MECHANISMS OF NITRATE REMOVAL WITHIN A MOUNTAIN WATERSHED, JEFFERSON COUNTY, COLORADO: CHEMICAL AND STABLE ISOTOPIC EVIDENCE

Previous studies have indicated that nitrate removal is a common phenomenon in aquifers, riparian zones and surface waters. In the Turkey Creek watershed (located in the Front Range foothills, Colorado, USA), which is underlain by igneous and metamorphic rocks of Precambrian age, chemical evidence suggests that large amounts of nitrate, derived from septic waste, are being removed through an unknown process. Evidence for nitrate removal includes anomalously low ratios of nitrate/chloride in surface water relative to groundwater. The most likely explanation for the removal of large amounts of nitrate from aquifers involves the process of microbial denitrification, by which bacteria convert nitrate principally into nitrogen gas. Although the importance of the riparian zone as a site of denitrification is well known, denitrification may also occur in other locations of the watershed as well, provided that sufficient sources of energy are available. Two such energy sources in the Turkey Creek watershed may include residual organic carbon from septic waste, as well as random disseminated or concentrated bands of pyrite that may cut through underlying rocks. Provided that these sources of energy are available, denitrification may occur deep in the crystalline bedrock or in the upper meter or so of unconsolidated regolith.

In order to elucidate mechanisms involved with nitrate removal in the Turkey Creek watershed, two approaches were taken. The first approach involved the collection of chemical data from both groundwaters and surface waters. Of particular interest, surface water data show strong seasonal trends, with notably elevated nitrate/chloride ratios in early spring and very low ratios at all other times. However, chemical data alone do not allow for definite conclusions regarding mechanisms. Consequently, the second approach involved the analysis of the stable isotope signatures of nitrogen in nitrate in order to gain a qualitative understanding of the extent to which denitrification may be occurring at different locations. In particular, the δ¹⁵N signature of residual nitrate affected by the process of denitrification shows a notably enriched δ¹⁵N value (i.e., δ¹⁵N> 25 ‰) that distinguishes it from all other processes.