LAKELINE ■ Spring 2005 11

Contaminants in Urban Runoff:

Xiaoping Zhou, Kimberly S. Zikmund, Peggy Roefer, and James F. LaBounty

Looking at Southern Nevada

Introduction

Las Vegas Wash (Wash) flows intoBoulder Basin of Lake Mead. Itserves as the sole drainage

channel for the entire 1600 square mileLas Vegas Valley Hydrographic Basinwith a population of 1.7 million. Flowsin the Wash include urban runoff fromtributaries, treated wastewater effluentsfrom the Valley’s wastewater treatmentplants, intercepted shallow groundwater,and stormwater. Although the Washprovides only about 1.5 percent of thetotal water inflow to Lake Mead, it is acritical element in the overallenvironmental and water resourcechallenge facing southern Nevada. LakeMead is the largest reservoir on theColorado River system and provideswater to Southern Nevada, Arizona,southern California, and Mexico.

Discharge of the Wash into LakeMead presents potential concerns due tothe presence of certain chemical andbiological constituents typical of urbaninfluence. The Urban Runoff WaterQuality Monitoring Project wasdesigned to provide data that are helpfulin managing urban flows. Six samplinglocations were chosen. Each site wassampled quarterly for chemical andbiological constituents.

As a first-time systematic study, thetributary monitoring program, whichbegan in 2000, provided a baseline ofwater quality data from urban runoffduring non-storm (dry) weatherconditions. Chemical loadings from thetributaries and their relativecontributions to Lake Mead were alsocalculated (LVWCC 2003; Zhou,Roefer, and Zikmund 2004). Thisinvestigation, along with other waterquality investigations of the Wash andLake Mead, improves our ability tomanage the Las Vegas Valley watershed.

MethodsIn situ physical parameters and

water samples were collected from thesix tributaries quarterly beginning inOctober 2000. The six tributaries are:Alta Channel at Meadows DetentionBasin (LVC_2), Sloan Channel (SC_1),Las Vegas Creek (LW12.1), FlamingoWash (FW_0), Monson Channel(MC_1), and Duck Creek (DC_1)(Figure 1). Samples were collected onlywhen there was dry weather. EPA-certified protocols were strictlyfollowed. We monitored the followingfield parameters: water temperature,dissolved oxygen, pH, specificconductance, and turbidity using amulti-parameter probe (HydrolabCorporation Model Surveyor® 4). Water

samples were analyzed for major ions,heavy metals, plant nutrients, and a fullsuite of organic constituents (derivedfrom drinking water standards) in orderfor us to fully evaluate water quality ofthe Las Vegas Wash and itsenvironmental impacts on drinkingwater in Lake Mead.

Water ChemistryField Parameters: Waters in all

tributaries to the Las Vegas Wash werealkaline, with pH ranging from 8.2 to8.6. Water temperature in the tributariesvaried with seasons with a rangebetween 1°C and 30°C. Dissolvedoxygen concentrations were alwayssaturated or supersaturated, rangingfrom 9 mg/L to 12 mg/L. Specific

Figure 1. Location map showing major tributaries to the Las Vegas Wash in the Las VegasValley and six sample sites (▲) for the tributary water quality monitoring program.

▲

12 Spring 2005 ■ LAKELINE

Parameters DC_1 FW_0 LW12.1 LVC_2 MC_1 SC_1Calcium 505 328 233 122 435 122Magnesium 287 225 255 103 321 151Sodium 566 289 293 157 425 169Potassium 63 32 55 18 32 16Bicarbonate 225 236 260 282 227 224Carbonate 1.9 3.0 4.8 10 3.5 5.0Sulfate 2469 1743 1715 608 2526 763Chloride 839 303 278 160 414 212Bromide 1.0 0.8 0.7 0.3 1.1 0.8Fluoride 1.3 0.6 0.5 0.5 0.7 1.1Silica 53 31 31 21 43 57

Maj

or I

ons

(mg/

L)

Total Dissolved Solids (TDS) 5048 3246 3147 1413 4505 1720

Aluminum 167.9 54.8 77.0 105.0 74.9 167.1Arsenic 50.5 6.6 5.5 3.7 16.8 21.4Barium 28.9 35.9 36.8 43.9 26.1 61.7Chromium 2.0 2.4 2.3 2.3 2.3 4.5Copper 10.8 8.7 7.3 6.9 8.7 5.5Iron 307.5 340.0 263.3 260.0 250.0 386.7Lead 0.6 0.8 0.9 1.0 1.0 0.7Manganese 31.7 8.7 11.4 10.2 5.3 43.8Nickel 21.1 12.6 10.2 6.7 15.6 6.3Zinc 15.2 16.6 18.7 19.7 13.6 12.4

