Urbanization Effects On Nitrate-Discharge Relationships And Nitrate Concentration In Bozeman Creek

Abstract

Water quality issues can arise in rapidly growing micropolitan cities like Bozeman, Montana. Bozeman has experienced a population growth rate ranging from 1 to 7% in the last decade, and that growth has been accompanied by an increase in impervious area and stormwater infrastructure. In this paper I investigate how urban growth can potentially impact local streams by means of altering discharge and runoff contamination. To examine how stream nitrateconcentration-to-discharge (N-Q) relationships and nitrate concentration relate to the expansion of urban area in Bozeman, I selected three sites in Bozeman Creek to collect and analyze data. I collected data using Montana State University Extension’s Water Quality Data Hub. My objectives were to assess nitrate concentration and discharge for three sites that represent an urbanization gradient along Bozeman Creek and to determine if nitrate concentrations and N-Q relationships have a positive, neutral, or negative trend. Analysis of Variance (AOV) tests for N-Q relationships identify a significant overall negative linear correlation between nitrate concentration and discharge (p < 0.001). The tests also suggest that this relationship varies by site (p = 0.04), with the upstream site having a more negative relationship than the downstream sites (p = 0.06). Sample size corrected Akaike Information Criterion (AICc) test scores suggest that a simple model that does not specify site-specific N-Q relationships is the optimal model, but also provides some support for a model that allows the N-Q relationship to vary by site. AOV tests for mean annual nitrate concentration suggest significant differences in mean values by site (p < 0.001) and year (p = 0.007). A Tukey’s HSD test suggests that mean annual nitrate concentration is higher at two downstream sites than at the upstream site (p < 0.001), but it does not provide clear evidence for trends across years. Therefore, the evidence suggests that all monitoring sites exhibit a clear dilution-type N-Q relationship, while the rate of nitrate dilution decreases from upstream to downstream. Additionally, the evidence suggests that mean annual downstream nitrate concentrations are higher than upstream. Because the data span from June October, inferences about patterns are constrained to that period. My study shows that urbanization may result in downstream consequences to water quality and freshwater habitat. Further studies using more robust data would help in more thoroughly understanding ecological impacts of urban growth on local streams, which would provide more accurate information on how to mitigate potentially negative consequences to water quality of freshwater habitat.