9626

• UCREFRP Catalog Number: 9626
• Author: Butler, D. L.
• Title: Effects of Piping Irrigation Laterals on Selenium and Salt Loads, Montrose Arroyo Basin, Western Colorado.
• USFW Year: 2001.
• USFW - Doc Type: Denver, CO.
• Copyright Material: NO

EFFECTS OF PROJECT ON SELENIUM
AND SALT LOADS
The placing of five laterals in pipe in the
Montrose Arroyo Basin significantly reduced sele-
nium loading and, to a lesser degree, reduced salt
loading. Results are summarized in tables 4 and 5.
More than 90 percent of the decrease in total-
selenium load at the four sites downgradient from the
lateral work (all sites in table 4 except site MA4) was
attributed to decreases in the ground-water component.
Because of the low selenium concentrations in the
incoming irrigation water (table 2), the selenium load
in the irrigation wastewater and tailwater was a small
part of the total selenium load (table 4). At the outflow
site at Niagara Street (site MA2), the total-selenium
load decreased about 194 lb/yr, or 28 percent (table 4;
fig. 3). Site MA2 was the only site in the Montrose
Arroyo Basin with selenium data prior to 1998 (fig. 4);
therefore, the pre-project loads for this site may be
subject to less uncertainty than loads at the other four
sites, where sampling did not begin until June 1998.
Also noteworthy is the greater than 50-percent
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d
Z 500
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400
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300
a
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100
decrease in selenium load in drainage ditch D2MA
(table 4; fig. 3), apparently the result of placing the
AME lateral in pipe (fig. 2). At control site MA4, sele-
nium loads essentially did not change (table 4; fig. 3).
From an inspection of figure 4, it appears that
the post-project selenium loads for individual samples
collected at site MA2 were less than the pre-project
loads. However, because of the variation in the loads,
it is difficult to know if the differences in loads
between periods are statistically significant. To deter-
mine if the loads were significantly different, the two-
sample t-test (Helsel and Hirsch, 1992) was used on
concentration and load data to compare the pre- and
post-project periods. Because some of the data sets do
not have a normal distribution, the t-tests were done on
the ranks of the data rather than on the original data.
The tests were done to compare the individual sample
concentrations and loads between the pre- and post-
project periods for selenium and salt. A significance
level of 0.05 was used to accept or reject the null
hypothesis that concentrations or loads between two
data sets are equal. The data for each period are sepa-
rated into irrigation-season (April-October) and
MA4
Figure 3. Comparison of selenium loads in Montrose Arroyo Basin before and after the lateral project.
EFFECTS OF PROJECT ON SELENIUM AND SALT LOADS
D21VIA MA3 D1MA MA2
SITE