A Time‐Dependent Model of Nitrogen Loading to Estuaries from Coastal Watersheds

Abstract

We have developed a dynamic nitrogen loading model (NLM) that incorporates temporal and spatial trends in land use with a three‐dimensional ground water model. In conjunction with historical patterns of land use in the Waquoit Bay (USA) watershed, we have modified an existing steady‐state watershed NLM to estimate historical and future rates of total dissolved nitrogen (TDN) to the coastal margins of Waquoit Bay and its subestuaries. The model simulations indicated a significant increase in nitrogen loading to these systems in recent decades. We estimated that the TDN loading rate to Waquoit Bay increased from approximately 5000 to 23 000 kg yr−1 (0.8 to 3.7 g N m−2 yr−1) from 1930 to 1990. We also compared the dynamic model with steady‐state simulations where the lag effect of ground water travel time was not considered. These results indicate occasional significant differences (up to 37%) between the two modeling methods, especially between 1950 and 1990, when large areas of naturally vegetated and agricultural land within the watershed were converted to unsewered residential housing. Although all subestuaries experienced similar temporal trends in nitrogen load, heterogeneity in the timing, source, and magnitude indicates that these factors are dependent upon watershed size, shape, and spatio‐temporal trends in land use.

Oyster and Macroalgae Bioindicators Detect Elevated δ15N in Maryland’s Coastal Bays

“…Due to small watershed areas (totaling 452 km 2 ) of Maryland's coastal bays, freshwater inputs and activities that result in anthropogenic nitrogen inputs generally occur within 6 km of shore as compared to larger ecosystems (e.g., Jordan et al 1997;Brawley et al 2000;Turner and Rabalais 2003). Freshwater base flow, transporting nitrate from terrestrial recharge areas, enters Delmarva Peninsula's coastal lagoons via both groundwater (Andres 1992;Bratton et al 2004;Krantz et al 2004;Manheim et al 2004) and overland sources that include riverine (Lung 1994;Schwartz 2003) and agricultural ditches (Schmidt et al 2007).…”

Modelling the impact of historical land uses on surface‐water quality using groundwater flow and solute‐transport models

“…2001) was used to simulate groundwater flow moving away from perennial streams instead of flowing to these discharge points. A similar approach was used to estimate historical nitrogen loading from the Waquoit Bay watershed (Brawley et al . 2000).…”

“…2000). Brawley et al . (2000) used reverse‐particle tracking to establish flow paths and groundwater travel times from streams, bay shoreline and ponds.…”

“…Second, the solute‐transport modelling with the MT3D code produced advective fronts that represented contour lines of travel time. Rather than tracking specific particles to establish travel times as was done by Brawley et al . (2000), all surface water‐bodies became sources of an unreactive species with a constant concentration of 1.…”

“…The regional flow and solute-transport model (Boutt et al 2001) was used to simulate groundwater flow moving away from perennial streams instead of flowing to these discharge points. A similar approach was used to estimate historical nitrogen loading from the Waquoit Bay watershed (Brawley et al 2000). Brawley et al (2000) used reverse-particle tracking to establish flow paths and groundwater travel times from streams, bay shoreline and ponds.…”