OVERVIEW
The Orange County Water District owns 2,150 acres behind Prado Dam in Riverside County, California. Within OCWD property and adjacent lands are nearly 465 acres of constructed wetlands, which have effectively demonstrated the ability to reduce nitrogen levels in Santa Ana River water. The Santa Ana River is the main source of recharge for the vast Orange County groundwater basin, and consists primarily of tertiary treated wastewater from upstream dischargers. The river also receives storm flows, natural run-off, and rising groundwater, especially during winter months.

The wetland consists of a system of 50 shallow ponds that have been utilized to remove nitrogen in river water since July 1992. The wetland system removes approximately 20 tons of nitrate a month, and during summer months reduces nitrate concentration from 10 milligrams per liter to less than 1 milligram per liter. More about denitrification efforts at the Prado Wetland.

Several modifications have been made to increase the hydraulic capacity of the Prado wetland pond system, in order to handle a potential increase in future baseflows from the Santa Ana River, and to improve the operational flexibility of the system. More information about the reconstruction of Prado Wetland.

Prado Dam is a key component for increasing local water supplies in Orange County. Historically, storm flows from the Santa Ana River have been lost to the ocean because flood control took precedence over water conservation. However, a series of agreements between Orange County Water District, the U.S. Army Corps of Engineers, and the U.S. Fish and Wildlife Service have allowed the District to conserve water behind the dam in a seasonal storage pool.

WATER CONSERVATION AT PRADO BASIN WETLAND
A landmark agreement in 1995 between OCWD, the U.S. Army Corps of Engineers (Corps), and the U.S. Fish and Wildlife Service, the water conservation level behind Prado Dam was raised, nearly doubling the amount of valuable water that can be stored behind the Dam. The agreement between the agencies was the culmination of years of cooperative efforts to enhance the water conservation and environmental values of Prado Basin, breeding grounds of the endangered least Bell's vireo.

Prado Dam, in Riverside County, California, is the primary flood control facility on the Santa Ana River. At spillway crest of 543 feet, the reservoir can hold up to 196,000 acre-feet of water. Based on a 1980's water control plan, the Corps set the maximum reservoir pool elevation at 494 feet between September 15 and March 1.

Upstream of the dam lies the largest riparian habitat in Southern California; it is rich in plant and animal life including rare, threatened and endangered species. It consists of a productive and rare ecosystem, in which more than 311 species of vascular plants, 7 species of amphibians, 13 species of reptiles, 47 breeding bird species, 11 raptor species and 23 mammal species can be found. Of major concern is the least Bell's vireo, a small endangered songbird that nests in the willows of Prado Basin. The Corps has allowed water conservation only as an incidental function of the dam, and previously limited the water conservation pool to elevation 490 feet after March 15 because of the potential negative impact on the endangered native species.

In 1988, after years of negotiation and studies, the Corps agreed to develop an Environmental Impact Statement (EIS) and operating plan to maximize water conservation behind Prado. A crucial factor in determining water storage elevations has been protection of the least Bell's vireo. The U.S. Fish and Wildlife Service (USFWS) required that habitat areas be reserved for the vireo as mitigation for higher levels of water conservation.

OCWD negotiated temporary agreements with the Corps in 1991, 1992, and 1993, while awaiting the final agreement for permanent storage at Prado, and increased the storage to elevation 500 feet. Numerous permits required by the Endangered Species Act and various water quality agencies to reach the maximum capacity were obtained during this period.

In December 1993, OCWD and the Corps signed the final agreement that provided for incremental increases in the conservation pool to eventually reach elevation 505 feet as habitat in the mitigation area matured. As part of the agreements, OCWD set aside more than 124 acres as protective habitat for the vireo and funded more than $1 million in mitigation costs for a vireo monitoring program. The mitigation program included restoration of habitat and the trapping of Cowbirds that parasitize the vireo nests. At the time of the agreement, due to efforts from the previous six years, the vireo population increased from 19 pairs in 1986 to 123 pairs in 1993.

