Introduction
Water quality problems are common in the surface waters and groundwater of North Carolina. North Carolinians often hear about algae thriving in estuaries and rivers, fish kills, and excess nutrients in our rivers. Water quality problems in North Carolina originate from numerous sources, both point and non- point. Point sources are those such as pipes discharging to streams and surface runoff. Non-point sources of pollution in the waters of the State are diffuse sources such as pesticides or nutrients that reach surface waters via runoff after their application in farm fields or sediments from construction sites that end up in rivers and lakes. Non-point source (NPS) pollution is a significant source of contamination.
On-site wastewater systems such as septic systems contribute to NPS pollution. On one hand, surface failing systems can contribute with significant concentrations of organic matter, nutrients, and pathogens entering surface water directly or by stormwater runoff. On the other hand, these systems release nutrients and pathogens through their drainfields into the ground, to the groundwater and ultimately to surface waters through groundwater discharge. These nutrients and pathogens can potentially contaminate both subsurface and/or surface waters if not properly treated prior to reaching the groundwater.
Many studies have been performed on OSWS such as system performance and design, nutrients and pathogen transport near and away from the drainfield. However, we still ask ourselves the questions of how much these systems really contribute to pollution? To what extent are these systems contaminating the groundwater and surface waters? In an effort to answer these questions, this review looks at the relevant studies conducted throughout the country and elsewhere in the past decades. These studies assess fate and transport of nutrients and pathogens that are introduced in the soils after leaving an OSWS.
Nitrogen contributors from OSWS to the soil and eventually groundwater or surface water depend on several factors including soil texture, water table level, organic matter content in the soil, dissolved oxygen, and soil temperature. Figure 1 shows schematically the typical N transformations that take place as the effluent form OSWS is applied on to the soil.
The fate of NH4+ in the soil depends greatly on the water table level. For conventional septic tank systems, the highest water table level in the soil should be kept at a distance of 48 inches when suitable soils are present. Suitable soils for OSWS are described as well drained and range from sand to loamy sands to sandy loams to loams. These soils do not exhibit high water tables and ensure proper functioning of the septic tank. When NH4+ is applied on to these soils the oxygen present in the soil pores facilitates the transformation of NH4+ into NO3-. This process is called nitrification and occurs only when aerobic conditions are present. Several studies performed on sandy unsaturated soils reported almost complete nitrification (Weiskel and Howes, 1992 and Robertson et al., 1991).
Weiskel and Howes (1992) monitored a 53 ha residential area in Buttermilk Bay, Massachusetts with septic tank systems draining on to sandy soils with less than 0.1% clay content. The soils contained a negligible background N concentration. The study documented the depth and surface extension of the plume of contaminants as it flowed away from the drainfield into the bay. Weiskel and Howes (1992) found that NO3- comprised 73% of the dissolved inorganic N in the plume before reaching the groundwater. Similarly Robertson et al. (1991) extensively monitored two individual households located on sandy soils in Cambridge, Ontario (Canada). This study also established a complex monitoring network of wells in order to document the depth, width and length of the plume as it flowed away from the drainfield. Robertson et al. (1991) reported that in one site 100% of the effluent NH4+ converted into NO3- and 67% was reported in the other site.
Nitrification may not take place when nitrifying bacteria are not present. In such case NH4+ may adsorb onto soil particles via cation exchange. Bicki et al. (1984) summarized that NH4+ adsorption can be significant. The primary factors that determine the extent of NH4+ adsorption are the number of soil cation exchange sites exposed to the septic effluent, the affinity of the sites for NH4+, and the composition of the effluent. Adsorption of NH4+ onto soils can reach an equilibrium once the cation exchange sites are filled with the cations from the effluent. Bicki et al. (1984) also explained that NH4+ can desorb and be subject to nitrification if the soil conditions allow it. Thus adsorption and desorption of NH4+ in soils can be a cyclic process driven towards finding an equilibrium.
