WSP07583

• File Number: 8270.200
• Description: Selenium
• State: CO
• Basin: Gunnison
• Water Division: 4
• Date: 1/1/1996
• Author: USGS
• Title: Detailed Study of Selenium and Other Constituents in Water-Bottom Sediment-Soil-Alfalfa and Biota Associated with Irrigation Drainage - Uncompahgre Project Area and in the Grand Valley - 1991-93
• Water Supply Pro - Doc Type: Report/Study

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Residual soils derived from Mancos Shale (Km) and
alluvium overlying Mancos Shale (Qm) contained the
highest geometric mean selenium concentrations of
2.2 and 2.91lglg (table 2). These values are 10 to
13 times the geometric mean for Western soils.
The geometric mean total-selenium concentrations
in soils derived from the Quaternary terrace deposits
(QI), recent flood-plain alluvium (Qr), and Dakota
Sandstone (Kd) were lower than the geometric mean
concentrations in soils derived from Mancos Shale
(Km), but exceeded the geometric mean for Western
soils.
The geometric mean concentrations of water-
extractable selenium for the soil samples, grouped by
geologic unit, are listed in table 2. Concentrations
ranged from 0.005 to 1.8 Ilg/g dry weight. As with
the results of total"selenium analysis, the geometric
mean concentration of water-extractable selenium was
higher in alluvium overlying Mancos Shale (Qm) than
in the residual soils derived from Mancos Shale (Km).
The percent of total selenium that was water extract-
able was unifonnly low for all geologic units and
ranged from 1.5 to 2.7 percent.
Total-selenium concentrations in ]28 alfalfa
samples from the Uncompahgre Project area ranged
from less than 0.03 to 9.5 Jlglg dry weight (table 2).

Data for individual samples are listed in Crock and
others (1994). The geometric mean selenium concen"
tration of 0.33 Jlglg (table 2) for all alfalfa samples
exceeded the recommended minimum for callIe feed
of 0.1 Jlglg (Fisher and others, 1987). Only five a]falfa
samples exceeded the recommended dietary tolerance
limit of 4 Jlglg (Byers, 1935). The alfalfa data from
the Uncompahgre Project area indicates lower
selenium concentrations compared to the Kendrick
Reclamation Project area in Wyoming, where
IS percent of alfalfa samples from irrigated fields
exceeded the tolerance limit (See and others, 1992).
As with total and water-extractable selenium in soil,
the highest geometric mean selenium concentrations
in alfa]fa were in plants growing on alluvium
overlying Mancos Shale (Qm).
Selenium concentrations in a]falfa do not
show the same trend as selenium concentrations in
soil when the geometric means for the geologic units
are compared. Residual soils derived from Mancos
Shale (Km) and Quaternary terrace deposits (Qt)
contained higher total"selenium concentrations in
soil than did recent flood-plain alluvium (Qr), but
selenium concentrations in alfalfa from units Km and
Qt were significantly lower (significance level 0.05)
than the concentrations in alfalfa from unit Qr.

Table 2. Geometric mean and observed range of total. and water"extractable selenium concentrations in soil and total.
selenium concentrations in alfalfa for all samples from the Uncompahgre Project area, by geologic unit

(Selenium concenlralions in soil and alfalfa, in micrograms per gram; geologic units are defined as follows: Kd. Upper Crelaceous Dakota Sandslone;
Km. Upper Cretaceous Mancos Shale; Qt. Qualemary terrace and alluvial. fan deposils; Qm. QU81emary (Holocene) alluvium overlying Mancos Shale;
Qr. Quaternary (recent) flood-plain alluvium deposited by lhe Uncompahgre River and olher slreams; <. less lhan; analytical duplicates nor included;
data for all samples includes one sample for geologic unit Qd. Quaternary alluvium overlying Oakola Sandstone)

Statistic All samples Kd Km QI Om Or
SOIL, TOTAL SELENIUM'
Detection ratio t42:142 11:11 43:43 28:28 36:36 23:23
Geometric mean 1.4 .5 2.2 1.0 2.9 .6
Observed range .1-8.6 .3-.6 .3--7.5 .3-3.4 .9-8.6 .1-2.9
SOIL, WATER-EXTRACTABLE SELENIUM'
Detection ratio 128:128 9:9 40:40 24:24 33:33 21:21
Geometric mean .027 .009 .043 .013 .048 .018
Observed range .005-1.8 .005-.019 .005-1.8 .005-.062 .008-.86 .005-1.0
ALFALFA, TOTAL SELENIUM'
Detection ratio 118:128 10:11 31:35 25:26 30:34 21:22
Geometric mean .33 .12 .28 .25 .56 .48
Observed range <.03-9.5 <.03-.69 <.03--1.6 <.03-1.8 <.03-6.3 <.03-9.5
IOala from Crock and others (1994). Site duplicates were considered independent samples.
20ata from Stewart and others (1993). Sile duplicales were averaged to obtain one value for each site.

SOILS AND ALFALFA INVESTIGATIONS 19