Six communities developed with on-site septic systems in New Mexico were investigated.  All six sites were located in areas with geochemical conditions favorable for the oxidation of nitrogen in septic system effluent to nitrate.  Five sites were developed with individual domestic water wells, and one had public water service.  Four sites were located on basin fill sediments, and two were underlain by fractured bedrock.  Depths to ground water ranged from approximately 30 to 400 feet.  Platted lot sizes ranged from approximately 0.33 to 5 acres.  Regional development densities, calculated to account for varying lot sizes and the acreage of undeveloped lots, roadways, utility easements and greenbelts, ranged from 0.43 to 4.46 acre/house.  Nitrate was measured in domestic supply wells throughout five communities, and in monitor wells installed in the community with a public water system, to establish background and increases caused by septic systems. 

Increases in nitrate above background levels were observed at all six sites, and ranged from approximately 2 to 22 mg/L as N.  At basin fill sites, nitrate-N increases exceeded the allowable limit of 10 mg/L for development densities less than 0.5 acre/house, but not for densities greater than 1 acre/house.  Nitrate contamination exceeded 10 mg/L as N at both fractured bedrock sites, which had development densities of 0.82 and 4.46 acre/house, and depths to ground water up to 200 feet.  Increasing depth to ground water appeared to correspond with some degree of increased natural attenuation in basin fill terrain.