The presence of high water table levels can cause NH4+ or NO3- to leach into the groundwater. Continuously flooded conditions can limit the amount of oxygen in the soil pores and therefore limit nitrification. In an anaerobic environment NH4+ is a stable cation that can diffuse towards groundwater and eventually move with the groundwater flow. When NH4+ leaches into the groundwater (as indicated in Error! Reference source not found.) insignificant nitrification may take place at the groundwater table interface as the dissolved oxygen concentration rapidly decreases in the water column. Cogger and Carlile (1984) and Carlile et al. (1981) reported evidence of high NH4+ content in groundwater from septic systems.
Cogger and Carlile (1984) described the performance of conventional and alternative septic tank systems built on seasonally and continuously flooded soils. The study sites were located in Craven, Hyde, and New Hanover counties in the lower Coastal Plain of North Carolina. The soils found in the study sites were of coarse, silty, fine, and organic texture. Cogger and Carlile (1984) showed that continuously saturated systems exhibited higher NH4+ concentrations and lower NO3- concentrations. In addition, they observed that seasonally saturated systems displayed higher NO3- concentrations and lower NH4+ concentrations of 8 mg/l and 5.7 mg/l [NH4+-N] on the average during the periods summer of 1979-summer of 1980 and fall 1980-winter 1981, respectively. Nitrate concentrations were rather low with averages of 0.3 mg/l and 2.1 mg/l [NO3--N] for the same periods of time. In contrast, seasonally saturated systems averaged 3 mg/l and 1.9 mg/l [NH4+-N] for the above mentioned periods of time, respectively. The average NO3- concentrations that the seasonally saturated systems reached were 2.1 mg/l and 4.9 mg/l [NO3--N] during the same seasons. These results suggested that continuously saturated systems experience little nitrification; however, water table variability and alternate aerobic/anaerobic conditions in the soil prompt nitrification in seasonally saturated systems.
Carlile et al. (1981) presented an expanded report on the same sites and systems as in Cogger and Carlile (1984). The septic tank systems included conventional, low-pressure pipe systems (LPP), mound and soil replacement. Some of they systems were exposed to continuously flooded conditions and some were seasonally flooded. Carlile et al. (1981) reported that NH4+ and NO3- concentrations decreased with distance from the drainfield for both continuously flooded and seasonally saturated systems. On continuously saturated soils, NH4+ concentrations ranged from average concentrations up to 26 mg/l [NH4+-N] for wells near the drainfield (5m) to up to 3.0 mg/l [NH4+-N] for wells located at 100 ft from the drainfield. Nitrate concentrations were much lower averaging from up to 7 mg/l [NO3--N] near the drainfield to 1 mg/l [NO3--N] for wells located away from the drainfield. Three out of five of the LPP systems were continuously flooded. These systems displayed a similar behavior to the conventional systems.
If nitrification takes place in unsaturated soils that lack organic matter NO3- usually leaches into the groundwater and eventually moves with the groundwater flow (see Figure 1). Numerous studies on OSWS have reported high NO3- (greater than 10 mg/l [NO3--N]) concentrations in groundwater and have indicated OSWS as the source. Bicki et al. (1984) summarized several of these studies and cited Preul (1966), Polkowski and Boyle (1970), Walker et al. (1973a, b), Childs et al. (1974), Wolterink et al. (1979), Rea and Upchurch (1980). These studies reported NO3- concentrations varying from 10 mg/l [NO3--N] to 70 mg/l [NO3--N]. These concentrations were observed at distances greater than 10 ft and up to 100 ft. Recent studies such as Robertson et al (1991) and Aravena et al. (1993) have also reported on significant NO3- movement in a septic plume.
Robertson et al. (1991) documented rapid nitrification in a septic plume that diffused downwards toward the groundwater in a sandy soil in Cambridge, Ontario. The study followed the plume using a sodium tracer through a complex set of monitoring wells. The plume initially moved vertically downwards through the unsaturated soil layer, but as it reached the groundwater it switched to horizontal movement. Nitrate concentrations in the plume core varied within a range from 21 to 48 mg/l [NO3-]. Background concentrations varied from 17 to 34 mg/l [NO3-]. High background concentrations were due to agricultural practices. Nitrate concentrations did not change as the septic plume moved downgradient through a distance of 330 ft.
