The Effects of Urban Land Use Type on Low Order Stream Geochemistry, Columbus, Ohio
The influence of urban land use on water quality in areas of high population density can be significant. Particular urban land use types each contributes its own suite of chemical constituents. Some previous studies on urban impacts to streams have primarily examined the bulk chemical contributions to high order rivers and streams, while not focusing so strongly on the tributaries that control their chemistry. Small scale analysis of solute concentrations in urban tributaries is necessary to understand the influence of urban land use types on stream chemistry because many sources of chemical constituents are non-point sources. The present study focuses on a storm flow outfall pipe and a set of tributaries to the Olentangy River in Columbus, Ohio that represent the lowest order streams in the Olentangy watershed. These tributaries were selected based on their diverse urban land use types and geographic extent and were sampled at four different times through the spring, summer, and autumn. The waters of these tributaries were analyzed for major ions (Li+, Na+, K+, Mg2+, Ca2+, F-, Cl-, SO42-), nutrients (NO3-, PO43-, NH4+, Si, TN, TP), and trace metals (Cr, Cu, Mo, Ni, Pb, Pt, V, Zn). Major ion concentrations for some elements were noticeably higher than typical natural waters with F- concentrations ranging from 9.7 to 36.0µM, Cl- from 1380 to 7280µM, Na+ from 1240 to 5840µM, and SO42- from 1020 to 5920µM. However, few significant relationships were discovered between major ions concentrations and urban land use type. Nutrients such as NO3-, NH4+, and PO43- varied in concentrations from 6.39 to 229µM, 0.37 to 8.96µM, and 0.03 to 10.6µM respectively. Significant correlations were found between nitorgen containing constituents and both residential and green space land uses. Dissolved Si concentrations (63.2 to 383µM) were highest in streams with high percentages of concrete and less vegetation, which is likely a combination of a lack of Si inclusion in biomass coupled with a leaching of Si from artificial surfaces. Trace metal concentrations were comparable to those in similar studies with values less than urban highway runoff, but greater than higher order rivers with significant agricultural influences. The highest dissolved trace metal concentrations were typically associated with watersheds containing the highest percentages of impermeable land surfaces, but atmospheric deposition appears to play a role in blending chemical signals from different land use types. Tributaries exhibited enrichment in major ions relative to the Olentangy River, especially for Na+ and Cl- that are likely sourced from de-icing salts. Nutrients concentrations were typically depleted in tributaries, while trace metals were often enriched as would be expected for elements with vehicle roadway and other urban sources. Seasonal trends were observed for Na+, Cl-, and K+, but not for dissolved trace metals. More frequent sampling through the year, sampling during precipitation events to capture peak discharge, and an examination of biological and metal adsorption processes is needed to understand these trends.
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