PARENT SESSION
Oral Session #94: Nutrient Cycling.
Presiding: R. Yanai
Thursday, August 8. 1:00 PM to 3:45 PM. Grand Ballroom East, Radisson.

Nutrient and metal loads exported from hydrologic catchments by storm runoff.

Lewis, David^*,1, Grimm, Nancy¹, ¹ david.lewis@asu.edu, Tempe, AZ

ABSTRACT- We investigated whether features of storms influenced the chemistry of runoff. Storms move materials and trigger biogeochemical processes in many ecosystems. Little is known, however, about how they accomplish this transport. This knowledge gap derives from the difficulty in quantifying storms and runoff from start to finish, and in determining the area over which runoff integrates material. We overcame these problems using instrumented, delineated catchments in arid cities. Within contiguous metropolitan areas (e.g., Phoenix, AZ) variation in runoff chemistry was greater among storms than among catchments. We therefore asked whether variation among storms in runoff chemistry corresponded with total precipitation, precipitation intensity (cm / 5 min), storm duration, antecedent dry days, year, and season (summer monsoon vs. winter cold front). All nitrogen species and total phosphorus correlated positively with precipitation intensity, but not with other features of storms. In runoff from single events, TN load ranged from 17.7 - 1,147 g / ha, and TP load ranged from 2.7 - 266 g / ha. No features of storms explain variation in molar N : P ratios, which varied between 4.0 and 279, with modal ratios near 16. Loads of arsenic, cadmium chromium, copper, lead, nickel, and zinc also correlated most strongly with storm intensity. Total load of metals in single events exhibited a log-normal distribution with a modal value of 10.0 g / ha. The molar ion balance of storm runoff showed a declining charge with increasing pH, suggesting that large amounts of H+ preclude a predominance of anion species. These data reflect the potential for storms to initiate great biogeochemical activity in disproportionately brief events. This potential is particularly true in arid cities, where material accumulates on an impermeable landscape during protracted dry periods.

KEY WORDS: storm runoff chemsitry, nutrient loads, metals, arid city biogeochemistry