Developing Criteria for Small On-Site Sewage Treatment Systems: Two Case Studies

Two sites in Dona Ana County, the Lyons Country Estates Evapotranspiration (ET) bed and the Mesa Village sewage lagoons, were chosen as case study sites for evaluating the design criteria, operation, and impacts on groundwater of troubled on-site sewage systems.

The failure of the Lyons Country ET bed was due to inadequate design. Specifically, the bed was too small to accommodate the low winter ET rates. The bed saturated during the winter months, killing off the vegetation, thereby reducing the summertime ET. The system could not recover, and remained under ponded effluent for several months each year. This ponding presents an immediate public health risk. Soil samples from near the ET bed showed extremely elevated levels of Total Kjeldahl Nitrogen (TKN), indicating a risk of nitrate contamination of nearby groundwater. Monitoring wells installed around the perimeter of the ET bed showed little or no detectable nitrogen; the causes of the conflicting results are unclear. Tierra del Sol Housing Corporation has applied for a discharge permit to allow the release of the ponded effluent into the surrounding soil. They are also pursuing funding for the installation of a constructed wetlands.

The Mesa Village lagoon system was not built as designed. The four cells were not lined as required. However, the limited results of this study indicate that the groundwater beneath the system was not impaired. The regional groundwater is deep (100-150 m), and a near-surface layer of fine-textured soil induced a lateral flow. Increased vegetative growth in the area suggested plant uptake of water and nutrients was significant. Unfortunately, the investigators were unable to obtain sufficient geohydrological data to model the subsurface movement of water and nutrients as intended.