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68 J. GARCIA-HERNANDEZ ET AL The Cienega de Santa Clara This marsh on the eastern edge of the delta was created in 1977 by brackish agricultural drain water from Yuma via the Main Outlet Drain Extension (MODE), The flow created a wetland of 20,000 ha water surface of which 4500 ha are thickly vegetated. The marsh is dominated by cattail (Typha domingensis) (Glenn et ai., 1996). EI Doctor These desert springs or pozos, located on the eastern portion of the delta are a separate system with little or no interaction with the Colorado River or with the Cienega de Santa Clara. Dissolved solids in the springs range from 100 to 3000 mg I-I which allows for a great diversity of plants (Glenn et ai., 1996). Intertidal wetlands Primarily marine area that consists of approximately 33,000 ha of-extensive tidal mudflats along the coast of the upper Gulf of California (Glenn et al., 1996). Sample collection A total of 41 bottom material cores (Table 1), nine soil samples, and 34 discrete water samples were collected from 12 locations in the delta in April 2000. Position was recorded at each site using a GPS unit (Garmin~i' 12XL). Water depth, temperature, dissolved oxygen (YSI~l' Model 55 oxygen meter), specific conductance (CON 5~' portable conductivity meter), water pH (Digi-sense~l digital pH/temp/mV/ORP meter with a general purpose electrode), water and bottom material redox potential (Digi- sense~l' digital pH/temp/mV/ORP meter with a platinum redox electrode), were mea- sured in the field. Bottom samples were collected using an AMSo" stainless steel sludge sampler with a core tip adapted with a butterfly valve to minimize loses of fines. A cleaned (previously rinsed with 5% nitric acid) butyrate plastic liner was inserted into the sampler and replaced with a clean liner after each sampling to prevent cross- contamination. The core obtained by this method measured 7.6 em diameter and 20 em long. Liners were capped and transported chilled to the laboratory, afterwards samples were kept at 40C until their analysis. Twelve Colorado River delta locations were visited on ten occasions from March 1998 to May 2000 for biota sampling (Table 1). We collected 98 samples of biota. All samples were analysed for selenium, 24 of the samples were analysed for metals, and 30 samples for organochlorine pesticides. Fish were collected using gillnets (0.5-cm mesh size), dip nets, or minnow traps baited with cat food. Invertebrates and aquatic insects, were collected using minnow traps. A sample consisted of a composite of more than ten organisms of the same species and similar size. Weight and length of each organism was recorded in the field. Composite samples for organochlorine analysis were stored in precleaned glass containers and composite samples for inorganic analysis were wrapped with aluminum foil inside plastic bags. All samples were transported chilled to the laboratory and stored frozen until chemical analysis. Chemical analysis Each sample of bottom material was homogenized and an aliquot was oven dried at 600C for 12 h, and ground. Prepared samples were analysed for free iron oxide, percent clay, silt and sand, percent organic carbon, and for water content at the Soil, Water and SELENIUM, SELECTED INORGANIC ELEMENTS FROM COLORADO RIVER DELTA 69 Plant Analysis Laboratory (SWPAL) of the University of Arizona. Water samples Were analyzed for their acid-neutralizing capacity (ANC) and dissolved solids, also at SWPAL. Another aliquot of the homogenized bottom material sample was used for selenium analysis. This aliquot was sieved through a 63-j.lIIl sieve over a 500 ml plastic bottle. The sample was wet-sieved using native water until the bottom material was approximately 1 em deep in the receiving bottle. The sample was allowed to settle for 3 days, afterwards the supernatant was decanted and the obtained bottom material was used for analysis. The samples were dried at 600C for 12 h and they were ground using mortar and pestle. Soil samples were also sieved through a 63-j.lIIl sieve (Shelton & Capel, 1994). Prepared bottom material samples were analyzed for selenium at V.I. Vernadsky Institute of Geochemistry and Analytical Chemistry using Instrumental Neutron Activa- tion Analysis (INAA). In this procedure, 100 mg of each sample and reference material were irradiated in a research reactor using a slow neutron flux. Induced radioactivity of the samples was then measured with a Nokialll' gamma ray spectrometer with 4096 channels and with a Ge(Li) high resolution detector. Six check samples were analysed at the Research Triangle Institute, RTI by Graphite Furnace Atomic Absorption (GFAA). Detection limit for selenium in bottom material samples using either method was 0.5l!gg-'. Each composite sample of biota (whole body) was homogenized using an industrial blender. Prepared samples were sent to RTI laboratory for the analysis of the following elements: AI, As, B, Ba, Be, Cd, Cr, Cu, Fe, Hg, Mg, Mn, Mo, Ni, Pb, Se, Sr, V, Zn. Analysis were done using Inducted Coupled Plasma (ICP) spectrometer except for selenium and mercury which were analysed by graphite furnace and by cold vapor atomic absorption, respectively. Additional biota samples were analysed at the SWPAL for selenium by graphite furnace atomic absorption. Animal tissue was analysed for organochlorine pesticides at Patuxent Analytical Control Facility, PACF. Pesticides .were quantified with a gas-liquid chromatograph (GLC), equipped with a 63Ni electron-capture detector. c.::" c.:::J CA.' i:'j i ... t:.'fl Quality control/quality assurance procedures Procedural blanks analyses were performed at each laboratory with no anomalies detected. Relative percent differences (RPD) for bottom material by INAA method averaged 9.5 (n = 2). In most of the trace elements analysed on fish and invertebrates samples, RPD resulted in < 15 (n = 6), with the exception of boron, lead, and mercury which had an arithmetic mean of 48,28 and 28 RPD, respectively. For organochlorine pesticides, RPD was 0 (n = 6) on fish and invertebrate samples. Percent recoveries of reference material (marine sediment IAEA-356, International Atomic Energy Agency-356) had a mean of 70%. The RPD range between samples analyzed by INAA at V.I. Vernadsky laboratory compared to samples analysed by GFAA at RTI laboratory varied from 0.7 to 11 (n = 6). NRCC TORT -2 Oobster hepatopancreas) was used as reference tissue for metal scan of biota samples. All samples analysed at RTI differed by less than 20% from the reference (n = 3 for each element). Spike recoveries obtained for metals were all greater than 90% (n = 3 for each element). Spike recoveries for organochlorines pesticides were 113%, 84% and 88% for DDD, DDE, and DDT respectively. Statistical analysis Statistical analyses were performed using ]MP<(l. software of the SAS Institute Ine, (Sail & Lehman, 1996). Only concentrations of selenium in bottom material were