The Shallow and Deep Hypothesis: Subsurface Vertical Chemical Contrasts Shape Nitrate Export Patterns from Different
1Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States.
Environmental Science & Technology
|August 20, 2020
Summary
Subsurface chemical differences between shallow and deep waters control nitrate export. Agricultural lands may see escalating nitrate levels during floods, worsening eutrophication.
Area of Science:
- Environmental Chemistry
- Hydrology
- Ecosystem Science
Background:
- Eutrophication poses a global threat to water resources.
- Mechanistic understanding of nutrient export controls is limited.
- Human activities impact water quality and nutrient cycling.
Purpose of the Study:
- To test the hypothesis that subsurface vertical chemical contrasts regulate nitrate export.
- To investigate how land use influences these chemical contrasts and export patterns.
- To challenge existing perceptions of nutrient dynamics in agricultural watersheds.
Main Methods:
- Synthesis of data from 228 watersheds.
- Reactive transport modeling with 500 simulations.
- Analysis across diverse land use, climate, and geological conditions.
Main Results:
- Human activities amplify chemical contrasts between shallow and deep waters.
- Flushing patterns (nitrate increases with streamflow) dominate agricultural lands.
- Dilution patterns (nitrate decreases with streamflow) are observed in urban watersheds.
- A quantitative link between export patterns and depth-wise chemical contrasts was found.
Conclusions:
- Nutrient distribution over depth is a critical, often overlooked, factor in nitrate export.
- Nitrate concentrations in agricultural lands are expected to rise with increased streamflow, especially during large hydrological events.
- Future climate intensification of flooding may exacerbate eutrophication due to increased nitrate export from agricultural areas.
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