EBTAG Annual Workshop and Field Trip
May 16-17, 2012

¹N.M. Environment Department, 1190 St. Francis Drive, Harold Runnels Building, Suite S-2100, Santa Fe, NM, 87505, dennis.mcquillan@state.nm.us

²N.M. Environment Department, 2540 Camino Edward Ortiz, Santa Fe, NM, 87507

Onsite wastewater systems provide treatment and decrease contaminant concentrations, but do not produce drinking-water quality effluent.  Natural attenuation mechanisms, therefore, must be able to decrease contaminants to acceptable concentrations as the effluent migrates from the drainfield, through soil, and into groundwater to the nearest down-gradient water-supply well.  Natural attenuation mechanisms that may occur in soil or groundwater include dispersion, diffusion, mechanical filtration, microbe death, cation exchange, adsorption, biodegradation, denitrification and volatilization.  A proper site evaluation is critical to designing an onsite wastewater system that, through a combination of treatment and natural attenuation, will not adversely affect drinking water wells.  Onsite wastewater systems that are not properly sited, designed, constructed, operated and maintained have contaminated water wells with nitrate, chloride, total dissolved solids, anaerobic respiration byproducts and with pathogenic microbes.  

Lot size, depth to groundwater, lithology, and redox conditions can influence the effectiveness of natural attenuation.  Lot size has no influence on the groundwater quality impact from a single wastewater system, but can strongly influence the cumulative impact of multiple systems in an area.  Increasing depth to groundwater can increase the migration distance and travel time of effluent percolating in the soil, thereby increasing the potential for natural attenuation and decreasing aquifer vulnerability.  Lithologic heterogeneities, such as a gravel layer or other capillary barrier inter-bedded with finer-grained sediments, also can increase migration distance, travel time and the potential for natural attenuation.  Fractured rock typically provides less opportunity for natural attenuation than do sedimentary deposits with inter-granular porosity.  Redox conditions can control the occurrence and transformation of nitrogen compounds.

Field studies in basin-fill areas utilizing onsite septic systems and domestic wells demonstrate that increasing lot size and depth to groundwater generally decrease the potential for water well contamination.  Water wells in Rancho San Marcos, with a minimum lot size of 10 acres and groundwater depths of approximately 175 to 250 feet, show little if any influence from onsite septic systems.  In Eldorado, with an average lot size of about 1 acre, significant greenbelt acreage, and groundwater depths in the range of 100 to 200 feet, nitrate-N has been detected above the drinking-water standard of 10 mg/L in only one well, and most nitrate-N levels are less than 5 mg/L.  Hernandez, with many small lots ½ acre or less, and groundwater depths of about 50 to 100 feet, onsite wastewater systems polluted at least 76 private domestic water wells with nitrate-N greater than 10 mg/L.  Public water was extended into Hernandez to eliminate the widespread health hazard from nitrate pollution. 

In some fractured bedrock areas, onsite wastewater systems have polluted drinking water wells with nitrate, chloride, total dissolved solids and E. Coli even though all regulatory requirements for lot size, set back, clearance and suitable soil were met.