Met

als

(ug/

L)

Selenium 23.3 15.0 11.0 5.0 22.6 6.6

Acetaldehyde 2.00 2.88 2.50 3.73 2.29 4.50Caffeine 0.06 0.11 0.12 0.20 0.08 0.66Formaldehyde 8.71 15.00 9.89 13.25 9.86 12.33Glyoxal 1.33 1.71 2.56 3.89 2.57 3.25O

rgan

ics

(ug/

L)

M-Glyoxal 1.75 1.50 2.63 5.29 2.00 3.33

Perchlorate ClO4, _g/L 19.87 10.52 11.13 11.06 17.01 8.65

NH4, _g N/L 96 89 94 222 91 188NO2, _g N/L 80 80 119 89 95 93NO3, _g N/L 4799 4075 2977 2346 4326 2448NO2 + NO3, _g N/L 4823 4091 3032 2491 4375 2498TKN, _g N/L 467 467 983 1817 833 975OP, _g P/L 21 24 66 73 15 38

Nut

rien

ts

TP, _g P/L 34 33 78 137 22 47

conductance was lower in Alta Channel(1900 µS/cm) and Sloan Channel (2350µS/cm) and much higher at Duck Creek(5900 mS/cm), Monson Channel (4600µS/cm), Flamingo Wash (3700 µS/cm),and Las Vegas Creek (3600 µS/cm). Ingeneral, higher specific conductancewas found in the tributaries with alonger flow path and/or shallowgroundwater inputs.

Interestingly, there is a large rangein the relationship of specificconductance and total dissolved solids(TDS) in the urban tributaries. Specificconductance is a measurement of theability of a substance toconduct an electricalcurrent. The presence ofcharged ionic species(dissolved solids) in watermakes the waterconductive. As dissolvedsolid concentrationsincrease, conductance ofthe water increases;therefore, the specificconductance measurementprovides an indication oftotal dissolved solid (TDS)concentration in water.TDS can be calculatedbased on the specificconductance. Among sixtributaries, Duck Creek,Flamingo Wash, Las VegasCreek, and MonsonChannel have higher totaldissolved solids (TDS)(ranging from 3246 mg/Lto 5048 mg/L), whereasSloan Channel and AltaChannel have lower TDSconcentrations(approximately 1720 mg/Land 1413 mg/L,respectively) (Table 1).

Major Ions: Waters inall tributaries to the LasVegas Wash are the Mg-Ca-Na-SO

4-Cl type. The

dominant cations arecalcium (Ca+2) rangingfrom 122 to 505 mg/L,magnesium (Mg+2) from103 to 321 mg/L, andsodium (Na+) from 157 to566 mg/L. The dominant

anions are sulfate (SO4

-2) ranging from608 to 2526 mg/L, chloride (Cl-) from160 to 839 mg/L, and bicarbonate(HCO

3-) from 224 to 282 mg/L (Table1).

Additionally, potassium (K+) and silica(SiO

2) concentrations range from 16 to

63 mg/L and from 21 to 57 mg/L,respectively. Carbonate (CO

3-2)

concentration ranges from 2 mg/L to 10mg/L. Bromide (Br-) and Fluoride (F-)concentrations are very low in alltributaries, less than 1.5 mg/L. Majorion (salt) concentrations from thetributaries to the Wash are generallyhigh due to high evaporation rates and

contacts with surface soils that arenaturally saline in the arid environment.Salts in the surface soils can readily bedissolved by irrigation water andleached into the tributaries.

Heavy Metals: Seventeen heavymetals were analyzed. Concentrations of11 metals from six tributaries are inTable 1. The metals not included inTable 1 were below detection limits.Average concentrations of aluminum,iron, and manganese varied from 5.3mg/L to 386.7 mg/L. Concentrations ofother trace metals, including arsenic,

Table 1. Average Chemistry Data of Water Samples from the Tributaries to the Las Vegas Wash,Nevada

LAKELINE ■ Spring 2005 13

barium, chromium, copper, lead,manganese, zinc, and selenium, werelower or much lower than the maximumcontamination level (MCL) for theprimary standards and the secondarystandards for drinking water. Of the sixtributaries, Duck Creek has relativelyhigher concentrations of aluminum(167.9 µg/L), iron (307.5 µg/L),manganese (31.7 µg/L), arsenic (50.5µg/L), nickel (21.1 µg/L), and selenium(23.3 µg/L). Monson Channel also has ahigher selenium concentration (22.6 µg/L).

