Planning for Compliance with the New Chromium-6 MCL


  • Nicole Blute, Eric Tambini, Lynn Grijalva, Joe DeMaggio, Trey Driscoll, and Chris Dahlstrom

Hazen and Sawyer has conducted several studies and developed tools to help utlities select a Cr(VI) treatment strategy.

The California Division of Drinking Water released a final MCL for hexavalent chromium (chromium-6) of 10 parts per billion (ppb) effective July 1, 2014. The total chromium (total Cr) MCL remains at 50 ppb. Santa Ynez River Water Conservation District, Irrigation District #1 (District) relies on a combination of groundwater wells (Upland basin and Riverbed basin) and treated surface water. Each of these supplies is subject to potential stressors, making a diverse portfolio necessary for risk management. For example, the riverbed groundwater wells have been destroyed in the past by flood events, requiring redrilling. The treated surface water originates from the State Water Project, which has been impacted by the persistent drought. Finally, half of the District’s Upland groundwater wells are near or above the new chromium-6 MCL.

Alternatives for complying with the new chromium-6 regulation were developed by the District in consultation with the team of engineers, hydrologists, and hydraulic modelers. Three primary goals for the District were identified and each alternative assessed with respect to the District’s ability to:

1. Comply with the water quality regulations, with high assurance and low risk
2. Meet current water production reliably, and look for opportunities to increase production
3. Minimize the cost impact to District customers

Non-treatment and treatment alternatives were evaluated. Non-treatment approaches included blending with other wells and distribution system water to decrease chromium-6 concentrations in well water near or above the MCL, infrastructure improvements to better use Riverbed wells or surface water sources if impacted wells were shut off, development of a separate irrigation distribution system for agriculture, and installation of well packers to isolate lower-chromium zones in the aquifer. Best available treatment technologies were evaluated to assess applicability of each to the District, focusing on water quality, residuals disposal, operations requirements, and cost. Alternatives were then assembled into “complete options” reflecting the combination of alternatives necessary to provide the water portfolio needed by the District.

This presentation will report on the methodology that an agency can undertake to lay out the array of alternatives, assemble complete options, and weigh decision factors important to the agency and its customers. Several levels of analysis are necessary to reach the end goal. For example, technologies were evaluated on the basis of seven criteria important to the District – treatment robustness, O&M complexity, water loss, residuals handling, removal of other constituents, footprint, and annualized cost. At the higher level, complete options were assessed based on water quality compliance assurance, water production reliability, and cost. The outcome of this project was a logical assessment of all reasonable options, which was presented to the District’s Board of Directors and communicated to the public. Work continues on implementation of the selected option.

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