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Journal of Environmental Hydrology
ISSN 1058-3912 |
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Electronic Journal of the International Association for
Environmental Hydrology
JEH Volume 18 (2010), Paper 6 Posted March 31, 2010 THE INFLUENCE OF URBAN CHANNEL INCISION AND WATER TABLE DECLINE ON FLOODPLAIN GROUNDWATER NITROGEN DYNAMICS; GREENVILLE, NC
David F. Harnsberger ABSTRACT The contact between groundwater and organic matter (OM) across floodplains of low-order streams has important implications for groundwater and stream nitrogen (N)-dynamics. Stormwater-induced channel incision along low-order urban streams has been linked to the decline of riparian groundwater levels (water table decline). Water table decline adjacent to incised urban streams has the potential to diminish contact between floodplain groundwater and OM concentrated in surface soils that may alter the nitrate-removal capacity of floodplain riparian zones. The objectives of this study were to determine if water table decline across riparian, floodplain areas of incised streams in Greenville, NC: i) diminishes contact between floodplain groundwater and surface soil horizons, ii) diminishes the NO3- attenuation (loss, removal, and/or retention) capacity of incised floodplain riparian zones and iii) drives an increase in the loading of NO3- from floodplain groundwater to surface water. Three floodplain study sites with similar catchment size were selected adjacent to low-order streams with varying degrees of stream channel incision. These sites were classified as urban, suburban, and rural, based on the extent of watershed total impervious area (TIA; 36.7, 22, 3.8% respectively). Piezometer nests were installed across floodplains to measure groundwater levels and concentrations of groundwater nitrate-N (NO3---N), ammonium-N (NH4+-N), dissolved organic carbon (DOC), dissolved oxygen (DO), and chloride (Cl-). Though water table decline resulted in diminished contact with organic surface soils at the urban and suburban sites, buried peats contributed to locally elevated DOC concentrations (> 4 mg/L) in shallow wells at both sites. The positive relationship observed between median annual near-stream groundwater levels and floodplain NO3- attenuation across the three study sites suggests that water table decline may cause a net decrease in the NO3- attenuation capacity of incised urban floodplains. Despite notably reduced NO3- attenuation across floodplain riparian areas, NO3- loading from groundwater to surface water was not consistently greater at the more incised sites. Differences in loading were influenced by variability in groundwater flux at each site due to differences in the median hydraulic permeability (K) of floodplain sediments at each site. This indicates that although land-use may influence floodplain groundwater NO3- attenuation, variability in the hydraulic properties of floodplain sediments both along individual streams, and from one stream to the next, also plays an important role in the loading of NO3- to low-order streams. Reference: Harnsberger, D.F., and M.A. O’Driscoll. 2010. The influence of urban channel incision and water table decline on floodplain groundwater nitrogen dynamics; Greenville, NC. Journal of Environmental Hydrology, Vol. 18, Paper 6. CONTACT: David F. Harnsberger 3774 Lois Lane Salt Lake City, UT 84124 E-mail: hydrochemdave@gmail.com |
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