Project Summary

The rapid growth of population and development in mountain watersheds caused Jefferson County of Colorado to begin collecting data in a pilot study of ground-water resources in the Turkey Creek Watershed. Located approximately 20 miles west of Denver, the local communities are served by a fractured-crystalline rock aquifer, typical of those in the western US that provide water through individual domestic wells and treat wastewater with individual sewage disposal systems.

Resource managers in the county commonly assume that 90% of water pumped by a residence is returned to the ground water system via individual septic treatment systems. This is inconsistent with the observation that the surface water quality has declined since 1975, while the ground-water quality has been relatively constant. This discrepancy suggests that high permeability regolith may support strong lateral flow in shallow zones between sewage disposal systems and streams. Such a short-circuiting of domestic sewage return flows is consistent with the fact that ground-water levels have been declining over the last few decades.

This project evaluates the possibility that septic system return flows are short-circuiting the deep ground water system via lateral flow through regolith on the bedrock surface and rapidly reaching streams. Four specific sites (two existing septic systems and two new septic systems) that are likely to support shallow lateral flow on the bedrock surface will be studied. Geophysical surveys will identify depth to and slope of bedrock, as well as water table conditions. Repeat surveys at the new sties will indicate whether the water table changes after extended (90 to 180 days) use. Piezometers will be installed to identify the occurrence of lateral flow and sample water quality in the zone.

If short-circuiting is occurring, alternative drain field designs will be recommended to facilitate percolation of return flows. If this process is not documented, alternative causes for the observed changes in water quality and water levels will be hypothesized and the observed behavior of return flows will be documented.