Las Vegas Wash Research Articles

Evaluation of bendway weir bank stabilization techniques for stream restoration

In this study, velocity distribution patterns in a stream in the presence of bendway weirs were assessed for their effectiveness in bank stabilization in a demonstration site in the Las Vegas Wash, Nevada, USA. Streambanks of the target stream had been stabilized previously by longitudinal stone toe protection (LSTP) and stone spur dikes. Stone toes are meant to protect the toe of streambanks. Similarly, stone spur dikes consist of a series of dikes (of a crest relatively higher than that of the bendway weirs) protruding at right angles from the convex side of the streambanks. Velocity measurements were taken to evaluate the velocity field in the channel in the following three different cases: Case 1, channel with LSTP and the pre-existing spur dikes; Case 2, channel with LSTP with no deflecting structures (spur dikes removed); and Case 3, channel with LSTP and bendway weirs. The results suggested that bendway weirs caused a reduction in the uncontrolled high-velocity flow near the outer bank (right bank), allowing only controlled flow. Thus, bendway weirs were found to be more effective than spur dikes in reducing the velocity of flow at the vulnerable weir zones near the outer banks, thus providing a means to stabilize the streambanks.

Short-term and diurnal patterns of salt secretion by Tamarix ramosissima and their relationships with climatic factors

Tamarisk (Tamarix spp.) is an invasive shrub or tree species in the United States. Glands within Tamarix leaves secrete salt that may increase surface soil salinity and consequently result in an unviable ecosystem for native riparian tree species. To increase our understanding of the effects of invasive Tamarix on the ecosystem and its native riparian vegetation, we investigated salt secretion of a stand of Tamarix ramosissima in the Las Vegas Wash, Nevada. Short-term and diurnal salt secretion rates were monitored in the stand from September to October 2009. Soil samples were collected beneath the canopy so that water and salinity conditions could be determined. We used weather data from a Las Vegas database in conjunction with soil temperature data collected for this study. The major cation secreted by salt glands was sodium. Diurnal variation in the salt secretion rate had a maximum during the day, suggesting that salt secretion is dependent on salt transportation to leaves. Variations in the salt secretion rates were mostly influenced by solar radiation, suggesting that salt secretion in T. ramosissima would be enhanced by light in natural conditions.

Bottom sediment as a source of organic contaminants in Lake Mead, Nevada, USA

Treated wastewater effluent from Las Vegas, Nevada and surrounding communities’ flow through Las Vegas Wash (LVW) into the Lake Mead National Recreational Area at Las Vegas Bay (LVB). Lake sediment is a likely sink for many hydrophobic synthetic organic compounds (SOCs); however, partitioning between the sediment and the overlying water could result in the sediment acting as a secondary contaminant source. Locating the chemical plumes may be important to understanding possible chemical stressors to aquatic organisms. Passive sampling devices (SPMDs and POCIS) were suspended in LVB at depths of 3.0, 4.7, and 6.7 (lake bottom) meters in June of 2008 to determine the vertical distribution of SOCs in the water column. A custom sediment probe was used to also bury the samplers in the sediment at depths of 0–10, 10–20, and 20–30cm. The greatest number of detections in samplers buried in the sediment was at the 0–10cm depth. Concentrations of many hydrophobic SOCs were twice as high at the sediment–water interface than in the mid and upper water column. Many SOCs related to wastewater effluents, including fragrances, insect repellants, sun block agents, and phosphate flame retardants, were found at highest concentrations in the middle and upper water column. There was evidence to suggest that the water infiltrated into the sediment had a different chemical composition than the rest of the water column and could be a potential risk exposure to bottom-dwelling aquatic organisms.

