon toxicity, and the speciestested are not the same. This may bias the additivityanalysis, because 10th centile points on the acutespecies sensitivity distributions were selected toestimate chlorpyrifos equivalent concentrations.

Sediment toxicity data is quite limited relativeto laboratory results reported from simple watersystems. Interpretation of sediment toxicity valuesgenerally relies on the equilibrium partition method,which appears to be valid for chlorpyrifos.(27)

Application of the additive toxicity model is notpossible in the absence of diazinon sediment toxicitytest data. In 1992, the site was also characterized byrelatively large concentrations of total DDT resi-dues in all of the sampled matrices (range of 24 ngL1 to 4,350 ng g1).(13) The contribution of legacy


DDT toxicity to benthic macroinvertebrates needsto be factored into the assessment.

Variability in the effects database reflectsinherent biological variation in the responses oftest organisms and interlaboratory variability rela-ted to conditions of cultured organisms, methods,and, possibly, poorly controlled parameters duringthe testing period. Fig. 3 shows the combinedeffects of all these contributing sources of vari-ation and quantifies the 95th centile predictionerror for a point estimate of probability in thedistribution of acute species sensitivity (4th to 19thcentile for the 10th centile point estimate). Theprediction interval accounts for the error of theregression coefficient, the error of the mean, andthe variation of individual data points around theestimated mean.

6.3. Risk Characterization

Comparison of the complex temporal pattern ofobserved exposures in Orestimba Creek to thesimple, short-duration laboratory toxicity tests gen-erates uncertainty in interpreting overlaps betweenthe exposure and effects distributions. A betterunderstanding of the dynamics of exposure, uptake,elimination, and acetylcholinesterase regenerationwould allow application of modeling to improve therisk characterization.

Repeated exposures of fish and invertebrateswere assumed to be independent, with full recoveryfrom cholinesterase inhibition occurring during eachrecovery period. The actual kinetics of depurationand enzyme regeneration may be more complex,thus violating this assumption. Depuration rates forchlorpyrifos and diazinon are quite rapid, and fishmay tolerate substantial levels of cholinesterasedepression without experiencing toxicity.(8) Recov-ery of invertebrate organisms from repeated chlor-pyrifos exposures has been demonstrated by Naddyet al.(28) In pulsed exposure work with Daphniamagna, a sensitive freshwater lentic invertebrate,these authors found recovery following pulsedexposures to chlorpyrifos, if the critical body burdenfor chlorpyrifos with the daphnid was not achieved;the required timeframe between acute exposures forrecovery appeared to be three days or more.

The 10th centile single-point estimate of speciesacute sensitivity benchmark assumes the LC50/EC50values are realistic predictors of population-leveleffects in the stream environment, and those specieslocated outside the sensitive tail of the distribution

would not be at risk for sufficient mortality to affectthe assessment endpoints. Research indicates thatthis is a conservative assumption for some sensitiveaquatic species, as studies on the effects of insecti-cides on zooplankton and miticides on terrestrialarthropods have shown that laboratory LC50 valuesmay significantly overestimate field effects at thepopulation level.(2931) If one therefore interprets theerror in the point estimate of the 10th centile as arange of possible concentrations impacting thesensitive grouping of species in the distribution tail(error bounded by prediction interval, Fig. 3), thenRADAR analysis can be performed to determinethe sensitivity of the exposure characterization tothis error.

RADAR runs for chlorpyrifos-equivalent con-centrations using the thresholds 63, 177, and 426ng L1 (Fig. 3) produced the following patterns ofexposure (reported by increasing concentration ofexposure threshold): 18, 11, and 7 events; 18, 9,and 4% of time above threshold; and 3, 2, and 1days average duration. The sets of event concen-trations, when ranked and considered as cumula-tive distributions of acute exposure events,generate 90th centile typical worst-case concentra-tions of 416, 766, and 964 ng L1, respectively, foreach threshold value (the last number is extrapo-lated on the regression line outside the datarange). Only one additional taxon, ranked 10 inTable II, is added to the list of organismspotentially impacted by the 964 ng L1 typicalworst-case exposure event. We conclude that thepoint estimate of the 10th centile is adequate todescribe the sensitive species grouping for riskcharacterization. Moreover, the prediction intervalaround the 10th centile regression estimate in-cludes both the 4th and 19th centiles; this bracketsthe 5th centile level of species sensitivity com-monly used in regulatory schemes as a brightlinefor ecosystem protection.(32,33)

Seasonality of acute-event occurrence was notrelated to invertebrate reproduction strategy. Moresite-specific information on invertebrate communitycomposition and knowledge of life histories wouldimprove understanding of the significance of expo-sure and recovery patterns.

Additive exposures based on a common point inthe species sensitivity distribution can predict gen-eral levels of effect but do not well characterizetoxicity to specific taxa, unless each component inthe mixture is similar in its activity. A more reliablepredictor for specific taxa would employ a ratio of


toxicity values derived from tests on the sameorganism.

6.4. Ecological Relevance

The principal uncertainty associated with theanalysis of ecological relevance of effects is the lackof knowledge of the specific ecological entitiesrequiring protection in a system dominated byagricultural activities. In particular, more informa-tion is necessary to determine whether sensitiveinvertebrate populations important in fish diets arepresent.

7. CONCLUSION

Analysis of a detailed chemical exposure dataset in combination with a relatively robust toxicitydatabase of surrogate species for chlorpyrifos anddiazinon suggests that no direct adverse effects onfish should have occurred during the year of mon-itoring. Certain genera of sensitive aquatic inverte-brates, if present, may have experienced populationreductions during acute-exposure events and subse-quent functional replacement by more tolerantspecies if recovery did not take place. These reduc-tions/replacements would not have had appreciableeffect on the diets of common fish species currentlyinhabiting the main stem San Joaquin River and itsagriculturally dominated tributaries. Neither of theassessment endpoints, fish population persistence orinvertebrate community productivity, appeared tobe adversely affected by the presence of thesechemical residues.

8. RESEARCH RECOMMENDATIONS

This risk assessment can be refined with addi-tional information on the site-specific nature of theaquatic community. Surveys of benthic macroinver-tebrates and fish in the lower reach of OrestimbaCreek and in an appropriate reference site canincrease understanding of the types of organismsexpected in this type of system in the absence of OPinsecticides. However, reference sites may be diffi-cult to identify, given the widespread use of chem-ical pest control in commercial agriculture. Surveysof physical habitat would also contribute to under-standing the relative roles different types of stressorsplay in determining species composition. Once thisbiological information is obtained, toxicity testing of

local species could provide additional data forinterpretation of site-specific effects.

ACKNOWLEDGMENTS

The authors thank the three anonymous re-viewers for the many constructive suggestions forimproving the manuscript, particularly in the area ofuncertainty and variability analysis.

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