An open access publication of the American Academy of Arts & Sciences
Summer 2015

Progress on Nonpoint Pollution: Barriers & Opportunities

Adena R. Rissman and Stephen Russell Carpenter
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Nonpoint source pollution is the runoff of pollutants (including soil and nutrients) from agricultural, urban, and other lands (as opposed to point source pollution, which comes directly from one outlet). Many efforts have been made to combat both types of pollution, so why are we making so little progress in improving water quality by reducing runoff of soil and nutrients into lakes and rivers? This essay examines the challenges inherent in: 1) producing science to predict and assess nonpoint management and policy effectiveness; and 2) using science for management and policy-making. Barriers to demonstrating causality include few experimental designs, different spatial scales for behaviors and measured outcomes, and lags between when policies are enacted and when their effects are seen. Primary obstacles to using science as evidence in nonpoint policy include disagreements about values and preferences, disputes over validity of assumptions, and institutional barriers to reconciling the supply and demand for science. We will illustrate some of these challenges and present possible solutions using examples from the Yahara Watershed in Wisconsin. Overcoming the barriers to nonpoint-pollution prevention may require policy-makers to gain a better understanding of existing scientific knowledge and act to protect public values in the face of remaining scientific uncertainty.

ADENA R. RISSMAN is an Assistant Professor of the Human Dimensions of Ecosystem Management in the Department of Forest and Wildlife Ecology at the University of Wisconsin–Madison. Her research has appeared in such journals as Conservation Letters, Journal of Environmental Management, Environmental Science and Policy, and Landscape and Urban Planning.

STEPHEN R. CARPENTER, a Fellow of the American Academy since 2006, is the Stephen Alfred Forbes Professor of Zoology and the Director of the Center for Limnology at the University of Wisconsin–Madison. He is the author of Princeton Guide to Ecology (with S. A. Levin et al., 2009) and Regime Shifts in Lake Ecosystems: Patterns and Variation (2003). His research has appeared in such journals as EcologySustainability, and Science.

Authors’ Note: We thank the Water Sustainability and Climate Team, which is funded by National Science Foundation DEB-1038759.

Water is an important, dwindling resource. Water and aquatic ecosystems support industry, agriculture, outdoor recreation, aesthetic pleasure, aquatic food sources, and livelihoods. Massive, expensive efforts have been made to improve water quality and “repair what has been impaired.”1 These efforts have led to some important gains, but water quality is still poor in many rivers, lakes, and coastal oceans. Runoff of soil, nutrients, and other chemicals from agricultural, urban, and other lands is called nonpoint source pollution. In contrast, point source pollution comes directly from a pipe, such as at an industrial or municipal facility. Runoff of phosphorus–also called nonpoint phosphorus pollution–is a major cause of toxic algae blooms, oxygen depletion, and fish kills in streams, lakes, and reservoirs.2 Why are we not making progress on nonpoint source pollution in water quality? What are the challenges of producing science to predict and assess nonpoint management and policy effectiveness, and of using this science in management and political decisions? Finally, what changes are needed to improve water quality?

A major scientific enterprise is devoted to producing scientific knowledge to inform nonpoint policy and management through long-term monitoring, statistical analysis, and modeling. But is scientific knowledge actually reducing uncertainty about the causes of water-quality impairment and the effectiveness of control measures? Researchers are increasingly vocal about the challenges facing nonpoint-pollution science on sediment, phosphorus, nitrogen, and other pollutants.3  For instance, it is well-established that end-of-pipe mitigation of phosphorus improves water quality, but proving the effectiveness of actions to control nonpoint-source phosphorus is challenging. It is extremely difficult to demonstrate causality when connecting water-quality conditions to policies and the behaviors of agricultural and urban residents. An increase in knowledge and data has therefore not always translated to more effective policy.