Patterns of metal composition and biological condition and their association in male common carp across an environmental contaminant gradient in Lake Mead National Recreation Area, Nevada and Arizona, USA

There is a contaminant gradient in Lake Mead National Recreation Area (LMNRA) that is partly driven by municipal and industrial runoff and wastewater inputs via Las Vegas Wash (LVW). Adult male common carp (Cyprinus carpio; 10 fish/site) were collected from LVW, Las Vegas Bay (receiving LVW flow), Overton Arm (OA, upstream reference), and Willow Beach (WB, downstream) in March 2008. Discriminant function analysis was used to describe differences in metal concentrations and biological condition of fish collected from the four study sites, and canonical correlation analysis was used to evaluate the association between metal and biological traits. Metal concentrations were determined in whole-body extracts. Of 63 metals screened, those initially used in the statistical analysis were Ag, As, Ba, Cd, Co, Fe, Hg, Pb, Se, Zn. Biological variables analyzed included total length (TL), Fulton's condition factor, gonadosomatic index (GSI), hematocrit (Hct), and plasma estradiol-17β and 11-ketotestosterone (11kt) concentrations. Analysis of metal composition and biological condition both yielded strong discrimination of fish by site (respective canonical model, p<0.0001). Compared to OA, pairwise Mahalanobis distances between group means were WB<LVB<LVW for metal concentrations and LVB<WB<LVW for biological traits. Respective primary drivers for these separations were Ag, As, Ba, Hg, Pb, Se and Zn; and TL, GSI, 11kt, and Hct. Canonical correlation analysis using the latter variable sets showed they are significantly associated (p<0.0003); with As, Ba, Hg, and Zn, and TL, 11kt, and Hct being the primary contributors to the association. In conclusion, male carp collected along a contaminant gradient in LMNRA have distinct, collection site-dependent metal and morpho-physiological profiles that are significantly associated with each other. These associations suggest that fish health and reproductive condition (as measured by the biological variables evaluated in this study) are influenced by levels of certain metals in the Lake Mead environment.

Restoring habitat for riparian birds in the lower Colorado River watershed: An example from the Las Vegas Wash, Nevada

The success of riparian restoration projects in the arid southwestern U.S. is often measured in terms of vegetation characteristics such as growth, cover, and structure. Among low-elevation riverine environments within the Colorado River watershed, restoration is typically conducted to improve degraded habitats for birds of conservation concern by replacing the exotic tamarisk ( Tamarix ramosissima) with native cottonwoods ( Populus spp.) and willows ( Salix spp.). The working assumption for many restoration practitioners is that replacing exotic plants with native plants will improve habitat quality and will, therefore, benefit birds. Based on data collected at exotic and restored (i.e., native) sites along the Las Vegas Wash, Nevada, not all birds benefit from restoration. Broad measures of community benefit, including benefits to birds of conservation concern and riparian obligate/dependent birds, were not detected. There were, however, some species-specific benefits. Some environmental variables that were associated with exotic and native sites were significant in explaining the composition of the bird community. For example, the richness of forbs and grass-like plants (a proxy of soil moisture), invertebrate mass, and percent shade (a proxy for canopy characteristics) were important. Considering our results and depending on restoration goals, tamarisk replacement projects may not inherently provide benefits to birds.

Systems dynamic model to forecast salinity load to the Colorado River due to urbanization within the Las Vegas Valley

This study evaluates the impact of urban growth in the Las Vegas Valley (LVV), Nevada, USA on salinity of the Colorado River. In the past thirty eight years the LVV population has grown from 273,288 (1970) to 1,986,146 (2008). The wastewater effluents and runoff from the valley are diverted back to the Colorado River through the Las Vegas Wash (LVW). With the growth of the valley, the salinity released from urban areas has increased the level of TDS in the wastewater effluents, ultimately increasing the TDS in the Colorado River. The increased usage of water softeners in residential and commercial locations is a major contributor of TDS in the wastewater effluents. Controlling TDS release to the Colorado River is important because of the 1944 Treaty signed between the USA and Mexico. In addition, the agriculture salinity damage cost for the Colorado River has been estimated to be more than $306 a million per year using 2004 salinity levels. With the expected growth of LVV in coming years the TDS release into Lake Mead will increase over time. For this purpose, it is important to investigate future TDS release into the Colorado in anticipation of potential TDS reducing measures to be adopted. In this research, a dynamic simulation model was developed using system dynamics modeling to carry out water and TDS mass balances over the entire LVV. The dynamic model output agreed with historic data with an average error of 2%. Forecasts revealed that conservation efforts can reduce TDS load by 16% in the year 2035 when compared to the current trend. If total population using water softeners can be limited to 10% in the year 2035, from the current 30% usage, TDS load in the LVW can be reduced by 7%.

