Groundwater Flow Cycling Between a Submarine Spring and an Inland Fresh Water Spring by J. Hal Davis1 and Richard Verdi2 Abstract Spring Creek Springs and Wakulla Springs are large first magnitude springs that derive water from the Upper Floridan Aquifer. The submarine Spring Creek Springs are located in a marine estuary and Wakulla Springs are located 18 km inland. Wakulla Springs has had a consistent increase in flow from the 1930s to the present. This increase is probably due to the rising sea level, which puts additional pressure head on the submarine Spring Creek Springs, reducing its fresh water flow and increasing flows in Wakulla Springs. To improve understanding of the complex relations between these springs, flow and salinity data were collected from June 25, 2007 to June 30, 2010. The flow in Spring Creek Springs was most sensitive to rainfall and salt water intrusion, and the flow in Wakulla Springs was most sensitive to rainfall and the flow in Spring Creek Springs. Flows from the springs were found to be connected, and composed of three repeating phases in a karst spring flow cycle: Phase 1 occurred during low rainfall periods and was characterized by salt water backflow into the Spring Creek Springs caves. The higher density salt water blocked fresh water flow and resulted in a higher equivalent fresh water head in Spring Creek Springs than in Wakulla Springs. The blocked fresh water was diverted to Wakulla Springs, approximately doubling its flow. Phase 2 occurred when heavy rainfall resulted in temporarily high creek flows to nearby sinkholes that purged the salt water from the Spring Creek Springs caves. Phase 3 occurred after streams returned to base flow. The Spring Creek Springs caves retained a lower equivalent fresh water head than Wakulla Springs, causing them to flow large amounts of fresh water while Wakulla Springs flow was reduced by about half. Introduction springs or spring groups have been identified in Florida, Salt water intrusion in fresh water aquifers has long most of which are near shore (Scott et al. 2004). been recognized as a serious threat to potable water The evolution of the Upper Floridan Aquifer (UFA) supplies and can occur by several different processes, one has resulted in the development of large conduits that are of which is the direct flow of sea water into submarine present both inland and at the present coastline. In the springs during backflow conditions. Submarine springs past, large parts of the Florida Platform were exposed are common around the world and are usually associated to karst formation processes during periods of lowered with outcrops of carbonate rocks (Fleury et al. 2007). The sea level, resulting in deep conduits that are now part Floridan Aquifer system, which is carbonate, underlies of submarine spring systems (Fleury et al. 2007). The parts of Florida, Georgia, Alabama, and South Carolina presence of submarine springs and conduits can allow (Miller 1986), and submarine springs occur in some areas salt water to move inland toward fresh water supplies. where the aquifer system extends offshore (Bush and However, the investigation of salt water intrusion, which Johnston 1988). More than 700 springs discharging water leads to brackish groundwater along the coast, rarely from the Floridan Aquifer system in Florida have been addresses the groundwater flow inside the karst aquifers identified, and 33 of these springs are first magnitude (Fleury et al. 2007). (Scott et al. 2004). A first magnitude spring is defined as Fresh water flows from and sea water intrusion into submarine springs have rarely been quantified due having an average flow rate of at least 2.83 m3/s using the to the difficulty of measurement, leading to a lack of classification system of Meinzer (1927). Thirty submarine understanding of the interaction of submarine springs with the marine environment. This interaction is affected 1Corresponding author: 2625 Vergie Ct, Tallahassee, FL 32303; by the rising sea level, which can increase the pressure [email protected] head on submarine springs, changing flow patterns and 2 U.S. Geological Survey, Northboro, MA 01532; groundwater movement in the aquifer. [email protected] Received April 2013, accepted September 2013. The purpose of this study is to describe how the Published 2013. This article is a U.S. Government work and is short-term dynamic hydrologic conditions occurring from in the public domain in the USA. June 2007 to June 2010 caused the flow in a group of doi: 10.1111/gwat.12125 coastal first-magnitude submarine springs to cycle from NGWA.org Vol. 52, No. 5–Groundwater–September-October 2014 (pages 705–716) 705 outflowing large quantities of fresh water to backflowing is often low during high flow, but rises as the flow rate large quantities