LAND USE CONTROLS ON GROUNDWATER INPUTS AND WATER QUALITY FOR SUBURBAN STREAMS

Impervious surface area (ISA) in suburban and urban streams alters hydrologic response, leading to higher peak discharges, shorter and sharper rising and recessional limbs of hydrographs, and lower baseflow contributions relative to their undisturbed, rural counterparts. However, surprisingly few studies attempt to resolve the relative contributions of pre-event (baseflow) and event (recent precipitation) water runoff fractions to suburban and urban streams. This study assesses changes in baseflow contributions to streams during flooding as well as stream water quality along a gradient of increasing urbanization. Our sampling sites feature a range of development in their watersheds and include a rural stream (1.5% ISA), two suburban streams (26.1% and 28.3% ISA), and two urban streams (39.5% and 41.2% ISA). We used high frequency monitoring of stable isotope ratios (i.e., δ²H and δ¹⁸O) and water chemistry (e.g., specific conductivity and Cl^- concentrations) for two-component hydrograph separations to characterize the changes in flow components as a function of land use and to address changes in water quality with increasing urbanization. Our hydrograph separation results showed that, on average, the rural stream had the highest baseflow contributions at 61%, but baseflow inputs decreased by 6% and 10% in the suburban streams and 27% and 37% in the urban streams. Importantly, we found that the relationship between baseflow contributions during floods and ISA is not linear, with the suburban streams featuring baseflow inputs similar to the rural stream. Thus, the hydrology of suburban streams is less impacted than might be predicted by impervious surface alone. Nevertheless, the suburban streams in this study featured water quality impairment comparable to highly urbanized streams. For example, average Cl^- levels were low at the rural stream (170 ± 261 mg/L), but concentrations at the two suburban streams sites were similar to (319 ± 682 mg/L) or even higher than (506 ± 757 mg/L) an urban stream site (372 ± 588 mg/L). Our findings show that while suburban streams may have similarly impaired water quality relative to highly urbanized streams, which negatively impacts aquatic life and environmental quality, the hydrologic responses of these streams are comparable to their rural counterparts.