WSP10284

• File Number: 8170
• Description: Arkansas Basin Water Quality Issues
• State: CO
• Basin: Arkansas
• Water Division: 2
• Date: 1/1/1998
• Author: USGS
• Title: Water-Quality Assessment of the Arkansas River Basin - Southeastern Colorado - 1990-93
• Water Supply Pro - Doc Type: Report/Study

Tabla 11. Number of samples that exceeded chronic iron water-quality standards In the upper
Arkansas River, April 199(}...March 1993

{--. no standard available]

Site name
(.ee "ble 1)

Number of
samples
analyzed

Number of samples that exceeded standard'
Chronic Chronic
stand8rd for standard for
dissolved Iro"2 tote I-recoverable Iron3

2
4
3
3
5
7
8
114

Leadville 28
Malta 26
Granite 27 2
Buena Vista 28 0
Nalhrop 27 0
Wellsville 27 0
Park dale 27 0
Portland 26 0
lColorado Department of Health, 1994.
2300 tTIlcrograms per liter.
3Unless nOled: Chronic standard = 1,000 micrograms per liter.
41,200 micrograms per liter.

Lead

Dissolved-lead concentrations in the upper
Arkansas River exhihited very different spatial and
temporal variability patterns compared to the other
trace elements of concern (fig. II). Median dissolved-
lead concentrations typically were largest during
snowmelt runoff; the largest median concentrations
were at Malta (1.1 ~g/L) and at Nathrop (1.0 ~g/L)
(fig. II). Moore and Ramamoorthy (1984) reported
that the dissolved-lead concentrations in unpolluted
freshwater typically are less than 3 ~g/L. During
early snowmelt and snowmelt runoff, dissolved-lead
concentrations increased substantially between
Leadville and Malta. During snowmelt runoff and
post-snowmelt runoff, concentrations increased
substantially between Buena Vista and Nathrop
(fig. 1I). DOMlstream from Leadville, concentrations
of dissolved lead were affected minimally by changes
in the partitioning oflead between the dissolved and
particulate phases; the dissolved-lead fraction
remained relatively constant (5 to IS percent)
throughout most of the upper basin (fig. 12). Total-
recoverable lead concentrations were largest during
snowmelt runoff, and the maximum median concen-
tration (51 ~g/L) occurred at Nathrop (fig. II). There
was little variability between total-recoverable lead
concentrations during the other three flow regimes;
concentrations were much smaller than during snow-
melt runoff (fig. II). Tbe elevated concentrations of
dissolved and total-recoverable lead at Malta (fig. II)

are probably attributable to mine drainage, primarily
from California Gulch. The elevated lead concentra-
tions at Nathrop cannot be accounted for by inflow
from the two major tributaries in the reach, Cotton-
wood Creek and Chalk Creek (pI. 1) because their
concentrations were generally less than those in the
main stem of the river (Dash and Ortiz, 1996). Resus-
pension of lead-enriched fluvial sediment and
nonpoint-source contributions are possible sources of
the elevated lead concentrations at Nathrop.
An analysis of instantaneous lead loads indi-
cates that the only tributaries that contributed a
~ubstantiallead load to the main-stem river throughout
the study period were California Gulch in reach I and
Lake Creek in reach 2 (table 12). Lead loading from
California Gulch was solely a function of extremely
elevated lead concentrations in California Gulch
(Dash and Ortiz, 1996). Streamflow typically was less
than 2 fi3is, but the median total-recoverable lead
concentration was 125 1!g!L (Dash and Ortiz, 1996).
Pre-treatment and post-treatment dissolved-
lead concentrations at the two mine-drainage sites
were not tested with the Wilcoxon rank-sum test
because more than 50 percent of the data were less
than the reporting level. Tbe median total-recoverable
lead concentration at the LMDT decreased signifi-
cantly from 7 ~g/L to less than I ~g/L (table 13).
However, lead concentrations in the pre-treatment and
post-treatment periods were not statistically different
at most main-stem Arkansas River sites (table (3).

26 Walar-Quallty Assessment 01 tha Arkansas River Basin, Southaastern Colorado, 1991l-93