In the mid-1990s, the City of Santa Monica, Calif., was forced to shut down its 10 million gallon per day (MGD) groundwater supply due to methyl tertiary-butyl ether (MTBE) — a gasoline additive — contamination in one of its wellfields. Since then, the city has relied on the Metropolitan Water District (MWD) for its water supply. Restoring the city’s well supply for sustainable, potable water production remained the key goal as studies, research and design evaluations were performed in the 12 years leading to the start of the Charnock Wellfield Restoration Project (CWRP) in late 2007.
The city, already familiar with design-build project delivery recognized that it was the best approach for the CWRP — particularly with its potential schedule benefits and capital cost savings. The initial target date for completion was December 2010; however, with new and pending limitations on freshwater supplies that could be delivered to southern California from the Colorado River and the State Water Project, the city and Black & Veatch are working together to optimize scheduled design and construction tasks to beat the target date for completion.
Contamination Detection
Before the city detected MTBE in three of the city’s 10 wells in 1995, groundwater was softened with ion exchange and aerated for volatile organic carbon (VOC) removal prior to distribution. Since 1995, MWD has provided the city with water, which is chloraminated for distribution.
After detection of MTBE, city staff was involved in research groups, task forces and numerous evaluations to help define the extent of contamination and predict the any future MTBE concentrations when production was restored. The studies indicated that three of the city’s wells would require treatment for MTBE removal. The studies also helped to establish treatment and facility requirements for future approval by the California Department of Public Health (DPH).
Although purchasing and delivering water from MWD has served the needs of Santa Monica’s customers well, the goal was always to expedite the city’s wellfield restoration to establish a sustainable water supply for the city. Not only will this reduce delivery costs, it also will foster sustainability for Santa Monica as well as the surrounding region and other parts of California.
The timing of the project is critical. In 2003, the Colorado River water allocation to southern California was reduced 1922 levels, with MWD receiving much less water than in years past. MWD, as well as many other utilities that historically have relied on the Colorado River, is looking at other water sources to replace this lost supply. Having the city’s water demand of approximately 10 to 15 MGD removed from MWD’s system helps sustain southern California water supply.
Drought has severely impacted fresh water supplies throughout California in recent years as well, and using the city’s groundwater wells is a sustainable way to provide more water. The drought has spawned additional supply shortages for utilities receiving water from the State Water Project (SWP), the water transfer system fed by northern California’s Sacramento-San Joaquin River Delta. On Oct. 30, 2008, water utilities in southern California that get water through SWP received 2009 allocation estimates that were about 15 percent of the requested amounts.
A recent biological opinion that highlights protective measures for the delta smelt (i.e., a fish uniquely adapted to the delta’s ecosystem) was published Dec. 15, 2008, by the U.S. Fish & Wildlife Service and will likely translate to seasonal delta water pumping restrictions — an additional strain on southern California water purveyors. Considering all of these factors, restoration of Santa Monica’s wellfield fosters sustainability not only for Santa Monica, but throughout the state.
The project’s importance for the city and its customers required a cost-effective and fast project delivery mechanism. With the city’s previous design-build experience for public buildings and parking structures, it was chosen as the delivery method for the CWRP.
The qualifications-based design-build entity selection occurred in December 2007. The CWRP will then allow design-build for procurement and construction after the guaranteed maximum price is approved this spring. Managed and executed in accordance with Project Management Institute (PMI) procedures, the management approach will focus on each of the nine knowledge areas of project management in accordance with PMI.
Contamination Solutions
Several wells feed into the pipeline that supplies the Arcadia Water Treatment Plant (WTP) near Santa Monica. Of these wells, the Department of Health considers three in the Charnock wellfield as contaminated. As a result, water from Charnock wells 13, 15 and 19 requires multi-barrier treatment (Figure 1, opposite page). This treated water will be blended with other supply wells, and then delivered to the Arcadia plant for softening.
