QUANTIFYING NUTRIENT LOADING FROM ONSITE WASTEWATER SYSTEMS TO SURFACE WATERS IN THE LAKE DILLON WATERSHED, SUMMIT COUNTY, COLORADO

This project focuses on surface water quality monitoring efforts in the Dillon Reservoir watershed, Summit County, Colorado. Since the creation of this reservoir in 1963, the area has undergone a large increase in development. The number of housing units has increased from about 2200 in 1970 to about 23,000 in 1998. Many of the homes in this area utilize on-site wastewater systems (OWS) for wastewater treatment and disposal. OWS have the potential to provide advanced treatment and be protective of public health and water quality. This is important as the Lake Dillon is reliant upon good water quality for several reasons. Dillon Reservoir is a drinking water source for the city of Denver, many people use the surface waters for recreational activities, and the water carries a high aesthetic value.

The study area for this project encompasses portions of the Blue River and Tenmile Creek upstream of Lake Dillon. Within this larger study area three smaller focus areas were chosen where there were developments that rely heavily on OWS. At the beginning of the project theoretical calculations were made to predict the levels of nutrients that would be anticipated in the surface waters. Then a total of 20 water quality stations were selected and monitoring was completed to screen the flow and water quality characteristics as well as refine field and laboratory methods. Detailed monitoring of selected sites was then conducted with the goal of obtaining low-level nutrient data as well as producing a dataset for a broad suite of other water quality parameters. The dataset produced is being used to enable seasonal and upstream to downstream trends of nutrients and indicators to be analyzed in an attempt to determine what, if any, water quality effects might be present that can be attributed to OWS. Data are also being compared to local water quality standards. The dataset is also being used to support watershed-scale modeling efforts being completed as part of the same project. This presentation will highlight the methods used and the results of nearly one year of monthly water quality monitoring efforts.