Characteristics of Lake Mead, Arizona–Nevada

Lake Mead was formed in the 1930s by the construction of Hoover Dam on the Colorado River. Hoover Dam was one of the first multipurpose dams constructed by the federal government. Lake Mead is the largest reservoir in the United States by volume (3.5479 × 1010 m3 at full pool) and is second only to Lake Powell in surface area (637.05 km2). Water from Lake Mead is used for municipal and industrial purposes and irrigation water by approximately 25,000,000 people. The Colorado River provides approximately 97% of the inflow, with the remainder coming primarily from Las Vegas Wash and the Virgin and Muddy rivers. Flow in Las Vegas Wash more than doubled over the past 30 years as a result of the rapid population growth in Las Vegas. Lake surface elevations have dropped by about 40 m since 1999 as a result of extended drought and increasing water demands brought about by population growth. This elevation change impacted water quality and recreation and is just one of many management challenges facing the lake. The discovery of adult quagga mussels (Dreissena rostriformis bugensis) in Lake Mead in February 2007 created infrastructure management problems and raised concerns about lake biology and water quality. The impacts of this invasion are still under investigation. This manuscript provides information on the history of Lake Mead, as well as descriptions of lake and watershed characteristics, hydrology, water quality, fisheries, and recreation that lake managers can use to guide future assessment and management of Lake Mead.

TDS and selenium projections for the Las Vegas Wash, post completion of the Systems Conveyance and Operations Program (SCOP)

The Las Vegas Wash (Wash) is the primary drainage channel for the Las Vegas Valley. Flows from the tributaries are a major source of contaminants of particular concern to the Wash, mainly total dissolved solids (TDS) and selenium (Se). Treated wastewater effluent discharged to the Wash currently provides enough dilution to lower the TDS and to maintain selenium concentrations below the current 5μg/L aquatic life criteria recommended by the Unites States Environmental Protection Agency (EPA; USEPA 1987); however, the Systems Conveyance Operations Program (SCOP) was intended to take most of this treated effluent through a pipeline directly into the Boulder Basin of Lake Mead. While construction of the pipeline may have had many water quality benefits for the Las Vegas Valley and Lake Mead, the dilution benefit to the Wash will be significantly reduced, primarily affecting TDS and Se concentrations in the Wash. The Wash enters Lake Mead at Las Vegas Bay, home to the endangered razorback sucker. The concern is the increased concentration of TDS and Se to the Las Vegas Bay, which potentially would impact the razorback sucker. Using data collected by the Southern Nevada Water Authority's (SNWA) environmental monitoring and management water quality team, projections have been made for expected TDS and Se concentrations post completion of the SCOP. These projections show 2 options for managing TDS and Se concentrations on the Wash: dilution and/or treatment of the tributaries. Discharging 2.08 × 105 m3/d to the Wash provides the necessary dilution to lower Se to the target concentration of 5 μg/L but will still leave TDS levels higher than desired. Treating one or more of the major tributaries will bring both Se and TDS concentrations within targeted levels.

Analysis of iodide and iodate in Lake Mead, Nevada using a headspace derivatization gas chromatography–mass spectrometry

We report here a derivatization headspace method for the analysis of inorganic iodine in water. Samples from Lake Mead, the Las Vegas Wash, and from Las Vegas tap water were examined. Lake Mead and the Las Vegas Wash contained a mixture of both iodide and iodate. The average concentration of total inorganic iodine (TII) for Lake Mead was approximately 90 nM with an iodide-to-iodate ratio of approximately 1. The TII concentration (approximately 160 nM) and the ratio of iodide to iodate were higher for the Las Vegas Wash (approximately 2). The TII concentration for tap water was close to that of Lake Mead (approximately 90 nM); however, tap water contained no detectable iodide as a result of ozonation and chlorine treatment which converts all of the iodide to iodate.