Treatment will include greensand filtration for iron and manganese removal, followed by two-stage, granular activated carbon (GAC) adsorption for MTBE removal. The GAC system uses a lead-lag configuration, so when the adsorption capacity of the GAC in the first vessel in series becomes exhausted, the media is replaced and the interconnecting valves are adjusted so that the second vessel in series becomes the first vessel in series. This lead-lag configuration provides two barriers for contaminant removal, while maximizing the capacity of the GAC in each vessel. The dissolved oxygen content of the raw well water will be increased by a side-stream air eductor system. Chlorine is used for oxidation of the well casings to help maintain water production and for oxidation of iron and manganese. Sodium bisulfite will be used to dechlorinate the water prior to GAC adsorption.
The treated water from the Charnock site is then blended with water from other wells and delivered to the Arcadia WTP. The water has a hardness of about 500 mg/L as CaCO3. Greensand filters will remove iron and manganese as a pretreatment process to reverse osmosis (RO). Four cartridge filters are planned upstream of four RO skids, which will lower the hardness to 120 mg/L as CaCO3.
The blended RO permeate and RO bypass flow will pass through decarbonators prior to final chemical dosing and stabilization with sodium hydroxide and fluoride to minimize the downstream sodium hydroxide dose required for stabilization. The finished water will be chlorinated and converted to combined chlorine prior to entering the existing 5 million gallon reservoir, which has internal surface aerators for VOC removal.
The Charnock site and Arcadia WTP site had limited space available for new structures due to site constraints (Figures 2 and 3). In addition, the surrounding community’s concerns about plant appearance, truck traffic, noise and chemical hazards design influenced the use of underground storage tanks for equalization, backwash supply and backwash holding.
The layout and process vessels were selected to minimize GAC deliveries at the Charnock site.
The facilities also will use sodium hypochlorite instead of the more hazardous chlorine gas for disinfection.
Charnock site waste disposal includes sewer disposal of the greensand and GAC backwash solids, which will be from the backwash that is treated with a packaged plate settler.
A similar system will be used at the Arcadia plant for the greensand backwash flow. Decant from the plate settlers will be recycled to the head of each treatment plant.
The citric acid and caustic cleaning solutions for RO cleans will be neutralized and disposed to the sewer. Although sewer disposal is also planned for the RO concentrate, conservation interests together with the estimated annual cost of disposal have prompted the city to conduct an evaluation of treating the concentrate flow (i.e., <1.25 mg per day) for metals and organics removal to determine whether a less costly surface discharge could be possible. To achieve the target operational date of December 2010, the city plans for sewer disposal of the concentrate and will investigate other treatment or disposal options for the future.
Schedule Considerations
In early 2008, the project team conducted well water quality sampling, both for RO feasibility evaluations and for DPH’s impaired water source assessment followed by a 12-week pilot study last summer to evaluate the performance of greensand upstream of RO. A 30 percent level preliminary design was then completed in October 2008.
The project required preparation and approval of the Environmental Impact Report, which outlined a conceptual facility and included estimates of chemical and GAC deliveries, generated significant community interest and support. Detailed design began in November 2008.
To expedite the schedule, the city and Black & Veatch are now evaluating ways to minimize the time for certain tasks and working to provide rapid response and turnaround of document reviews and approval processes.
Green Issues
In addition to cost-benefit assessments of design alternatives, the city requested that the project include environmentally friendly aspects to complement its sustainability program, which includes consideration of roof-mounted solar panels for the process buildings or shades, as well as LED lighting. The RO bid packages also include energy recovery devices to help scavenge excess pressure in the concentrate pipe of the RO skid, thus reducing the facility’s overall power operations cost.
The city also is planning to study options for treating the RO concentrate stream, while possibly using processes that can reduce the waste volume from the Arcadia plant. This “concentrating the concentrate” approach would not only reduce operating costs for sewer disposal but could also provide additional potable water for supply.
A sustainable water supply is the primary goal for the CWRP. By restoring an existing wellfield to potable production, the city can help sustain its water needs for years to come and lessen the burden on surface water supplies in California.
Holly Shorney-Darby is a process engineering manager with Black & Veatch in Kansas City, Mo. Tony Antich and Spiros Lazaris are with the City of Santa Monica, Calif., and Hala Titus is a project manager with Black & Veatch in Los Angeles: www.BV.com.