Quantifying Land Use and Land Cover Effects on Urban Runoff Water Quality.

The impact of non-point source pollution on urban storm runoff is of major concern in the Southwest where water resources are scarce, episodic rainfall is intense and runoff recharge is a water management strategy. The objectives of this study are to 1) determine the extent to which specific types of urban land use impact the quality of monsoonal rainfall-runoff, and 2) identify pollutant source and modification during transport within urban washes of different types. We installed autosamplers at the outlet of four watersheds in the Tucson, AZ basin, with land uses representative of growing urban centers in the southwest U.S.: 1) commercial; 2) medium and high density residential; 3) low density residential; and 4) mixed use. At each outlet, storm runoff samples were collected at 20 minute intervals during several monsoonal storms. To characterize how pollutants were modified during transport, we installed autosamplers at upstream and downstream locations of a wash. Samples were analyzed for nutrients, organic pollutants, metals, anions, cations and fecal indicator bacteria (E. coli). Preliminary data show that nitrate concentrations were highest in the commercial and low density watersheds (median = 2.53 mg/L and 2.81 mg/L NO3-N, respectively) and lowest in the medium density watershed (median = 1.68 mg/L). Ammonium concentrations were also highest in the commercial and low density watersheds (median = 1.84 mg/L and 1.75 mg/L NH4-N, respectively) and lowest in the medium density watershed (1.28 mg/L). E. coli counts were highest in the commercial (median = 4500 CFU/ml) and lowest in the medium density watershed (median = 61.26 CFU/ml). Over the season, E. coli concentrations decreased in all except the mixed density watershed where they increased as the monsoon progressed. We observed distinct pollutant concentration response patterns to storm events among watersheds. Pollutant concentrations in runoff from commercial and low density watersheds peaked within the first 40 minutes of a storm event and subsequently tapered, whereas concentrations in the middle density watershed increased throughout a storm event. Our study demonstrates that land use type directly and distinctly impacts storm runoff chemical composition, which has significant implications for basin wide pollutant fate and transport. Our data also suggests that the type of runoff drainage system may play an important role in contaminant degradation and subsequent transport.