Effluent flux prediction in variably saturated soil zones within a septic tank–soil absorption trench
Cara D. Beal A D , Ted Gardner A , David W. Rassam B , Alison M. Vieritz C and Neal W. Menzies AA School of Land & Food Sciences, University of Queensland; and Coastal CRC, Indooroopilly, Qld 4068, Australia.
B Qld Department of Natural Resources, Mines & Water, Indooroopilly, Qld 4068, Australia.
C CSIRO Land and Water, Indooroopilly, Qld 4068, Australia.
D Corresponding author. Email: c.beal@uq.edu.au
Australian Journal of Soil Research 44(7) 677-686 https://doi.org/10.1071/SR06007
Submitted: 11 January 2006 Accepted: 25 July 2006 Published: 20 October 2006
Abstract
The treatment and hydraulic mechanisms in a septic tank–soil absorption system (SAS) are highly influenced by the clogging layer or biomat zone which develops on bottom and lower sidewall surfaces within the trench. Flow rates through the biomat and sub-biomat zones are governed largely by the biomat hydraulic properties (resistance and hydraulic conductivity) and the unsaturated hydraulic conductivity of the underlying soil. One- and 2-dimensional models were used to investigate the relative importance of sidewall and vertical flow rates and pathways in SAS. Results of 1-dimensional modelling show that several orders of magnitude variation in saturated hydraulic conductivity (Ks) reduce to a 1 order of magnitude variation in long-term flow rates.
To increase the reliability of prediction of septic trench hydrology, HYDRUS-2D was used to model 2-dimensional flow. In the permeable soils, under high trench loading, effluent preferentially flowed in the upper region of the trench where no resistant biomat was present (the exfiltration zone). By comparison, flow was more evenly partitioned between the biomat zones and the exfiltration zones of the low permeability soil. An increase in effluent infiltration corresponded with a greater availability of exfiltration zone, rather than a lower resistance of biomat. Results of modelling simulations demonstrated the important role that a permeable A horizon may play in limiting surface surcharge of effluent under high trench hydraulic loading.
Additional keywords: biomat zone, septic system, unsaturated flow, HYDRUS-2D, modelling, on-site wastewater.
Ahmed W,
Neller R, Katouli M
(2005) Evidence of septic system failure determined by a bacterial biochemical fingerprinting method. Journal of Applied Microbiology 98, 910–920.
| Crossref | GoogleScholarGoogle Scholar | PubMed |
Beach D,
McCray J,
Lowe K, Siegrist R
(2005) Temporal changes in hydraulic conductivity of sand porous media biofilters during wastewater infiltration due to biomat formation. Journal of Hydrology 311, 230–243.
Beach DNH, McCray JE
(2003) Numerical modeling of unsaturated flow in wastewater soil absorption systems. Ground Water Monitoring and Remediation 23, 64–72.
Beal CD,
Gardner EA, Menzies NW
(2005) Process, performance and pollution potential: A review of septic tank–soil absorption systems. Australian Journal of Soil Research 43, 781–802.
| Crossref | GoogleScholarGoogle Scholar |
Beal CD,
Gardner T,
Kirchhof G, Menzies NW
(2006) Long term flow rates and biomat zone hydrology in soil columns receiving septic tank effluent. Water Research 40, 2327–2338.
| Crossref |
PubMed |
Bopp DJ,
Sauders BD,
Waring AL,
Ackelsberg J,
Dumas N,
Braun-Howland E,
Dziewulski D,
Wallace BJ,
Kelly M,
Halse T,
Musser KA,
Smith PF,
Morse DL, Limberger RJ
(2003) Detection, isolation and molecular subtyping of Escherichia coli O157:H7 and Campylobacter jejuni associated with a large waterborne outbreak. Journal of Clinical Microbiology 41, 174–180.
| Crossref | GoogleScholarGoogle Scholar | PubMed |
Bouma J
(1975) Unsaturated flow during soil treatment of septic tank effluent. Journal of Environmental Engineering [ASCE] 101, 967–983.