Once scientific knowledge is produced, why is it so difficult to use it as evidence in nonpoint pollution-related policy-making and management? Science does not determine public interests and values, but it can serve important purposes in policy-making and resource management.4  It can identify problems, prioritize the location or type of interventions, identify the likely effects of actions before they are taken (including anticipating unintended effects), and evaluate the effects of actions after they are taken.5  Science and society affect each other deeply.6  It is important to understand how scientific evidence, models, uncertainty, and risk enter into the decisions of actors such as the Environmental Protection Agency (EPA), county conservationists, farmers, urban homeowners, and lake managers. We will illustrate how scientific information has been created and used to improve water quality in Wisconsin’s Yahara Watershed, focusing on watershed nonpoint-pollution reduction and in-lake biomanipulation.

Water pollution is typically viewed as an externality that does not directly subtract from the productivity of those responsible for the pollution, except indirectly or through social limits. This means that producers of pollution are not inherently incentivized to remedy it; the issue of assigning responsibility becomes even more difficult with the diffuse nature of nonpoint source pollution. The difficult issue of nonpoint source pollution has led to a proliferation of blended regulatory, incentive, and collaborative efforts to engage homeowners, municipal stormwater systems, and farmers in reducing nutrient and sediment runoff.7

Building scientific evidence for nonpoint pollution is long, slow, and scale-dependent. Given the rapid changes taking place in ecological and social systems, is the baseline moving faster than we can learn? We suggest that, in addition to science, political will and public value should play a greater role in decision-making to improve environmental outcomes.

There are a number of difficulties inherent in producing knowledge about nonpoint-pollution control. First, a growing number of studies from around the world show that it is extremely difficult to determine the efficacy of interventions aiming to reduce nutrient runoff from watersheds. In many cases, freshwater quality has not been found to have recovered even after decades of nutrient management,8  and the divergent explanations for lack of success reflect the complexity of watersheds as social-ecological systems.9  Despite the urgent need for management interventions to protect freshwaters, there is a high level of uncertainty about the efficacy of methods; indeed, there may be fundamental limits to our knowledge of this subject. It is not clear whether watershed management is making progress on uncertainty; for now, the success or failure of policies may be a matter of luck rather than knowledge. For this reason, it is important to consider the barriers to the production of knowledge about nutrient policy and management and the opportunities to improve scientific understanding in this area. We will explore the reasons for the difficulty of demonstrating causal effects of nutrient-management policies in large watersheds, including: long time lags between intervention and response, spatial heterogeneity (that is, a solution that works in one site may not work in another), simultaneous changes in multiple pollution drivers, and lack of monitoring.

Nonpoint pollution–management programs involve large areas with multiple nutrient sources; many individual land managers; spatially heterogeneous topography, soils, and ecosystems; and diverse streams and lakes. Specific practices for ameliorating pollution–such as buffer strips, cover crops, tillage practices, and wetland restoration–are usually tested on relatively homogenous sites at scales of a few hectares for a few years. While these methods are effective in short-term, small-scale field trials, little is known about how they scale up to whole watersheds.10  At the watershed scale, new sources or sinks for phosphorus and new interactions along flowpaths could emerge and lead to surprising outcomes. It is plausible that spatial interactions (such as movement of soil from one area to another) contribute to the observed failures of large-scale nonpoint-pollution management.

Interventions to mitigate nutrient inputs also have delayed effects because of the slow response of nutrients in the environment.11  Time lags ranging from one to more than fifty years have been measured between the initiation of a management intervention and the observation of an environmental response.12  Projections estimate that interventions to cut off phosphorus fertilization of soil will take two hundred and fifty years to produce a new, low-phosphorus equilibrium in the agricultural lands of a Wisconsin watershed.13  In a diverse set of watersheds, response times for nutrient interventions ranged from less than one year to more than one thousand.14  Such long time lags pose serious difficulties for scientific inference and for sustaining the engagement of the public and policy-makers.

