Becca Hedges
Geo 4070-01
Dr. Kaiser
27 Apr. 2017
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Geochemistry Final Exam Proposal
*Clearly state the problem(s) to be addressed and/or to be resolved; the hypothesis or hypotheses to be tested; and the overall objectives of your proposed project. (1,000 character limit):
The Grand Canyon is a premier geologic location to study the earth’s history drawing nearly 5 million visitors a year (nps.gov). Many of the water bearing units are limestones and sandstones. There are two major aquifers in the Grand Canyon watershed, the Coconino and the Redwall. I will test the hypothesis that the Coconino aquifer is interconnected with the Redwall aquifer through weaknesses of the Hermit Shale in the Grand Canyon. These discharge through weaknesses like faults. I plan to use the radiogenic isotopes of 14C and 3H (tritium) to determine the ages of the groundwater. Aquifers provide about 95% of accessible fresh water globally (Aggarwal, 2013). The Redwall aquifer is the primary source of water in the South Rim (Hart). In addition other isotopic information is useful and easily gained through this study. Using the stable isotopes of 13C and 12C determines recharge areas, radiogenic isotopes 87Sr and 86Sr identify flow paths, and stable isotopes 18O and 16O coupled with 2H and 1H provide meteoric data for sources of recharge (Hart). I propose that the ages and origin of these water sources will allow us to understand the movements of groundwater through time. (1000 Characters)
*Discuss the disciplinary and, if appropriate, regional context, and document the importance of your project (2,500 character limit):
This canyon began as the ocean floor followed by the uplift of the Colorado Plateau created by the Laramide Orogeny 65 million years ago. Since then water has carved its way downward in places about 3000 m exposing nearly 2 million years of the earth’s life. It is one of few natural features that can be seen on the earth from space. Very little is known about the movement of ground water through the rocks of this plateau (Hart). The city of Flagstaff, AZ is highly dependent on this water source (azwater.gov). Understanding the age and behavior of groundwater can help understand how long a particular system will take to generate a fresh water supply. It can also help us better understand potential recharge project locations. This is important because a vast majority of aquifers around the word are being depleted so quickly that the natural recharge rates cannot keep up. When the waters from these aquifers are released through natural features like faults they create seeps and springs. These water ways empty into the Colorado River. The Colorado River serves as a major water source for communities all the way into Mexico.
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Carbon-14 has a half-life of 5,730 years, and tritium has a half-life of 12.3 years. These are important due to high quantities of each being released during the 1950-60’s nuclear bomb testing’s. (Fritz, 1980). Baseline water chemistry gained from viewing the major ions and other constituents will give any studies done after this the ability to track changes in chemistry over time and even be able to identify factors that may have influenced the changes. The carbon isotopes used to determine recharge areas are influenced by photosynthesis, biological, and environmental factors. Nutrient analysis provides information on the general quality of the spring water. This value in the waters rely on the isotopic composition of carbon dioxide in the soils and rocks. It will also vary due to different reactions with carbonate minerals along the flow paths (Lopes, 1997). The strontium isotopes are needed for information on the composition of the rocks the water is traveling through. This is important because limestones will have a heavier 87Sr/86Sr signature than volcanic rocks. The limestones and volcanic material in this area are very permeable which can influence the flow rates as well as water quality. These are needed because topography can influence the rates of movement within the system. The amounts of trace and rare earth elements give us data that will potentially provide information useful for identifying recharge areas. This will include if it was largely from rain, snow and if the water was altered by evaporation (Hart). This information can be helpful to see palaeowaters and palaeoclimatic recharge conditions. Using a tracer model we can gain estimates of the waters age to aid in determining recharge rates. Data from the O and H isotopes gives us insight as to the source of the recharge (Aggarwal, 2013). (2489 characters)
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*Concisely state how you plan to address your problem(s) and test your hypothesis or hypotheses (2,500 character limit):
One of the hardest parts of this project will be sample collection. Gathering samples that are unaltered from human activities is difficult. Springs are generally not ideal locations to gather aquifer samples. The most accepted method used for this is done by placing a low capacity piston pump at depths within or just above a well opening that is either man made, or natural springs and seeps. A second pump operating on a high capacity will be placed at a more shallow depth of the aquifer. This helps maintain a fresh, untouched supply from the formation water as it enters the well bore near the inserted pump (Aggarwal, 2013). This will be done with both aquifers. This method lessens the potential for contaminants from human use of the aquifers.
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The samples gathered from the pumps will then be placed into a mass spectrometer. This will separate different isotopes of elements based off their mass to charge ratio. Analyses of the radiogenic isotopes of 14C and 3H (tritium) used to determine ground-water age will show that younger waters will contain a higher level of 3H and older waters will contain higher amounts of 14C (water.usgs.gov). The mass spectrometer will also test the other isotopes discussed above to gain an overall picture of the ground water systems (Aggarwal, 2013).
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Once analysis of the samples has occurred a characterization of groundwater flow systems can be created. This is composed of three classes; geological, hydrological, and geochemical. The geological system will be determined by creating a three dimensional description with the physical features of the formations detailed lithologies, thickness, depth, and geologic structures. The hydrological must contain details of the surface elevation, locations of rivers and drainage locations from the aquifer, local precipitation distributions, and recharge areas. This section can be view through cross-sections that also show the geologic framework. Geochemical factors like chemical reactions of groundwater flow paths can give regional variations in water composition and how it can change with flow direction which can influence the age of the water. Data on the concentrations of dissolved solutes, gases and isotopes are needed here. This data is then plotted on maps and contoured with the known depth intervals (Aggarwal, 2013). This will be done for each aquifer.
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After the above information is gathered and analyzed, computer software such as The MODFLOW and MODPATH will be used by placing area parameters such as the length, depth, and impermeable layers of the aquifers. These models will then give us predictions of the movements of the water underground. These modeling software will also predict the flow rates and travel times using equations built into the software. The model simulations will predict the ages and water sources that can then be compared to the age data gained from the mass spectrometer estimations (Aggarwal, 2013). (2477 characters)
*References cited in proposal (2,500 character limit):
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Hart, Robert. J, Howard E Taylor, Stephen A. Monroe. “Assessment of Spring Chemistry along the South Rim of Grand Canyon in Grand Canyon National Park, Arizona”. NATIONAL PARK SERVICE (2013). Web. 22 Apr. 2017.
Lopes, T.J., and Hoffmann, J.P., 1997, “Geochemical analyses of groundwater ages, recharge rates, and hydraulic conductivity of the N aquifer, Black Mesa Area, Arizona”, U.S. Geological Survey Water Resources Investigations Report 96 – 4190, 42 p. Web. 22 Apr. 2017.
Buschatzke, Thomas. “Western Plateau Planning Area Hydrology-Groundwater”. Arizona Department of Water Resources. http://www.azwater.gov/AzDWR/StatewidePlanning/WaterAtlas/WesternPlateau/PlanningAreaOverview/Hydrology.htm. Web. 21 Apr. 2017.
Aggarwal, P.K. “ISOTOPE METHODS FOR DATING OLD GROUNDWATER”. International atomic energy agency. (2013). Google Scholar. Web. 22 Apr. 2017.
Fritz, P., Jean-Charles Fontes. 1980, Handbook of environmental isotope geochemistry, v. 1–2: Amsterdam, Elsevier Scientific Publishing Company (1980): 557 p. Google Scholar. Web. 21 Apr. 2017.
https://water.usgs.gov/lab/3h3he/background/. Web 23 Apr. 2017.
http://nps.gov. Web 23 Apr. 2017.
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