Identification of methyl triclosan and halogenated analogues in male common carp ( Cyprinus carpio) from Las Vegas Bay and semipermeable membrane devices from Las Vegas Wash, Nevada

Methyl triclosan and four halogenated analogues have been identified in extracts of individual whole-body male carp ( Cyprinus carpio) tissue that were collected from Las Vegas Bay, Nevada, and Semipermeable Membrane Devices (SPMD) that were deployed in Las Vegas Wash, Nevada. Methyl triclosan is believed to be the microbially methylated product of the antibacterial agent triclosan (2, 4, 4'-trichloro-4-hydroxydiphenyl ether, Chemical Abstract Service Registry Number 3380-34-5, Irgasan DP300). The presence of methyl triclosan and four halogenated analogues was confirmed in SPMD extracts by comparing low- and high-resolution mass spectral data and Kovats retention indices of methyl triclosan with commercially obtained triclosan that was derivatized to the methyl ether with ethereal diazomethane. The four halogenated analogues of methyl triclosan detected in both whole-body tissue and SPMD extracts were tentatively identified by high resolution mass spectrometry. Methyl triclosan was detected in all 29 male common carp from Las Vegas Bay with a mean concentration of 596 μg kg − 1 wet weight (ww) which is more than an order of magnitude higher than previously reported concentrations in the literature. The halogenated analogs were detected less frequently (21%–76%) and at much lower concentrations (< 51 μg kg − 1 ww). None of these compounds were detected in common carp from a Lake Mead reference site in Overton Arm, Nevada.

Reassessing the Risks of Tamiflu Use during a Pandemic to the Lower Colorado River

We wish to highlight an error in the article “Potential Risks Associated with the Proposed Widespread Use of Tamiflu” (Singer et al. 2007) in which we predicted environmental concentrations of Tamiflu (influenza antiviral) in several catchments in the United States and the United Kingdom. An incorrect assumption was made in the hydrology of one of these catchments, the Lower Colorado River (LC).

Perchlorate exposure from food crops produced in the lower Colorado River region

The Colorado River shows low levels of perchlorate derived from aerospace- and defense-related fuel industries once located near the Las Vegas Wash. At sufficiently high dosages perchlorate can disrupt thyroid function by inhibiting uptake of iodide. The Colorado River is the primary source of irrigation water for most food crops grown in Southern California and Southwestern Arizona. The objective of this study was to evaluate potential perchlorate exposure from food crops produced in the lower Colorado River region (LCRR). The major food commodities produced in the region were sampled and perchlorate levels were determined by ion chromatography followed by detection using either conductivity or tandem mass spectrometry, depending on analyte levels. The Monte Carlo module of the Dietary Exposure Evaluation Model (DEEM) was used to derive an estimate of the 2-day average perchlorate intakes. Data were derived assuming that individuals residing in the LCRR get their fruits and vegetables from within the LCRR as well as from other areas in the United States, or assuming individuals living in the LCRR get their fruits and vegetables from the LCRR only. Perchlorate exposure estimates derived in this study are comparable to exploratory estimates by the US Food and Drug Administration. For infants and children, over 50% of the estimated perchlorate exposure was from milk. The relative impact of vegetables and fruit toward perchlorate exposure increased by age through adulthood. Cumulative perchlorate exposure estimates based on this hypothetical analysis could approach or exceed the NAS reference dose (RfD) for some population groups as drinking water levels exceeded 6 microg/l. However, few individuals are exposed to perchlorate in drinking water at levels above 4 microg/l in the United States and very few would be exposed to perchlorate levels exceeding the RfD, whether consuming food crops from within or outside the LCRR.