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  • 1Charles J. Vörösmarty, Michel Meybeck, and Chistopher L. Pastore, “Impair-then-Repair: A Brief History & Global-Scale Hypothesis Regarding Human-Water Interactions in the Anthropocene” Dædalus 144 (3) (2015): 94–109.
  • 2Stephen R. Carpenter, Nina F. Caraco, David L. Correll, Robert W. Howarth, Andrew N. Sharpley, and Val H. Smith, “Nonpoint Pollution of Surface Waters with Phosphorus and Nitrogen,” Ecological Applications 8 (3) (1998): 559–568.
  • 3Graham P. Harris and A. Louise Heathwaite, “Why is Achieving Good Ecological Outcomes in Rivers so Difficult?” Freshwater Biology 57 (1) (2012): 91–107.
  • 4Daniel Sarewitz and Roger A. Pielke, Jr., “The Neglected Heart of Science Policy: Reconciling Supply of and Demand for Science,” Environmental Science & Policy 10 (1) (2007): 5–16.
  • 5Kenneth Prewitt, Thomas A. Schwandt, and Miron L. Straf, Using Science as Evidence in Public Policy (Washington, D.C.: National Academies Press, 2012).
  • 6Sheila Jasanoff, ed., States of Knowledge: The Co-Production of Science and the Social Order (London: Routledge, 2004): 317.
  • 7Winston Harrington, Alan J. Krupnick, and Henry M. Peskin, “Policies for Nonpoint-Source Water Pollution Control,” Journal of Soil and Water Conservation 40 (1) (1985): 27–32; Paul A. Sabatier, Will Focht, Mark Lubell, Zev Trachtenberg, Arnold Vedlitz, and Marty Matlock, eds., Swimming Upstream: Collaborative Approaches to Watershed Management (Cambridge, Mass.: The MIT Press, 2005): 327.
  • 8Donald W. Meals, Steven A. Dressing, and Thomas E. Davenport, “Lag Time in Water Quality Response to Best Management Practices: A Review,” Journal of Environmental Quality 39 (1) (2010): 85–96, doi:10.2134/jeq2009.0108.
  • 9Harris and Heathwaite, “Why is Achieving Good Ecological Outcomes in Rivers So Difficult?”; and Helen P. Jarvie, Andrew N. Sharpley, Paul J. A. Withers, J. Thad Scott, Brian E. Haggard, and Colin Neal, “Phosphorus Mitigation to Control River Eutrophication: Murky Waters, Inconvenient Truths, and ‘Postnormal’ Science,” Journal of Environmental Quality 42 (2) (2013): 295–304, doi:10.2134/jeq2012.0085.
  • 10Andrew N. Sharpley, Peter J.A. Kleinman, Philip Jordan, Lars Bergström, and Arthur L. Allen, “Evaluating the Success of Phosphorus Management from Field to Watershed,” Journal of Environmental Quality 38 (5) (2009): 1981–1988, doi:10.2134/jeq2008.0056.
  • 11Meals, Dressing, and Davenport, “Lag Time in Water Quality Response to Best Management Practices: A Review”; Stephen K. Hamilton, “Biogeochemical Time Lags May Delay Responses of Streams to Ecological Restoration,” Freshwater Biology 57 (2012): 43–57, doi:10.1111/j.1365-2427.2011.02685.x; and Andrew Sharpley, Helen P. Jarvie, Anthony Buda, Linda May, Bryan Spears, and Peter Kleinman, “Phosphorus Legacy: Overcoming the Effects of Past Management Practices to Mitigate Future Water Quality Impairment,” Journal of Environmental Quality 42 (5) (2013): 1308–1326, doi:10.2134/jeq2013.03.0098.
  • 12Meals, Dressing, and Davenport, “Lag Time in Water Quality Response to Best Management Practices: A Review.”
  • 13Stephen R. Carpenter, “Eutrophication of Aquatic Ecosystems: Bistability and Soil Phosphorus,” Proceedings of the National Academy of Sciences 102 (29) (2005): 10002–10005, doi: 10.1073/pnas.0503959102.
  • 14Hamilton, “Biogeochemical Time Lags May Delay Responses of Streams to Ecological Restoration.”