WSP03721

• File Number: 8276.300
• Description: Lower Gunnison Basin Unit - Colorado River Basin Salinity Control Project
• State: CO
• Basin: Colorado Mainstem
• Water Division: 4
• Date: 12/1/1989
• Title: Colorado River Water Quality Improvement Program Lower Gunnison Basin Unit North Fork Area Preliminary Findings Report December 1989
• Water Supply Pro - Doc Type: Report/Study

~'..'.:
~!
!I'.'~.,
,
.

?i.'.~.
1!i
~~'

F\
C,.)
, \

~.;}
,"::.;'j

>1
:",',1
.5.;-1

~~.
".'
,"
~,4

~i]
~~i

;.,~

~<~

t.c?l
~ '-4
~-7~

f}~
~.'A
~-::~

?"::~
~:i
...;:~

r:1
~.'
g;l

~"
'''}
,~~~
~,J

;;:~
~-:.
~J

CHAPTER II

PIAN FORMULATIOO

Hydrosalinity

Salinity investigations for the North Delta area were conducted in
accordance with recommendations generated from a review of the EC profiles.
The recommendations were that further field investigations were warranted to
account for an observed rise in the EC profile from 440 to 540 umhos in one
specific reach on the right side of the Gunnison River, at cross section 94.

Initially it was believed that seepage from the North Delta Canal caused
the EC profile rise in this reach because the canal is located on a weathered
Mancos Shale hillside with visible surface salt deposits. However, further
inveftigation of the area indicated that a side inflow of approximately
4 ft Is with an associated water quality of 2,362 mg/L was discharging from
the north bank of the river at cross section 95.2. The most probable source
of this inflow was determined to be from Alfalfa Run and Cedar Run, two minor
drainages on the north side of the Gunnison River.

To determine if the observed rise in the EC profile was primarily due to
canal seepage, estimates were made of seepage amounts necessary to yield and
EC rise from 440 to 540 umhos. Computations were made assuming a range of
river discharges with which dilution could occur and an assumed water quality
of 3,500 mg/L for the seep water entering the river from the North Delta
Canal.

To determine if the rise in EC values could be attributed solely to the
Alfalfa Run inflows, calculations were conducted to determine the river flow
rate required to dilute this inflow and account for the EC profile rise.

A final set of calculations were performed, incorporating the effects of
the observed Alfalfa Run inflows, to determine the required canal seepage
rates that would account for the increase in EC values using the previously
assumed base flow concentration.

The calculations indicated that the observed rise in the EC profile can
be sol~lly attributed to Alfalfa Run inflows mixing with a river flow equal to
113 ft Is. However, the same rise in EC values can also be attributed to
Alfalfa Run inqows plus 2 fe Is of canal seepage mixing with a river flow
equal to 200 ft Is. The 2 fe Is canal seepage rate is extremely high,
approximately 2.5 feet per square foot per day. Also, it appears unlikely
that canal seepage could reach the assumed base flow concentration of
3,500 mg/L because of the close proximity of the canal to the river along this
reach and the observed surface emergence of canal seepage water. Based on
this analysis, it appears that the Alfalfa Run inflows may reasonably explain
the EC profile rise. Therefore, canal lining alternatives in this specific
reach were dropped from further consideration.

Additionally, a salt loading analysis was conducted to evaluate the
salinity reduction potential of lining the entire North Delta Canal and
Hartland Ditch systems. All of the irrigated lands served from the two
irrigation delivery systems are located on the north side of the
Gunnison River.

l4

000940