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Septic Systems: The Real Poop. Dennis McQuillan, New Mexico Environment Department, September 2003. Septic Tanks. The purpose of a septic tank is to: separate solids and oil/scum (sinkers and floaters) from the waste water;

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septic systems the real poop
Septic Systems: The Real Poop

Dennis McQuillan, New Mexico Environment Department, September 2003

septic tanks
Septic Tanks

The purpose of a septic tank is to:

  • separate solids and oil/scum (sinkers and floaters) from the waste water;
  • provide some degree of anaerobic digestion of organic matter; and
  • deliver the waste water to the subsurface
septic tank
Septic Tank

Inside Tank: phase separation; digestion of organic matter; gas generation (methane and hydrogen sulfide)

This and previous slide courtesy of New Jersey Department of Environmental Protection

septic tanks4
Septic Tanks
  • Septic tanks are a suitable means of waste disposal in many areas of New Mexico where lot sizes are large enough, and subsurface conditions are favorable enough, for contaminants to be naturally attenuated to acceptable levels.
  • Non-biodegradable materials should not be disposed of to septic tanks.
  • Septic tanks must be pumped periodically to remove solids and scum.
new mexico
New Mexico
  • ~220,000 household septic systems (septic tanks and cesspools), with ~6500 new systems/year
  • ~75 million gallons per day of wastewater discharged to the subsurface
  • most of this wastewater eventually percolates to ground water
  • many areas using septic systems also rely on private wells for domestic supply
environmental problems
Environmental Problems
  • Leachfield failure: surfacing sewage, backed up toilets and sinks;
  • Surface-water contamination: elevated nutrients, algae blooms;
  • Ground-water contamination: nitrate or anoxic conditions; septic systems are the leading cause of GWC in N.M.
regulatory controls
Regulatory Controls
  • < 2000 gpd, domestic waste only, EIB Liquid Waste;
  • > 2000 gpd domestic, or any non-domestic, WQCC.

Meth Lab Cesspool

Ground Water

what s wrong with this picture
What’s Wrong with this Picture?


Montana State University

drinking your neighbor s sewage
Drinking Your Neighbor’s Sewage

ammonia, urea



ground-water flow


ABQ West Mesa NO3-N (mg/L)




Ground-Water Flow















Atmospheric Nitrogen

Atmospheric Nitrogen

Animal &Human Manure



Nitrogen Fixation

Plant Decay

N2 Gas

Organic nitrogen







Plant Uptake



Water Table

Nitrate Contamination

Environmental Nitrogen Cycle


anoxic ground water conditions
Anoxic Ground-Water Conditions
  • Oxygen deficiency
  • Nitrate is chemically unstable, ammonia in sewage will not oxidize to nitrate
  • Manganese, iron and hydrogen sulfide may be present, and can cause taste, odor and staining problems
redox reactions
REDOX Reactions
  • Aerobic respiration
  • Denitrification
  • Manganese reduction
  • Iron reduction
  • Sulfate reduction
  • Methanogenesis




(to ground-water bacteria)

three domains of life on earth
Three Domains of Life on Earth
  • Bacteria: lack true cell nucleus
  • Archaea: morphologically same as bacteria but have different RNA sequence
  • Eucarya: complex cell structure & nucleus; protozoans, algae, plants & animals
  • Viruses: tiny, parasitic, non-cellular lifeforms?; no independent metabolism; cannot reproduce w/o host
ground water microbes
Ground Water Microbes
  • Bacteria: Pseudomonas (common aerobes in GW),Escherichi, (E. coli), Shigella, Salmonella
  • Archaea: geologically important; methanogens
  • Eucaryotes: less abundant in GW; algae, fungi & protozoa (Giardia, Cryptosporidium); low mobility; can prey on bacteria
  • Viruses: enteroviruses detected in GW; pathogen survival 1-3 months?; hepatitis
contamination factors
Contamination Factors
  • Wastewater flow
  • Vadose-zone conditions (redox, % saturation, hydraulic conductivity)
  • Depth to ground water
  • Ground-water conditions (redox, hydraulic conductivity, gradient)
  • Lot size
  • Nearest down-gradient supply well

Alamogordo, Albuquerque, Anthony, Arenas, Belen, Bernalillo, Bosque Farms, Caballo, Carlsbad, Carnuel, Chamita, Chilili, Chimayo, Clayton, Corralles, Cuyamungue, Deming, Des Moines, Dexter, Elephant Butte, El Rancho, Espanola, Folsom, Gabaldon, Glorieta, Hernandez, Hobbs, Holman, Hondo, Jacona, Jal, Jarales, Lamy, La Puebla, Las Placitas, Lordsburg, Los Lunas, Lovington, Magdalena, Medanales, Monticello, Nambe, Nara Visa, Pojoaque, Portales, Quartales, Quemado, Questa, Raton, Reserve, Roswell, Santa Cruz, Santa Fe, Sedillo, Seneca, Silver City, Tesuque, Tome, Tucumcari, Velarde

ground water modeling
Ground-Water Modeling
  • N.J. nitrate dilution model: uses population density and soil type to calculate acceptable lot size
  • Nitrate dispersion simulation: after 50 years, nitrate dilutes to safe level in 600 feet; low level plume extends ~1 mile
  • Lot size simulations: minimum lot sizes of 2, 5, and 40 acres to protect valley-fill, karts and fractured rock aquifers, respectively in Bernalillo County
health effects
Health Effects

Nitrate – blue-baby case in Carnuel

Pathogens –

  • 1980’s study in ABQ South Valley found no relationship between diarrheal illness and consumption of private well water
  • Additional human health studies are needed /w emphasis on Colonias’
alternatives to conventional septic tanks
Alternatives to Conventional Septic Tanks
  • Composting toilets
  • Holding tanks
  • Evapotranspiration (ET) mounds
  • Aerobic treatment systems
  • Centralized management of on-site systems
  • Regional collection/treatment systems
small lot policy
Small Lot Policy
  • Permit applications for conventional septic tanks on lots smaller than ¾ acre will be scrutinized to make sure that highly vulnerable bodies of water are protected.
  • NMED may require that advanced treatment systems be installed on these small lots.
highly vulnerable bodies of water
Highly Vulnerable Bodies of Water
  • water-table aquifer with a vadose zone thickness of 100 foot or less containing no soil or rock formation that would act as a barrier to saturated or unsaturated flow;
  • aquifer with known anthropogenic anoxic or nitrate contamination;
  • aquifer overlain by fractured bedrock;
  • aquifer in karst terrain; or
  • gaining stream known to be impacted by nutrients from liquid waste systems