UC Irvine

As water imports dwindle, and climate changes, southern California is progressively turning to non-traditional water resources, including water reclamation, ocean water desalination, and the capture and fit-for-purpose use of dry and wet weather urban runoff. All three approaches are being employed to various degrees in the region, but capture and use of urban runoff has the potential to not only increase water supply but also provide habitat for rare and endangered flora and fauna, pollutant removal services, urban greening, and mitigation of the urban heat island effect.

This thesis explores both the water supply and water quality challenges and opportunities associated with using green stormwater infrastructure (GSI) to capture and treat urban runoff in Southern California. On the supply side, Chapter 2 presents an analysis of streamflow trends across the region, with the goal of assessing how reliable this source of water is likely to be into the future, given current trajectories in water conservation and climate change. The results suggest that, generally speaking, streamflow was increasing across the region until around 1990, and has been steadily falling since. In most of the 37 streams analyzed here, current summertime flows are less than 50% of 1990 levels. This precipitous decline reflects a combination of human and hydrological factors, including reduced water imports to the region and outdoor water conservation measures initiated during, and following, the drought of 2011-2016.

On the water quality side, two complementary mathematical fate and transport frameworks are presented for assessing, and predicting, the removal of pollutants in a form of GSI called bioretention systems. The two frameworks take distinctly different approaches for addressing a primary challenge for modeling pollutant removal in these systems; namely, the intermittent nature of storms implies that flow through these systems is inherently unsteady.

The first mathematical framework (Chapter 3) hypothesizes that, by flow weighting time, pollutant breakthrough in these systems can be represented by a standard one-dimensional advective-dispersion model for pollutant transport through porous media. This hypothesis is validated using bromide breakthrough data collected during a pilot-scale bioretention experiment at Orange County Public Works (OCPW). This experiment, which was conducted in Spring of 2019, explicitly accounts for the highly variable (flashy) storm flows generated from impervious areas in Southern California.

The second mathematical framework (Chapter 4) hypothesizes that transient transit-time distribution theory (T-TTD) can be coupled to a biokinetic model of nitrogen cycling, to estimate the fate and transport of nitrogen species (ammonium and nitrate) in the same pilot-scale bioretention system described above. A particular advantage of T-TTD theory is that it can account for loss of water by both gravitational drainage and evapotranspiration--both of which can exert significant controls on nitrogen cycling during the long antecedent dry periods typical of Southern California's summers. The breakthrough of both ammonium and nitrate in OCPW's pilot scale facility can be reproduced, after accounting for adsorption and nitrification (ammonium) and nitrification (nitrate) in the soil media component of this system.

By shedding light on both the supply and water quality dimensions of urban stormwater runoff, this thesis supports Southern California's long-term goal of reducing its reliance on imported sources of water. It also informs adaptive water management approaches under changing conditions and guides bioretention design enhancements for improved stormwater quality.