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U.S. GEOLOGICAL SURVEY
Data Series 286

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Introduction

Hoover Dam was built between 1931 and 1936 on the Colorado River in Black Canyon along the Nevada-Arizona border. Lake Mead, the reservoir created behind the 221.4-m high Hoover Dam (fig. 1), is the largest manmade reservoir by volume in the United States (Covay and Beck, 2001). Hoover Dam created a new water supply, which is used for hydroelectric power, irrigation, and municipal and industrial use. Lake Mead stores Colorado River water for municipal-water supply to the Las Vegas Valley area and is used to store and regulate flows for downstream users along the Colorado River. Lake Mead also is well known for its year-round recreational activities, including sport fishing, boating, swimming, and scuba diving. Several species of fish are present in Lake Mead including striped bass (Morone saxatalis), channel catfish (Ictalurus punctatus), largemouth bass (Micropterus salmoides), rainbow trout (Oncorhynchus mykiss), and the endangered razorback sucker (Xyrauchen texanus). Lake Mead National Recreation Area was established in 1964 and is managed by the U.S. Department of the Interior through a cooperative agreement between the National Park Service and Bureau of Reclamation.

Major inflows to Lake Mead are from the Colorado, Virgin, and Muddy Rivers, and Las Vegas Wash. Las Vegas Wash is perennial because of inflows from municipal wastewater-treatment plants (Covay and Leiker, 1998) and excess irrigation water from urban landscapes, which flows into the wash through surface- and ground-water discharge. Population within the Las Vegas Valley has significantly increased since the 1940s and the volume of treated effluent, which in 1993 constituted about 96 percent of the annual discharge of Las Vegas Wash, has increased concurrently (Bevans and others, 1996). The mean flow of Las Vegas Wash into Las Vegas Bay from 1992 to 1998 was about 490,000 m³/d (Preissler and others, 1999); in 2001, mean flow increased to 606,000 m³/d (Bureau of Reclamation, 2001). Other sources of environmental contaminants like perchlorate, a rocket fuel component, have been seeping into Las Vegas Wash from former manufacturing sites in Henderson, Nev. (National Park Service, 2006).

In 1995, the U.S. Geological Survey (USGS) in cooperation with other Federal agencies including the U.S. Fish and Wildlife Service, the National Park Service, the Bureau of Reclamation, and the Nevada Department of Wildlife, began a study to determine the potential for endocrine disruption in common carp (Cyprinus carpio) from Lake Mead. In that study, male and female common carp were collected from Las Vegas Wash, Las Vegas Bay, and Callville Bay (fig. 1) during the spawning season and analyzed for several reproductive biomarkers. The results showed that common carp from Las Vegas Wash and Las Vegas Bay exhibited evidence of endocrine disruption (Bevans and others, 1996). Significantly lower concentrations of the sex steroid hormone 11-ketotestosterone4, the major androgen responsible for spermatogenisis (Kime, 1999) were found in male common carp from Las Vegas Wash and Bay than in reference male common carp from Callville Bay.

Low concentrations of sex steroid hormones can affect reproductive development and cause, reduced gonad size and altered secondary sex characteristics (Munkittrick and others, 1992). Another indication of endocrine disruption was the induction of vitellogenin (Vtg) in male common carp, which was present in high concentrations in fish collected at Las Vegas Wash and Las Vegas Bay (Bevans and others, 1996). Vitellogenin, a phospholipoprotein produced in the liver under control of 17β-estradiol, is the precursor of egg yolk normally only detected in female oviparous vertebrates. However, male fish do have estrogen receptors in the liver and can produce Vtg when exposed to estrogenic compounds (Purdom and others, 1994). Some researchers have proposed that low concentrations of Vtg are normally present in males in some fish species (Sepulveda and others, 2004).

