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  • 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

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  • 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

    3

  • 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

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