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<br />Sample Collection and
<br />Analytical Methods
<br />
<br />Many of the shallow wells used for the ground-
<br />water investigation were installed by the BOR as part
<br />of the salinity-control program in the Uncompahgre
<br />Project area and in the Grand Valley (Bureau of
<br />Reclamation, 1986a, b). Five shallow monitoring
<br />wells were installed by the USGS for this study using
<br />the hollow-stem auger method-three wells were
<br />installed in alluvium (Reed Wash type area) and two
<br />wells were installed in shale residuum (Sweitzer Lake
<br />type area). Aquifer-sediment samples were collected
<br />at 5-ft intervals using a split-spoon sampler.
<br />Sediment-core samples for trace-element analysis
<br />were air dried, and samples for selenium-speciation
<br />analysis were frozen. After installation of 2-in.
<br />polyviny] chloride (PVC) casing with 5-ft screens,
<br />the wells were developed by purging about 30 well
<br />volumes of water from the wells. Depths of wells in
<br />the study area (BOR, USGS, and private) ranged from
<br />8 to ] ,000 ft below land surface.
<br />Boreho]e samples were collected during insta]-
<br />]ation of the shallow wells, topical samples were
<br />collected (outcrop of ash layers), and Mancos Shale
<br />bedrock-core samples were oblained from cores
<br />archived at the USGS Core Research Center in
<br />Denver, Colo. Selected sediment-core, bedrock-core,
<br />and topical samples were analyzed for (I) major and
<br />trace constiluents by ICP-AES (Crock and others,
<br />1983); (2) selenium and arsenic by continuous-flow
<br />HGAAS (Crock and Lichte, 1982; Sanzolone and
<br />Chao, 1987); (3) uranium and thorium by delayed
<br />neutron-activation counting (McKown and MaI]ard,
<br />]987); and (4) total organic carbon by combustion-
<br />infrared detection (Jackson and others, 1987).
<br />Sequential extractions were performed on selected
<br />sediment-core samples (Fio and Fujii, 1990), and
<br />selenium specialion of the extraction fluids was
<br />performed by column chromatography and HGAAS
<br />(Makita and Fujii, ] 992). X-ray diffraction (XRD)
<br />analyses were performed on bedrock, shale residuum,
<br />alluvium, and ground-water-filter residuum samples
<br />by L.M. Fukui (Senior Petrologist, Chern-RUST
<br />Geotech, Grand Junction, Colo.). Bedrock, shale
<br />residuum, alluvium, and ground-water-filter
<br />residuum samples were analyzed by scanning
<br />electron microscopy (SEM).
<br />Water samples from wells and one spring
<br />were analyzed for (l) major ions, (2) selected trace
<br />constituents (including uranium), (3) nitrogen species,
<br />
<br />(4) dissolved organic carbon, (5) stable isotopes of
<br />oxygen and hydrogen, (6) nitrogen isotopes of
<br />dissolved nitrate. (7) sulfur isotopes of dissolved
<br />sulfate, and (8) selenium species. Three to six
<br />well volumes of water were purged from the wells
<br />prior to sample collection, using a centrifugal
<br />pump. Samples were collected using a rinsed
<br />Teflon bailer and were carefully decanted into
<br />clean I -L polyethylene bOllles, using a bollom-
<br />emptying device on the bailer to prevent sample
<br />agitation and loss of volatile components (Barcelona
<br />and others, 1984). Disso]ved-oxygen measurements
<br />for ground water were performed within the borehole
<br />by lowering the oxygen probe (Yellow Springs
<br />Instrumenls, Inc., dissolved-oxygen probe) on a
<br />cable and moving the probe vertically about I ftls
<br />in the middle of the screened interval; the disso]ved-
<br />oxygen meter was calibraled according to atmospheric
<br />pressure and the ambient ground-water temperature,
<br />and the meter was set to zero using a solution devoid
<br />of oxygen. Sample filtration and preservation,
<br />field-alkalinity titration, and measurement of water
<br />properties (pH, Eh, and specific conductance) were
<br />performed in a field vehicle. Oxidation-reduction
<br />potential (Eh, or redox potential) was measured
<br />using a platinum electrode, which was checked
<br />against ZoBell's solution at the ambient ground-
<br />water temperature. Water samples were filtered
<br />through a 0.45-j!m filter for a]kalinity titrations,
<br />dissolved major-ion, nitrogen species, and trace-
<br />constituent analyses. Samples were filtered through
<br />a 0.20-j!m filter for sulfur-isotope and selenium-
<br />speciation analyses. Samples collected for dissolved-
<br />organic-carbon analyses were filtered through a
<br />0.45-j!m silver filter. Samples collected for uranium
<br />and oxygen- and hydrogen-isotope analyses were
<br />unfiltered. Water samples collected for cation
<br />analyses were acidified using nitric acid to pH < 2;
<br />samples collecled for nilrogen species analysis were
<br />preserved using liquid mercuric chloride; samples
<br />collected for mercury analyses were preserved using
<br />potassium dichromate; and samples collecled for
<br />oxygen and hydrogen stable isotopes were preserved
<br />using mercuric chloride tablets. Water-sample filters
<br />were saved and dried for SEM and XRD analyses to
<br />describe the particulate phases that might affect
<br />ground-water quality.
<br />A data logger was installed in one well in
<br />the Grand Valley for continuous recording of the
<br />water leve] and specific conductance of the ground
<br />
<br />24 Detailed Study 01 Selenium end Other Conellluents In Water, Bottom Sedlmant, Soli, Alfalfa, and Blola Associated with
<br />Irrigation Drainage In tha Uncompahgre Project Area and In the Grand Valley, West-Centrel COloredo, 1991-93
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