Western San Joaquin Valley
Cover Photo: San Joaquin-Sacramento River Delta, California, NASA, 2006
The western San Joaquin Valley is one of the most productive farming areas in the United States, but salt-buildup in soils and shallow groundwater aquifers threatens this area’s productivity. Elevated selenium concentrations in soils and groundwater complicate drainage management and salt disposal. In this document, we evaluate constraints on drainage management and implications of various approaches to management considered in:
The alternatives developed in the SLDFRE EIS and other recently proposed drainage plans (refer to appendix A for details) differ from the strategies proposed by the San Joaquin Valley Drainage Program (1990a). The Bureau of Reclamation (USBR) in March 2007 signed a record of decision for an in-valley disposal option that would retire 194,000 acres of land, build 1,900 acres of evaporation ponds, and develop a treatment system to remove salt and selenium from drainwater. The recently proposed SLU Plans emphasize pumping drainage to the surface, storing approximately 33% in agricultural water re-use areas, treating selenium through biotechnology, enhancing the evaporation of water to concentrate salt, and identifying ultimate storage facilities for the remaining approximately 67% of waste selenium and salt. The treatment sequence of reuse, reverse osmosis, selenium bio-treatment, and enhanced solar evaporation is unprecedented and untested at the scale needed to meet plan requirements.
All drainage management strategies that have been proposed seek to reduce the amount of drainage water produced. One approach is to reduce the amount of drainage per irrigated acre. From modeling simulations performed for the SLDFRE EIS of the Westlands Area of the SLU, theoretical minimums that can be achieved range from approximately 0.16 to 0.25 acre-feet per acre per year (AF/acre/year). Minimum production rates from the Northerly Area of the SLU are theorized as being much higher, approximately 0. 42 to 0.28 AF/acre/year. Rates shown in the SLU Plans for drained acres from the two areas combined are 0.5 AF/acre/year at the subsurface drain stage and 0.37 AF/acre/year after a series of on-farm and regional measures are instituted.
Land retirement is a key strategy to reduce drainage because it can effectively reduce drainage to zero if all drainage-impaired lands are retired. Land retirement alternatives considered in the SLDFRE EIS differ for the two areas analyzed in the SLU. The Northerly Area is to retire a nominal 10,000 acres and Westlands is to retire up to 300,000 acres. The initial land retirement option recently put forth in the SLU Plans predicted drainage volume reductions that are consistent with 200,000 acres of land retirement, but only 100,000 acres of land retirement was proposed.
Within the proposed area of drainage there are, for all practical purposes, unlimited reservoirs of selenium and salt stored within the aquifers and soils of the valley and upslope in the Coast Ranges. Salt imported in irrigation water is estimated to be at least 1.5 million tons per year for the Westlands and Northerly Areas (SJVDIP, 1998). Analysis of the land retirement alternatives presented in the SLDFRE EIS indicates that land retirement of a minimum of only 100,000 acres results in the annual pumping to the surface of 20,142 pounds of selenium or about a million pounds of selenium over a 50 year period. Retiring 200,000 acres results in an annual pumping of 14,750 pounds of selenium; and retiring 300,000 acres reduces selenium pumped to the surface annually to 8,756 pounds, almost all of which is produced in the Northerly Area.
A selenium mass balance analysis by USGS quantifies the amount of selenium, in general, exposed on the landscape and specifically contained in each waste-stream component (e.g., regional collector, reuse area, reverse osmosis facility, selenium bio-treatment plant, and enhanced solar evaporator system) for the SLDFRE EIS land retirement alternatives and recently proposed SLU Plans. A third of the selenium is lost in the first step at the agriculture water reuse areas. Selenium bio-treatment, if successful, would remove another 66% of the selenium from the waste stream, leaving a waste-stream of 10 µg/L to be evaporated. Salt produced and stored at the surface in solar evaporators in the 100,000-acre, 200,000-acre and 300,000-acre alternatives totals 412,000, 307,000 and 181,000 tons per year. At 50 years, the 100,000-acre land retirement option will require salt storage of 20 million tons in these evaporators or landfills. This salt will be contaminated with a variety of trace elements common in drainage waters including selenium, boron, molybdenum, chromium, and arsenic.
Storage of salts in the aquifer below irrigated lands will also occur. Useable groundwater may be defined by the amount of total dissolved solids it contains. Regardless of what drainage plan is implemented, the amount of salt in groundwater will increase. Based on projections of future total dissolved solids in groundwater of the Westland and Northerly Areas, the useable life of the aquifer under various irrigation and drainage management goals is estimated to be between 25 and 220 years.
The hydrologic imbalance in the western San Joaquin Valley can be partly addressed through a program that substitutes groundwater pumping for surface water delivery, thus helping to shift the groundwater budget from large surplus to small deficit and to stem any expansion of the drainage problem through time with continued irrigation. USGS models estimate that if pumped ground water is substituted for surface water deliveries there are several significant benefits: (1) the water table will be lowered under existing impaired lands; (2) the future area of land impacted by bare soil evaporation will be reduced; (3) up to 400,000 acre feet of surface water deliveries will be offset by groundwater pumping of about 320,000 AF/year and 80,000 AF/year by improvements in ground-water efficiency; and (4) the amount of drainwater that would need to be brought to the surface and treated will be reduced, thus reducing both cost and potential ecological risk. Coupling this type of ground-water flow model with salt and selenium biogeochemical models would yield an integrated approach to predicting water, salt, and selenium transport and identifying any potential degradation of aquifer resources that might accompany such a regional program.
Given the amount of analysis and documentation available from the SJVDP and recent re-evaluations of drainage management, the USGS identifies not a lack of information, but rather a lack of decision analysis tools to enable meeting the combined need of sustaining agriculture, providing drainage service, and minimizing impacts to the environment. A more formal decision-making process may better address uncertainties (e g., the scaling up of re-use areas and enhanced solar evaporators; the feasibility of bio-treatment of drainwater containing 32,500 mg/L salt); help optimize combinations of specific drainage management strategies; and document underlying data analysis for future use. The benefits of such a process of decision analysis (refer to appendix B for details) are that it provides the flexibility to move forward in the face of uncertainty. It does, however, require long-term collaboration among stakeholders and a commitment to formalized adaptive management.
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Page Contact Information: Michael Diggles
Page Created: June 13, 2008
Page Last Modified: June 19, 2008
