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

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Figure 10. Median percentages of dissolved iron in percentage of total. recoverable iron
concentrations in the upper Arkansas River, April 199O-March 1993.

42 percent at Leadville to a minimum of 3 percent at
Portland (fig. 10). The median total-recoverable
iron concentrations were largest during snowmelt
runoff and increased from 665 !,g/L at Leadville
to 4,600 !,g/L at Portland, an increase of almost
600 percent (fig. 9). Total-recoverable iron concentra-
tions were much smaller during the other three flow
regimes, and tbey were not substantially di fferent from
one another (fig. 9). The downstream increase in total-
recoverable iron concentrations during snowmelt
runoff probably was caused by the resuspension and
downstream transport of iron-enricbed fluvial sedi-
ment. The elevated streamflow during snowmelt
runoff has the largest potential effect on the resuspen-
sion and downstream transport of sediment. An
analysis of total-recoverable iron-load contributions
indicates that tributary contributions of iron typically
were small (<10 percent) compared to the main-stem
loads (table 9) with the exception of California Gulch,
Lake Fork above Halfmoon Creek, and Lake Creek
below Twin Lakes Reservoir.

Dissolved and total-recoverable iron concentra-
tions decreased significantly at the LMDT during the
post-treatment period, but concentrations at California
Gulch showed no significant change (table 10).

Nonpoint-source loading of iron to California Gulcb
from sources otber tban the Yak Tunnel probably
contributed a substantial amount of iron, which
negated the effects of the treatment plant. Iron concen-
trations at most main-stem Arkansas River sites were
not statistically different during the pre-treatment and
post-treatment periods (table 10). The results of the
statistical tests of pre-treatment and post-treatment
concentrations were not unexpected owing to the wide
distribution of iron-enriched mine tailings and the
ubiquitous nature of iron throughout the basin.
Chronic stream-water-quality standards for
dissolved and total-recoverable iron were exceeded
in many samples throughout the upper basin during
the study period (table II). There was no acute
standard for iron. Two samples collected at Granite
exceeded the chronic standard for dissolved iron
(300 !,g/L). The chronic total-recoverable iron
standard (1,000 !,g/L) was exceeded in about 7 percent
of the samples collected at Leadville. The percent
exceedance increased downstream to a maximum of
about 42 percent at Portland (table II). Most chronic
iron-standard exceedances occurred during snowmelt
runoff.

24 W8t8r-Qu8l1ly Assessment 01 the Arkansas River Basin, Southeastern Colorado, 1991l-93