WSP03775

• File Number: 8507
• Description: Rio Grande Project
• State: CO
• Basin: Rio Grande
• Date: 6/1/1996
• Title: Assessment of Metal Transport into and out of Terrace Rervoir/ Conejos County/ Colorado/ April 994 through March 1995
• Water Supply Pro - Doc Type: Publication

Upstream from Terrace Reservoir, total-zinc loads
increased substantially during rainstorms during the
summer period. Most of the total-zinc loads that
entered the reservoir during rainstorm runoff did not
appear to have been transported out of the reservoir
(fig. 15). The smallest daily zinc loads occurred
during the base-flow period between November 1994
and February 1995 (fig. 15).
During most of the study, zinc was transported
into and out of the reservoir mostly in the dissolved
fraction (fig. 10). However, during the pre-peak and
peak snowmelt periods, about 30 percent of the zinc
generally was transported into the reservoir in the
suspended fraction (fig. 10).
During the study, about 90 percent of the
20 tons of total zinc that entered the reservoir was
transported out of the reservoir, indicating that the
reservoir was a sink for only a small fraction of zinc
(table 5; fig. 15). About 60 percent of the annual
total-zinc load was transported out of the reservoir
between May 15 and July 14,1994, during the peak
and post-peak snowmelt periods. Another 23 percent
of the annual total-zinc load was transported out of the
reservoir during the summer-flow period from July IS
through the end of September 1994.

SUMMARY

Terrace Reservoir is a small irrigation reservoir
located on the Alamosa River at an elevation of about
8,550 ft above sea level in the San Juan Mountains in
Conejos County, Colorado. The Alamosa River and
Terrace Reservoir are the primary sources of water
for crops and livestock in the southwestern part of
the San Luis Valley. Much of the drainage basin
upstream from Terrace Reservoir contains extensive
areas of mineralized rocks that in some places have
been mined and that contribute a substantial metal
load to Terrace Reservoir. Gold, silver, copper,
and lead mining have occurred in the basin for more
than 100 years, and extensive mining activities have
occurred intermittently at the Summitville Mine.
Historically, the Summitville Mine site has produced
highly acidic, metal-enriched water that drained from
the mine site into Wightman Fork and flowed to the
Alamosa River and Terrace Reservoir.
A comparison of the streamflow hydrographs
upstream and downstream from Terrace Reservoir
indicated that from April I through June 8, 1994,
the volume of water released from Terrace Reservoir

was within 5 percent of the volume of water that
entered the reservoir. During the post-peak snowmelt
and summer-flow periods, more water was released
from the reservoir than entered the reservoir due
to downstream irrigation demand. During the
base-flow and early spring snowmelt periods from
October I, 1994, through March 31, 1995,46 percent
more water entered the reservoir than was released
from the reservoir. Over the entire study period, there
was only a small difference between the volume of
water that entered the reservoir and the volume of
water that was released from the reservoir.
During the study period, pH immediately
upstream from Terrace Reservoir ranged from 4.3
to 7.8. The highest pH occurred during the pre-peak
snowmelt period; the lowest pH occurred during storm
runoff during summer. Downstream from Terrace
Reservoir, pH ranged from 4.6 to 7.6. The highest pH
occurred during the pre-peak snowmelt period, and the
lowest pH occurred during summer in mid-July.
Large spatial and temporal variations in concen-
trations of the metals of concern occurred during the
study. The median and maximum concentrations of
dissolved and total aluminum, iron, copper, cadmium,
manganese, and zinc were larger upstream from the
reservoir than downstream from the reservoir.
Upstream and downstream from Terrace Reservoir, the
largest concentrations of dissolved aluminum, iron,
copper, cadmium, manganese, and zinc generally
occurred between mid-June and November. Upstream
from the reservoir, the largest concentrations of total
aluminum, iron, copper, cadmium, manganese, and
zinc generally occurred between July and September
during storm runoff. After November, dissolved-
and total-metal concentrations generally decreased
downstream from the reservoir after the closure of
the reservoir outlet works.
Throughout the study, aluminum was
transported into the reservoir predominantly in
the particulate or suspended form. Downstream
from the reservoir, the suspended-aluminum fraction
was predominant only during the pre-peak snowmelt
and peak snowmelt periods. After June 8, most of
the aluminum that was transported out of Terrace
Reservoir was in the dissolved fraction. During the
pre-peak snowmelt and peak snowmelt periods, more
than 90 percent of the iron was transported into the
reservoir in the suspended fraction. However, the
dissolved fraction of iron generally increased to more
than 50 percent during the post-peak snowmelt and

36 Assessment 01 Metal Tnlnsport Into Ind Out 01 TlrrlCl RISlrvolr, Conllos County, Colorodo,
April 1994 Through Mlrch 1995

r'0319~1