On March 3, 1995, the original agreement's timeline was accelerated and OCWD, the Corps, and the USFWS increased the water conservation pool to its current elevation of 505 feet. In exchange, OCWD agreed to contribute $1 million to the USFWS for removal of the invasive plant species, Arundo donax, along the River. Arundo is a major threat to the ecosystem of not only Prado Basin but the entire Santa Ana River watershed, and a dedicated Arundo removal program will ultimately be as effective in enhancing the environment as vireo mitigation. Arundo is a heavy water user and its removal will reduce water losses along the River.

OCWD originally agreed to set aside a total of 506 acres as wildlife and vireo habitat to qualify for the 505-foot conservation level. However, due to the success of the monitoring program, Fish and Wildlife approved the increase to the current maximum elevation on March 3, 1995 without increasing the habitat area. Of the 217 territorial male vireos detected in the area, 164 were found to be paired as of that date. A minimum of 355 known fledged young were produced by the breeding pairs, resulting in a 9% increase over 1994 and a 44% increase over 1993. By the end of 1996, the count stood at 195 nesting pairs.

In addition, an Environmental Assessment (EA) was required by the National Environmental Policy Act. The EA was completed in 10 days on March 3, 1995. Immediately after the 1995 agreement was implemented, a large storm hit southern California, and OCWD was able to capture and save more than $3.5 million in storm flows during the month of March 1995. The added storage capacity of 27,500 acre-feet will eventually allow the District to increase its usage of more reliable local water resources. Without these annual negotiated agreements, this water would have been lost to the Pacific Ocean.

The water captured behind the Dam is high-quality storm water that is used to recharge Orange County's vast groundwater basin, saving the District million of dollars in water purchases. All three participating agencies recognize the value of cooperatively working together to pursue a holistic approach in managing Prado Dam to benefit Orange County's economy, citizenry, and natural resources.

NITRATE REMOVAL AT PRADO BASIN WETLAND
In cooperation with Mother Nature, OCWD has found an economical way to treat wastewater discharged into the Santa Ana River. By routing river water through a network of constructed wetland ponds behind Prado Dam in Riverside County, the District has created a natural, cost-effective process that naturally reduces nitrate levels to well below current drinking water requirements.

Supported by OCWD and the National Water Research Institute (NWRI), scientists from Northwestern University and the University of California at Berkeley conducted studies from 1992 through 1996. The initial research investigated the effectiveness of the wetland at removing nitrate and the fate of the nitrogen. Following completion of that study, which determined that nitrate is effectively removed, subsequent studies focused on the effects of various wetland manipulations upon river water nitrate removal. Specifically the researchers evaluated the effects of wetland age, vegetation type, hydraulic residence time (HRT), and water depth on nitrate removal rates.

The results from the studies indicate that the wetlands is a very efficient means for the removal of nitrate from the Santa Ana River. The primary mechanism for nitrate removal is denitrification. It is commonly thought that plants or soil are responsible for nitrogen removal, however, the research has shown that neither plant uptake nor sediment uptake were significant to the nitrate removal.

Wetland plant regimes were important factors in controlling nitrate removal rates because of the quality and quantity of organic carbon that they provide to the water-sediment interface, and because of the effects they have on water temperature. Plant organic matter provided both a short-term and a long-term organic source for denitrifiers. This is key to denitrification in the Prado Basin because denitrification was found to be organic carbon limited. Vegetation may also have increased microbial habitat and the amount of anoxic zones, thereby further improving denitrification rates.

Other results determined that longer hydraulic residence times (HRT) did lower outflow nitrate concentrations, but varying HRT did not affect removal rates. Nitrate removal rates were seasonally dependent. The ponds are most effective during the warm summer months, due to warm temperatures. This dependence may have resulted from temperature effects both on denitrifiers and the fermentative bacteria which made organic carbon more readily available to denitrifiers.

Nitrate removal rates also increase with wetland age. More mature wetlands provide increased litter which in turn provide additional organic carbon, greater anoxic zones, and improved habitat for the denitrifiers and for the entire microbial community. The magnitude of improvements in nitrate removal rates varied with plant types.

