Preliminary Report On The Groundwater Isotope Study In The Brentwood Region

Author : Nur Ahmed
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.:/5 (134 download)

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Download or read book Isotope Analysis and Groundwater Modeling for Advanced Understanding of Lake Water and Groundwater Mixing Through Lake Sinkholes in North Florida written by Nur Ahmed and published by . This book was released on 2020 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: The dissertation presents a series of hydrogeological studies related to mixing of lake water and groundwater in the Upper Florida aquifer system. In the study area located in the Northern Highland of Florida, two sinkholes were developed on lakebed of Lake Miccosukee in late January or early February of 2018. Lake water directly drained into the sinkholes, and discharged at downgradient springs. A dye tracing experiment conducted in April 2018 indicates that lake water discharges at Natural Bridge Spring, a first-magnitude spring located 32 km downgradient from the lake. The objective of this dissertation research is to use isotope analysis and groundwater modeling to advance our understanding of the mixing of lake water and groundwater in the aquifer. Oxygen and hydrogen isotopes were used to detect the hydraulic connection between the sinkhole lake and the karst spring. By collecting water samples from the lake, the spring, and groundwater wells during 10/2019 - 1/2020, it was found that, after removing rainfall effects on isotopic signature in spring water, mixing of heavy-isotope-enriched lake water into groundwater explains increased isotope ratios of spring water. This detects the hydraulic connection between the lake and the spring, and such a detection at the scale of tens of kilometers and for a first-magnitude spring has not been reported in the literature. Spring hydrograph mixing fraction calculation during storm indicate that relative contribution of different sources decreases in the order of groundwater (10-93 %), birdsink flow (45-76 %), rainwater (2-41 %) and lake water (4 -14%). The isotope analysis was extended to include the other two sinkhole lakes, id est, Lake Jackson and Lake Iamonia, in the Tallahassee area, to study lake water and groundwater mixing at a larger scale. Based on [delta]18O isotopes in lake water and groundwater and using mass-balance mixing calculations, it is estimated that the proportion of lake water that mixes with groundwater ranges from 2 to 96 % at the wells downgradient to Lake Miccosukee. In the wells downgradient to Lake Jackson and Lake Iamonia, the mixing ratios range from 7 to 39%. The spatial pattern of lake water and groundwater mixing was consistent with that simulated by using a regional MODFLOW model developed by the Northwest Florida Water Management District. An indirect evidence of lake water draining into lakebed sinkhole can be found by analyzing the relation between lake stage and hydraulic head downgradient to the lake, because the lake water drainage can potentially impact the groundwater head through groundwater flows from karst conduits to matrix. The processes governing the interactions between sinkhole lake water and groundwater vary in time and space. To capture the dynamics of the interaction, it is necessary to derive indicator parameters that can serve as proxies to lake water recharge via lakebed sinkholes. Principal component analysis (PCA) was used to identify the indicator parameters for groundwater wells downgradient of Lake Miccosukee. The PCA approach highlighted differences between hydraulic heads at observation wells and the response of lake stage fluctuations. Three observation wells were shown to be representative of dynamics of lake water and groundwater interaction in the study area. The PCA results also indicated that the magnitude of hydraulic connection was greater in the downgradient from Lake Miccosukee due to presence of karst conduits. Groundwater head and lake stage are recommended as a combined proxy for understanding the dynamics of lake water and groundwater interaction in the study area.