As one of those metalloids that havea potential to be bioaccumulated inaquatic environment, selenium can betoxic to some animals at elevatedconcentrations. The EPA seleniumcriterium for wildlife in freshwater is 5µg/L. Selenium is widely distributed innature and abundant with sulfideminerals of various metals, such as iron,lead, and copper. Weathering of rocks,including volcanic and sedimentaryrocks, is the major source ofenvironmental selenium. These types ofrocks are common in southern Nevadaand around the Las Vegas valley.

Plant Nutrients: Analysis fornitrogen and phosphorus includedammonia (NH

4-N), nitrite (NO

2-N),

nitrate (NO3-N), total Kjeldahl nitrogen

(TKN), orthophosphorus (OP4-P), and

total phosphorus (TP) (Table 1, Figure2). Ammonia concentrations in mosttributaries were lower than the detectionlimit (80 µg N/L). The exceptions wereAlta Channel at Meadows DetentionBasin and Sloan Channel, in whichaverage ammonia concentrations were222 and 188 µg N/L, respectively. As anunstable species of nitrogen in aeratedwater, nitrite concentrations weregenerally below the detection limit in alltributaries. Nitrate nitrogen, as a stablespecies in natural water, was alwaysdetected in all tributaries in highconcentrations. Nitrate concentrations inthe tributaries ranged from 2346 µg N/Lto 4799 µg N/L. The average TKNconcentrations varied from 467 µg N/Lto 1817 µg N/L, with some highconcentrations (1200 – 1600 µg N/L) inAlta Channel at Meadows DetentionBasin, Monson Channel, and SloanChannel. Both orthophosphorus and

0.0

5.0

10.0

15.0

20.0

LVC_2 SC_1 FW_0 LW12.1 MC_1 DC_1

g

Acetaldehyde Caffeine Formaldehyde Glyoxal M-Glyoxal

0.00

0.05

0.10

0.15

0.20

0.25

LVC_2 SC_1 FW_0 LW12.1 MC_1 DC_1

TPOP

0

2000

4000

6000

LVC_2 SC_1 FW_0 LW12.1 MC_1 DC_1

Figure 2. Average concentrations of total dissolved solids (TDS), phosphorus nutrients,and five organic pollutants in water from six tributaries.

Co

nce

ntr

atio

n, u

g/L

Ave

rag

e C

on

cen

trat

ion

, mg

P/L

TD

S C

on

cen

trat

ion

, mg

/L

14 Spring 2005 ■ LAKELINE

total phosphorus concentrations in thetributaries ranged from 15 µg P/L to 137µg P/L.

Organic Compounds: One hundredand seventy-seven priority organiccompounds were analyzed for allcollected samples. Most of thesecompounds were below the analyticaldetection limits. However, five organiccompounds were detected from alltributaries and 35 organic compoundswere detected at least once. Twenty-onecompounds were detected from only onesample site. There were 15 compoundsdetected at Meadows Detention Basin(LVC_2), 12 at Sloan Channel (SC_1),11 at Monson Channel, 9 at FlamingoWash (FW_0), and 7 at Duck Creek(DC_1) and Las Vegas Creek (LW12.1),respectively. The five most commonorganic compounds detected wereacetaldehyde, caffeine, formaldehyde,glyoxal, and M-glyoxal (pyruvicaldehyde). They were detected at all sixtributaries (Table 1, Figure 2).Concentrations of caffeine were lessthan 1 µg/L and concentrations ofacetaldehyde, formaldehyde, glyoxal,and M-glyoxal (pyruvic aldehyde) wereless than 15 µg/L. Less common organiccompounds, such as di(2-ethylhexyl)phthalate, propanal, and 2,4-D, werealso found at most sample sites.Acetaldehyde and formaldehyde arehazardous pollutants and toxic atelevated levels. Sources of acetaldehydeand formaldehyde include emissionfrom combustion processes, buildingmaterials, food preservative, tobaccosmoke, paper mills, crude oil and naturalgas mining, and petroleum refining.

