* Releases may be closed after they are investigated, and if necessary, cleaned up, and the remaining contaminant levels in soil and groundwater are determined not to cause a further threat to human health or the environment.

The Holbrook Project got underway with an initial public meeting that was held at the Holbrook City Council Chambers on June 23, 2004. Since that time, ADEQ has held meetings with interested property owners in Holbrook to discuss site specific issues. In late October, ADEQ met with consultants working in the area and Holbrook officials to discuss remedial (cleanup) options related specifically to the unique soil and groundwater conditions in Holbrook, and options for the disposal of groundwater that has been cleaned up. ADEQ has assigned a case manager (Chiou Chen) who is dedicated to overseeing the Holbrook Project. In addition, ADEQ has a liaison for the northeastern area of Arizona (Byron James) who lives in St. Johns, and frequently makes trips to Holbrook to meet with local officials and property owners.

Past and Current Activities

LUST Site Information

• If you would like to review the current site status, please refer to the summary table.
• Map of Central Holbrook UST Sites
• Map of North Holbrook UST Sites
• More photos of remedial activities in Holbrook

Groundwater/Soil and Fuel Related Contaminants in Holbrook

Although all aquifers in Arizona are subject to State and Federal drinking water standards, groundwater in the shallow aquifer in downtown Holbrook (along Hopi Drive and Navajo Boulevard) is not typically used as drinking water. The municipal drinking water supply in Holbrook is provided by wells located along McLaws Road on the opposite side (south) of the Little Colorado River.

Although the shallow groundwater is not part of the municipal water supply, human exposure to contaminants can occur from contaminated soil and groundwater through dermal (skin) contact or the inhalation of vapors from volatile contaminants in soil or groundwater. This is a potential risk if a site is excavated during construction, or vapors migrate upward through shallow soils and into buildings. For these reasons, the State has also set cleanup standards for soil.

Gasoline is a complex mixture of chemicals. However, cleanup standards have been set for a few of the more hazardous chemicals, including benzene, toluene, ethylbenzene, and xylenes (BTEX) and gasoline additives such as methyl tertiary butyl ether (MTBE).

Petroleum products, like gasoline and diesel fuel, are called LNAPLs (light non-aqueous phase liquids), and as a pure product, float on top of the groundwater. Not all leaks result in product being released in a manner that promotes a buildup of "free product" that floats on the groundwater. When a leak (release) occurs, the product migrates downward through the soil (unsaturated zone). When the released product reaches the water table, the fuel slowly dissolves into the groundwater, and a plume develops. This plume migrates with the groundwater in the direction of groundwater flow. When groundwater samples are collected, they are analyzed for the presence of chemicals of concern that are typically associated with the product that was released. For example, gasoline is a complex mixture of chemicals. However, cleanup standards have been set for a few of the more hazardous chemicals, including benzene, toluene, ethylbenzene, and xylenes (BTEX) and gasoline additives such as MTBE.

A map of central Holbrook, along old Route 66 shows the general maximum historical distribution of contamination in shallow groundwater. Although many discrete plumes of contaminated groundwater are evident on the map, because many gas stations were operated in close proximity to one another, the plume boundaries are sometimes co-mingled. Over time, and following remediation, these plumes have and will be reduced (shrink). The map shows the maximum extent of plumes as they were defined by drilling. Because the plumes migrate with groundwater, and the flow direction is generally toward the west, the plumes are generally elongated toward the west, with some variation from northwest to southwest, depending on the local direction of groundwater flow.

• Map of central Holbrook
• Map of Hopi Drive plume
• Holbrook plume map legend

A north-south cross-section along Navajo Boulevard and an east-west cross-section along Hopi Drive depict the variation in the type of sediments that occur in the unsaturated zone and in the sediments where groundwater is present. Although the groundwater level is not shown on the cross-sections, between the late 1990s and the present, since most of the monitoring wells were drilled, the depth to groundwater has ranged from 10 to 12 feet below ground surface. The depth to groundwater may have been higher in the past.

• North-east cross-section: Navajo Boulevard
• East-west cross-section: Hopi Drive

Cleanup Technology

Ongoing and approved soil and groundwater cleanup activities in Holbrook include a variety of methods or technologies, such as soil excavation, multi-phase extraction, vapor extraction, free product removal (where present) from the top of the groundwater table, hydrogen peroxide injection, and natural attenuation.

Groundwater is generally shallow along Navajo Boulevard and Hopi Drive (10 to 20 feet below the ground surface). In general, along Navajo Boulevard, the soil above groundwater is composed primarily of clay and silt. Along Hopi Drive, the soil is generally sandier, and is composed of sands, silts, and clays.

Because of the presence of finer grained material at many of the sites, cleanup will require the use of more innovative technologies. One such cleanup technology that is being used in Holbrook is multi-phase extraction. In this technology, contaminated groundwater, free product (if present) and vapors are slowly withdrawn, separated, and treated on the surface.

At some sites, contaminated soil can be excavated and replaced with clean fill. Where the soil is sandier, vapor extraction may be used as the primary technology for removing contaminants from soil. More than one cleanup method is often used to remove contaminants. The effectiveness of each method depends on the nature of the site.

The cleanup work must be technically feasible, reasonable, necessary and cost effective to actively remove contaminants. When "active" cleanup technologies don't meet these criteria, a more "passive" approach can be taken by allowing the existing bacteria in the ground reduce contaminant levels. This is called the natural attenuation process, in which the contaminants act as a food source for the existing bacteria. Natural attenuation can be monitored to make sure contaminants are degrading, and can either be used as a stand alone technology or used after "active" cleanup technologies have removed the majority of the contamination.

If you would like more information about the UST program and the details of corrective action technologies, please visit the US Environmental Protection Agency's Web site .

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