Characterization of the lignin signature in Lake Mead, NV, sediment: comparison of on-line flash chemopyrolysis (600°C) and off-line chemolysis (250°C)

The distribution of lignin in sediment is a useful tool for tracing the transport of land-derived organic matter in an aquatic environment. Tetramethylammonium hydroxide (TMAH) flash chemopyrolysis, or chemolysis followed by GC-MS analysis can be used for evaluating the origin of organic carbon in sediments. TMAH chemopyrolysis or chemolysis of organic matter produces a myriad of semi-volatile products. Among these products are methylated phenols which are an indirect measure of lignin in sediment. In this study, total organic carbon, elemental carbon, and lignin were measured in Lake Mead sediments. This study indicates that terrestrial runoff makes a contribution to Lake Mead sediments, and that this contribution is most apparent in sediment that is close to the Las Vegas Wash. Two chemolysis methods (on-line and off-line) were examined and compared for detection of lignin phenols. The results from these sediment cores indicate that comparable results can be obtained from the two approaches, although detection levels are significantly lower for the off-line approach.

Uptake of perchlorate by vegetation growing at field sites in arid and subhumid climates

AbstractPrevious greenhouse and field studies show that terrestrial and aquatic vegetation, including trees, grasses, and agricultural produce grown on perchlorate‐contaminated soil or with perchlorate‐contaminated irrigation water, accumulate perchlorate mainly in their leaf tissue. The phytoaccumulated perchlorate poses potential ecological risk by either contaminating the food chain of humans and animals or recycling in the ecosystem as leaf litter fall that accumulates on topsoil. In this study, the uptake and phytoaccumulation of perchlorate in terrestrial and aquatic vegetation growing at two perchlorate‐contaminated sites (the Longhorn Army Ammunition Plant [LHAAP] in Karnack, Texas, and the Las Vegas Wash [LVW], Nevada) was monitored during multiple growing seasons. The LHAAP site is located in a subhumid climate, while the LVW site is located in an arid climate. All vegetation species collected from both sites contained quantifiable levels of perchlorate. The detected concentrations varied with the type of plant species, amount of perchlorate concentration in soil, and season and stage of plant maturity. The highest perchlorate concentrations were measured in willows (Salix nigra), crabgrass (Digitaria spp.), and Bermuda grass (Cynodon dactylon) at the LHAAP, while salt cedar (Tamarix ramosissima) at the LVW phytoaccumulated the highest mass of perchlorate. The concentrations of perchlorate measured in plant leaves growing over contaminated soils at multiple LHAAP locations did not reveal the strong seasonal variability observed at the LVW site. The slow rate of phytodegradation of the perchlorate fraction taken up by plants during the growing season explained the detection of higher perchlorate concentrations in leaves collected later in the growing season (fall) and in senesced leaves compared to younger, live leaves. This proves that senesced plant leaves potentially recycle perchlorate back into the soil on which plant litter collects. To minimize the potential recycling of perchlorate during phytoremediation, it is recommended that senesced leaves be collected and composted or phytoremediation be designed to enhance rapid rhizodegradation (rhizoremediation). © 2007 Wiley Periodicals, Inc.

Selective bed-load transport in Las Vegas Wash, a gravel-bed stream

Summary Bed-load measurements collected at the Las Vegas Wash, a gravel-bed stream near Las Vegas, Nevada, were used to study selective transport of sand and gravel in uni-modal or weakly bi-modal river sediment. Measurements showed that size selectivity in a sediment mixture decreases as shear stress increases. Transport of variously sized sediment particles approaches equal mobility as the transported bed load is composed approximately of the same size particles as surface-bed material. Consequently, a hiding function was derived to account for the increase or reduction in reference shear stress for an individual size class in a sediment mixture as compared with that in an uniformly sized sediment. An empirical equation for determining fractional bed-load transport rate was then formulated by correlating the dimensionless, fractional bed-load transport rate with the dimensionless bed-shear stress. This equation indicated that the hiding function depends not only on the size of individual size class but also on the flow depth used to quantify the magnitude of shear stress. The present study contributes to the body of knowledge used in predicting selective transport of sediment mixtures in gravel-bed streams.

