Influence of lithology and climate on spring chemistry in
the Upper Deschutes River watershed, Oregon
Julia E. Schwarz
Senior Integrative Exercise
March 10, 2010
Submitted in partial fulfillment of the requirements for a
Bachelor of Arts degree from Carleton College, Northfield, Minnesota
Table of Contents Abstract Introduction . . . . . . 1 Quantifying weathering controls . . 1 Constraining groundwater age . . 3 Geologic Setting . . . . . 3 Methods . . . . . . 9 Results . . . . . . 11 Geologic Map . . . . 11 Chlorofluorocarbons(CFCs) . . . 11 Field Data . . . . . 14 Trace Element Data and Lab Analyses . 16 Discussion . . . . . . 21 pH. . . . . . . 21
Conductivity and Alkalinity. . . . 23 Spring Temperature. . . . . 24 Dissolved Oxygen Content (DO) . . 25 Dissolved Organic Carbon (DOC) . . 26 Trace Elements . . . . 26 Rare Earth Elements (REEs) . . 26 Other Trace Elements . . . 28 Fe, Al, Mn, Sm, and Nd . . 28 Lithium and Arsenic . . 29 Strontium . . . 30 Vanadium . . . 30 Groundwater Residence Time . . . 30 Weathering Rates . . . . 32 Trace elements and climatic influences . 33 Lithologic influences on trace elements . 33 Conclusions . . . . . . 35 Future Work . . . . . 36 Acknowledgements . . . . . 37 References Cited . . . . . 37
Influence of lithology and climate on spring chemistry in the Upper Deschutes River watershed, Oregon
Julia E. Schwarz
Senior Integrative Exercise
March 10, 2010
Advisors:
Bereket Haileab and Cameron Davidson, Carleton College Department of Geology ABSTRACT Fourteen springs in the Upper Deschutes River watershed, Oregon, were sampled to determine factors contributing to trace element distribution within the watershed. In particular, two areas were compared for differing climate and lithology; the Ochoco Mountains in the east had higher concentrations of trace elements compared to springs within 30 km of the Cascades Mountains crest. Specific trace elements (Fe, Al, Mn, Sm, Nd, Li, As, V) within these spring waters were examined for links with precipitation, air temperature, and catchment lithology. Rock composition plays a large role in element concentrations. For example, vanadium concentrations act as a tracer for certain rock units, particularly the youngest Quaternary basalts and Tertiary andesites. Bedrock age may also play a role in element concentrations, with older rocks in the Ochocos contributing to higher trace element concentrations. Throughout all springs, the weathering relationships between Fe, Al, Mn, Sm, and Nd are constant, though the weathering rate is higher in the Ochocos. High spring temperature and arsenic concentrations indicate that some springs are part of a deeper regional groundwater flow.
Keywords: Deschutes River, ground water, springs, geochemistry, chemical weathering, trace elements, rare earth elements
INTRODUCTION
Knowledge of silicate mineral chemical weathering rates is important to
understanding of individual watersheds and the global carbon cycle, because silicate
weathering creates bicarbonate from the uptake of atmospheric CO2 (Walling and Webb,
1992a; Bluth and Kump, 1994; Suchet et al., 2003; Velbel and Price, 2007). Within
watersheds, silicate rock weathering is essential to soil development and water quality
(White and Brantley, 1995), with silicate weathering contributing an estimated 60% of
major dissolved constituents (e.g. Ca2+
, Na+, Si) in rivers (Walling and Webb, 1992b).
Clearer understanding of silicate mineral weathering rates will help quantify its role in
the global CO2 budget and silicate mineral contributions to solute fluxes within
watersheds.
This paper examines silicate weathering fluxes from springs as part of a larger
project that aims to better understand the interplay between weathering reactions and
products in rocks, soil, and water in the Upper Deschutes River watershed in Oregon.
This study investigates weathering reactions on mineral and outcrop scale, soil formation
from rock and tephra deposits, and the role of regional rock chemistry on water chemistry.
The purpose of this paper is to explore factors affecting weathering rates of volcanic
rocks in aquifers by looking at the influence of climate, lithology, and residence time on
trace element concentrations in springs.
