Downstream Effects of Upstream Causes

The United States Environmental Protection Agency considers nutrient pollution in stream ecosystems one of the United States' most pressing environmental challenges. But limited independent replicates, lack of experimental randomization, and space- and time-varying confounding handicap causal i...

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Bibliographic Details
Published in:Journal of the American Statistical Association 2019-10, Vol.114 (528), p.1493-1504
Main Authors: Saul, Bradley C., Hudgens, Michael G., Mallin, Michael A.
Format: Article
Language:English
Subjects:
Applications and Case Studies
Chlorophyll
Computer simulation
Ecosystems
Environmental monitoring
Environmental protection
g-formula
Marginal structural models
Pollution
Potential outcomes
Regression analysis
Simulation
Statistical methods
Statistics
Stream pollution
Structural nested models
Upstream
Wastewater
Water pollution
Water quality
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Summary:The United States Environmental Protection Agency considers nutrient pollution in stream ecosystems one of the United States' most pressing environmental challenges. But limited independent replicates, lack of experimental randomization, and space- and time-varying confounding handicap causal inference on effects of nutrient pollution. In this article, the causal g-methods are extended to allow for exposures to vary in time and space in order to assess the effects of nutrient pollution on chlorophyll a-a proxy for algal production. Publicly available data from North Carolina's Cape Fear River and a simulation study are used to show how causal effects of upstream nutrient concentrations on downstream chlorophyll a levels may be estimated from typical water quality monitoring data. Estimates obtained from the parametric g-formula, a marginal structural model, and a structural nested model indicate that chlorophyll a concentrations at Lock and Dam 1 were influenced by nitrate concentrations measured 86 to 109 km upstream, an area where four major industrial and municipal point sources discharge wastewater. Supplementary materials for this article, including a standardized description of the materials available for reproducing the work, are available as an online supplement.
ISSN:0162-1459
1537-274X
DOI:10.1080/01621459.2019.1574226