U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2004-5146
Prepared in cooperation with the
NATIONAL PARK SERVICE
Chemical Characteristics of Ground-Water Discharge along the
South Rim of Grand Canyon in Grand Canyon National Park, Arizona, 2000-2001
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By Stephen A. Monroe, Ronald C. Antweiler, Robert J. Hart, Howard E.
Taylor, Margot Truini,
John R. Rihs, and Tracey J. Felger
ABSTRACT
Springs flowing from the south rim of Grand Canyon are an important resource of
Grand Canyon National Park, offering refuge to endemic and exotic terrestrial
wildlife species and maintaining riparian areas. Population growth on the
Coconino Plateau has increased the demand for additional development of
ground-water resources, and such development could reduce spring discharge and
affect the sustainability of riparian areas within the park. In addition,
springs are an important source of drinking water for hikers and are culturally
and economically important to Native Americans living in the region.
Water samples were collected from May 2000 to September 2001 from 20 spring and
creek sites that discharge water from the Redwall-Muav Limestone aquifer along
the south rim of Grand Canyon. Sample collection sites were described and
samples were analyzed for major ions, nutrients, trace elements, radioactivity,
and selected isotopes, and potential sources of ground-water flow to the
springs. Rock samples representing the major stratigraphic units of Grand Canyon
were collected near the Bright Angel Fault and analyzed for mineralogy,
strontium-87/strontium-86, and
carbon-13/carbon-12.
The chemical composition of water samples collected from a given spring did not
vary appreciably over the course of the study. Although water at each spring had
a temporally constant composition, the composition was chemically distinct from
that of every other spring sampled, indicating spatial variability in the
ground-water composition. Most samples had a calcium magnesium bicarbonate
composition; a few had a substantial sulfate component. Concentrations of
arsenic, nitrate, selenium, uranium, and gross alpha approached or exceeded U.S.
Environmental Protection Agency Maximum Contaminant Levels in water discharging
from some springs. Oxygen and hydrogen isotopic compositions varied little among
samples, and for most sites the isotopic data plot close to the global meteoric
water line or below the local meteoric water line. Isotopic enrichment indicates
fractionation due to evaporation occurs at some sites. The evaporative process
may occur prior to recharge or post-discharge. Flow paths are differentiated
between the eastern part of the study area where strontium-87/strontium-86
values for water from springs and creeks are more radiogenic than
strontium-87/strontium-86 values for water that discharges from sites farther
west. Tritium and carbon isotope analyses indicate that residence time of
ground-water discharge from springs and creeks ranges from less than 50 years to
about 3,400 years. Water with a residence time of less than 50 years is absent
at several sites. Discharge of most springs and creeks is a mixture of younger
and older waters.
CONTENTS
Abstract
Introduction
Investigation methods
Site descriptions
Chemical characteristics of ground-water discharge at springs
Future studies
Summary and conclusions
References cited
Physical and chemical data
Appendix
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