Flows in the TributariesIn order to quantify mass loading

rates of different chemical constituents,flow rates of six tributaries weremeasured monthly since April of 2001.The U.S. Geological Survey (USGS)Parshall flume and the Price pygmyflow meter were used depending on thehydrological conditions of flowchannels. The methods for measurementand computation of streamflowdeveloped by USGS (Rantz et al. 1982)were followed.

Based on our measurements, the sixtributaries contribute approximately 18 0

2000

4000

6000

LVC_2 FW_0 LW12.1 SC_1 MC_1 DC_1

Ave

rag

e A

nn

ual

Flo

w, A

cre-

feet

All tributaries contribute 13,344 acre-feet flow to the wash annually

Figure 3. Average annual flow (acre feet per year) from each tributary to the Las VegasWash.

cubic feet per second (approximately12million gallons per day) of flow to theWash under dry weather condition. Asone of four flow components in the LasVegas Wash, urban runoff from the LasVegas Valley contributes morethan13,000 acre feet per year (LVWCC,2003) of water to the Wash,approximately 7% of the total Washflow (Figure 3). As stated previously,the Las Vegas Wash accounts for 1.5%of the total water inflow to Lake Mead.Therefore, flows from tributaries areequivalent to 0.11% of the total waterinflow to Lake Mead.

Mass Loading from the Tributaries tothe Wash

The daily mass loading rates (lbs/day or tons/day) of TDS, heavy metals,and nutrients from each tributary werecalculated using the averageconcentrations of these chemicalconstituents and the average flow rate ineach tributary. The total daily load(TDL) from all six tributaries to the LasVegas Wash was obtained by summingthe mass loading of each tributary. Therelative percentages of the TDL of eachchemical constituent from all sixtributaries vs. that in the Las VegasWash were also computed (Table 2).With approximately 7% of the Washtotal flow, six tributaries togethercontribute approximately 17% of totalTDS, 40% of selenium, 15% of arsenic,2% of total metals, 2% of nitrogennutrient, and 1% of phosphorus nutrient

mass loading to the Wash (Table 2,Figure 4).

DiscussionIn general, tributary water has

higher TDS, higher trace metals (copper,nickel, selenium, and arsenic), andlower nutrients compared with themainstream Wash water that consistsmainly (> 85%) of treated wastewater.This is particularly true for thosetributaries that flow through the highlyurbanized areas and are located in thelower portion of the Las Vegas Valley.These variations in water quality aremost likely due to the high evaporationrates in the Las Vegas Valley, urbanactivities, and shallow groundwaterinputs.

Comparisons of concentrations andmass loading rates of different chemicalconstituents in the six tributaries showedthat Duck Creek (DC_1) and FlamingoWash (FW_0) are the two major sourcesof TDS, heavy metals (especiallyselenium, arsenic, copper, and nickel),organic compounds, and nutrients to theWash. Recent studies of selenium in theLas Vegas Wash and its tributariesindicated that more than 40% ofselenium entering the Wash, andeventually to Lake Mead, comes fromDuck Creek. Duck Creek interceptslocally resurfacing shallow groundwater,which is the major source of selenium(Figure 4). Las Vegas Creek (LW12.1)contributes the third greatest massloading of each chemical constituent.

LAKELINE ■ Spring 2005 15

Tributaries with less flow, such as SloanChannel, Alta Channel, and MonsonChannel, play a very limited role interms of mass loading of compounds tothe Wash.

Almost three dozen organiccompounds were detected in tributaries.The organic compounds found generallycome from both urban and industrialsources, such as pesticides, herbicides,automobile oils, and chemical spills andwastes. Las Vegas Wash tributaries drainseveral sub-basin flows that come fromstreets and roads. Over-watering oflawns, industrial water uses, carwashing, pool water draining, andresurfacing shallow groundwater aresources of urban runoff. Water qualityand quantity of these tributary flows,like other urbanized areas in the WesternUnited States, are greatly affected by

quality, and to help better manage theWash and Lake Mead. As the fastest-growing city in U.S.,Las Vegas will continue to facechallenges of urban runoff water quality.There are no easy solutions to thesecomplicated problems. The long-termand comprehensive management plansthat have been implemented by federal,state, and local governmentorganizations and agencies since theearly of 1990s greatly helped maintainor improve water quality in thetributaries, the Las Vegas Wash, andLake Mead. Specifically, the BestManagement Practice (BMP) within theLas Vegas Valley watershed, such assub-basin and drain maintenance, streetsweeping, illegal/illicit connectionelimination, hazardous material disposalmanagement, water conservation, andpublic education, are having positiveimpacts on water quality in thetributaries, the Wash, and Lake Mead.

