Type of Thesis
G. Lang Farmer
Strontium isotopic composition and major and minor trace element compositional data allows us to classify waters and identify sources of dissolved solutes to deduce history of surface waters. In this study, such data were used to investigate the compositions and sources of solute found in selected surface waters from the Colorado Front Range and vicinity. The major ion compositions of surface waters draining from the eastern slope of the central Front Range are similar. Bulk water composition of upper Boulder Creek, Left Hand Creek and Coal Creek are all classified as Ca-Mg-Cl type. Most of the solutes in these waters were likely derived from weathering of local Precambrian granitic rocks and gneisses, but with the additional Ca from surface dust and rainwater deposition. Strontium compositions of creeks in this study reflect weathering and mixing from underlying bedrock. Coal Creek and upper Boulder Creek have strontium isotopic compositions (87Sr/86Sr>0.720) consistent with the weathering of Precambrian crystalline rocks, but strontium in Left Hand Creek is significantly less radiogenic 87Sr/86Sr~0.712877) and may reflect weathering of early Cenozoic alkaline intrusive rocks present only in this subwatershed. Lower Boulder Creek is also a Ca-Mg-Cl type water, but has lower 87Sr/86Sr than the upper reaches of this stream and likely reflect the addition of low 87Sr/86Sr derived from Cretaceous sedimentary rocks across which the creek traverses immediately to the east of the Front Range. Finally, Lake Creek, at high elevation in the Sawatch Range of central Colorado is a Ca-Mg-Cl water but has significantly higher dissolved metal concentrations than any of the Front Range creeks likely reflecting weathering of hydrothermal sulfide deposits found in the creek headwaters. The strontium characteristics and rare earth element concentrations normalized to chondrite in Lake Creek waters are consistent with what we see in bedrock tuffs and so are consistent with the weathering of altered, Oligocene ash flow tuff in the headwaters. These results illustrate again how the compositions of the surface waters are largely controlled by local bedrock compositions.
Cristo, Natalie P., "SURFACE WATER GEOCHEMISTRY AND STRONTIUM CHARACTERISTICS OF COLORADO CREEKS" (2016). Undergraduate Honors Theses. 1101.