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<br />increased concentration of major dissolved constitu- <br />ents, as wel1 as dissolved uranium in the lower <br />Arkansas River Basin. Zielinski and others (1995) <br />identified increased uranium concentrations as the <br />river flowed out onto agricultural1y developed lands <br />underlain by marine shale and limestone. A marked <br />increase in uranium also was observed along the reach <br />of the river where irrigation return flow can make up <br />most of the streamflow in the Arkansas River. <br />Uranium samples col1ected as part of this study <br />showed the same downstream trend (fig. 29). <br />Adverse effects associated with exposure to <br />uranium are a concern to water users because of its <br />radioactive nature and its ubiquitous occurrence in the <br />environment. The proposed drinking-water standard <br /> <br />45 <br /> <br />0: <br />W <br />t:: <br />~ 35 <br />0: <br />w <br />"- <br />"' <br />" <br /><< <br />ffi 30 <br />0 <br />0: <br />" <br />:> <br />z <br />;. 25 <br />0 <br />~ <br />I- <br />Z <br />~ 20 <br />z <br />0 <br />" <br />" <br />::> <br />~ 15 <br />0: <br />::> <br />6 <br />w <br />> <br />~ <br />@ 10 <br />"' <br />Ci <br />5 ~ <br /> 0 <br /> z <br /> <br />o <br />.J>.j,*-6 <br />\J> <br /> <br />Upper Arkansas River Basin <br /> <br />40 <br /> <br />,a) <br />o <br /> <br />NumtJer 01 observabons <br />OI.lllier data value more than 3 times Ihe <br />interquanlle range outside the quartile <br />OJI~81 dtIU1 value I6S5 than or equal to 3 <br />and more than 1 5 times ttle <br />Interquar1lle ran~ oulSide the quartile <br />Data ...akA less than or equal to 1.5 M'lOS thla <br />Int8~artlle range outsioe the quartile <br />75th percenlila <br />MedIan <br /> <br />. <br />for natural uranium is 20 I'gIL and is based on protec- <br />tion from kidney toxicity (U.S. Environmental Protec- <br />tion Agency, 1991). The Colorado Water Quality <br />Control Commission (Colorado Department of Health, <br />1994) adopted an instream uranium standard of <br />40 pCiIL (58.8 I'gIL) or natural background where <br />natural concentrations exceed 40 pCiIL. The standard <br />was based on approximate routine human exposures <br />given the background concentrations present in Colo- <br />rado and the smal1 statistical risk of human health <br />hazards at this concentration. Instantaneous dissolved <br />natural uranium concentrations exceeded the proposed <br />drinking-water standard at two sites, once at Las <br />Animas (36 I'gIL) and twice at Below John Martin <br />Reservoir (29 I'gIL). The median value for dissolved <br /> <br />Lower Arkansas River Basin <br /> <br />(5) <br /> <br />(4) <br /> <br /> (5) <br /> (7) (5) ~ <br /> (8) (8) * e <br /> 0 * (7) <br />... . . ... ... ~ . ~ ... ~ ... ... <br />. {j 1; . ~ . 1; ~ . ~ <br />" " " ~ " " " " " <br />0 0 0 0 0 0 0 0 0 0 0 <br />z z z z z z z z z z z <br /> <br />#'tJ. ~'lJ.(..~6 ..s.~,'lJ. .....~~~ ~~t; ~::.'lb __,q/J G~-4.. ~ ,.rO\\ iJ.o ~ rP~e ...? <;)#' ~~'lJ> .~'lJ.? ~o"4- ~'i> <br />o .;Jb(\'lJ. ~ '5 ~0~ ,?r".tfF qO ~~ <?.:P ~~ l--"\O ~rJr ~\(;- '->~ t-~ <(.~e -t# <br />~ c;.,,).o ~{j v~..y,j>~ pO. <br />~.,. ~ of' <br />~~ "irFf:o <br />~ ~ <br />'?I' <br />Figure 29. Variations in dissolved-uranium concentrations in the Arkansas River, April 199(}--March 1993. <br /> <br />"* <br /> <br />~ <br /> <br />25th percenllle <br /> <br />58 Wltlr-QUlllty A.8I8Smont oIthl Arkonso. River BI.ln, Southolatlm Colorado, 199tH13 <br />