A later study performed at the Cambridge site by Aravena et al. (1993) confirmed Roberston et al (1991) plume characterization and used stable isotopes of oxygen (18O) and nitrogen (15N) to identify NO3- contributions to groundwater that originated in septic systems. As did Robertson et al. (1991). Aravena et al. (1993) identified the septic tank plume using a sodium tracer and described the plume as diffusing downwards right below the drainfield. The plume moved through a medium coarse sand aquifer until it reached a zone of compact pebbly sandy silt with lower hydraulic conductivity. Once the plume reached the more impermeable layer it switched from vertical movement to horizontal movement parallel to the groundwater flow. Araena et al. (1993) concluded that the isotope 15N values in the plume reflected values reported for animal waste sources while the values of the non-plum NO3- were within the values reported for organic N in soils. The study also showed that the isotope 18OH2O values in the plume were smaller from those of the non-plume (- 10.6±0.3% versus -9.4±0.4%, respectively). An analysis of the 18O isotope in the well water showed a value of -10.8%, which is within the range of the plume 18OH2O isotope. Aravena et al. (1993) concluded that this isotope was a good tool to trace the plume. Aravena et al. (1993) was able to track the plume for 130 m from the tile bed. Nitrate remained dissolved in the groundwater and no significant denitrification took place.
Other N removal paths are ammonia volatilization, conversion of NH4+ into ammonia gas (NH3) plant root uptake of NH4+ and NO3-, and microorganism incorporation of NH4+ and NO3- (See Figure 1). These removal paths are considered non-significant and not permanent in the case of microorganism in the septic tank. However, some studies have found up to 45% N removal through plant uptake (Bicki et al. 1984). Plant uptake is not an important N removal path for conventional systems because the amount of N released by these OSWS greatly exceeds that which can normally be utilized by nearby plants. However, innovative and alternative OSWS such as recirculating sand hills and constructed wetlands can offer great pre-ground absorption N removal (Spooner et al., 1998).
Under anaerobic conditions denitrifying bacteria convert NO3- into N gases. This reaction is the most significant N removal mechanism in soil environments. For denitrification to take place several conditions must be present in the soils: nitrification of NH4+, anaerobic conditions, presence of denitrifying bacteria, and adequate carbon source for the denitrifying bacteria must be present in the anaerobic soil. A study by Hinson et al. (1994) reported that these conditions might not always be present in the soils. Hinson et al. (1994) discussed on the performance of sand lined trench systems on wet clayey soils in North Carolina. This study monitored N at a site where drainage is used in conjunction with a septic system. The groundwater flow direction changed through out the year due to the complex drainage network in the area. The authors did not attempt to follow the N plume, nevertheless, high NO3- concentrations were observed in some wells during the winter high water table. During low water table months, the authors believed that both NH4+ and NO3- were held in the unsaturated soil column. The study found that high water table months displayed high redox levels (Eh > 500 mV) and low water table months were associated with Eh values <100 mV. Values for Eh between 100 to 500 mV (sub- oxic redox) are a sign of the anaerobiosis needed for denitrification to occur. Anoxic redox values with Eh <100 mV occurred through late summer to early fall. Subsequently, sub- oxic redox values preceded oxic redox values (Eh>500 mV) which started in early winter.
Other studies such as Robertson et al. (1991), Anderson (1998), and Gerritse et al. (1995) followed the septic plume and documented nitrate disappearance in soils. These studies did not report on redox levels, yet, organic matter content and low dissolved oxygen levels were pointed at denitrification as the cause of septic N disappearance of soils.
