Using in-situ spectrometry and isotopic tracers to assess nutrient loading along a rural to urban gradient
Abstract
Urbanization creates novel ecosystems wherein hydrologic controls, sources and position of solutes are altered. Many intentional and unintentional decisions create heterogeneous landscapes across watersheds that span from rural to urban. Along this gradient, nutrient sources vary greatly (e.g. septic systems, sewers, urban runoff), as does the hydrologic regime altered by impervious surface cover and hydrologic connectivity (e.g. pipes, roads, ditches). Existing models of nutrient loading rarely account for patch scale processes needed to resolve nutrient loading of heterogeneous watersheds in space and time. Ecohydrologic models informed by fine scale data will be necessary to identify hot spots and hot moments in nutrient loading that can act as control points for management and inform urban design. To this end, we are leveraging water chemistry data from 40+ watersheds with in-situ spectrometry at five land use specific watershed pour points to build a process level understanding of nutrient loading and solute transport.
In-situ spectrometry revealed strong patterns in event NO3-, Br-, Cl-, and SO42- concentration with land-use and wetness gradients. Hysteresis of [NO3-] through storms shifted from generally counter clockwise at urban study sites with sanitary sewers, to counterclockwise at the rural site on septic systems. Urban study sites generally had larger [NO3-] peaks on the rising limb of event flows while the rural site had larger peaks on the falling limb. This suggests source limitation of NO3- loading in urban communities from fast moving surficial flow paths and transport limitation in rural communities from slower moving subsurface flow paths. However, as antecedent wetness increased, falling limb [NO3-] peaks increased at urban sites. The slope of this relationship decreased at higher development intensity and with lower baseflows, suggesting that rising groundwater levels mobilized different NO3- sources in urban environments, possibly from inundated sewers, and the loading is partially controlled by urbanization's effect on hydrologic routing. Ongoing work includes triple NO3- isotope analysis of storm samples to identify and separate NO3- sources in time and analysis of continuous Cl-, Br-, and SO42- concentrations to separate surface and subsurface flow paths.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.H31M2133D
- Keywords:
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- 0432 Contaminant and organic biogeochemistry;
- BIOGEOSCIENCESDE: 0481 Restoration;
- BIOGEOSCIENCESDE: 1830 Groundwater/surface water interaction;
- HYDROLOGYDE: 1871 Surface water quality;
- HYDROLOGY