INVESTIGATING THE IMPACTS OF BRACKISH FLOODING ON PATHOGEN FATE AND TRANSPORT IN SANDY SOIL TREATMENT AREAS OF COASTAL SEPTIC SYSTEMS

Abstract

Many of Maryland’s coastal communities rely on private septic systems for wastewater treatment, instead of public wastewater treatment facilities. Septic systems depend on soil treatment areas (STA’s) for removal of bacteria from wastewater. Climate change is driving rising groundwater tables and increased flooding, threatening the performance of these systems by inducing saturation and introducing brackish or saline waters. While the negative effects of soil saturation on bacteria removal in STA’s are well documented, the influence of brackish flooding remains largely unknown. This study used a series of laboratory-scale sand columns to simulate new (clean sand) and mature (sand with an established biofilm) STA’s. All columns were fed syn. STE containing a model pathogen, E. coli. A sub-set of columns were subjected to repeated flood simulations with brackish water. Influent and effluent E. coli concentration, pH, and electrical conductivity were monitored throughout the experiment. Post-experiment analyses characterized sand properties including total organic content, moisture content, and adsorbed bacterial abundance. Scanning electron microscopy was used to visualize bacterial adsorption and biofilm formation on the surface of sand particles. Over the duration of the experiment, the weekly E. coli removal rate increased by 232% in control columns and 23% in flooded columns. The increased removal that was observed in all columns can be attributed to column media maturation. The removal rate increased significantly (p<0.05) more in the control columns than those subjected to floods, as the benefits of maturation were largely offset by brackish flooding in the flooded columns. Biofilm inoculation of column media enhanced E. coli adsorption in control and experimental columns, but it did not have a significant impact on overall E. coli removal rates. These findings demonstrate the vulnerability of sandy-STA’s to coastal flooding, providing critical insight into how coastal flooding may negatively impact bacterial removal. Results were used to generate estimates of bacterial loads from properly functioning and failed septic systems, for inclusion in Maryland’s Bacteria Total Maximum Daily Loads.