Robertson et al. (1991) monitored a site located on the edge of the Muskoka River in Ontario, Canada. The site was a conventional septic system located on a 10-m fine fluvial sand layer overlying granitic bedrock. The septic system discharged to a tile bed of 80 m2 in area and 20 m away from the Muskoka River. The water table was located at about 3 m from the surface at the drainfield. The land use was mainly residential and no important N background levels were reported. This study followed the NO3- plume as it moved away from the drainfield. The average NO3- concentration reported in the drainfield was 0.1 mg/l [NO3--N] while the average NH4+ concentration was 59 mg/l [NH4+-N]. the average concentration of NO3- and NH4+ found at the plume core were 39 mg/l [NO3--N] and 0.5 mg/l [NH4+-N] respectively. Thus, nitrification was almost complete and NO3- diffused in the groundwater. The NO3- plume was extensively documented using a network of wells that captured NO3- concentrations at several depths. As NO3- approached the river bed sediment the average concentrations reported declined from 30 mg/l (in the center of the plume) to 10 mg/l (before reaching the riverbed). In addition, Robertson et al. (1991) reported that the fraction of organic carbon (foc) in the aquifer was 0.0003 while the foc in the riverbed sediments was 0.017, 60 times larger. Moreover, the dissolved oxygen concentrations were depressed in the plume but increased at the riverbed interface. The readings from seepage meters located at the riverbed interface indicate that NO3- remained under 0.5 mg/l [NO3--N]. These data strongly suggested that denitrification occurred in the aquifer riverbed interface. The most vigorous zone of denitrification found through the seepage meters was at 0.5 below the riverbed.
Anderson (1998) monitored a septic system located on a raised lot that drained to the Indian River Lagoon (an estuary), in Florida. The soils were described as sandy soils consisting of dark brown fine sand with some organic material and varying to reddish brown fine sands with organic material. The sands contained a foc varying from 0.5 to 1.5%. The dissolved oxygen varied from 3.2 to 5.1 mg/l. The water table was generally established using a tracer study and a network of wells was installed to follow this gradient. The study reported NO3- concentrations in the wells varying from 9 mg/l [NO3--N] near the drainfield to <0.05 mg/l [NO3--N] 50 ft away from the drainfield. Ammonium concentrations were not reported, however, total N concentrations did not differ significantly from those reported for NO3-. the author discussed that literature values for denitrification rates were positively correlated with reported values of foc. Using this argument, Anderson (1998) concluded that this site had a great potential for denitrification as the mechanism responsible for NO3- removal from the soil near the lagoon.
Gerritse et al. (1995) measured N concentrations from a modified Australian septic tank system on the Darling Plateau in Western Australia located in a residential area. The septic system was located in a 0.4-ha residential lot that had only a residential land use. The site drained to a nearby creek located 262 ft away from the system. The septic tank drained into a 40-ft long leach drain made of an open-ended concrete slab or leach drain embedded in sandy soils. the soil was described as gently undulating terrain with well-drained, shallow to moderately deep (up to 3 ft) sands, overlying lateritic duricrust. The lateritic layer was saturated during three months at the end of the summer. A network of wells covering depths from 0.5-3 ft and a surface area of 130-ft wide x 230-ft long was used to monitor N species. Groundwater movement was monitored using a bromide tracer. The groundwater followed the topographic gradient. Tracer concentrations decreased with distance indicating a high septic plume dilution in groundwater. High NO3- and NH4+ concentrations near the leach drain were reported. However, negligible amounts of both NO3- and NH4+ were found in the wells located 32 ft away from the leach drain and those following downgradient towards the creek. The mass ratio of inorganic N to excess bromide declined with distance indicating that a factor additional to groundwater dilution was responsible for low N concentrations. Gerriste et al. (1995) concluded that denitrification was a likely cause of NO3- disappearance. The authors argued that pH (6.1- 7.7) and temperature (18°C) conditions were ideal for denitrification. Furthermore, the changing water table above the lateritic layer and the temperature and pH conditions in the soils were conducive to fluctuating anaerobiosis in the soil. This study did not report any amount of organic matter or dissolved oxygen in the soil.
Please address any questions to Dr. David Lindbo.
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