Simulation of Groundwater Flow and Chloride Transport in the “2,000-foot” Sand of the Baton Rouge Area, Louisiana, with Scenarios to Mitigate Saltwater Migration

Charles E. Heywood, 2131 Pleasant Hills Ct., Ridgeway, Colo. 81432, cheywood@usgs.gov; John K. Lovelace, 3535 S. Sherwood Forest Blvd., Suite 120, Baton Rouge, La. 70816, jlovelac@usgs.gov

Groundwater withdrawals have caused saltwater to encroach into freshwater-bearing aquifers beneath Baton Rouge, Louisiana. Groundwater investigations in the 1960s identified a freshwater-saltwater interface located at the Baton Rouge Fault, across which abrupt changes in water levels occur. Aquifers south of the fault generally contain saltwater, and aquifers north of the fault contain freshwater, though limited saltwater encroachment has been detected within 7 of the 10 aquifers north of the fault. The 10 aquifers beneath the Baton Rouge area, which includes East and West Baton Rouge Parishes, Pointe Coupee Parish, and East and West Feliciana Parishes, provided about 184 million gallons per day (Mgal/d) (700 million liters per day [ML/d]) for public supply and industrial use in 2012. Groundwater withdrawals from the “2,000-foot” sand in East Baton Rouge Parish have caused water-level drawdown as great as 356 feet (108 meters) and induced saltwater movement northward across the fault. Saltwater encroachment threatens industrial wells that are located about 3 miles north of the fault. Constant and variable-density groundwater models were developed with the MODFLOW and SEAWAT groundwater modeling codes to evaluate strategies to control saltwater migration, including changes in the distribution of groundwater withdrawals and installation of “scavenger” wells to intercept saltwater before it reaches existing production wells. The models were created and calibrated using well construction information, pumpage data, groundwater levels, and chloride concentrations that the USGS has collected and maintained since the early 1900s.

Five hypothetical scenarios predict the effects of different groundwater withdrawal options on groundwater levels and the transport of chloride within the “2,000-foot” sand during 2015–2112. Amongst these five scenarios, three of the scenarios simulate only various withdrawal reductions, whereas the two others also incorporate withdrawals from a scavenger well that is designed to extract salty water from the base of the “2,000-foot” sand. Two alternative pumping rates (2.5 Mgal/d and 1.25 Mgal/d [9.5ML/d and 4.7 ML/d]) 0re simulated in each of the scavenger-well scenarios. For the “2,000-foot” sand scenarios, comparison of the predicted effects of the scenarios is facilitated by graphs of predicted chloride concentrations through time at selected observation wells, plots of salt mass in the aquifer through time, and a summary of the predicted plume area and average concentration. In all scenarios, water levels essentially equilibrate by 2047, after 30 years of simulated constant withdrawal rates. Although predicted water-level recovery within the “2,000-foot” sand is greatest for the scenario with the greatest reduction in groundwater withdrawal from that aquifer, the scavenger-well scenarios are most effective in mitigating the future extent and concentration of the chloride plume. The simulated scavenger-well withdrawal rate has more influence on the plume area and concentration than do differences among the scenarios in industrial and public-supply withdrawal rates.