ReferencesHem, J. D., 1992. Study and Interpretation

of the Chemical Characteristics of NaturalWater. United States Geological SurveyWater-Supply Paper 2254, 263 p.

Las Vegas Wash Coordination Committee(LVWCC), 2003. 2002 Year-end Report,Chapter 4, Water Quality: 13-37.

Rantz, S. E., and others, 1982. Measurementand computation of streamflow: Volume1. Measurement of stage and discharge:

TDSTotal Metals As

Se TKN NO2+NO3

NH4OP TP

Tributarie Las Vegas Wash

Figure 4. Pie diagrams showing the relative percentages of the daily mass loadings ofdifferent chemical constituents (tributaries vs. Las Vegas Wash).

Table 2. Daily Mass Loading of TDS, Heavy Metals,and Nutrients from Tributaries (TRB) and

the Las Vegas Wash (LVW)

Parameter TRB LVW %, TRBvs. LVW

TDS (tons/day) 186.6 1092.7 17.1

Aluminum 8.9 863.6 1.0

Arsenic 2.1 14.4 14.5

Barium 3.3 70.4 4.7

Chromium 0.2 110.0 0.2

Copper 1.1 8.3 13.2

Iron 28.4 1281.3 2.2

Manganese 1.9 96.5 1.9

Nickel 1.6 15.2 10.3

Lead 0.1 2.7 3.2

Selenium 1.6 4.1 39.7

Zinc 2.1 52.9 3.9

Hea

vy M

etal

s (lb

s/da

y)

Total Metals 51.3 2519.4 2.0

NH4 8.1 142.2 5.7

NO2+NO3 378.6 20039.2 1.9

TKN 36.6 796.3 4.6

OP 3.0 262.4 1.1

Nut

rient

s (lb

s/da

y)

TP 4.9 366.4 1.3

Table 2. Daily Mass Loading of TDS, HeavyMetals, and Nutrients from Tributaries (TRB)and the Las Vegas Wash (LVW).

urban development,human activity, wateruse habit and land usepattern, and localweather and geology.However, there were nosevere or massivechemical contaminationsto the tributaries, exceptfor an elevated seleniumconcentration at DuckCreek, Flamingo Wash,and Monson Channel.

Summary The long-termmonitoring in thetributaries permitscharacterization of thewater quality of urbanrunoff within the LasVegas Valley watershed,and it provides usinformation to quantifypotential environmentalimpacts. Data from thisstudy were used by theLas Vegas WashCoordination Committee(LVWCC) to evaluatethe current state of heathof urban runoff from thetributaries to the Wash,to monitor variationsover time in water

16 Spring 2005 ■ LAKELINE

U.S. Geological Survey Water-SupplyPaper 2175, 284 p.

Zhou, X., Roefer, P., and K. Zikmund, 2004.Las Vegas Wash Monitoring andCharacterization Study: Results for WaterQuality in the Wash and Tributaries. FinalReport to USEPA, Region IX, 42 p.

Xiaoping Zhou is ahydrologist II withSouthern Nevada WaterAuthority (SNWA),Watershed ManagementDivision. He has morethan 15 years ofexperience working onthe geology,hydrogeology, and

hydrology. His previous and currentresearch focuses on geochemistry ofgroundwater and water quality monitoringin the Las Vegas Wash and its tributaries,Lake Mead, and constructed wetlands. Hecan be reached atxiaoping.zhou@snwa.com.

Kimberly S. Zikmund isthe manager ofWatershed ManagementDivision, SNWA. Shehas over 15 years ofexperience in waterresource management,environmentalrestoration, and waterquality research.

Previously serving as the Las Vegas WashProject manager, she played a key role indeveloping and implementing the Las VegasWash Comprehensive Adaptive ManagementPlan. She can be reached atkimberly.zikmund@snwa.com.

Peggy Roefer is theregional water qualitysupervisor of WatershedManagement Division,SNWA. She has over 25years of experience inmicrobiology, waterquality, and waterresource management.She was previously the

principal microbiologist with SouthernNevada Water System (SNWS), SNWA. Shecan be reached at peggy.roefer@snwa.com.

James F. LaBounty iscurrently a researchscientist with SNWAand the editor of theLake and ReservoirManagement. He hasmore than 35 years ofexperience in limnologyand ecology research.Jim can be reached atjim.labounty@snwa.com.

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