Karst aquifers, which supply potable water to about 25% of the world?s population, are vulnerable to contaminants from surface water. One reason is that contaminants on land surface (e.g., nitrogen due to fertilizer uses) and in surface waterbodies (e.g., dissolved organic matters) can enter the aquifers directly through sinkholes, which are a common geomorphologic landscape in karst terrains. This project studies lake sinkholes, i.e., sinkholes on lakebeds. Since the sinkholes are connected to karst conduits that end at karst springs, lake water that drains into the sinkholes discharges at karst springs, and contaminants in lake water may affect not only groundwater geochemistry but also spring water quality and ecosystems of rivers that receive spring water. Studying the impacts of surface water contaminants on groundwater and spring water quality requires identifying hydraulic connections between surface water and spring water, i.e., which surface water body flows to which spring. The identification is challenging, because karst conduits are complex networks. As a result, surface water that drains into one sinkhole may discharge at multiple springs, and one spring may receive surface water from multiple sinkholes.  

One way to identify the hydraulic connections is to conduct a dye tracing experiment by injecting a dye (e.g., rhodamine) at a sinkhole and monitoring where the dye appears. Conducting a dye tracing experiment is time consuming and labor intensive, and it is impractical to conduct many dye tracing experiments at many locations or at one location for many times. An alternative to the dye tracing experiments is to use environmental tracers that are ubiquitous such as hydrogen and oxygen isotopes of water. This project is to explore whether we can turn a lake sinkhole event into a natural tracer experiment.

At the study site located in northwest Florida, USA, two sinkholes occurred on the lakebed of Lake Miccosukee in 2010, and lake water drained through the sinkholes to karst conduits in the upper Floridan aquifer underlying the lake. This happened again in 2018, and it provided a unique opportunity of research for advancing our understanding of karst hydrogeology and hydrology, especially for using environmental tracers to identify hydraulic connections between the lake and downgradient karst springs. A dye tracing experiment was conducted in April, 2018 at one sinkhole, and charcoal packets were deployed at a number of springs to investigate to which springs the dye discharged. After the dye tracing experiment, the sinkholes were covered by lake water after a heavy rainfall in May, 2018. Based on laboratory measurements of dye concentration absorbed on the charcoal packets, we found that the lake water discharged at Natural Bridge Spring, a first-magnitude spring about 32 km (20 miles) downgradient from the lake. This is the first time that the hydraulic connection between the lake and the spring is identified, and this identification may potentially change the existing groundwater models developed for Northwest Florida including the study site.

We developed a new method that uses hydrogen and oxygen isotopes in water to identify the hydraulic connection between the lake and the spring. Because light isotopes are evaporated more easily than heavy isotopes, lake water is enriched by heavy isotopes, and there is a contrast in the isotope signature between lake water and groundwater. By analyzing isotope ratios in water samples collected from the lake, the spring, and groundwater wells during 10/2019 ? 1/2020, the hydraulic connection between the lake and the spring is detected, and such a detection at the scale of tens of kilometers and for a first-magnitude spring has not been reported in the literature.

This project supported a doctoral student, who graduated in 2020 and published a first-authored paper in Groundwater in 2021. This research was recently reported in Eos, a news magazine of the American Geophysical Union (AGU) that is delivered electronically to more than 100,000 AGU members worldwide. The Eos media coverage is available at https://eos.org/articles/dyes-and-isotopes-track-groundwater-from-sink-to-spring. The PI has collaborated with scientists worldwide, and published a total of six peer-reviewed journal articles and one book chapter on various topics related to karst hydrogeology such as spring hydrograph separation and sinkhole identification and water quality assessment using machine learning approaches. Conference presentations were also given at national and international meetings. The research was also presented to the Florida Geological Survey, a state agency in charge of sinkhole mapping and involving in environmental protection. The PI is collaborating with Florida Geological Survey to study another lake sinkhole event that occurred on June 6 at Lake Jackson, Florida. This research has supported the PI?s teaching of two courses. Undergraduate and graduate students of the two courses were brought to the study site to learn karst geology, hydrology, and hydrogeology. The students also practiced various field methods such as water sample collection and flow rate measurement. The isotope data used in this study are available online at https://figshare.com/articles/dataset/Hydrogen_and_oxygen_isotope_ratios_for_Lake_Miccosukee_and_Natural_Bridge_Spring/13087856.

Last Modified: 07/01/2021
Modified by: Ming Ye