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<br />72
<br />
<br />J. GARciA-HERNANDEZ ET AL.
<br />
<br />transformed to their namrallogarithm to normalize the distribution, the rest of the data
<br />had a normal distribution and no transformation was applied. One-sample 1- or z-tests
<br />were used to compare mean selenium concentrations to a specific threshold. One-way
<br />ANOV A was used to compare .mean selenium concentrations among the different
<br />wetlands of the delta. In order to detect differences between the means of two
<br />groups of samples (either geometric means or arithmetic means), we used two sample
<br />I-tests. These statistics were used to compare the concentrations of selenium in bottom
<br />material to the concentrations in soil; to compare redox potential, pH, content of clay,
<br />silt and sand, organic carbon and dissolved solids among sites influenced by agriculmral
<br />runoff to sites influenced by river waters; and to compare concentrations of
<br />selenium in fish from sites influenced by the Colorado River to sites influenced by
<br />agriculmral runoff. Simple linear regression statistic analysis was used to identify
<br />relations between concentration of selenium in bottom material and the physical and
<br />chemical determinations measured at the field and laboratory.
<br />
<br />Results
<br />
<br />Selenium in bOllom malerial
<br />
<br />Distribution of Se concentrations in bottom material ( < 63 J.lm in size) cores from the
<br />Colorado River delta is shown in Table 2 and Fig. 2. Individual concentrations of
<br />selenium in bottom material ranged from 0.6 to 5.0 J.lg g -I, and the 90% confidence
<br />interval of the mean was between 0.7 and 3.1 J.lg g -I .
<br />The baseline selenium concentration for western soils is estimated to be < 1.4 J.lg g - 1
<br />dry wt. (Shacklette & Boerngen, 1984; Radtke el al., 1988). Half of the bottom material
<br />samples (21 samples) from the Colorado River delta exceeded the baseline for western
<br />soils. The sites that had 100% of their samples above the baseline were Bocana, Laguna
<br />del Indio, Zacatecas drain, Campo Rafael, and El Mayor. Sites with selenium concentra-
<br />tion in all samples below the baseline, were El Doctor, Ayala drain, and Hardy River.
<br />The threshold where sedimentary selenium can. cause adverse biological effects in
<br />10% of exposed fish and birds (EC10) is 2.5 J.lgg-I. Adverse effects are always
<br />observed at concentrations greater than 4.0 J.lg g -I (BC 1 00) (Skorupa et al., 1996;
<br />USDI, 1998). The mean Se concentration in bottom material from all sites in the delta
<br />(n = 41, geom. mean"" 1.5 J.lgg-I) was lower than the EC10 threshold (one sided
<br />p-value < 0.0001 from one-sample l-test, 1= 5.8 df. "" 40). Nevertheless, 22% (nine
<br />samples) exceeded the EC I 0 toxicity threshold. A hundred percent of the samples from
<br />Laguna del Indio exceeded this threshold, 67% from El Mayor, 30% from the Cienega
<br />de Santa Clara, 17% from Colorado River and 13% from the Cucapa complex also
<br />exceeded the threshold. Only 5% (two samples) exceeded the EC100 threshold in the
<br />delta and these were from El Mayor wetland. In soils, the mean selenium concentration
<br />(n = 10, geom. mean"" 1.03 J.1g g-I) was below the EC10 (one sided p-value = 0.001
<br />from one-sample I-test, I"" 4.1 df. = 9), although a sample from Laguna del Indio
<br />exceeded the EC10 threshold.
<br />No difference was found between soil and bottom material samples from the
<br />Colorado River sites (one-sided p-value = 0.12 from two-sample I-test, I = 1.6,
<br />dr. = 16) nor from El Indio location (one-sided p-value = 0.6 from two-sample I-test,
<br />1=0.3, df. = 1). The mouth of the river site (Bocana) did have a difference
<br />between soil and bottom material samples (one-sided p-value = 0.006 from two-sample
<br />I-test, I = 12, df. = 2), Se concentration in soil was lower than concentration in bottom
<br />material. '
<br />No differences were found in the concentrations of selenium in bottom material
<br />samples among the different locations in the Colorado River delta listed in Table 2
<br />(one-way ANOVA FlI.29 = 1.73; p-value = 0.11).
<br />
<br /> SELENIUM, SELECTED INORGANIC ELEMENTS FROM COLORADO RIVER DELTA 73
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