Characterization of dissolved organic matter in drinking water sources impacted by multiple tributaries

The characterization of dissolved organic matter (DOM) in drinking water sources is important as this material contributes to the formation of disinfection by-products (DBPs) and affects how water treatment unit operations are optimized. Drinking water utilities often draw water from sources impacted by multiple tributaries, with possible shifts in DOM concentrations and reactivity over time, depending on specific environmental conditions. In this study, results are presented on the characterization of DOM under varying ambient conditions from the four main tributaries of Lake Mead, a large reservoir in the southwest United States. The tributaries include the Las Vegas Wash (LVW), Muddy River (MR), Virgin River (VR) and the upper Colorado River (UCR). One additional sample was collected at the outflow of the reservoir (lower Colorado River (LCR)). The DOM was characterized by both bulk parameters (specific ultraviolet absorbance (SUVA)) and specific physicochemical properties, i.e. size, polarity and fluorescence. The analyses were performed emphasizing limited changes in its natural configuration by eliminating analytical preparation steps, excluding sample filtration (0.45 μm filter). Results indicate that each tributary had a different molecular weight distribution, as well as fluorescence properties, which helped in the identification of the relative source of DOM (allochthonous versus autochthonous). The largest apparent molecular weight distribution was observed for DOM samples collected at the MR site, which is fed mostly by groundwater seepage. The smallest apparent molecular weight was observed for DOM collected at the LCR site, suggesting that retention in the reservoir resulted in a decrease in molecular weight as a probable result of photo oxidation and microbial processes. Fluorescence analysis aided the differentiation of DOM by clearly identifying waters that were affected by microbial activity (LVW, UCR, and LCR), either by wastewater influence or by autochthonous processes, versus limited microbial influence (MR and VR). Polarity analysis revealed clear differences in the hydrophobic/hydrophilic nature between waters, including temporal differences within individual waters at a particular site. The DOM from the LVW and VR sites had higher hydrophobic character, as measured by retention onto non-polar sorbents. Additionally, the DOM collected at the LCR had the least hydrophobic character. This type of analysis would be beneficial to utilities who want to better understand and manage their source waters, especially in the evaluation of temporal variation within a watershed.

Perchlorate Accumulation and Potential Exposure from Durum Wheat Irrigated with Colorado River Water

Abstract The Colorado River is contaminated with low levels of perchlorate derived from aerospace-and defense-related fuel industries once located near the Las Vegas Wash. At sufficiently high doses, perchlorate can disrupt thyroid function by inhibiting uptake of iodide. Researchers have reported on the presence of perchlorate in lettuce (Lactuca sativa L.) grown in the lower Colorado River region. Recent work has demonstrated the potential for wheat (Triticum aestivum L.) to accumulate perchlorate when irrigated with perchlorate contaminated irrigation water in Texas. Over 45 000 ha of durum wheat (Triticum turgidum var. durum) are irrigated with Colorado River water in Arizona and California for the national and international pasta industries. Studies were conducted to determine perchlorate accumulation of wheat irrigated with Colorado River water. Perchlorate did accumulate in the above-ground plant components (982 to 3924 µg/kg dry weight (dw)) and roots (96 to 650 µg/kg dw) of immature durum wheat. For mature wheat, concentrations were greater in the stalk (490 µg/kg dw) compared to the spike (80 µ/kg dw). Most of the detectable perchlorate in the spike was in the rachis (125 µ/kg dw) and glumes (236 µ/kg dw). A majority of grain samples collected had perchlorate concentrations below levels of quantification by ion chromatography with tandem mass spectrometry (IC/MS/MS) (25&amp;lt;25 µg/kg dw. These results indicate potential exposure from the consumption of pasta products derived from durum wheat irrigated with Colorado River water are low relative to the reference dose adopted by USEPA.

