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In Oak Ridge, Tennessee the Department of Energy and its predecessor agencies have had a mission over the past fifty years of uranium enrichment, weapons production, and energy research. As a result of this mission a legacy of hundreds of contaminated sites remains on the Oak Ridge Reservation. The remediation strategy for these sites is based on watershed management.

The Clinch River bounds the Oak Ridge Reservation on three sides, and there are active creeks that flow down the valleys to the River. These surface water systems are fed by runoff from rainfall and by the groundwater that continually discharges to the surface streams. As much as 90% of the water entering the ground flows rapidly through highly porous and shallow soil, which contains most of the contaminated sites, before discharging to nearby surface water. Consequently, the primary pathway for offsite contaminant migration is through shallow groundwater to surface water. Because of abundant rainfall, contaminant transport by shallow subsurface flow to surface waters, and the presence of contaminated sites in defined watersheds, a watershed strategy became the basis for environmental restoration.

Watershed management is an integrated, holistic approach to restore and protect ecosystems and to protect human health by focusing on hydrologically defined drainage basins. Watershed management is applied to the environmental restoration of the Oak Ridge Reservation by grouping contaminated sites into five watersheds. The watersheds are used to:

identify, assess, and prioritize contaminant releases

make remedial decisions, and

evaluate remedial effectiveness

When the watershed strategy first was initiated, existing groundwater, surface water, soil and sediment, and biological data was assembled and reviewed in conjunction with what was known about contaminated sites. From this review it was apparent that there was insufficient data to manage the program. A comprehensive monitoring program was developed that was capable of:

identifying contaminant releases,

understanding contaminant transport pathways and mechanisms,

assessing the relative human health and ecological risk of each contaminant release,

prioritizing releases based on risk and flux, and

developing a baseline for performance assessments of remedial actions

Contaminants released from the contaminated sites accumulate in floodplain soils and aquatic sediments. Contaminants not retained, or those remobilized, are released to the surface waters and subsequently offsite to the Clinch River. Therefore, the surface water acts as an integrator of contaminant flux, and an integration point was identified in each watershed at which contaminant releases can be tracked, assessed, and prioritized.

Remedial decisions reflect tradeoffs among protection of human health and the environment, compliance with environmental standards, and implementation criteria, primarily cost and constructability. A preferred alternative is selected that represents the optimum solution among these factors. For the Oak Ridge Reservation the optimum solution needs to be determined at the watershed scale to ensure that the evaluation considers the cumulative resources needed for cleanup and the resource implications for alternate land uses. The optimum decision for a single contaminated site may not be the same as when other contaminated sites in the same watershed are considered as well. For this reason the optimum decision for each contaminated site is made in the context of the optimum solution for the entire watershed. By focusing on future land use and then the appropriate level of cleanup, an objectives driven program results that remediates a watershed sufficiently to meet the objectives and not to meet a standard.

Once the baseline monitoring and characterization are completed and the cleanup objectives are defined, the contribution of each remedial action toward achieving the objectives can be estimated and assessed at the watershed integration point. Through surface water monitoring both the specific performance of each action and the cumulative progress toward achieving the cleanup objectives can be assessed.

The advantages of watershed management as a basis for the environmental restoration of the Oak Ridge Reservation are:

watersheds are integrators of contaminant releases and are appropriate for their identification, characterization, and prioritization,

remedial action objectives can be defined based on the desired health of the watershed instead of meeting regulations,

remedial action objectives have measurable end points,

remedy selection is consistent and appropriate, and

performance assessment can be accomplished through watershed monitoring
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Document Type: Research Article

Publication date: 01 January 2000

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