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

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1993 1994 1995 1996 1997 1998 1999 2000
Figure 4. Selenium loads in Montrose Arroyo at Niagara Street (MA2), 1993-2000.
nonirrigation-season data sets (table 3) for the statis-
tical test. The selenium loads for the post-project
samples were significantly less than the pre-project
loads for both seasons (p values 0.001 for both
seasons). Mean selenium concentrations and loads in
the nonirrigation-season samples at site MA2 (tables 1
and 3) were significantly less in the post-project period
than in the pre-project period (p values 0.005 and
0.010).
Salt loads also decreased at all sites except at the
control site (table 5; fig. 5). However, the relative
percentage of change in total salt loads was consider-
ably less than the percentage of change in total sele-
nium loads. The maximum percentage of change in
salt load was the 32-percent decrease in drainage ditch
D2MA. A greater percentage of the decrease in salt
load was caused by reduced lateral waste discharges
(the change in the surface-water component)
compared to selenium load decreases; and at the
outflow site (MA2), about 37 percent of the salt-load
decrease (1,070 divided by 2,900) was in the surface-
water component (table 5). By comparison, only 8
percent of the decrease in selenium load at site MA2
was in the surface-water component (16 divided by
194, table 4). Based on the loading analysis in tables 4
and 5, a greater percentage of the total salt load is in
the surface-water component compared to the total
selenium loads. For example, for the pre-project
period at site MA2, the surface-water component is 5
percent of the total selenium load but 14 percent of the
total salt load. Therefore, the elimination of the waste-
water flows from the laterals that were piped had a
proportionately greater effect on the salt loads than on
selenium loads.
Statistical tests indicate that the post-project salt
loads are significantly less than pre-project loads,
especially during the irrigation season. The mean irri-
gation-season salt load decreased from 62.1 tons/d to
52.8 tons/d (p value 0.001). Although irrigation-
season salt loads were significantly less in the post-
project period, salt concentrations in the irrigation
season (table 1) were significantly higher in the post-
project period than in the pre-project period (p value
0.001). The increase in salt concentrations in the irri-
gation season was caused by the elimination of waste-
water flows from the laterals that were piped, which
results in less dilution of the much higher salt concen-
trations (about 4,800-4,900 mg/L based on nonirriga-
tion salinity concentrations for site MA2, table 1) in
ground-water inflow into Montrose Arroyo upstream
from site MA2.
10 Effects of Piping Irrigation Laterals on Selenium and Salt Loads, Montrose Arroyo Basin, Western Colorado