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

20,000
18,000
16,000
14,000
z
O
F
z 12,000
Pre-project load
Post-project load
0
Q
0 10,000
J
Q
Q 8,000
Z
Z
Q 6,000
4,000
2,000
MA4 D2MA MA3 D1 MA MA2
SITE
Figure 5. Comparison of salt loads in Montrose Arroyo Basin before and after the lateral project.
Before the demonstration project began, the
projected salinity reduction from the lateral project
was about 2,300 tons/yr (Michael Baker, Bureau of
Reclamation, written commun., 1998). That projection
was based on some general canal and lateral seepage
rates for the Uncompahgre Valley that were estimated
for previous salinity studies (Bureau of Reclamation,
1982), the size of the laterals, and the length of time
water was in the laterals. The annual ground-water
load at site MA2 decreased about 1,800 tons (table 5),
which is the part of the salt-load change that is
assumed to be directly attributable to eliminating
ground-water recharge from laterals that were previ-
ously open ditches. At the control site, salt load
increased about 180 tons/yr in the post-project period
(table 5). If that increase in salt load into the study
basin is accounted for, then the salt-load decrease
attributable to the lateral piping was about 1,980
tons/yr (1,800 plus 180), which is about 86 percent of
the projected salt reduction.
Data from the demonstration project provided
the NIWQP with information on whether a lateral
improvement project has the same effect on selenium
loads as it does on salt loads. In remediation planning,
some simplifying assumptions previously have been
made that use ratios of selenium and salt loads to esti-
mate selenium reductions that might occur for salinity-
control type projects. Ratios of selenium load to salt
load were computed using samples collected in
November through March that were assumed to be
composed solely of ground-water discharge, and those
ratios were used to calculate a selenium reduction
based on the estimated salinity reduction for a partic-
ular project. For Montrose Arroyo at site MA2, the
load ratio was 0.041 (using only pre-demonstration
project data), which implies that 0.041 pound of sele-
nium is proportional to 1 ton of salt. If the original
2,300-ton salt-reduction estimate is used, then the
lateral project should have produced about a 94-pound
reduction in selenium at site MA2 (2,300 times 0.041).
However, the selenium load (in the ground-water
component) decreased 178 pounds (table 4) at site
MA2, almost double what was predicted using the
load ratio. The ratio for the selenium and salt load
reductions measured for this study (tables 4 and 5) is
about 0.10 (178 pounds of selenium divided by
1,800 tons of salt). It is not known how applicable the
results of the demonstration project are to other areas
EFFECTS OF PROJECT ON SELENIUM AND SALT LOADS 11