GSA Connects 2021 in Portland, Oregon

Paper No. 71-5
Presentation Time: 9:25 AM


WALTER, Donald, US Geological Survey, 10 Bearfoot Road, Northborough, MA 01532

Coastal eutrophication from excess nitrogen (N) input in urbanizing watersheds is an ecological and economic problem that often requires communities to undertake costly nitrogen-management actions. Nitrogen loading rates often are estimated using current land uses within a watershed boundary. These are instantaneous loads that assume that (1) N sources do not change over time, (2) the flow system is in a steady state, and (3) transport times through the aquifer are negligible. These assumptions likely are not valid in some regions, such as Long Island, New York; a 1,400 square mile island underlain by a regional aquifer system that is more than 2,000 feet in some areas. Precipitation is the sole source of aquifer recharge and most ground water discharges to the surrounding coastal waters. Travel times in the aquifer can exceed several thousand years. A large amount of groundwater is withdrawn to meet the public-supply demand for over 7 million people; most of the pumped water reenters the aquifer as wastewater return flow resulting in the discharge of excess N and eutrophication in coastal waters. Estimating N loads and the response to N-management actions in this system is complex because (1) the amount and spatial distribution of N discharge from residential, agricultural, and atmospheric sources has changed dramatically over the last two centuries, (2) the hydrologic system changes in response to changes in natural recharge and has been highly perturbed by water withdrawals and wastewater management, both of which have changed dramatically over the last several decades, and (3) the large range of travel times from land surface to coastal waters encompasses the periods over which these changes have occurred. Understanding current loading to Long Island’s coastal waters, and the often non-intuitive response of loading to management actions, requires the compilation and analysis of very large, diverse data sets—including geologic, hydrologic, atmospheric, climatologic, and anthropogenic data—and the synthesis of these data into process models capable of simulating N transport from the water table underlying surficial N sources to coastal waters. This talk describes the coupling of such large data sets with numerical models to fully account for the dynamic nature of the hydrologic system and Long Island’s complex land-use history.