Further measures in 1988, including drainage of evaporation ponds as well as covering of the deeper ponds with fill soil, led to the elimination of aquatic habitat at the Kesterson Reservoir, thus preventing any additional waterfowl from being exposed to selenium at the location . Despite the closure of the Kesterson Reservoir, problems of excessive selenium concentrations persisted in the greater surrounding Grassland wetland area , the largest freshwater wetland ecosystem in California . Farmers of the Grassland drainage area had historically discharged their surface and subsurface runoff through the natural channels of the Grassland wetlands to the San Joaquin River . As a result of increased scrutiny following the events at Kesterson , 33 km2 of the Grasslands were added to California’s Clean Water Act section 303 list of impaired waters due to excessive selenium concentrations in 1988. The wetland’s two major flow channels Salt and Mud Slough followed in 1990 . In 1996, the Grassland Bypass Project was created to amend this situation . The GBP consists of a series of measures to reduce selenium loads in the Grassland marshes and the San Joaquin River, including the reopening of a stretch of the San Luis Drain bypassing the wetlands . The GBP is analyzed below in the section entitled “Selenium load reduction coupled to conveyance into the San Joaquin River”.Two fundamental approaches have been used to manage seleniferous runoff in the San Joaquin Valley: local disposal and conveyance out of the San Joaquin Valley. Geographically,procona buckets the separation between these two approaches aligns with the drainage areas defined under the San Joaquin Valley Drainage Program’s grand management plan . The southern subareas Kern, Tulare and, after the San Luis drain closure, also the Westlands subarea dispose of runoff locally, while the Northern and Grassland subareas convey drainage to the San Joaquin River .
The two approaches share common elements . In either case, methods are employed to decrease the disposal load by first decreasing drainage production and then decreasing the volume and selenium concentrations of the drainage itself . Fundamentally, only the final disposal step differs, with drainage either being evaporated and the salts disposed, or channeled to be diluted in a larger water body .The debate on how to sustainably manage selenium loads and drainage needs in the Central Valley is far from over. Mass balance analysis by the USGS reveals that the drainage needs of the Westlands subarea, the greatest near surface selenium reservoir among the subareas, cannot be met without the retirement of at least one third of the 1,200 km2 of agricultural lands and the use of treatment methods for selenium and salt removal that are as of yet unproven . For a detailed discussion of proposed management scenarios for this subarea the reader is referred to the Final Environmental Impact Statement of the San Luis Drainage Feature Re-evaluation and the technical analysis of proposed plans by Presser and Schwartzman . The current plans under discussion for the Westlands in terms of selenium removal include reverse osmosis and reductive precipitation in microbial bioreactors that have so far only been tested at the pilot scale . It is uncertain whether the proposed bioreactors will be effective with the high salinity inputs expected to result from reverse osmosis . Given such uncertainty I focus on the proven management methods by which seleniferous drainage is actively being managed in the San Joaquin Valley .Locally, seleniferous runoff can be treated using technologies that physically, chemically or biologically remove selenium from water, reused through irrigation of designated land planted with salt tolerant crops, or disposed of in evaporation facilities. A number of removal technologies have been proposed post-Kesterson , however the physical and chemical methods are too costly and the biological methods fail to reduce selenium concentrations in treated drainage to below 2 µg/L at relevant scales . Thus, for lack of better alternatives, drainage reuse and evaporation ponds are so far the local remediation and disposal methods of choice. The reuse of seleniferous drainage as irrigation water on designated reuse plots thus reducing the water volume requiring final disposal, has seen a marked expansion over the last decade, with areas of reuse in the Grassland subarea alone increasing from 7.4 km2 to more than 20 km2 from 1996 to 2009 .
A number of innovative approaches often involving a sequence of increasingly salt tolerant crops in either time or space have been developed and tested in the San Joaquin Valley . Of the 2,600 kg selenium produced in the Grassland subarea in 2009, about half were disposed through drainage reuse . The exact fate of this selenium is as of yet unclear. In particular, there are concerns about the long term sustainability of drainage reuse, after increasing concentrations of dissolved selenium were observed at all monitored soil depths in the only study monitoring soil selenium for an extended time period on a reuse plot in the San Joaquin Valley . Additionally, there are concerns that endangered wildlife such as the San Joaquin kit fox , the kangaroo rat , and the blunt-nosed leopard lizard may be adversely affected if their ranges overlap with reuse areas . Evaporation ponds, which are shallow basins used for the evaporative disposal of drainage water, are deployed primarily in the Tulare and Kern subareas . Since there are no drainage channels or rivers to convey drainage out of these two subareas, evaporation is the only option for final disposal . The design of evaporation ponds has been optimized post-Kesterson to reduce wildlife use and thus the risk of exposure to elevated selenium concentrations in pond water . Specifically, steep levee slopes, elimination of windbreaks, and a minimum water depth of 0.6 m are used to deter waterfowl. Enhanced solar evaporator designs that use sprinklers to ideally eliminate standing water altogether have been tested at the pilot scale, but have not seen wide-spread deployment to date . Evaporation ponds are currently exempt from water quality guidelines that apply to natural waters, however management needs to include active and continuous measures to limit wildlife use through hazing and removal of pond vegetation. Additionally, the provision of nearby alternative habitat for waterfowl is recommended when selenium concentrations exceed 2 µg/L . While these management procedures are necessary during the operation of evaporation ponds to avoid ecological damage, they also greatly increase the cost of operation. In fact, the owners of many of the privately operated evaporation ponds in the San Joaquin Valley have decided to cease operation as a result of imposed requirements .