Lake Mead is mildly mesotrophic (Carlson, 1977) with generally good water quality. However, the water quality at Las Vegas Bay has significantly higher numbers and higher concentrations of environmental contaminants (Bevans and others, 1996) and higher nutrient concentrations (LaBounty and Horn, 1997) than other parts of Lake Mead. More organochlorine pesticides, polyaromatic hydrocarbons (PAHs), furans, phthalates, phenols, and polychlorinated biphenyls (PCBs) were detected in bed sediment, fish tissue, and passive sampler extracts from Las Vegas Bay compared to Callville Bay, which has no point source discharges (Bevans and others, 1996). Sediment from the bottom-sediment core-sampling study done in 1998 by Covay and Beck (2001) showed twice as many organic contaminants detected from Las Vegas Bay compared to the sediment from Overton Arm where maximum concentrations of pesticides and PCBs were one order of magnitude higher in Las Vegas Bay compared to sediment in Overton Arm. Covay and Beck (2001) also detected several dioxins and furans in sediment from Las Vegas Bay, but no dioxins and furans were detected in sediment from Overton Arm.

Recent studies have documented the presence of emerging chemical contaminants and endocrine disrupting compounds in Lake Mead subsequent to the evidence of endocrine disruption in fish reported by Bevans and others (1996). Water samples collected from Las Vegas Bay in April 1997 contained four xenoestrogens: octylphenol at 0.027 µg/L; nonlyphenol at 0.750 µg/L, polyethoxylates at 4.85 µg/L; 17a-ethinyl estradiol at 0.0005 µg/L; and one natural estrogen, 17β-estradiol at 0.0022 µg/L (Snyder and others, 1999).

Another study by Snyder and others (2001) determined that the compounds with the highest estrogenic activity at Las Vegas Bay were two steroid hormones, 17β-estradiol, a naturally occurring estrogen, and 17a-ethinyl estradiol, a synthetic estrogen used in oral contraception. Fish exposed to ethinyl estradiol in concentrations as low as 0.0005 µg/L have shown evidence of endocrine disruption (Purdom and others, 1994; Routledge and others, 1998; Bayley and others, 1999; Doyle and Lim, 2002; Thorpe and others, 2003). Boyd and Furlong (2002) detected 13 human-health pharmaceutical compounds in surface water in Las Vegas Wash between October 2000 and August 2001. Caffeine, carbamazepine (used to treat epilepsy), cotinine (a metabolite of nicotine), and dehydronifedipine (a metabolite of the antianginal Procardia) were the most frequently detected compounds of the 33 they targeted for analysis. The effects of pharmaceuticals and personal care products on aquatic biota are largely unknown, but several examples indicate effects have been detected at low concentrations (Daughton and Ternes, 1999).

Purpose and Scope

Concern about initial 1995 study results from Lake Mead prompted a more thorough and comprehensive study which began in 1999 by the U.S. Department of the Interior. This study, led by USGS, included the U.S. Fish and Wildlife Service; National Park Service; Bureau of Reclamation; University of Nevada, Las Vegas; University of Florida, Gainesville; Texas Tech University, and the Nevada Department of Wildlife. The purpose of this study, partially reported here, was to determine if the evidence of endocrine disruption observed in 1995 could be confirmed during a 1-year common carp reproductive cycle from May 1999 through May 2000. Results from this study can serve as a baseline for the reproductive health of Lake Mead fish and a comparison for other ongoing research and monitoring of fish reproductive studies.

The scope of work was to characterize the environmental contaminants in Lake Mead including estrogenic and other emerging compounds in pharmaceuticals and personnel care products, and examine possible correlations with changes in endocrine and reproductive biomarkers. Specific results for the current 1999–2000 study pertaining to morphometric and histopathological biomarkers were reported by Patiño and others (2003). This part of the study compared male and female common carp from Las Vegas Bay with those from Overton Arm. The Las Vegas Bay male common carp had lower gonadosomatic indices (a measure of testicular development) throughout the study, a higher proportion of spermatocytes (a less mature stage of sperm) for one sampling date, and higher incidence of gonadal macrophages aggregates (biomarkers of contaminant exposure). Differences in biomarkers from female common carp between the two sampling areas are most likely strongly influenced by differences in water temperature and could not be attributed to environmental contaminants.

This report presents organic chemical compound concentrations in fish tissue and biomarker data for 83 male common carp collected from Las Vegas Bay, similar organic chemistry results for 15 male common carp, and similar biomarker measures for 80 male common carp collected from Overton Arm. Tissue chemistry results also are presented for 16 female common carp and biomarker measures for 79 female common carp collected from Las Vegas Bay, and tissue chemistry results for 15 female common carp and biomarker measures for 81 female common carp collected from Overton Arm.

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