Predicting the Occurence of Deep Aquifier

Unaweep Canyon (western Colorado, USA) bisects the northwestward-trending Uncompahgre Plateau on the northern Colorado Plateau and is the only major canyon in the Colorado River drainage not occupied by a river (Soreghan et al., 2015). The Colorado and Uncompahgre Plateaus form part of the greater Rocky Mountain orogenic plateau, a large region of high elevation in the United States (McMillan et al., 2006). Studies done on this area have mainly focused on the history and drainage evolution of the Unaweep Canyon; however, the hydrochemistry characteristics of the area is underdeveloped. My research is to provide more insight into the chemical characteristics of the various aquifer systems present at Unaweep Canyon. A shallow fluvial aquifer system has already been identified by domestic well construction activities (as seen in Figure 1), but the probability of a deeper lacustrine aquifer has not been examined. So, I hypothesize that if the Unaweep Canyon has a deeper aquifer, then the hydrochemical characteristics of the Lacustrine deposits should match that of the seeps and springs found at bedrock fractures/faults. The significance of this research is to provide an alternative source for irrigation and also a source for drinking water for humankind. Usage of the deeper aquifers for domestic water consumption appears to be sustainable since the recharge rate of the deeper aquifers calculated from residence times is of comparable magnitude (Zheng et al., 2005). Proving the presence of a deeper aquifer can assist with the understanding of the interaction between groundwater and bedrock faults in mineral deposits. For example, Uranium fixed onto mineral grain boundaries or present in less-resistant minerals such as biotite or hornblende can be readily leached by groundwater (Gascoyne et al., 2002).

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Water resources are closely linked to the wellbeing of humankind (Huang et al., 2019). Groundwater is a crucial aspect of life as it is essential as a source of drinking water supply, irrigation supply, and also supporting ecosystems. However, before it can be qualified to be safe for use and consumption, its water quality analyses must be carried out. Hydrochemical indices are commonly used to ascertain aquifer characteristics, salinity problems, anthropogenic inputs, and resource management, among others (Litaor et al., 2010). Hydrochemistry is an interdisciplinary field combining the expertise of hydrology, the study of the earth’s water with chemistry, which identifies the chemical composition of water. In my thesis, water samples from domestic wells seeps, and springs would be collected and analyzed to reveal its hydrochemical properties and possibly identify any differences or similarities from the different sampling locations, which would then be interpreted as accurate as possible using other analytical methods. Dissolved Organic matter concentrations and characterization, a hydrochemical measurement, for each sample would also be measured using a TOC-Analyzer and Spectrofluorometer (as seen in Figure 2), respectively, which can then be used as a tracer. Conversely, given its fluorescence properties, DOM could be used as a natural tracer in place of artificial dyes if its characteristics change with either space or time (Baker & Lamont-Black, 2001). If the DOM constituents differ in the wells and the seeps and springs, then it would highlight the probability of there being two different sources of water for both outlets.
The use of stable isotope analysis, oxygen, and hydrogen isotopes, in particular, is a handy tool in locating deep aquifers. Generally, waters originating from deep aquifers should have a lighter isotopic signature compared to surface water as a result of their reduced interaction with meteoric water. Thus, the source and evolution process of groundwater in a specific location can be determined based on the groundwater’s relationship with hydrogen and oxygen isotopes in meteoric water (Yeh & Lee, 2018). The presence of seeps and springs originating from the Precambrian rock at Unaweep Canyon provides an excellent source of groundwater that can be analyzed for its isotopic composition using a mass spectrometer and then compared to the isotopic composition of the domestic wells drilled into the shallow aquifer. If there is a considerable difference in the

18O and ∂2H values in both water, then it hints the presence of a confined aquifer that forces water to the surface of the earth; perhaps, a deeper aquifer system. If this deeper aquifer is proven, an additional step would be obtaining its age using ∂3H values. The measurement of the relationship between helium (the daughter isotope produced by the decay of tritium) and tritium (the parent isotope) can be used to calculate the groundwater age using the following equation (Moran & Hudson, 2006):
Groundwater Age (years) = -17.8 x ln (1 + 3Hetrit/3H)
The groundwater obtained would highlight the last time it was in contact with the atmosphere as meteoric water or as surface water being it got into the subsurface. The calculation of the groundwater age is only feasible if He is present and can be proven to be the daughter isotope of tritium. Therefore, performing this step is still being considered and dependent on cost, and the results gotten from an XRD analysis showing the occurrence of He.
Resistivity, a geophysical method, is another approach that can be used in identifying groundwater by measuring the resistivity/conductivity of the subsurface. The resistivity methods and especially the vertical electrical sounding method have been used successfully for investigating groundwater quality in different lithological settings because the instrumentation is simple, field logistics are easy, and the analysis of data is straight forward compared to other methods (Zohdy et al. 1974). At Unaweep Canyon, a resistivity investigation has been carried out (as seen in Figure 3) and then inverted to translate raw geophysical measurements to spatial patterns of the geophysical parameter (Binley et al., 2015). Examining this inverted data has shown possible areas of low high conductivity that might highlight water in the earth’s subsurface. However, because a medium’s resistivity is dependent on porosity, water content, and the concentration of salts, the information gotten from the VES survey needs to be correlated with the chemistry of the groundwater as well as lithological studies done in the area of interest. By correlating the inverted geophysical data with hydrochemical analyses, I would be able to examine the existence of a deeper aquifer system.
My thesis research at Unaweep Canyon, western Colorado, would involve mostly hydrochemical and isotope analysis to try to predict the occurrence of a deep aquifer. The age and source of the deeper groundwater would be inferred as well as its water quality. Thus, possibly providing an alternative to supply the drinking and irrigation needs of people.

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Figure 2. Digital elevation model of research area and surroundings. The mouth of Unaweep Canyon that hosts the Cutler rocks lies near Gateway along the Dolores River (Soreghan et al., 2015)

Figure 3. Resistivity profile of Unaweep Canyon, Colorado (Behm M., 2019, slide 17)