Campbell GS
(1974) A simple method for determining unsaturated conductivity from moisture retention data. Soil Science 117, 311–314.
van Cuyk S,
Siegrist R,
Logan A,
Masson S,
Fischer E, Figueroa L
(2001) Hydraulic and purification behaviours and their interactions during wastewater treatment in soil infiltration systems. Water Research 35, 953–964.
| Crossref | GoogleScholarGoogle Scholar | PubMed |
van Cuyk S,
Siegrist RL,
Lowe K, Harvey R
(2004) Evaluating microbial purification during soil treatment of wastewater with multicomponent tracer and surrogate tests. Journal of Environmental Quality 33, 316–329.
| PubMed |
van Genuchten MT
(1980) A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Science Society of America Journal 44, 892–898.
Hansen EC, Mansell RS
(1986) Simulated unsaturated-saturated water flow from a septic tank drain to groundwater. Soil and Crop Science Society of Florida 45, 21–29.
Janni KA,
Nye JC,
Jones DD,
Krutz GW, Yahner JE
(1980) Finite element analysis of effluent flow from subsurface sewage disposal fields. Transactions of the American Society of Agricultural Engineers 23, 336–342.
Kristiansen R
(1981) Sand-filter trenches for purification of septic tank effluent: I. The clogging mechanism and soil physical environment. Journal of Environmental Quality 10, 353–357.
Magdoff FR,
Bouma J, Keeney DR
(1974) Columns representing mound-type disposal systems for septic tank effluent: I. Soil-water and gas relations. Journal of Environmental Quality 3, 223–228.
McGauhney PH, Winneberger JH
(1964) Studies of the failure of septic tank percolation systems. Journal – Water Pollution Control Federation 36, 593–606.
Mualem Y
(1976) A new model for predicting the hydraulic conductivity of unsaturated porous media. Water Resources Research 12, 513–522.
Ptacek CJ
(1998) Geochemistry of a septic-system plume in a coastal barrier bar, Point Pelee, Ontario, Canada. Journal of Contaminant Hydrology 33, 293–312.
| Crossref | GoogleScholarGoogle Scholar |
Radcliffe DE,
West LT, Singer J
(2005) Gravel effect on wastewater infiltration from septic system trenches. Soil Science Society of America Journal 69, 1217–1224.
| Crossref | GoogleScholarGoogle Scholar |
Richards SJ
(1938) Soil moisture content calculations from capillary tension records. Soil Science Society of America Proceedings 3, 57–64.
Sherlock MD,
McDonnell JJ,
Curry DS, Zumbuhl AT
(2002) Physical controls on septic leachate movement in the vadose zone at hillslope scale, Putnam County, New York, USA. Hydrological Processes 16, 2559–2575.
| Crossref | GoogleScholarGoogle Scholar |
Siegrist R
(1987) Soil clogging during subsurface wastewater infiltration as affected by effluent composition and loading rate. Journal of Environmental Quality 16, 181–187.
Siegrist R, Boyle WC
(1987) Wastewater-induced soil clogging development. Journal of Environmental Engineering 113, 550–566.
Talsma T
(1983) Soils of the Cotter catchment area, A.C.T.: distribution, chemical and physical properties. Australian Journal of Soil Research 21, 241–255.
| Crossref | GoogleScholarGoogle Scholar |
Whitehead JH, Geary PM
(2000) Geotechnical aspects of domestic on-site effluent management systems. Australian Journal of Earth Sciences 47, 75–82.
| Crossref | GoogleScholarGoogle Scholar |
Wilhelm SR,
Schiff SL, Robertson WD
(1994) Chemical fate and transport in a domestic septic system: unsaturated and saturated zone geochemistry. Environmental Toxicology and Chemistry 13, 193–223.
