## 5 Discussion  
During the 2016 dry season, the apparent conductivity measurements at 1 KHz and 4 KHz increased from February to April (Figures 4 and 5) in the Seven Palm system at monthly intervals. This reflects a general increase in apparent conductivities (salinity) in the groundwater over the dry season. This change is in response to decreased precipitation (Figure 13). Similarly, the apparent conductivity of the 16 KHz data indicates a general increase in apparent conductivities (salinity) in the surface water over the dry season and from north to south along the profile line of the Seven Palm system. This change is due to reduced precipitation and increased potential evapotranspiration, which results in a sharp decrease in the water level as well (Figure 13). The surface water salinities of the coastal lakes of ENP showed brackish to hypersaline salinities with higher salinities in the summer following the dry season [22; 23]. Salinity variation in time and space in the Florida Bay is governed by the influence of precipitation [35] evapotranspiration, runoff, mass exchange with the surrounding basins [5], proximity of upstream freshwater sources, and exchange with Florida Bay marine waters downstream [22; 23]. Between February 2016 and January 2017, the EM data show a considerable decrease in apparent conductivity in both the Seven Palm and West Lake system. A general decrease of 150 mS/m in Seven Palm system and 300 mS/m in Middle Lake (Figure 6a,b) suggests a freshening of the groundwater and may reflect the effects of increased flow due to  
Figure 13: Water level, rainfall, and evapotranspiration data between January 2015 and December 2017 at the NCL station, Everglades National Park. The red rectangle indicates the decrease in rainfall during the EM survey period in 2016 and 2017. The data source is the Everglades Depth Estimation Network (EDEN), USGS website [[http://sofia.usgs.gov/eden/station.php](http://sofia.usgs.gov/eden/station.php)]([http://sofia.usgs.gov/eden/station.php](http://sofia.usgs.gov/eden/station.php)) (accessed on 15 July 2020). restoration efforts. Lower conductivities observed in Cuthbert Lake compared to West Lake and Long Lake may reflect increases in freshwater flows from upstream sources, possibly from increased flows due to restoration efforts. The apparent conductivity change between 2016 and 2017 in the Seven Palm system is more significant than the West Lake system. The apparent conductivity change decreased on average by 30% in the Seven Palm system and 10% in the West Lake system (Figure 8) at all frequencies. In general, in the coastal lakes of ENP, the decrease in apparent conductivity change is pronounced from west to east at all frequencies. This suggests the restoration efforts may have made a more observable contribution to the Seven Palm system but not yet in the West Lake system. Similarly, other studies conducted by the U.S. Geological Survey to map saltwater intrusion using helicopter electromagnetic (HEM) surveys identified variations in the apparent conductivity that were associated with changes in groundwater quality [39]. The apparent resistivity map (inversely related to conductivity) from the HEM survey showed a transition from low to high resistivity from West Lake to Seven Palm, i.e., high to low conductivity from west to east, which is consistent with the results of this study. The authors concluded the change is due to the influence of freshwater flowing in the Taylor Slough which pushes the transition in a southwesterly direction in the area of maximum flow [40]. A mean local formation factor of 10.7 \(\pm\) 1.8 is estimated for the Mangrove Lakes of ENP. This estimated formation factor is similar to previously published estimates of formation factor 9.65 for the ENP [19]. We used the estimated formation factor 10.7 in this study to calculate the salinity of the groundwater. This approach has the advantage of forcing the geophysics derived salinities to be consistent with the in situ well measurements.