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Geochemical Attributes of Hydraulically Active Fractures and Their Influence on Groundwater Quality

This study utilized discrete interval diffusion sampling of a fractured bedrock well completed in schist to investigate if a natural weathering signal can be used to identify hydraulically active fractures. The open borehole well MFS-1, is the focus of the study, which is in close proximity to the recharge zone making it ideal for testing the hypotheses of this study. The hydraulically active fractures of Well MFS-1 were identified, and the dominant mixing force of the water column of the well was determined to be thermal convection in the upper portion of the well and upward gradients. The isotopic data collected from the well concludes that these forces are sufficient to fully mix the water in Well MFS-1 over the one month diffusion sampler deployment period. Iron in groundwater is in the reduced form while flowing through the fractured system, and becomes oxygenated when it enters the well, resulting in a significant amount of iron hydroxide precipitate. The best signal of natural weathering and the systems response to changes in recharge is shown by the changes in silicon concentration in solution. The highest silicon concentrations measured in the groundwater samples were related to waters recharged during the highest precipitation period of the year. The greatest response to system perturbations were observed at the hydraulically active fractures, as shown in the changes in measured concentrations in this portion of Well MFS-1. Even though in-well mixing causes the concentrations of natural weathering products to be measureable throughout the water column, the signal and response to changes in system chemistry are greatest at the hydraulically active fractures. Studying the rock core collected from the fractured bedrock well showed that the hydraulically active fractures are all associated with red staining on the fracture surface and in the matrix of the rock adjacent to the fracture, but the data does not support the conclusion that iron oxide is currently being precipitated. Other observations about the rock, such as the presence and nature of garnet crystals show that the current zone of advective flow in Well MFS‚Äë1 was previously active during a period of hydrothermal activity.
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