2000 BES Abstracts

Interactions amongst natural, engineered and human processes influencing the spatial and temporal variability of nitrate concentrations in the Gwynns Falls watershed.

Neely L. Law, Lawrence Band

Land use and demographic characteristics have been shown to correlate well with nitrate concentrations for watersheds in which human inputs from agricultural and residential land uses dominate (Howarth et al. 1996, Jordon et al. 1997, Pontius et al 2000). For example, studies have shown a positive linear relationship between percent residential and agricultural land use and nitrate concentration. A similar relationship is shown for population density and nitrate concentration. These relationships are not strongly evident in the new data collected for the Gwynns Falls and Oregon Ridge watersheds. The non-monotonic relationship of nitrate concentration versus discharge moving downstream in the Gwynns Falls watershed suggests that other processes about the landscape may dominate. Nitrate concentration increases with discharge at low flow events, but then begins to decrease at discharges of approximately 1mm/d for catchments downstream of Glyndon. A key issue arising from the water resource infrastructure workshop held in June 1999 that may help to explain this nonlinearity, is the connectivity between natural and engineered hydrological pathways. Further the influence of human activity/behavior through their use and disposition of water and nutrients may help explain the pattern of nutrient fluxes in urban watersheds. It is suggested that there may be a discharge threshold in which natural and engineered flowpaths are hydrologically connected (e.g. exchanges between groundwater and sewer lines) and begin to dilute pollutant concentrations. At lower discharges the systems are hydrologically separated and with the continuous added input from human activity, there is positive relationship between nitrate concentration and discharge. It is suggested that such smaller scale processes relevant to planning level studies may be useful in furthering our understanding of nutrient dynamics in urban watersheds.