Potential perchlorate exposure from Citrus sp. irrigated with contaminated water

Citrus produced in the southwestern United States is often irrigated with perchlorate-contaminated water. This irrigation water includes Colorado River water which is contaminated with perchlorate from a manufacturing plant previously located near the Las Vegas Wash, and ground water from wells in Riverside and San Bernardino counties of California which are affected by a perchlorate plume associated with an aerospace facility once located near Redlands, California. Studies were conducted to evaluate the uptake and distribution of perchlorate in citrus irrigated with contaminated water, and estimate potential human exposure to perchlorate from the various citrus types including lemon ( Citrus limon), grapefruit ( Citrus paradise), and orange ( Citrus sinensis) produced in the region. Perchlorate concentrations ranged from less than 2–9 μg/L for Colorado River water and from below detection to approximately 18 μg/L for water samples from wells used to irrigate citrus. Destructive sampling of lemon trees produced with Colorado River water show perchlorate concentrations larger in the leaves (1835 μg/kg dry weight (dw)) followed by the fruit (128 μg/kg dw). Mean perchlorate concentrations in roots, trunk, and branches were all less than 30 μg/kg dw. Fruit pulp analyzed in the survey show perchlorate concentrations ranged from below detection limit to 38 μg/kg fresh weight (fw), and were related to the perchlorate concentration of irrigation water. Mean hypothetical exposures (μg/person/day) of children and adults from lemons (0.005 and 0.009), grapefruit (0.03 and 0.24), and oranges (0.51 and 1.20) were estimated. These data show that potential perchlorate exposures from citrus in the southwestern United States are negligible relative to the reference dose recommended by the National Academy of Sciences.

A case for using adaptive platforms in the development and implementation of urban-centred adaptive management plans

The recurring challenges in developing and coordinating social frameworks for the effective adaptive management of environmental resources in urban environments may be addressed by developing platforms, similar to those promoted by collective action in the management of common pool resources. This paper compares how these two frameworks perceive and foster social organization vis-a-vis management action, particularly at the local scale of resource use. The significant potential for incorporation of platforms into the adaptive management framework is discussed, based on a set of issues identified in collective action research as important in the development of platforms. An urban-centred case study, the Las Vegas Wash Comprehensive Adaptive Management Plan, illustrates how an adaptive management framework, when its social components come together in a mutually supportive, heuristic way, begins to generate what in collective action is called a platform. The paper further explores how platforms may be more explicitly incorporated into the adaptive management framework to address some of the more persistent social organization difficulties for which adaptive management has been faulted, especially regarding scale issues and the urban context. A revised adaptive management model including a platform is provided, and suggestions made about issues that may be studied using this model.

Sources and Concentrations of Mercury and Selenium In Compartments within the Las Vegas Wash During A Period of Rapid Change

The Las Vegas Wash, which drains the Las Vegas valley watershed and provides the second largest inflow to Lake Mead, is being dramatically altered with erosion control structures and wetland restoration efforts. The impact of these changes on the cycling and distribution of Hg and Se is of particular interest because of their tendency to bioaccumulate and because of a lack of information on these contaminants in the Wash. In this study, we determined concentrations of Hg and Se in surface water (monthly), groundwater (once) and sediments (quarterly) from strategic locations within and along the Wash during 2002 and 2003. The data was used to characterize Se sources and loading into the Wash. Samples containing resurfacing groundwater and urban runoff (LW10.75 and Duck Creek) had significantly higher yearly means (13.7 +/- 4.4 and 23.8 +/- 4.1 microg/L, respectively) compared with mainstream samples containing primarily treated wastewater (2.8 +/- 0.8 microg/L). Investigation of Se in tributaries, street runoff and rain suggest that the source of the elevated Se is likely groundwater seeps located within a relatively narrow geographic band on the southeast side of the valley. Se content of sediments was similar, except for LW10.75 which was rich in organic matter. Hg concentrations in the water and sediments were low, averaging 4 +/- 5 ng/L and 34 +/- 20 ng/g, dw, respectively. Overall, this study suggests that water quality remains relatively stable despite changes in the Wash and managers of developing wetlands should not use tributary water as source water.