Water table dynamics beneath onsite wastewater systems in eastern North Carolina in response to Hurricane Florence

Onsite wastewater treatment systems (OWSs) are commonly used in eastern North Carolina. A vadose zone or vertical separation distance (VSD) between the OWS drain ﬁ eld trenches and groundwater is required for effective aerobic wastewater treatment. Extreme weather events, including hurricanes, can deliver signi ﬁ cant rainfall that in ﬂ uences groundwater levels and reduces the VSD, thus also in ﬂ uencing the treatment of wastewater by the OWS. Few studies have quanti ﬁ ed the effects of storms on the VSD. Groundwater levels at three sites with the OWS 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 drain ﬁ eld 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 ﬂ ow direction shifted, leading to greater dispersion of wastewater impacted groundwater. The wastewater treatment ef ﬁ ciency of the soil-based OWS in coastal areas may lessen over time because of rising water tables and reduced VSD. Individual pretreatment OWSs, elevated drain ﬁ elds, or centralized sewage treatment may be required in regions with shrinking VSDs.


NITROGEN TREATMENT BY ONSITE WASTEWATER SYSTEMS
Nitrogen is one of the most commonly observed pollutants in groundwater (Naylor et al. ) and surface waters (Paerl et al. ) and may be attributed to various sources, including wastewater. Nitrogen concentrations in wastewater typically range between 33 and 171 mg/L (Lowe et al. ). Organic nitrogen and ammonium (NH 4 þ ) are the dominant forms of nitrogen found in raw wastewater that enters the septic tank. were monitored for each system to provide insights into system performance during an intense hydrologic episode.

Site selection
The study sites were located in Craven County (n ¼ 2) and Pitt County ( Table 1.

Instrumentation of sites
The groundwater wells were constructed using a 5-cm diameter solid PVC pipe connected to 0.9 m of well screen. Well sand was poured around the screen portion of the wells, and a mix of sand and bentonite was used to seal the annular space between the well casing and the outside edge of the borehole for each well. A topographic map was used to determine the approximate elevation of the surface at each site, and a laser level was used to determine the relative elevation of the top of each well casing. The depth to groundwater at each well was measured using a Solinst temperature, level, and conductivity meter (Solinst, Canada). The depth to water was subtracted from the relative elevation of the well casing to determine groundwater elevation. The distances between wells were measured using pull tapes, and well locations were plotted on aerial photographs. Manual, discrete groundwater depths from the wells were determined over the past several years, and these data were used to show typical groundwater-level ranges for comparison to water levels during an extreme event ( Figure 2).

Groundwater monitoring
Hurricane were noted and used along with barometric pressure data to calibrate and correct the water-level logger readings (Spane ). One of the loggers failed during the measurement period at Site 1 (Well C), so data were gathered and

RESULTS
Groundwater flow direction and hydraulic gradients The groundwater flow direction and the hydraulic gradient changed at each site in response to recharge associated with rainfall from the hurricane. At Site 1, the groundwater elevation was initially higher at Well A in comparison to Well B, suggesting that the groundwater flow direction was north. However, during the storm and the rising limb of the hydrograph, groundwater was higher at Well B relative to Well A for 24 h (south) before ultimately shifting back for the duration of the study (Figures 1 and 3). also increased during that period.

Groundwater-level dynamics
Groundwater levels rose over 1.5, 1.7, and 1.8 m at Sites 1, 2, and 3, respectively, within 9 h in response to rainfall associ-   The prestorm water table depths at Sites 2 and 3 were both less than 1.6 m, and these sites experienced direct discharge of septic tank effluent to groundwater. The prestorm water ), the incorporation of advanced pretreatment (Amador et al. ), and the extension of centralized sewer to the most impacted areas may be necessary in regions projected to experience rising seas, shallower groundwater tables, and/or more frequent intense rains.

DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.