Another drawback of this method of local disposal is that the evaporate needs to be stored at dedicated disposal sites. This is particularly problematic if evaporation is chosen to continuously dispose drainage from large source areas. For example, it has been estimated that if the drainage program in the Westlands district is expanded as planned, up to 400,000 tons of salt may need to be disposed yearly,procona florida containers which would require dedicated dumpsites covering an area of around 1 km2 every 50 years .The Northern and the Grassland subareas channel a large portion of their runoff to the San Joaquin River. Among the two, only the Grassland subarea has been marked by problematic selenium loads . Due to the Grassland Bypass Project , it is also the subarea in which management of seleniferous runoff has made the greatest progress over the last 20 years . The GBP was created in 1996 through an agreement between the U.S. Bureau of Reclamation and the regional drainage entity of the Grassland Area Farmers under the legal umbrella of the San Luis and Delta-Mendota Water Authority. It consists of three central measures. First, a 45 km stretch of the San Luis drain was opened to convey subsurface drainage from the GAF to Mud Slough, thereby bypassing most of the wetlands . Second, limits were imposed on the total allowable selenium discharge by the GAF and these limits were set to decrease over time . Finally, ongoing monitoring of water quality and quantity was initiated across the project area to enforce limits and gage impact . The original 5 year project was extended by 8 years in 2001 and then by 10 years in 2009 . Whereas the use agreement specified load dependent incentive fees for exceedance of the specified selenium load limits, the true innovation consisted in enabling the GAF to develop an internal selenium load trading program . This trading program represents the first ever cap-and-trade style program for any pollutant allowing trades directly between non-point sources. It is also the first instance in which “total maximum daily load” requirements under the Clean Water Act have been successfully enforced against a non-point source . Since 2001 the drainage management efforts of the GAF have shifted away from the load trading program and towards centralized management for the region , however the overall strategy has remained consistent . Funds obtained as part of the GBP are used to support programs and actions aimed at reducing selenium loads , such as the local drainage reuse measures described above. The direct aid provided by the GBP as well as the economic incentives for load reduction and conservation initially created by its load trading program have led to an overall reduction in selenium loads .
In fact, the total annual loads of selenium discharged through the San Luis Drain have decreased continuously over the course of the project, from 3,110 kg in 1997 to 560 kg in 2009. This has been reflected in a reduction of selenium loads in the San Joaquin River from a pre-project annual average of 3,690 kg to 690 kg in 2009 . Weekly selenium concentration monitoring data collected below the confluence with the Merced between 1996 and 2012, reveal seasonality in selenium loads , but reaffirm the overall load reductions reported by the GBP . However, the reduction is primarily due to a reduction in total discharge from 6.1×107 to 1.6×107 m3 in 2009. Average selenium concentrations of discharge decreased from 67 to 33 µg/L in the same time period and were still significantly above the water quality criterion of 5 µg/L. Much of the reduction in selenium loads can be credited directly to the impact of the projects and measures implemented as part of the GBP. For example, through the San Joaquin River Water Quality Improvement Project, more than 20 km2 of land have been purchased and planted with salt tolerant crops providing the capacity to reuse up to 1.9×107 m3 of drainage water per year . A GBP unrelated factor in the successful load reduction was the prevalence of drought during the early 1990s which incentivized investments in efficient irrigation technology and other conservation measures on the side of the GAF . Therefore, considerations of best management practices and technology aside, the creation of quantitative economic incentives for selenium load reduction should be a priority of any seleniferous drainage management program. Unfortunately, comprehensive incentive programs such as the GAF load trading program remain the exception rather than the rule, for selenium as they do for many other pollutants.The primary motivation of the GBP was the protection of the sensitive wetland habitat in the Grassland area. Through circumvention, the GBP has effectively removed all seleniferous drainage water from 145 km of channels that supply water to more than 650 km2 of sensitive wetlands and wildlife areas . This led to a rapid decrease in selenium concentrations at the monitoring stations in this area after the GBP’s implementation in 1996. In Salt Slough for example , concentrations have dropped from 16 µg/L in water year 1996 to below 2 µg/L for most of the project duration . In addition, selenium concentrations monitored in the tissue of various fish species collected at the slough , minnows , and others) have dropped from toxic levels between 1992 and 1995 to below levels of concern after 1998 . Accordingly, the slough was removed from California’s 303 list of impaired waters in 2008 .