Baltimore Ecosystem Study Institute of Ecosystem Studies

2014 BES Annual Meeting Presentation and Poster Abstracts

Numerical simulation of subsurface nitrogen transport and transformation in a Red Run subwatershed served by septic systems
Cui, Zhengtao
Co-Authors: Zhengtao Cui1, Claire Welty, Elvis Andino1, Michael Barnes, and Andrew J. Miller

Abstract: Few modeling efforts have focused on watershed-scale assessment of subsurface nitrogen fate and transport where homes are served by septic systems for on-site wastewater treatment. We present results from a study in which three-dimensional numerical modeling is used to investigate the spatial pattern and flow paths of nitrogen from septic tank effluent from 94 household sources in a small (0.23 sq km) watershed in suburban Baltimore, MD. The subwatershed is served by wells and septic and has no stormwater management. To carry out the simulations, we utilized a recently developed multispecies nitrogen transport and transformation numerical model for variably-saturated aquifers that incorporates modeled ParFlow groundwater flow fields as input. The size of the simulation domain is 480 m x 960 m x 60 m with a spatial resolution of 10 m x 10 m x 1 m. Simulation results indicate that groundwater travel times from the septic tanks to nearby streams range from 3 to 100 years with a mean value of 34 years. Due to modeled hydraulic properties of the Piedmont hydrogeology (saprolite underlain by fractured crystalline rocks), the spatial distribution of nitrogen species shows heterogeneity in both horizontal and vertical directions. Nitrate and ammonium exhibit high concentrations at stream locations where groundwater flow paths converge. Overall stream nitrate and ammonium concentrations increase from downstream to upstream. Stream nitrate and ammonium fluxes reach steady-state (maximum) values at about 80 and 40 years respectively, indicating that stream nitrate fluxes are expected to increase if houses were built less than 80 years ago.