GEOCHEMICAL AND GEOPHYSICAL DETERMINATION OF THE FATE OF SEPTIC TANK EFFLUENT IN A MOUNTAIN WATERSHED, COLORADO

With the rapid development and population growth of the Turkey Creek Basin of Jefferson County, Colorado, the degradation of the water quality has become a pressing issue. 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 an individual domestic well and treat wastewater with an individual sewage disposal system (ISDS). Jefferson County estimations suggest that 90% of well water usage is returned to the aquifer via the ISDS. However, when recent data are compared to data collected in 1975—when the basin was less developed—we find surface water quality has declined by a factor of 3, while ground water quality has declined by a factor of 1.5. We hypothesize that, after filtering through the high-permeability regolith, some of the ISDS effluent is “perching” on the bedrock surface and flowing laterally down-gradient along the regolith-bedrock interface, discharging into streams.

Alternative geophysical methods are evaluated and tested to identify perched effluent on the regolith-bedrock interface down-gradient of the ISDS. Where perched water is detected, small holes are drilled to the interface and water samples are taken. These samples are analyzed with respect to major ions and “human indicators” (i.e. caffeine, ibuprofen, estrogen). This chemical signature is compared to the signature of the well water for the home and effluent taken directly from the ISDS drain field. Once identified, specific chemical parameters can serve as fingerprints in stream water samples to detect ISDS effluent influence on stream water quality.