Spatial and temporal dynamics of groundwater salinity in south Florida: The role of climate change, sea level rise, and anthropogenic groundwater use

Background/Question/Methods: Sea level rise (SLR) has been observed throughout the 20^thcentury, and it is among the most prominent threats to coastal systems. Recent projections show that by 2100 SLR will exceed the 1990 level by 0.5- to 1.4-m. In south Florida where most lands are < 1.5-m above the current sea level, even small increases in sea level may greatly advance saltwater intrusion and affect groundwater salinity. Besides SLR, other factors like precipitation and groundwater extraction may also lead to changes in groundwater salinity. With 60% of Florida’s total freshwater coming from groundwater extraction, it is thus crucial to understand spatial and temporal changes in groundwater salinity and what social and biophysical factors (e.g., climate change, groundwater extraction and use, SLR) may drive these patterns. To address these questions, we collated long-term (from 1964 to 2016) measurements of shallow groundwater (i.e., < 76-m) chloride concentrations and specific conductance from U.S. Geological Survey wells for five counties in south Florida. We performed geospatial analysis to identify where changes in groundwater salinity were most profound, and used a linear-mixed effects model to test for the effects of meteorological, sea level rise, and aggregate groundwater use on groundwater salinity.

Results/Conclusions: Our preliminary analysis of chloride concentrations revealed that among the initial groundwater wells examined for Broward County, 66.7% of wells experienced a statistically significant increase in chloride concentrations between 1994 and 2016 (all P< 0.05). The average rate of change in chloride levels per year across all wells was 79.74 mg/L. Additionally, the average chloride concentration across all wells during 2016 was 2,356.50 mg/L, among which 41.7% of wells had average chloride concentrations greater than 1,000 mg/L. Such high chloride concentrations also seemed to correspond to periods of frequent drought, indicating the potential effects of droughts on groundwater level and thus salinity. Further analysis will tease apart the relative importance and interactions of climate, groundwater use, and SLR on groundwater salinity. As the population of south Florida grows coupled with associated increases in freshwater demand, the increasing trends in chloride concentrations may be intensified in the future due to changes in climate, sea level rise, and groundwater withdrawals, which may pose a threat to the security of dominant residential and agricultural freshwater sources in coastal systems.