|Ground Water – Surface Water Exchange
A View from Inside the Unconfined Karstified Floridan Aquifer with Implications for Ground Water Protection
River Water Intrusion to the Devil’s Ear Cave System
Detailed discharge measurements collected in the field area during the investigation indicate that this part of the Santa Fe river is a gaining stream. During the February 1992 sampling period, stream discharge increased from 16 m3/s at Rum Island to 23 m3/s at July spring. The June 1993 sampling period reflects a higher stage of the Santa Fe river where stream discharge increased from 19.1 m3/s at Rum Island to 26.2 m3/s at July spring. However, the radon data collected from the cave system clearly demonstrates that recently intruded river water accounts for a significant part of the spring discharge along this reach of the river.
Figure 6 provides river water intrusion contour maps from the 1992 and 1993 sampling periods. Both maps delineate distinct zones of river water intrusion to the Devil’s Ear cave system. The maps show that intruded river water enters the cave system from conduits that trend toward the river. However, conduits that emanate from the recharge area north of the river consistently provide the sources of background aquifer water.
Low 222Rn concentrations ranging between 2.5 and 5.8 Bq/L measured in sections A, B, and C clearly demonstrate that the conduits in these sections provide direct pathways for river water to intrude into the Floridan aquifer. As shown on Figure 6, these conduits trend south of the main cave toward and, in the case of those in sections B and C, beneath the Santa Fe river. During both sampling periods, turbid water was observed entering the main conduit from these three regions. It also emanated from fractures in the limestone on the south side of the main conduit. More turbid water was observed during the 1992 sampling period. During both sampling periods, the water clarity in the main conduit improved upstream of the section A junction and clear water was observed in sections D, E, and F.
High 222Rn concentrations ranging between 11.7 and 14.2 Bq/L indicate that the three most prominent sources of background aquifer water in the cave system are found in sections D, E, and F. Water clarity consistently improved and the 222Rn concentration dramatically increased upstream of section C shown on Figure 7. These conditions indicate that section C is the upstream extent of river water intrusion to the main conduit of the cave system.
Variations in the 222Rn concentration shown on Figure 7 reflect junctions where waters of different sources enter the main conduit. The percentage of background aquifer water in the main conduit decreases as the water moves toward Devil’s Ear and Devil’s Eye springs. The mixing model demonstrates that 62% of the discharge at Devil’s Ear spring was recently intruded river water during the lower recorded river stage in February 1992. During the June 1993 sampling period, which corresponds to the higher recorded river stage, 49% of the discharge at Devil’s Ear spring was recently intruded river water. Notice that the percentage of river water in the Floridan aquifer decreased during the higher stage of the Santa Fe river.
As shown on Figure 8, the variations in d18O in the cave system during the June 1993 sampling period qualitatively support the 222Rn data. More positive d18O values, between -3.55 and -3.45, were recorded in sections A and B of the cave system which were identified by the 222Rn mixing model as regions of high river water intrusion. More negative d18O values ranging from -3.65 to -3.75 were recorded in section F which corresponds to one of the principal sources of background aquifer water identified by the 222Rn mixing model. The mixing model was not applied to the d18O values because the d18O signature of the river was not determined.