Nitrogen Transformations During Subsurface Disposal of Septic Tank Effluent in Sands: II. Ground Water Quality

Abstract

Ground water observation wells were installed in the immediate vicinity of four septic tank effluent soil disposal systems. Potentiometric maps were constructed from measurements of the groundwater level at each site to establish the direction of movement. Ground‐water samples were pumped from each well to establish patterns of N enrichment in the ground water around the seepage beds and to evaluate the performance of these disposal systems in sands in terms of N removal. Soil disposal systems of septic tank effluent in sands were found to add significant quantities of nitrate (NO3‐N) , formed by nitrification of NH4‐N , the dominant N form in the effluent, to underlying ground water. The data obtained suggest that in sands, the only active mechanism of lowering the NO3‐N content is by dilution with uncontaminated ground water. Relatively large areas of 0.2 ha (0.5 acre) down gradient were needed the studied systems before concentrations in the top layer of the ground water were lower than 10 mg/liter. The average N‐input per person was 8 kg (10 Ib) per year. Essentially complete nitrification in the soil results in addition of approximately 33 kg NO3‐N (73 lb) to the ground water per year for an average family of four.

Septic Tank Impacts on Ground Water Quality and Nearshore Sediment Nutrient Flux

“…Descriptive statistics of NH 4 + , NO 3 -, and DIP for septic tank drainfield and ground water wells are provided in Figure 2. Dissolved inorganic nitrogen (DIN) (range 2852 to 8132 µmol/L) and DIP (range 189 to 530 µmol/L) levels were consistent with reported values (Walker et al 1973b;Reneau Jr. 1977;Sawhney and Starr 1977;Cogger et al 1988;Gold et al 1990). Limited dissolved oxygen data indicated anaerobic or near anaerobic conditions (< 1.0 mg/L) within the drainfields.…”

“…In contrast to phosphorus, nitrogen removal within the drainfield soil system is relatively low (zero to 40%) (Ritter and Eastburn 1988;Gold et al 1990;Wilhelm et al 1996;Valiela et al 1997). Subsequently, nitrogen contamination of ground water has been commonly observed in both shallow (Cogger et al 1988;Stewart and Reneau 1988;Gold et al 1990;Robertson et al 1991;Weiskel and Howes 1992;Wilhelm et al 1994) and deeper water table (> 5 m in depth) (Walker et al 1973b) sandy aquifer systems. As with nutrients, there is only limited fecal coliform purification of waste water within a septic tank, but high and near complete removal efficiencies in properly functioning drainfields (Hagedorn et al 1981;Reneau Jr. et al 1989).…”

Nitrogen Removal in Laboratory Model Leachfields with Organic‐Rich Layers

“…In ground water environments, the lack of usable organic carbon has been cited as the most common limitation to denitrification (Walker et al, 1973; Wilhelm et al, 1994). Nitrate is formed from the conversion of organic and ammonia N in the septic tank effluent after the majority of the carbonaceous biochemical oxygen demand is depleted.…”

“…The largest discharge, by volume, of domestic wastewater into the ground water is from septic tanks (USEPA, 1977; Perkins, 1984). Nitrate N released from septic system drainfields often exceeds the drinking water maximum contaminant level (MCL) of 10 mg L −1 NO 3 –N and may threaten ground water drinking water sources (Robertson et al, 1991; USEPA, 1999; Walker et al, 1973). As a result, septic systems have been listed as one of the top 10 major sources of ground water pollution by 31 out of the 52 reporting states, tribes, and territories in the USEPA's 2000 national water quality inventory (USEPA, 2001).…”

“…Previous research on N movement has been conducted at sites with septic tank systems placed within sandy soils (Walker et al, 1973; Childs et al, 1974; Robertson et al, 1991). Nitrate N concentrations were found to exceed the MCL in ground water up to 170 m from the drainfield.…”