Solute Sourcing and Hydrologic Response to Monsoon Precipitation Along a Gradient of Urban Land Use
Abstract
Urban storm runoff in arid and semiarid areas is used as a potential groundwater recharge resource, but knowledge gaps remain in our understanding on the underlying hydrologic and biogeochemical processes that control the water quality of urban runoff. This study addresses this gap by evaluating how hydrologic and biogeochemical processes interact to produce distinct storm runoff chemistry. We hypothesized that transport processes dominate the solute chemistry of highly urbanized watersheds with large impervious cover; whereas biogeochemical reactions dominate solute responses in less urbanized watersheds with potentially more vegetation and longer flow paths. Utilizing automatic water samplers, we collected urban storm runoff from five distinct urban land use watersheds: 1) low density residential (least urbanized), 2) old medium density residential, 3) new medium density residential, 4) mixed land use and 5) commercial (most urbanized). We coupled a conservative tracer (chloride, Cl-) with stable isotope data (δD and δ18O) to infer physical and biogeochemical processes contributing to the solute chemistry observed. Solute response was similar in the least and most urbanized watersheds, which had the highest mean seasonal concentrations of Cl-, DOC, fecal indicator bacteria (E. coli), Na, Hg and Cu among others, and had the lowest As, Ca and Ni concentrations. The low density site exhibited weak seasonal chloride flushing, contrasting with the commercial site's stronger flushing response. Coupling of Cl-, δD and δ18O data, and comparing it across sites demonstrates solute flushing and evapoconcentration in the commercial site as inferred by δ18O and δD values that plot along an evaporation trend (from -34 to -24 ‰ δD, and -5.3 to -3.5 ‰ δ18O) with increasing Cl- concentrations (from 1.8 to 7.4 mg L-1) during the runoff event. In contrast, high δD values (-27 to -22 ‰) of runoff and a simultaneous decrease in Cl- concentrations (from 11.5 to 3.7 mg L-1) at the low density site suggest watershed solute retention despite runoff evaporation which should concentrate Cl-. Lower δD values of runoff, closer to the meteoric water line, in the commercial site may indicate a shorter flow path when compared to the higher δD values (more evaporated signature) in the low density site. Our study demonstrates that the urban storm runoff quality can not be predicted by land use alone, and supports the study's hypothesis of transport controls on solutes at the most urbanized sites, and flow path and biogeochemical controls at least impervious sites.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.H13I..04G
- Keywords:
-
- 0493 Urban systems;
- 0496 Water quality;
- 1871 Surface water quality;
- 1879 Watershed