Wednesday, August 6, 2008
Exhibit Hall CD, Midwest Airlines Center
Background/Question/Methods Few studies have investigated the relationship between hydrology and nitrogen biogeochemistry in hyporheic zones of degraded urban streams despite significant national efforts to restore such streams in attempts to improve the nutrient uptake functions in these ecosystems. We examined relationships between stream flow, water table level, nitrate (NO3¯), dissolved organic carbon (DOC), dissolved oxygen (DO), chloride (Cl) and other ancillary water quality parameters in Minebank Run, a geomorphically degraded urban stream near Baltimore , Maryland, USA in the Chesapeake Bay watershed. Minebank Run exhibited “urban stream syndrome” characteristics including steep bank incision and flashy hydrology stemming from altered runoff from surrounding impervious surfaces.
Results/Conclusions NO3¯, DOC, and DO declined with depth below the stream bed (P<0.05). NO3¯ was inversely related to DOC (R2 = 0.64, P = 0.01) but not to any other chemical variable. NO3¯ was positively related to stream flow (R2 = 0.51, P = 0.03) and to mean water table level (R2 = 0.69, P = 0.01), whereas Cl, a conservative natural tracer in this system, was not. Lower stream flow and water table levels are related to longer ground water residence time. Thus, NO3¯ concentrations in the hyporheic zone of this degraded stream are strongly influenced by both DOC availability and ground water residence time, results that are corroborated by studies at Minebank Run demonstrating higher denitrification enzyme activity (DEA) in zones with greater organic matter (Groffman et al. 2005; Gift et al. in press) and a study at Minebank Run demonstrating a positive relationship between mass removal of NO3¯ measured in situ and ground water residence times (Kaushal et al. 2008). Stream restoration efforts that increase DOC availability to denitrifiers and which reduce stream flow velocities and/or increase ground water residence time are likely to improve the nutrient uptake capacity of urban stream channels.
Results/Conclusions NO3¯, DOC, and DO declined with depth below the stream bed (P<0.05). NO3¯ was inversely related to DOC (R2 = 0.64, P = 0.01) but not to any other chemical variable. NO3¯ was positively related to stream flow (R2 = 0.51, P = 0.03) and to mean water table level (R2 = 0.69, P = 0.01), whereas Cl, a conservative natural tracer in this system, was not. Lower stream flow and water table levels are related to longer ground water residence time. Thus, NO3¯ concentrations in the hyporheic zone of this degraded stream are strongly influenced by both DOC availability and ground water residence time, results that are corroborated by studies at Minebank Run demonstrating higher denitrification enzyme activity (DEA) in zones with greater organic matter (Groffman et al. 2005; Gift et al. in press) and a study at Minebank Run demonstrating a positive relationship between mass removal of NO3¯ measured in situ and ground water residence times (Kaushal et al. 2008). Stream restoration efforts that increase DOC availability to denitrifiers and which reduce stream flow velocities and/or increase ground water residence time are likely to improve the nutrient uptake capacity of urban stream channels.