The effects of vegetation on removal rates was investigated using monocultures of bulrush and cattail. The study found that some vegetation types provided a better environment for denitrification, hence better denitrification rates, but it was most effective to have a very diverse plant culture. In addition to maintaining a varied vegetation regime, harvesting improved denitrification by providing an immediate organic carbon source and may possibly improve long-term plant productivity by diminishing light limitations.

Nitrate removal rates were affected by changing water depth in a number of ways including: the decrease in wetted area, the decrease in organic carbon substrate, channelization and effects on water temperature. In shallow wetlands (<0.2 m) with open water or floating vegetation, daytime temperature increased markedly (>2-3 C) compared to either deep (>0.5 m) or densely vegetated wetlands, which increased denitrification rates.

The studies conducted by Northwestern and UC Berkeley have provided valuable information on the natural purification of Santa Ana River water. The District has developed operational guidelines incorporating the recommendations of the research to maximize the treatment potential of the Prado Wetlands. The research was also utilized as a design aid for the recent wetlands reconstruction, which will increase capacity and efficiency to effectively treat increasing future Santa Ana River flows. By taking this innovative approach to water quality issues, the Orange County Water District will be able to improve groundwater quality while enhancing the environment and controlling treatment costs.

RECONSTRUCTION OF PRADO BASIN WETLAND

A U.S. Army Corps of Engineers permit allows fifty-percent of the river's flows to be diverted through the wetland. Currently, the baseflow of the river is approximately 120 cubic feet per second (cfs), with 60 cfs traveling through the wetland. It is anticipated that the baseflow of the river will increase beyond 200 cfs due to an increase in population upstream in both Riverside and San Bernardino Counties.
This increase in population will create an increase in the amount of wastewater discharged into the river. At its previous a capacity of 60 cfs, the wetland would not be able to adequately handle a fifty-percent diversion rate of future flow increases. Because this natural wetland treatment process is crucial to improving the quality of water received for groundwater recharge downstream of Prado Dam, the wetland system was reconstructed in anticipation of increased river flows. In addition, there was a need to improve the operational flexibility of the pond system. The modifications to the system included dividing the wetland into four subsystems - North, South, East, and West. Larger ponds were divided into smaller areas by adding levees. Conveyance channels, which allow the District to manifold water to any combination of ponds, were constructed. Weir boxes and pipes were replaced with higher capacity more durable concrete boxes and composite pipes.

In order to repair storm damage or maintain an individual pond prior to reconstruction, the entire system had to be shut down. This repair process would often times take four to eight months, leaving the District without a treatment system for extended periods of time. To alleviate extended repair delays, the District designed a system of conveyance channels and control gates which allow the flows to be manifold to different sections of the wetland. Operation of the wetland treatment process can now continue without interruption during maintenance and repair activities.

Modifications to the system also included the conversion of 35 acres of existing pond area to high-quality habitat for the endangered least Bell's vireo, and the construction of nesting islands for other waterfowl. The project was constructed in a two-phased approach to eliminate potential effects on vireo habitat. During vireo nesting season, which is March 15 through September 15, construction was prohibited within a boundary of 250 feet around the songbirds habitat.

The modifications to the wetland system were completed in March 1997, at a cost of $2.2 million. The reconstruction of the wetland has enabled the capacity of the system to increase to 100 cubic feet per second and will improve the treatment levels by enhancing the natural denitrification process. The channels were designed to carry 200 cfs, however, the current capacity is limited to 100 cfs by the size and number of weir boxes and pipes. If diversions from the Santa Ana River should increase above 100 cfs in the future, the wetland capacity can be increased relatively inexpensively by adding pipe culverts and weir boxes.

By increasing the nitrate removal capability of the wetlands, increased flows from upstream dischargers can be treated sufficiently to meet the water quality requirements at Prado Dam. Because high nitrogen levels pose a threat to the groundwater basin, the Regional Water Quality Control Board is requiring reduced levels of nitrogen in waste discharges to the river. The cost to expand existing treatment facilities or construct new ones to comply with these limits would be quite high.The reconstruction has increased the hydraulic capacity of the pond system allowing more river water to be diverted. It has enhanced the natural biochemical nitrogen removal processes, thereby improving water quality. And, it has improved the operational flexibility of the wetland system allowing for more efficient maintenance.