WSPC00560

• File Number: 8270.200
• Description: Colorado River - Colorado River Basin Salinity Control Program - CRBSCP
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 10/1/1989
• Title: Selenium Contamination from Irrigation Drainage in the Western United States with Emphasis on Utah
• Water Supply Pro - Doc Type: Report/Study

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"IU.. ..~l':r peoW...-1A.,"'..tern
8011. and ri..~.. Over 70 perc.nt of
the 10 .il1ion ton. of di.aolved .elt.
in tb. Colorado Riv.r at Soov.r Da.
ar. fro. .vaporite. in 1708ing,
Colorado, and Utah (Whittig and oth-
.r., 1'82). Th. Manco. Shal., a
Cretac.ou. marine forution coaaon in
the Uinta Basin, eentral Utah, and
....t.rn Colorado, is a principl.
contributor of salts to the Colorado
River.
Th. pr.s.nce of s.l.niua in
geologie mat.rials does not guarantee
that the element is present in hazard-
ous coneentrations in soils derived
from these strata. Tbe~eolubility of
selenium in-solls.1s d.termined.~y the'
pH, moisture. .cntidati6ii'-t'i1duetion
conditions, and--degree -of -a.rft~Oh.
Unde.r. _alkaline, arid. and generally
well a.rated conditions pre..nt - -il:l
many soils of tb. "'.stern United
States, eonsiderable selenium ..y be
mobilized in the oxidiz.d and readily
soluble .s.l.aat..for~. Seleniferous
soils in arid areas average about 5
parts per million but may contain as
much as 80 parts per million (Tre-
lease, 1945). In seleniferous areas
where rainfall exceeds 25 inches
annually, leaching removes selenium at
a slow constant rate, reducing the
potential for evaporative conc.ntra-
tion (Lakin, 1961). Because most mape
.how precipitation contours in incre-
ments of 10 inChes, the-20-inch con-
tour has. been cbosen to repraa.nt the
annual precipitation n.ce..ary to
slowly l.ach s.laniu. fro. soila
(figs. 2 and 3).
Total-Ielenium content of soi~s
ie DOt a reliable indicator of seleni-
. um that is available to plants. In
moist, acidic, and r.ducing soils,
selenium may be present as insoluble
lelenides, elemental I.leniua, or
inloluble pyritic selenites whieh are
not taken up by plants. Selenates and
organic-selenium complexes produced by
microorganisms are ths most soluble
for.s and are comaon in seleniferoul,
alkaline s_o,il.1 found in tbe weltern
United State.. Wat.r draining th..e

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189

lOil. l.ach.. .olubl. ..l.nit.. and
..l.nat.. and al.o carr i.. di..olv.d
and .u.p.nd.d organic for.. of
..l.nillll.
Irrigation of ..l.nif.roul eoils
CIn gr..tl:r-iaer.... .th. conc.ntra-
tiGll.--of....twa1a 1n river. down.treaa
fra'-iu1g.Eid areBl. Williaml and
Byar. (1'35) reported that tha Colora-
do Riv.r had a a.leniu. concentration
of oaly 1 .icrogr.. par liter (ugIL)
upatre.. troa tb. Cunni Ion and Uncom-
pabgre "'1ver -baains (200 .Ues south-
ea.t of tbe Uinta Ba.inl, both of
which had considerable irrigation.
Th. Colorado River dowaatreaa from the
Gun~iSOD aDd.UDe~pahgre,R~er basins
contained 30 -ug/L. of ....l.niru.. at-
tributed to irrigation returns. Flood
irrigation, poDding, and the relultant
evaporative concentration in poorly
drained soils can result in large
concentrations of selenium in surface
runoff and partieularly in shallow
ground water. Bowever, a .ubatantial
quantity of dhsolved ....hnium in
water. ..y be pr.cipitated by basie
oxida. such as hydrous f.rric oxide
(Combs aDd Combs, 1986, p. 18).
Mlny agricultural areas in Utah,
partieularly the Uinta Basin, are
underlain by s.leniferous deposits.
r.ceive far le.s than 20 inehes of
pr.cipitation annually, and ara irri-
gated. In so.e of the.e potential
probl.. ar.aa. little or no reliable
data are available on eoneentrations
of sel.nium in surface or ground
vatar. Where data are available. so..
area. have been identified where water
contain. concantrations of dil.olved
aeleniu. which exeeed the Utah-stand-
ard-of 50 ug/L ..tablished to protect
aquatic wildlife (Utah Department of
Social Service.. 1978, a.andad 1983).
These areas generally coincide with
the presence of g.ologie sources of
seleniu. and annual precipitation of
less than 20 inches (fig. 3). Within
the Uinta Basin, there is considerable
irrigation upstrea. fro. several of
the sites where conta.ination is most
evident.