Bed sediments of streams and rivers may store high concentrations of fecal indicator bacteria (FIB) and pathogens. Due to resuspension events, these contaminants can be mobilized into the water column and affect overall water quality. Other bacterial indicators such as microbial source tracking (MST) markers, developed to determine potential sources of fecal contamination, can also be resuspended from bed sediments. The primary objective of this study was to predict occurrence of waterborne pathogens in water and streambed sediments using a simple statistical model that includes traditionally measured FIB, environmental parameters and source allocation, using MST markers as predictor variables. Synoptic sampling events were conducted during baseflow conditions downstream from agricultural (AG), forested (FORS), and wastewater pollution control plant (WPCP) land uses. Concentrations of FIB and MST markers were measured in water and sediments, along with occurrences of the enteric pathogens Campylobacter, Listeria and Salmonella, and the virulence gene that carries Shiga toxin, stx2. Pathogens were detected in water more often than in underlying sediments. Shiga toxin was significantly related to land use, with concentrations of the ruminant marker selected as an independent variable that could correctly classify 76% and 64% of observed Shiga toxin occurrences in water and sediment, respectively. FIB concentrations and water quality parameters were also selected as independent variables that correctly classified Shiga toxin occurrences in water and sediment (54%-87%), and Salmonella occurrences in water (96%). Relationships between pathogens and indicator variables were generally inconsistent and no single indicator adequately described occurrence of all pathogens. Because of inconsistent relationships between individual pathogens and FIB/MST markers, incorporating a combination of FIB, water quality measurements, and MST markers may be the best way to assess microbial water quality in mixed land use systems.

Relationships between N transport and hydraulic function of onsite wastewater treatment systems (OWTSs) in clay‐textured soils are largely unknown. The objectives of this study were to quantify N concentrations in a clay soil and estimate denitrification in the vadose zone beneath a conventional OWTS. An OWTS installed in the Piedmont region of Georgia was dosed with residential strength wastewater three times per day for 2 yr. Soil near trench bottoms cycled between saturated and unsaturated conditions under the dosing schedule we used. Mean NH4–N concentrations decreased to <1 mg L−1 within 90 cm of trench bottoms due to adsorption, immobilization, and nitrification. Mean NO3–N concentrations increased as the drainfield matured and ranged from 10 to 25 mg L−1 near the end of the two year study period. Mean pressure heads and Cl− concentrations measured at 3.3 m and 6.6 m from trench inlets indicated that significantly more water infiltrated the drainfield at the 3.3 m position. Wet conditions at the 3.3 m position may have restricted nitrification, while dry conditions at the 6.6 m caused rapid nitrification. Nitrogen:chloride ratios suggested that 61% of N entering the drainfield was lost, potentially as volatile N‐species. The 30‐cm zone directly beneath the trenches had high biomass N and C contents (15 mg kg−1 N and 60 mg kg−1 C) and was presumably the most biologically active zone. The fate and transport of N was dynamic in this system due to variable moisture conditions near the trench‐soil interface which may have intermittently stimulated denitrification.

Nitrogen cycling in clay‐textured soils with onsite wastewater treatment systems (OWTS) is studied and modeled much less often than sand‐ and loam‐textured soils because there is little data on OWTS performance in these soils. Information on the nitrogen loads from these systems is needed for quantification of total maximum daily loads (TMDLs). The objective of this study was to calibrate a 2D HYDRUS model using experimental soil pressure head and vadose zone nitrogen (N) and chloride (Cl) data from a conventional OWTS that was installed in a clay soil in the Piedmont region of Georgia. An N chain model with water‐content dependent first‐order transformation rates for nitrification and denitrification was developed. The overall predicted soil pressure heads and solute concentrations were similar to data collected from the field experiment. The calibrated model made it possible to estimate water and solute fluxes in the drainfield and N losses from the OWTS. The estimated annual N loss from leaching at the lower boundary of the experimental drainfield was 3.8 kg yr−1. Scaled up to an OWTS size typical for GA and a zoning density of 5 homes ha−1, the N load to groundwater would be 57.4 kg ha−1 yr−1, which is comparable to agricultural production losses to groundwater. The model predicted 52% of the N removal in the system was from denitrification, whereas plant uptake and change in N storage accounted for ≤5% of the N loss. These estimates were specific to clay‐textured soils and should be valuable to TMDL developers who need to predict load allocations for nonpoint sources in the Piedmont.

On-site wastewater treatment systems (OWTS) work by first storing the wastewater in a septic tank before releasing it to soils for treatment that is generally effective and sustainable. However, it is not clear how the abundance of E. coli changes during its passage through the tank. In this study, which was conducted under the UGA young Scholar Program in summer of 2010, we examined the change in wastewater quality parameters during the passage of the wastewater through the tank and after its release into soil. We collected wastewater samples at the inlet and outlet of an experimental septic tank in addition to obtaining water samples from lysimeters below trenches where the drainpipes were buried. We report that E. coli concentration was higher by 100-fold in the septic tank effluent than influent wastewater samples, indicating the growth of E. coli inside the tank under typical Georgian summer weather. This is contrary to the assumption that E. coli cells do not grow outside their host and suggests that the microbial load of the wastewater is potentially enhanced during its storage in the tank. Electrical conductivity, pH and nitrogen were similar between the influent and effluent wastewater samples. E. coli and total coliform concentrations were mainly below detection in lysimeter samples, indicating the effectiveness of the soil in treating the wastewater.