Abstract
AbstractOnsite wastewater treatment systems (OWTSs) are commonly used in eastern North Carolina. A vadose zone or vertical separation distance (VSD) between the OWTS drainfield trenches and groundwater is required for effective aerobic wastewater treatment. Extreme weather events, including hurricanes, can deliver significant rainfall that influences groundwater levels and reduces the VSD, thus also influencing the treatment of wastewater by the OWTS. Few studies have quantified the effects of storms on the VSD. Groundwater levels at three sites with the OWTS were monitored before, during, and after Hurricane Florence. Groundwater rose over 1.5 m within 9 h at the sites in response to rain from the hurricane but took more than 3.5 weeks to return to prestorm levels. Groundwater inundated the drainfield trenches for several days at two sites leading to direct discharge of wastewater to groundwater. The hydraulic gradient and the groundwater velocity increased during the storm and the groundwater flow direction shifted, leading to greater dispersion of wastewater impacted groundwater. The wastewater treatment efficiency of the soil-based OWTS in coastal areas may lessen over time because of rising water tables and reduced VSD. Individual pretreatment OWTSs, elevated drainfields, or centralized sewage treatment may be required in regions with shrinking VSDs.
Highlights
Onsite wastewater treatment systems (OWTSs) are a common means of wastewater treatment and dispersal in rural regions of many countries, including the USA (US EPA ), Australia (Gunady et al ; Geary & Lucas ), Canada (Harman et al ), and Ireland (Gill et al )
Previous research (Cox et al ) has shown that OWTSs in low-elevation, waterfront areas are vulnerable to the effects of sea-level rise and coastal storms, and this study has shown that regions with elevations greater than 20 m above sea level and on broad, flat, interstream divides may be negatively affected by intense rain events
The Hurricane Florence groundwater data in this study reveal that the OWTS treatment was negatively influenced by the storm
Summary
Onsite wastewater treatment systems (OWTSs) are a common means of wastewater treatment and dispersal in rural regions of many countries, including the USA (US EPA ), Australia (Gunady et al ; Geary & Lucas ), Canada (Harman et al ), and Ireland (Gill et al ). Most OWTSs include a septic tank, drainfield trenches, and soil beneath the trenches. While some pollutants in wastewater are removed in the septic tank, most wastewater treatment occurs in the vadose zone (aerated soil) beneath the drainfield trenches. The vertical separation distance (VSD) is the thickness of unsaturated soil between the OWTS drainfield trenches and the water table. Research has shown that when the VSD is reduced to less than 60 cm, bacteria reductions (Karathanasis et al ; Conn et al ; Humphrey et al , a; Stall et al ) and nutrient transformations and removal (Del Rosario et al ; O’Driscoll et al ; Humphrey et al b, ; Cooper et al ) are negatively influenced. The excess loading of nitrogen, phosphorus, and bacteria often causes impairment of water resources (US EPA ; Lusk et al ), and understanding the factors that control the fate and transport of these pollutants is an important step in developing effective environmental policies
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