Abstract
Many brackish-water reverse osmosis (BWRO) water treatment facilities that use groundwater typically display a linear increase in salinity over time based on upwards recharge of the production aquifer. The City of Fort Myers reverse osmosis facility pumps feed water from the upper part of the Floridan aquifer system which is a leaky aquifer. Aquifers containing saline water that is semi-confined or leaky tend to exhibit long-term changes in salinity. Aquifer characterization is an important component in the design and operation of a BWRO facility. The characteristics of the production aquifer feature an upper confining unit that is much thicker than the lower confining unit and when the aquifer is pumped, the recharge to the pumped aquifer is from the bottom upwards. Typically, salinity increases with depth so the pumped aquifer will take on the characteristics of the underlying aquifer over time. Data were collected and analyzed from production wells from May 2002 through August 2018 that includes pumping rates, total dissolved solids (TDS) concentrations, and dissolved chloride concentrations. The average wellfield TDS concentrations in May 2002 and August 2018 were 2,359 and 5,417 mg/L, respectively. A regression analysis was performed using the most recent five-year data and projected out 20 years. The average wellfield TDS concentrations are predicted to be 6,433 mg/L at 5 years and 7,683 mg/L estimated at 10 years. The 20-year projection shows an increase to 10,184 mg/L which is approximately 1.9 times greater than today. These are very large variations in the rate of salinity increase between production wells which may indicate that enhanced corridors of vertical permeability exist within the wellfield which could influence future facility operation.
