Increasing Stormwater Capture and Recharge Using Forecast Informed Reservoir Operations, Prado Dam.

The United States Army Corps of Engineers (USACE) operates Prado Dam in southern California for flood risk management and to capture stormwater for groundwater recharge. USACE and the Orange County Water District (OCWD) have collaborated for over 30 years to temporarily store Santa Ana River (SAR) stormflow at Prado Dam for groundwater recharge in the Orange County Groundwater Basin (Basin). USACE, OCWD, and other stakeholders are assessing Forecast Informed Reservoir Operations (FIRO) at Prado Dam as a new operational approach to capture additional supplies of SAR water for groundwater recharge without affecting Prado Dam's primary flood risk management purpose. Many dams, including Prado Dam, do not directly incorporate precipitation and streamflow forecasting in their operations. FIRO is an innovative research and operations partnership that uses weather forecasting, streamflow modeling, and watershed monitoring to help water managers selectively retain or release water from reservoirs in a manner that reflects current and forecasted conditions. A recently completed study, called a Preliminary Viability Assessment of FIRO at Prado Dam, determined that increased stormwater capture, beyond the current program, is viable subject to completion of additional studies. The ultimate increase in stormwater capture is anticipated to largely be a function of community and environmental tolerance for more frequent inundation rather than operational constraints of the dam. FIRO is a promising approach to operating Prado Dam that can increase SAR stormwater capture for recharge to the Basin, reducing the need for imported water and contributing to sustainable groundwater management.

Biomonitoring recycled water in the Santa Ana River Basin in southern California.

The Santa Ana River (SAR) is the primary source of groundwater recharge for the Orange County Groundwater Basin in coastal southern California. Approximately 85% base flow in the SAR originates from wastewater treatment plants operated by three dischargers. An on-line, flow-through bioassay using Japanese medaka (Oryzias latipes) as a means of judging potential public health impacts was employed to evaluate the water quality of the surface water and shallow groundwater originating from the SAR. Three chronic (3-4.5 mo) exposures using orange-red (outbred, OR) and see-through (color mutant, ST-II) Japanese medaka as bioindicators were conducted to evaluate endocrinologic, reproductive, and morphologic endpoints. No statistically significant differences in gross morphological endpoints, mortality, gender ratios, and vitellogenin induction were observed in fish from SAR groundwater treatment compared to the group tested in solute reconstituted reverse osmosis-treated or granular activated carbon (GAC)-treated control waters. Significant differences were observed in egg reproduction and the time to hatch in SAR groundwater; however, total hatchability was not significantly lower. To evaluate the estrogenic activity of the surfacewater source of the groundwater, SAR surface water was evaluated for vitellogenin and gonadal histopathology in juvenile medaka with no effects observed. These results demonstrate that OR Japanese medaka may be a sensitive strain as an on-line monitor to predict potential impacts of water quality, but further studies are needed to elicit causative agents within the water mixture.

Geochemical imaging of flow near an artificial recharge facility, Orange County, California.

Critical for the management of artificial recharge operations is detailed knowledge of ground water dynamics near spreading areas. Geochemical tracer techniques including stable isotopes of water, tritium/helium-3 (T/3He) dating, and deliberate gas tracer experiments are ideally suited for these investigations. These tracers were used to evaluate flow near an artificial recharge site in northern Orange County, California, where approximately 2.5 x 10(8) m3 (200,000 acre-feet) of water are recharged annually. T/3He ages show that most of the relatively shallow ground water within 3 km of the recharge facilities have apparent ages < 2 years; further downgradient apparent ages increase, reaching > 20 years at approximately 6 km. Gas tracer experiments using sulfur hexafluoride and xenon isotopes were conducted from the Santa Ana River and two spreading basins. These tracers were followed in the ground water for more than two years, allowing subsurface flow patterns and flow times to be quantified. Results demonstrate that mean horizontal ground water velocities range from < 1 to > 4 km/year. The leading edges of the tracer patch moved at velocities about twice as fast as the center of mass. Leading edge velocities are important when considering the potential transport of microbes and other "time sensitive" contaminants and cannot be determined easily with other methods. T/3He apparent ages and tracer travel times agreed within the analytical uncertainty at 16 of 19 narrow screened monitoring wells. By combining these techniques, ground water flow was imaged with time scales on the order of weeks to decades.