Liquid Waste (Septic Tank) Program
Program Guidance
- Bituminous Coatings on Advanced Treatment Units
- Drainfield Additions and Replacements
- Evapotranspiration Systems
Existing Unpermitted Liquid Waste Systems
- Single Lot Policy (link to policy)
- Split Flow Systems
- Tank Water-Tightness Testing
- Septic Tank Reuse Guidelines (link to guidance)
- Variances
Advanced Treatment Systems
Advanced treatment includes secondary (reduction of Biological Oxygen Demand, BOD and total suspended solids, TSS) and tertiary (treatment beyond secondary that includes nitrogen removal). The performance standard for secondary treatment is a reduction in BOD to 30 mg/L or less. The performance standard for tertiary treatment is a reduction in total nitrogen to an average of 20 mg/l or less. For lots less than 3/4 acre, tertiary treatment or, in areas identified by the Department as having anoxic ground water, secondary treatment may be required. In addition, permit conditions shall include: a supplier’s operation and maintenance agreement for the life of the system, acceptable to the Department; and quarterly monitoring requirements for the first year, semiannual monitoring for the second year and annual monitoring for the remainder of the system's life.
Advanced treatment systems are generally more expensive than conventional systems. Advanced treatment systems also require more maintenance than conventional systems. The maintenance for advanced systems must be provided by a factory-authorized individual, and has an associated cost. Anyone interested in purchasing a lot or building on a lot that is smaller than ¾ acre, should contact a licensed installer/contractor for the increased costs associated with the installation and maintenance of advanced treatment systems.
The list of current approved advanced systems in New Mexico, which identifies secondary and tertiary treatment is posted on the NMED Liquid Waste web page here. (Return to top of page)
Bituminous Coatings on Concrete Advanced Treatment Units (ATUs)
Pursuant to Liquid Waste Disposal Regulation 20.7.3.501.J.5, NMAC, a bituminous coating shall be required on all concrete ATU chambers (except those composed of Type V concrete) that will be in contact with hydrogen sulfide gas including, at a minimum, the anaerobic chamber in aerobic treatment units. The coating shall extend to at least six inches below the water line and shall cover all of the internal area, including inlets, outlets, and baffles, above that point. (Return to top of page)
Drainfield Additions and Replacements
If the design flow on an existing liquid waste system is increased, additional square footage may be added to the existing drainfield to achieve the total square footage required for the increased flow, provided that the existing drainfield appears to be functioning properly. In cases of drainfield failure, a new drainfield sized to dispose of the total design flow for the system shall be installed. This can be accomplished by either:
- replacing the failed drainfield; or
- installing a distribution box and a new drainfield of the proper size, and discontinuing flow to the failed drainfield to allow for potential rehabilitation. (Return to top of page)
Evapotranspiration (ET) Systems
Liquid Waste Disposal Regulations specifically define and recognize ET systems (20.7.3.7.E.6, 20.7.3.403 and 20.7.3.806, NMAC). ET systems, therefore, are alternative systems as administered in Regulations 20.7.3.403 and 20.7.3.806. (Return to top of page)
Existing Unpermitted Liquid Waste Systems
Current regulations 20.7.3.401.J and 20.7.3.401.K address existing liquid waste systems that are not permitted. Unpermitted systems may be issued a certificate of registration if:
1) after pumping and an inspection, the system is found to be functioning properly; and
The Environment Department has implemented Liquid Waste Regulations, effective September 1, 2005, and amended February 21, 2007, to ensure that liquid waste permits, for undeveloped lots of less than ¾ acre, are protective of public health and the environment. The new regulations will also ensure that the Department’s mission, to provide the highest quality of life throughout the State by promoting a safe, clean and productive environment, will be met.
While the minimum lot size for conventional septic systems remains ¾ acre, lots platted before February 1, 1990 on less than ¾ acre, but no less than 1/2 acre, and greater than 600 feet to groundwater, may be developed with a conventional septic system, provided all other requirements of the regulations are met. NMED studies have demonstrated that regional pollution of highly vulnerable bodies of ground water have occurred in areas developed with conventional septic systems on lots less than ¾ acre.
Some municipal authorities and the New Mexico Construction Industries (Building and Mobile Home) require a liquid waste permit number before they will approve their respective permits. All builders and installers should plan for the anticipated scrutiny and review to process liquid waste permits on lots that are less than ¾ acres. If the lot in question is less than ¾ acres, a traditional septic system may be inappropriate and an advanced-treatment, evapotranspiration, or split-flow system may be required pursuant to the Liquid Waste Regulations. (Return to top of page)
LWDR 20.7.3.201.M (NMAC) states:
"M. Nothing contained in 20.7.3 NMAC shall be construed to prevent the department from requiring compliance with more stringent requirements than those contained herein, where the department finds that such more stringent requirements are necessary to prevent a hazard to public health or the degradation of a body of water."
The Department has determined, based on hydrogeologic studies and case investigations in New Mexico, that areas where more stringent standards may exist include the following:
- A water-table aquifer (includes both unconfined and semi-confined conditions) 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 wastewater flow;
- sites within 1/4 mile of a known ground-water plume of anthropogenic anoxic or nitrate contamination caused by migration through undisturbed vadose zone, provided that the site overlies the same aquifer;
- an aquifer overlain by fractured bedrock;
- an aquifer in karst terrain; or
- a gaining stream impacted by nutrients from liquid waste systems. (Return to top of page)
In lieu of installing an advanced treatment system, NMED will allow, depending on lot size and site conditions, the installation of black-water/gray-water split-flow systems. Black water is diverted to a holding tank for periodic pumping and off-site disposal, and gray water is discharged to a conventional septic system that is sized to handle the combined flow of black and gray water. NMED also has allowed, depending on lot size and site conditions, toilet waste to be discharged to a holding tank with all other liquid waste going to a conventional system. While these are not true black-water/gray-water split-flow systems, they do reduce contaminant loading to adequately provide equal or better protection relative to advanced systems. (Return to top of page)
Pursuant to Liquid Waste Disposal Regulation 20.7.3.501.B.5, NMAC, NMED may require that septic tanks or other primary components (including ATUs) be tested for water tightness. The tank must be filled with water for at least 24 hours prior to inspection for leaks. Field Inspectors shall give priority to testing tank products that have a history of leakage, and to tanks that show evidence of potential leakage vulnerability during inspections. Additionally, District Managers shall establish a policy for random tank testing that is consistent with the staff workloads and local water conservation issues in each Field Office. Random testing may be waived at sites where water must be trucked in to perform the test, where the on-site supply well produces a low yield, or where residents are under household restrictions or rationing mandated by their public water-supply entity. (Return to top of page)
A variance is a request to vary from regulatory requirements if an applicant is unable to comply with the current regulations. A variance may be granted if the applicant demonstrates that the variance provides equivalent protection of the environmental and health standards set forth in the liquid waste regulations. The burden to demonstrate equal protection is upon the applicant and the granting of a variance is not automatic. A variance request needs to be completed and submitted by the applicant to the area district office of the Field Operations Division for review and processing. Regulation 20.7.3.405 NMAC provides that the applicant must demonstrate that:
“ (a) the proposed on-site liquid waste system will, by itself or in combination with other on-site liquid waste systems, or other discharges subject to 20.6.2.3000 through 20.6.2.3114 NMAC, neither cause a hazard to public health or degrade any body of water; and
(b) granting the variance will result in public health and environmental protection equal to or greater than the minimum protection provided by the varianced requirement.”
Economic hardship is not a basis for granting a variance. (Return to top of page)