A HYDROGEOCHEMICAL APPROACH FOR DETERMINING SOLUTE SOURCES IN THE BOULDER CREEK WATERSHED, COLORADO

During 2000, a hydrogeochemical investigation of the 1160 km² Boulder Creek Watershed was undertaken to evaluate natural and anthropogenic sources of solutes and geochemical processes that affect water quality. Located in the Rocky Mountain Front Range, Boulder Creek originates as snowmelt at the continental divide, which is underlain by crystalline bedrock, then flows through historical mining districts and mountain communities to the mouth of Boulder Canyon. From there, Boulder Cr. flows over Mesozoic sedimentary rocks and through the city of Boulder and into the plains. A 70-km reach of Boulder Cr. (16 sites) and its major inflows (13 sites) were sampled during high and low flow. Discharge was determined at all sites, and water samples were collected for major, trace element, and organic carbon analyses. At selected sites, analyses also included a suite of pesticides, pharmaceuticals, and wastewater-derived organic compounds and strontium isotopes.

In the headwaters, Boulder Cr. is a dilute, Ca/Mg-HCO₃/SO₄ water, and slight increases in solutes were observed in the upper basin. Mass balance calculations suggest that weathering, minor mineral dissolution of crystalline bedrock, is the primary source of solutes. Upon exiting Boulder Canyon, solute concentrations increase at a greater rate, with the most notable increase below Boulder’s wastewater treatment plant (WWTP). In the urban reach (upstream of the WWTP), increasing Ca, Mg, and SO₄ concentrations reflect the change in bedrock geology. The strontium isotopic composition of Boulder Cr. decreases from > 0.7190 in the upper basin to 0.7153 in the urban reach, also reflecting the change in geology. Treated effluent from Boulder’s WWTP dominates the chemistry of lower Boulder Cr. because of elevated concentrations of most constituents and because upstream flow has been diverted for agricultural uses and cannot provide instream dilution. The few pesticides detected were likely derived from urban and agricultural land uses. Forty wastewater-derived organic compounds were identified, indicating that these compounds may not be removed during secondary wastewater treatment. Water quality of Boulder Cr. is affected by discharge variations from snowmelt, diversions, and WWTPs, as well as land use, bedrock geology, and wastewater effluent composition.