Quantifying weathering controls
The rate of mineral dissolution reactions is governed by thermodynamics,
kinetics and the exposure time of the water to the soil or rock body (Walling and Webb,
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1992b). The reaction rate depends on the solute concentrations already in the water, and
the residence time determines if the reaction runs to equilibrium.
The rate of weathering is also determined by lithology (Walling and Webb, 1992b;
Bluth and Kump, 1994; Bowser and Jones, 2002), because varying mineral compositions
will lead to variable weathering rates. Spatial variation in weathering rates are found due
to a variety of climactic factors, including elevation, temperature (ambient and mean
annual), precipitation, and topographic features (Walling and Webb, 1992b; Bluth and
Kump, 1994).
One of the most reliable and straightforward ways of quantifying weathering
reactions in the field is a geochemical mass-balance model, sometimes referred to as
input-output or solute budgeting (Garrels and Mackenzie, 1967; Bricker and Jones, 2005;
Velbel and Price, 2007). Mass-balance model objectives include quantifying fluxes in
and out of the system, interpreting mineral weathering reactions, and determining
weathering rates of minerals (Bricker and Jones, 2005). Mass-balance models generally
use concentrations (this study) or mass flux of solutes (Bricker and Jones, 2005). Studies
based on concentration of elements in the system are useful in determining the
weathering reactions that are occurring, but they do not provide sufficient information to
determine quantitative weathering rates. In mass-balance models, a closed system is
assumed and all system inputs and outputs must be defined (White and Brantley, 1995;
Bricker and Jones, 2005; Velbel and Price, 2007).
System inputs include all processes that introduce minerals or elements into the
system. Precipitation is one of the major system inputs. Spring chemistry is affected by
2
precipitation chemistry, since groundwater is recharged by surface water. Further system
inputs include dissolution of minerals through weathering processes (Velbel and Price,
2007). The number of weathering reactions depends on the mineral assemblage and thus
the lithology (Bluth and Kump, 1994). Using local rock chemistry instead of idealized
chemistry is necessary to obtain more accurate results (Bowser and Jones, 2002; Bricker
and Jones, 2005). Rare earth elements (REEs) have been increasingly used in hydrologic
studies, and the source of dissolved REEs in ground water is generally assumed to be
weathering of the substrate rock (Garcia et al., 2007).
Constraining groundwater age
The residence time of water in the subsurface also plays an important role in
mineral weathering. Since the 1930s, chlorofluorocarbons (CFCs) have been used in
refrigeration and other industries. CFCs are released into the atmosphere, where they last
from 50 to 100 years. Waters exposed to the atmosphere have CFC concentrations
corresponding to the year the water was last exposed (Phillips and Castro, 2005). CFCs
are useful for dating young groundwaters, though the method is insensitive to dispersion
and mixing of waters (Phillips and Castro, 2005). Since the groundwater in the Deschutes
area is known to be fairly young (Gannett et al., 2001), CFC dating is a useful tool to
determine the underground residence time of these groundwaters.
GEOLOGIC SETTING
The Deschutes River and tributaries drain an area of over 28,000 km2 (Gannett et
al., 2001) from central Oregon to the Oregon-Washington border. This area is drained
from the west by the Deschutes and Metolius Rivers, and from the east by the Crooked
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River (Fig. 1). The watershed is bounded on the west by the Oregon Cascade Mountains
crest, on the south between the drainage divide with the Klamath Basin, and on the east
by the Ochoco Mountains (Fig. 1). The northern boundary of the Deschutes watershed is
its confluence with the Columbia River near The Dalles, Oregon. This study concentrates
on the area south of the Metolius-Crooked-Deschutes River confluence area.
The majority of water in the western part of the watershed is from precipitation
falling on the Cascade Range, the principal recharge area for the Deschutes River
(Gannett et al., 2001). Highly permeable igneous rocks of the high Cascades allow
precipitation to enter the subsurface and flow eastward through the basin (Gannett et al.,
2001). Much of this groundwater is released via springs into surface streams at the edge
of the La Pine basin, a structural graben filled with low permeability sediments (James et
al., 2000; Gannett et al., 2001). The La Pine basin is the main source of water into the
Deschutes River. The majority of the remaining groundwater is released into surface
streams near the confluence of the Metolius, Crooked, and Deschutes Rivers, where the
Green Ridge fault juxtaposes high permeability Quaternary and Tertiary
