| Summary |
Urbanization has been shown to alter stream and groundwater hydrology. In this study, the effects of urban channel incision on stream-groundwater interactions were evaluated along low-order Coastal Plain streams. Five low-order Coastal Plain streams and adjacent floodplains were studied across various degrees of urbanization (4-37% watershed total impervious area) in Greenville, NC. Stable isotopes (D, ¹⁸O) and specific conductivity were monitored monthly in surface water and groundwater systems during baseflow conditions and during five storm events from August 2007-2008. Increases in stormwater inputs were directly related to catchment impervious area and linked with stream channel incision. Annual stormwater runoff comprised 62% of the total discharge at the most urbanized site and 22% of total discharge at the least urbanized site. Although stormwater runoff was greatest at the most urbanized site, channel incision (2.13 m) resulted in no overbank flow during the year of study. The least urbanized site exhibited little channel incision (0.5 m) and flooded 7 times. The reduction in overbank events at the urban site led to a reduction in floodplain recharge. None of the floodplain recharge at the most urban site could be attributed to overbank flow, whereas at the least urban site, 23.3% of the total recharge was contributed by overbank flow. Floodplain effective recharge (recharge that was held for at least 48 hours) at the most urban site was 8.35 cm/yr while the least urban site retained 11.08 cm/yr. Groundwater ¹⁸O data during storm events also suggested that less recharge was occurring in urban floodplains. Due to reduced recharge and channel incision, floodplain groundwater levels were as much as 2 m deeper for urban sites. Groundwater isotope data (d-excess) suggested that evaporation was reduced along urban floodplains adjacent to incised channels with deep water tables. Although greater amounts of stormwater runoff were observed in the most urbanized catchment (Pomes Branch), which should indicate decreased catchment recharge, measurements of annual baseflow indicated greater catchment recharge occurring compared to the least urbanized catchment (Phillippi Branch). Although the baseflow percent was lower at the most urban catchment (Pomes Branch) annual baseflow was greater relative to less urbanized catchments. Possible explanations for increased baseflow in Pomes Branch include: decreased evapotranspiration, increased floodplain drainage into the stream channel, upwelling of Yorktown aquifer groundwater where channel incision into the Yorktown confining unit has occurred, and leaking municipal wastewater and water supply pipes. Overall, the results showed that urbanization affects stream-groundwater interactions by increasing stormwater runoff and channel incision, which results in a decline of stream-floodplain connections and floodplain recharge. These changes can affect the stream and floodplain's ability to enhance water quality and to store floodwaters. |
