GEOCHEMICAL AND GEOPHYSICAL DETERMINATION OF THE FATE OF SEPTIC TANK EFFLUENT IN TURKEY CREEK BASIN, 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 to residential users through individual domestic wells and treat wastewater with individual sewage disposal systems (ISDS). Jefferson County estimates that 90% of water pumped by a well is returned to the aquifer via the ISDS. However, when recent data were compared to data collected in 1975—-when the basin was relatively undeveloped—-the surface water quality had declined at a rate 3 times faster than that of the ground water. It is the hypothesis of this team that after leaving the ISDS, some of the effluent filters through the high-permeability regolith until it reaches the regolith-bedrock interface. This “perched” effluent then flows laterally down-gradient and discharges into surface water with only limited treatment.

Various geophysical methods were used to locate this effluent at the regolith-bedrock interface. Where areas of saturation were detected, shallow piezometers were installed to sample the water. The samples were chemically analyzed with respect to major ions and stable isotopes. This chemical signature was compared to the signature of effluent taken directly from the system to identify it as septic in origin. Surface water samples from the basin were then analyzed for the chemical fingerprint determined for the field site. Some surface water samples do appear to be chemically impacted by septic effluent. A water budget is being completed for the system, allowing the velocity and other parameters of the septic effluent flowing along the regolith-bedrock interface to be calculated.