EVOLUTION OF SALINITY IN THE COASTAL CHICOT AQUIFER SYSTEM, LOUISIANA, USA

Persistent overdrafting of groundwater in the Gulf and Atlantic Coastal regions of the U.S. has led to increased intrusion of salt water in many coastal aquifers. In order to effectively manage these aquifer systems we need to better understand the dynamics of salinity changes in relationship to evolving water supply and demand challenges. In this study, we evaluated the long-term geochemical changes associated with the 23,000 km² Chicot aquifer system in Louisiana, USA. The Chicot is currently being overdrafted by about 350 million gallons per day. Bulk geochemical data (pH, T, salinity, Na, Mg, Ca, HCO₃, Cl) were compiled from samples collected from 20 wells in the Chicot aquifer over a 10 to 20 year period. Oxygen and H isotope measurements were additionally completed for samples collected in 2014. Using these data we identified three zones of groundwater with distinctive geochemical character; (1) A groundwater recharge zone in the north of the study area with low pH, low salinity, and low temperature relative to other groundwater samples, (2) A groundwater recharge zone in the southeast of the study area with low temperature, high alkalinity, and higher Ca and Mg concentrations compared to the other groundwater samples, and (3) Groundwater in the south and central part of the aquifer system that is impacted by varying degrees of salt water intrusion with higher salinity, temperature, and a ~1:1 Na/Cl ratio. The extent of salt water intrusion in the Chicot aquifer is limited and has remained relatively stable over the last ~20 years. However, the extent of saltier water did increase measurably during the drought period of 2011. This was likely in response to infiltration of deeper, saltier waters into the overlying aquifer units. These observations suggest that the sustainability of the Chicot aquifer system could be compromised by increased stress from additional pumping and/or periods of extreme drought. Continued monitoring and evaluation of the groundwater geochemistry in the Chicot will be an invaluable tool for informing future water management decisions.