of sea water. During periods when the decreases. submarine springs were outflowing fresh water, another Spring Creek, wherein the Spring Creek Springs first-magnitude spring located 18 km inland from the occur, is relatively short, extending only about 3.2 km coast showed a reduction in outflow of approximately inland from the Gulf. The stage in Spring Creek rises 50%; when the submarine springs were backflowing sea and falls with the tides and there is little or no inflow of water, the inland spring outflow approximately doubled. water occurring at the headwaters. During a typical tidal A long-term increase in outflow of the inland spring cycle, the rising tide causes sea water to flow upstream from the 1930s to present can be explained by rising sea in Spring Creek. During the higher part of the high tide, level, which caused fresh water outflow to shift from the water commonly backflows through the spring caves and submarine springs to the inland spring. into the UFA; this is especially common during periods of low rainfall. At times, whirlpools can be observed above Study Area the spring vents due to backflows. All the Spring Creek Spring Creek Springs and Wakulla Springs (Figure 1) Springs have been observed to occasionally reverse flow are both regional groundwater discharge points in northern at high tide (Lane 2001; Scott et al. 2004). During lower Florida with a combined springshed covering about tidal stages, water typically flows out of the UFA and into 3000 km2 (Davis and Katz 2007). Both are first magnitude Spring Creek. During some high rainfall periods (when springs (Rosenau et al. 1977; Scott et al. 2002, 2004) that aquifer water levels are high) Spring Creek Springs may derive water from the UFA, which is the primary source discharge fresh water during entire tidal cycles. Dimova of potable water supply for Leon and Wakulla Counties, et al. (2011) found that the discharge of the Spring Creek Florida (Water Supply 2008), and other surrounding Springs responded very quickly to large rainfall events, counties that are upgradient of the springs. The occurrence requiring only 1 or 2 d for the salinity to decrease from of the Spring Creek Springs in a tidal estuary facilitated 27 to 2 ppm in one case. the measurement of sea water intrusion into and fresh Several of the caves at Spring Creek Springs have water flow out of the UFA. These measurements were been explored and described by Kincaid and Werner possible because flow between the springs and the Gulf (2008). They described the cave associated with Spring of Mexico (Gulf) is funneled through the relatively narrow Creek Spring 1 (Figure 1) as trending approximately 45 m Spring Creek channel. The following measurements were to the north of the spring vent, where it reached a depth of made: flow rate and salinity in Spring Creek, flow rate in about 50 m below water surface (bws) and was observed the Wakulla River, and flow rate in Lost Creek—a small to continue trending deeper. This vent was the largest of creek that flows into Lost Creek Sink and is a source of the conduits explored, and measured approximately 15 m water to both springs. in diameter. The cave associated with Spring Creek Spring 2 was the longest of the surveyed caves (more than 300 m Spring Creek Springs in length) and trended to the southwest at an average depth Spring Creek Springs consists of 14 individual of 45 m bws. The cave associated with the Spring Creek springs in the tidally affected Spring Creek (Rosenau Spring 10 was mapped for approximately 230 m, and it et al. 1977); however, Rosenau et al. (1977) reported the trended to the west-northwest where it connected to the location of only eight of the springs and the present study cave associated with Spring Creek Spring 8; these two was able to verify the location of only springs no. 1 to no. caves averaged between 4.5 and 9 m in diameter. 11 (Figure 1). The location of springs no. 12 and no. 13 could not be determined, but they are reported to occur Wakulla Springs seaward of the U.S. Geological Survey (USGS) Spring Wakulla Springs is located about 23 river km Creek gauge (Lane 2001). Only two flow measurements upstream from the Gulf (Figure 1) and is the headwaters of Spring Creek Springs were reported prior to 2002 of the Wakulla River; the straight line distance to the (Scott et al. 2002, 2004): 56.6 m3/s, measured on May 30, Gulf is 18 km. Almost all of the flow in the river comes 1974 (Rosenau et al. 1977), and 8.69 m3/s, measured on from a single large spring vent (Rosenau et al. 1977). November 1, 1991 (Davis 1996). So the flow from these Wakulla Springs does not reverse flow and sea water springs was poorly characterized. does not move upstream in the Wakulla River to the Spring Creek Springs is a type 2 submarine spring spring.