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When and where to intervene? Coastal nutrient loading, groundwater travel times, and estuary dynamics
Citation:
Merrill, N., S. Balogh, A. Piscopo, K. Mulvaney, AND T. Hill. When and where to intervene? Coastal nutrient loading, groundwater travel times, and estuary dynamics. 4th Water Research Conference: The Role of Water Technology Innovation in the Blue Economy, Kitchener-Waterloo, Ontario, CANADA, September 10 - 13, 2017.
Impact/Purpose:
This paper presents the planning problem of addressing coastal nutrient pollution with an example of an estuary system on Cape Cod (Barnstable County, MA). Due to long groundwater travel times in the aquifer, the primary transportation mechanism for nutrient pollution in this system, there is a connection between the spatial location and the effect of pollution control efforts based on their time to impact. We present a simplified model of groundwater transport and nutrient loading, followed by an analytical description showing the trade-offs through time and efficiency of source and in-estuary pollution control efforts using concepts from optimal control theory.
Description:
There is often a difference between the timing of an intervention in a natural system and the resultant impact. Delays in the transport of pollutants from sources, inertia in the effects of policy on economic actors, and long-term memory in natural systems, turn environmental management problems into dynamic decision problems. This makes the pertinent management questions not just where and how to intervene, but also when and to what extent? This paper presents the dynamic planning problem of addressing coastal eutrophication with an example and parameterization for an estuary system on Cape Cod (Barnstable County, MA). Due to long groundwater travel times in the aquifer, the primary transportation mechanism for nitrogen pollution in this system, there is a connection between the spatial location and the effect of abatement efforts defined along this travel time gradient. Interventions under consideration vary from source control, upgrading sceptic systems for example, to in-estuary approaches, aquiculture being popularly cited. For a community, finding the right balance of these choices is more complicated than simply implementing the fastest impact option first or prioritizing just the cheapest. This is because travel time, marginal costs, scalability, and estuary dynamics combine to make efficient choices more nuanced in space and time. We present the conceptual model of the groundwater transport and estuary nutrient loading, followed by an analytical description of the inter-temporal tradeoffs and efficiency of source and in-estuary nutrient abatement efforts using concepts from optimal control theory. This model is then parameterized for an estuary system, Three Bays, on Cape Cod, MA and the associated watershed. Our results inform the timing, spatial allocation and combination of treatment options by simulating the optimal control paths. These control paths are compared in terms of efficiency against heuristics and policy relevant simplifications.