The China Lake CSM was built as a decision making tool during the Basewide Hydrogeologic Characterization (BHC). The construction of the CSM resulted in a better understanding of the geology and hydrogeology beneath NAWS China Lake and the surrounding region. The CSM facilitated decision-making during investigations of specific sites at the base, supporting fate and transport evaluations and risk assessments with less need for site-specific data.
The BHC was performed in two phases. Phase I occurred in 1999-2000 and involved:
Phase II occurred in 2001-2002 and involved:
|Site Name||Naval Air Weapons Station (NAWS) China Lake|
|Location||Ridgecrest (Kern County), CA|
|Site Type||Defense Research and Training Installation|
|Project Lead Type||U.S. Navy Lead|
|Regulatory Lead Program||Superfund Remedial|
|Triad Project Status||Field Program Completed|
|Reuse Objective Identified||No|
NAWS China Lake is located in the upper Mojave Desert, 150 miles northeast of Los Angeles. The facility comprises more than a million acres of land with restricted airspace several times that size extending over the surrounding area. The base has operated from 1943 to the present, supporting research and development of Naval aircraft and ordinance, as well as training of Navy staff. NAWS China Lake is currently assessing more than 300 sites, AOCs, and operable units (OUs) under its IRP where releases of contaminants may have occurred.
Developing and refining the CSM led to a fundamental understanding of the hydrogeology across the NAWS China Lake complex. An extensive review of the existing data and literature was used to form a general understanding of the site hydrogeology. The CSM delineated three subsurface zones of interest: the shallow hydrogeologic zone (SHZ), the intermediate hydrogeologic zone (IHZ), and the deep hydrogeologic zone (DHZ). The SHZ is predominantly composed of alluvial deposits. Groundwater in the SHZ occurs under unconfined (water table) conditions. The base of the SHZ is marked by the occurrence of low-permeability lacustrine deposits of the IHZ. These deposits are primarily silts and clays. Water bearing zones within the IHZ generally occur within sand stringers that are interbedded with the low-permeability sediments. The DHZ is primarily composed of coarse sand and gravel deposits. The SHZ is of greatest concern for releases of contaminants from NAWS China Lake activities, but the DHZ is of greatest concern for receptors as a drinking water source for the surrounding towns. (Groundwater from the SHZ and IHZ is not in use.)
The mapping of the SHZ and DHZ potentiometric surfaces and the geologic descriptions from exploratory borelogs was fundamental to the CSM. In conjunction with isotopic analyses (oxygen-18, deuterium, tritium, carbon-14 and strontium-87), the mapping process also indicated groundwater recharge zones and travel times. Groundwater travel times and flow paths from recharge zones in the Sierra Nevada to the municipal pumping well fields in the IWV were assessed along with the locations of good and poor quality water in the region.
Stiff and Piper plots of the major ion geochemistry of the ground and surface waters from IWV were created to evaluate groundwater quality and to distinguish water types based on geochemical characteristics. In addition, isotopic signatures of the SHZ, IHZ, and DHZ were identified by creating scatter plots of the isotopic values versus the total concentration of the parameter or versus the sample elevation. With the signatures of the different hydrologic zones identified, the amount of mixing between zones was evaluated. The influence of DHZ pumping on groundwater flow and quality was further investigated by plotting groundwater elevations against oxygen and deuterium isotopic ratios.
These evaluations found that deeper screen elevations in the DHZ correlated with deuterium values that were more depleted; indicating that groundwater pumping was pulling water from greater depths rather than from the SHZ. In fact, over much of the facility, less permeable zones within the IHZ appeared to insulate the SHZ from the DHZ, and inhibited communication between these two hydrogeologic zones. This finding was significant because most groundwater contamination is located in portions of the SHZ. In essence, the BHC found that this contamination would not reach the DHZ and its associated drinking water receptors.
A detailed geologic and hydrogeologic conceptual site model (CSM) was constructed for NAWS China Lake as part of the BHC. The objectives for development of the CSM were to:
The program was designed to use a network of new and existing monitoring wells and soil borings to support closure of the over 300 sites and areas of concern (AOCs) identified at the facility. The BHC included the collection of data to support contaminant fate and transport evaluations for IRP sites, AOCs, and operable units (OUs). In compiling new as well as historical and literature data for the region, the BHC assessed long-term trends in groundwater quality, flow, and use. Water resources are limited in IWV and demand for water is growing, thus the local community was very supportive of the BHC effort.
The construction of the CSM resulted in a better understanding of the groundwater exposure pathway and evaluation of potential risk to groundwater receptors.
The cost savings using this approach are estimated at 50% relative to traditional methods involving discrete and repetitive geology and hydrogeology investigations at specific IRP sites and AOCs.
In the early stages of the construction of a revised CSM for NAWS China Lake, an extensive literature review was conducted. Geologic, hydrogeologic, structural, and geochemical data were obtained for nearly 2000 existing wells in the area during the literature review. Data were used from nearly 300 of these wells to create maps, cross-sections, and geochemical plots (Stiff and Piper diagrams). Where available, borehole logs were used to create geologic cross-section, structure contour, and isopach maps. A structure contour map was made on the top elevation of a low permeability lacustrine clay that dominates the IHZ, and an isopach map was made of its thickness. The examination of these diagrams and maps helped the project team define the orientation and extent of the three hydrogeologic zones in building the CSM.
The Navy Remedial Project Manager (RPM) was the head of the project team, with oversight from the California Department of Toxic Substances Control (DTSC) and the California Regional Water Quality Control Board (RWQCB). Project management and technical support was provided to the Navy by a contractor, Tetra Tech EM Inc (Tetra Tech). Tetra Tech provided the technical and field teams, including geologists, hydrogeologists, isotope geochemists, and analytical chemists. Other contractors included drilling and laboratory subcontractors.
Although a specific field-based decision logic was not followed for the BHC, the resulting CSM has guided the project team's decisions and actions for multiple subsequent investigations at IRP sites and AOCs. Key decisions made during these investigations include the risk each site poses to groundwater receptors, and whether additional data are necessary to characterize these risks. When additional data are necessary, the CSM can then be used as a tool to plan additional sampling locations and field activities.
As additional data are gathered at NAWS China Lake, the CSM will be further refined, and the presence and characteristics of hydraulic connections between hydrogeologic zones will be further established. Site prioritization of IRP sites will be determined by using the CSM as an interactive and dynamic decision-making tool. Additionally, the CSM process will provide a clear vision on how to most effectively allocate funds for the eventual closure of all IRP sites at the base.
Because of the specialized laboratory methods required, no real-time analytical methods were used for the BHC. Off-site methods are briefly discussed in the fields below. The reader is referred to the Case Study [Case Study Under Development] or the Points of Contact (POCs) for further information.
TQRS not prepared
QA/QC included the analysis of blanks, replicates, and laboratory instrument checks. Geochemical data underwent validation by independent reviewers versus the method standard operating procedures (SOPs).
BHC data were uploaded and managed in the NAWS China Lake basewide database and GIS query station.
Phase I field program: June 1999 - March 2000.
Phase II field program: August 2001 - February 2002.
Additional groundwater monitoring events occurred in March, June, September, and December 2002.
To update this profile, contact Cheryl T. Johnson at Johnson.Cheryl@epa.gov or (703) 603-9045.