Moapa Valley Water District Arsenic Treatment Plants
BACKGROUND
The Moapa Valley Water District (MVWD) was established under the Moapa Valley Water District Act, Chapter 477 of the Nevada Statutes and became effective July 22, 1983, with the formal dissolution and combining of the respective water systems and assets of the Moapa Valley Water Company with the Overton Water District. The MVWD service area is located in Clark County approximately 52 miles northeast of Las Vegas along Interstate 15. The service area of the MVWD covers some 79 square miles of unincorporated areas and several rural communities with less than 6,000 persons each and includes the Moapa, Glendale, Logandale, and Overton areas.
The MVWD is currently served from four source water sites: the MX-6 Well, the two Arrow Canyon Wells, Baldwin Springs, and Jones Springs. Each supply source generally provides dedicated service to a geographic area and a certain number of customers. The Arrow Canyon Wells were drilled in 1991 and 2003. The Arrow Canyon facility is equipped with SCADA control, a sodium hypochlorite generator for disinfection, and variable frequency drives for motor control and system pressure/flow optimization. Jones Springs was originally developed in 1905 as an irrigation supply and, in 1982, was converted to a municipal source. Jones Springs has been upgraded over time with the addition of booster pump equipment, chlorine disinfection, and SCADA control. Baldwin Springs was converted to a municipal supply in 1974. Both the Jones and Baldwin Springs sites include spring collection boxes that convey water to the respective pump stations which currently pump the water into the distribution system. All sources within the area originate from the same general carbonate aquifer system and have similar water quality characteristics.
The total storage capacity for the MVWD is 8.3 million gallons with facilities distributed throughout the service area. All of the existing facilities are well maintained and serviced regularly to provide continuous and reliable service for the production of drinking water.
The MVWD has a comprehensive maintenance program in which it inspects and exercises all valves in the service area and performs a unidirectional flushing of the system twice per year. The MVWD also maintains all fire hydrants in the service area. There is an updated hydraulic model for the water system and a comprehensive well head protection program in place. The system is inspected every three years by the Southern Nevada Health District under the Sanitary Survey Program and is in compliance with the USEPA Safe Drinking Water Act and respective Nevada State Drinking Water regulations. The MVWD updated their water conservation plan in 2007, and it is on file with the Division of Water Resources and the Nevada Division of Environmental Protection (NDEP), Office of Financial Assistance.
A Preliminary Engineering Report was completed in July 2007 to evaluate alternatives for arsenic mitigation and propose the most cost effective solution. The source water sites considered for treatment included the MX-6 Well, the Arrow Canyon Wells, Baldwin Springs, and Jones Springs. These sources previously met the 50 micrograms per liter (µg/L) (also referred to as parts per billion [ppb]) maximum contaminant level (MCL) for arsenic; however, they do not meet the new MCL of 10 µg/L. The average arsenic concentration for the four sources ranges from 15 - 17 µg/L. The MWVD operated these sources under an exemption to the arsenic rule granted by the State Environmental Commission and issued by the Bureau of Safe Drinking Water in September 2006. The exemption expired in January 2009.
The MVWD wells and springs have average fluoride concentrations of 2.27 milligrams per liter (mg/L) (also referred to as parts per million [ppm]). The MVWD received a variance to the state standard of 2.0 mg/L for fluoride (NAC 445.248 [4]) in December 1989 and is required to provide public notice to customers on an annual basis.
Speciation of arsenic from the four sources of supply showed only traces of arsenite (As [III]) with the majority of the arsenic in the water residing in the arsenate (As [V]) state. As (III) is high solubility whereas As (V) is less soluble, has strong adsorption potential, and is more easily removed from water by many processes. The Southern Nevada Water Authority and other regional utilities (including the MVWD) conducted pilot evaluation of adsorption technologies for arsenic removal. This pilot study was conducted at the Baldwin Springs site.
Based upon the results of the analysis and pilot testing, a granular iron-based media (GIM) treatment process was selected as the preferred treatment by the MVWD and its engineers. In this process, raw water is filtered through the media, arsenic present in the water adsorbs onto the media, and treated water is discharged into the distribution system. The Arrow Canyon wells (4,500 gpm capacity) are treated at a new arsenic treatment plant located at the Arrow Canyon site. Water from the MX-6 Well (450 gpm capacity) is blended with the treated water from the Arrow Canyon site. The Baldwin Springs (1,300 gpm capacity) source is treated at a new arsenic treatment plant located at the Baldwin Springs site. The Jones Spring source (450 gpm capacity) is piped to the Baldwin Spring site for treatment.
During the pilot testing, bypassing of a percentage of raw water flow at both the Arrow Canyon and Baldwin Springs treatment facilities was evaluated and is anticipated to extend media life. An on-line arsenic analyzer in the Arrow Canyon facility and a bench-scale manual arsenic analyzer at the Baldwin Springs facility provide data to assist with the streamlining of the split-stream operation. Adsorption vessel redundancy is an important part of the operational strategy to assure that both treatment sites can handle the production output requirements while service (e.g., media change-out) is conducted on one or more of the adsorption vessels.
A form of backwash called 'fluffing' of the media is required as the differential pressure across the media increases past a predetermined point due to residual silts, other waterborne particulates, and general compaction of the media. In order to prevent raw water channeling through the media and a premature arsenic breakthrough, fluffing is expected to occur approximately twice a month resulting in a minimum of water (either raw or finished) going to rapid infiltration basins.
Media life at current production rates is anticipated to be 1 - 2 years. The exhausted GIM media is expected to pass Toxic Characteristic Leaching Procedure (commonly referred to as TCLP) testing and be disposed of as non-hazardous waste.
 Both of the treatment facilities were constructed on land owned by the MVWD and pipeline construction to and from Jones Spring occurred within the public right-of-way and utility easements. A general schematic of the planned treatment was included in the PER in section 7 and is included in this summary in the figure to the right.
PROJECT DESCRIPTION
The Moapa Valley Water District applied for a grant under the AB-198 program to assist with the funding of the arsenic treatment system project. The grant was originally approved by the Board for Financing Water Projects on December 13, 2008, for an amount not to exceed $4,000,000 (approximately 38.75% of eligible project costs originally estimated to be $10,323,000). The scope of this project was to install two arsenic removal plants and pipeline between Jones and Baldwin Springs. Match funding is provided through the State of Nevada Revolving Loan Fund and the U S Department of Agriculture - Rural Development.
PROJECT STATUS
The project was bid in January 2008 and awarded to Wiser Construction. In December 2008, the Board for Financing Water Projects made a site visit to the Moapa Valley Water District project sites. The Arrow Canyon arsenic treatment facility was completed and place in service in January 2009. The Baldwin Springs arsenic treatment facility came on-line in April 2009.
Go to project photo page for the Arrow Canyon arsenic treatment facility --> click here
Go to project photo page for the Baldwin Springs arsenic treatment facility --> click here
Board visit to the project site --> click here
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