Implications of Past Agricultural Practices on Mobilization and Transport of Selenium and Nutrients from Groundwater to Surface Waters
The former Tustin Marine Corps Air Station (MCAS) is located in the coastal alluvial plain of Orange County California. The air station is situated in an area that historically was a swampland and ephemeral lake known as the ?Swamp of the Frogs.? This area was a large natural retention area that would occasionally overflow to the ocean during periods of high rainfall. Highly reducing conditions in the swamp provided an environment for long-term accumulation of selenium and other minerals that were leached and washed down from marine deposits in the surrounding hillsides. In the late 1800s, the swamp was drained by creating a series of drainage channels and the area was converted to orchards, grazing and other types of farming. Orchards were planted along the margins of the former marshland, and associated fertilization resulted in high nitrate levels in the shallow soils and groundwater. Additionally, drainage of the swamp increased the oxidation state, which promotes conversion of selenium from reduced forms (e.g., selenite) to oxidized forms (selenate). The oxidized forms of selenium are more soluble and more easily transported to drainage channels. The aquifer surrounding the drainage channels in the former ?Swamp of the Frogs? is a shallow pluvial aquifer that is highly responsive to precipitation. Groundwater elevations typically peak in the spring following winter rains, and are lowest in the fall at the end of the dry season. Groundwater flows are a major source of dry weather base flows in the drainage channels which flow downstream to Newport Bay. The dry weather base flows are impaired by elevated levels of nitrate and selenium, and TMDLs have been established. Discharge of shallow groundwater to the drainage channels within the historical swamp region is the primary source of elevated levels of nitrate and selenium. The Tustin MCAS is a former 1600-acre naval installation location in the historic swamp region. It was established in 1942 to support navel blimp operations and was later converted to a Marine helicopter base. During its operation the Navy leased about 500 acres to farmers for commercial crop development, and for many years, agricultural lands surrounded the facility. Beginning in the 1980s residential and light industrial/manufacturing areas developed adjacent to the station. The Tustin MCAS was slated for closure in 1991 and operational closure occurred in July 1999. Much of the area has been conveyed to the city of Tustin, and is being redeveloped with residential, commercial, educational, and recreational land uses. Runoff management planning for the base redevelopment was constrained by elevated levels of selenium and nitrate, a byproduct of legacy agriculture, as well as flat topography and the presence of shallow groundwater. A main objective was to design and manage the storm drain facilities in a manner that does not exacerbate discharges of nitrogen and selenium to the impaired receiving streams. To accomplish this goal the following principals were used to guide design of storm drain facilities and treatment BMPs: 1) Use distributed treatment BMPs that will treat runoff before it enters the storm drain system. 2) Avoid infiltration BMPs and groundwater recharge to the extent possible. 3) Use at-grade vegetated systems such as swales, bioretention, flow-through planters that provide effective treatment, aesthetics, and avoid ponding.4) Avoid infiltrations into storm drain facilities.