UC Irvine

Fecal indicator bacteria (FIB) impairment of coastal water bodies is a wide-spread problem that impacts recreational water contact and shellfish harvesting. Storm water runoff is a major contributor to FIB during wet weather periods leading to beach closures or warnings to stay out of the water for several days after a storm event. However, FIB impairments also occur during the dry season when the levels of water-contact recreation are highest, especially at enclosed beaches. The causes and remedies of these problems can be difficult to identify.

This dissertation addresses the impacts of dry-weather runoff on FIB impairment in Newport Bay, California. The vast majority of runoff (95%) enters the bay at two creeks located several kilometers from recreational waters, while only about 1% of the runoff enters the bay through over a hundred small drains located within the recreational waters. This dissertation examines the relative impact of these large and small runoff sources based on bay-wide mixing and transport processes. Dry weather flow rates are reported as a function of drainage area based on limited field sampling and a regression analysis, and loads of FIB into the bay area estimated based on measured FIB concentrations in runoff and the volumetric flow rate. A 2D flow and scalar transport model is developed and calibrated for prediction of bay-wide salinity, and subsequently applied to simulate the relative impact of large and small drains on FIB impairment. Results show that small drains with minimal fresh water discharges contribute disproportionally to FIB impairment due to their proximity to recreational waters and minimal mixing therein. Further, a "trap and release" mechanism whereby runoff accumulates in drain pipes during rising tides and is released at low tide is modeled and shown to also contribute to the number of FIB exceedances.

This dissertation also presents analysis of the erosion thresholds and erosion rates for sediments in Newport Bay which indicate that sediment resuspension occurs only in the main channel and only during energetic spring tide conditions, during the dry weather months. This result suggests that resuspension of contaminated sediments is most likely not a significant contributor to FIB impairment during dry-weather periods.