Conceptual Models for Migration of Key Groundwater Contaminants Through the Vadose Zone and Into the Upper Unconfined Aquifer Below the B-Complex

Abstract

The B-Complex contains three major crib and trench disposal sites and three single-shell tank farms that have released nearly 346 million-liters of waste liquids containing the following groundwater contaminants: ~14,000 kg of cyanide, 29,000 kg of chromium, 12,000 kg of uranium and 145 Ci of technetium-99. After a thorough review of available vadose zone sediment and pore water data, groundwater plume information, field gamma logging results, and field electrical resistivity information, conceptual models were developed for which waste sites have been the significant sources of the contaminants in the groundwater. This included estimating the masses of these contaminants remaining in the vadose zone and currently present in the groundwater in comparison to the totals released. This allowed mass balance calculations to be made on how consistent our knowledge is on the deep vadose zone and groundwater distribution of contaminants. Strengths and weaknesses of the conceptual models are discussed as well as implications on future groundwater and deep vadose zone remediation alternatives. The hypothesized conceptual models attribute the source of all of the cyanide and most of the technetium-99 currently in the groundwater to the BY Cribs. The source of the uranium is the 1951 BX-102 tank overfill event and the source of most of the chromium is the B-7-A&B Cribs and B-8 Crib and Tile Field. Our mass-balance estimates suggest that there are much larger masses of uranium, and technetium-99 remaining in the deep vadose zone within ~20 ft of the water table than is currently in the groundwater plumes below the B-Complex. This hypothesis needs to be carefully considered before future remediation efforts are chosen. The masses of these contaminants in the groundwater plumes have been increasing over the last decade, and the groundwater plumes are migrating to the northwest towards the Gable Gap. The groundwater flow rate appears to fluctuate in response to seasonal changes in hydraulic gradient. The flux of contaminants out of the deep vadose zone from the three proposed sources also appears to be transient such that the evolution of the contaminant plumes is transient. PNNL-19277 v Executive SummaryThis report offers detailed conceptual models for the distribution of two key groundwater contaminants, technetium-99 and uranium, and in less detail addresses cyanide, nitrate, and chromium within the B-Complex region. The current distribution of technetium-99, uranium, and cyanide in the vadose zone are evaluated. The conceptual models identify the most likely sources (disposal facilities or single-shell tanks) and hypothesize the migration pathways that the contaminants took to reach the water table at evaluated locations. In addition, the report provides estimates for the activity of technetium-99 and mass of uranium that still remain in the vadose zone (from ground surface to the water table; with the deep vadose zone portion highlighted) and within the current groundwater plumes. The mass of cyanide in groun...

Hydrogeologic Model for the Gable Gap Area, Hanford Site

“…These core samples were described in detail within the laboratory to more accurately evaluate the types and vertical distribution of contaminants within the vadose zone (Brown et al 2007;Lindenmeier et al 2002Lindenmeier et al , 2003 2.7 Serne et al 2002Serne et al , 2003Serne et al , 2010. Much of what researchers know about the intact character of the postbasalt sediments was derived from these recent, more-detailed characterization studies.…”

“…Figure 2.5 shows the distribution of two contaminant plumes (uranium and 99 Tc) in the vicinity of Gable Gap. The plumes appear to emanate from waste facilities associated with the B-BX-BY Tank Farms (Serne et al 2010), and move in a northwesterly direction via at least one paleochannel across a buried divide in the basalt bedrock.…”

“…Within the vadose zone, movement of moisture and liquid effluent from the 200 East Area is strongly influenced by anisotropic sedimentary layering within the Hanford formation and the underlying Cold Creek unit (Serne et al 2010). This includes a sizable, elongated, and streamlined mound of CCUz, up to 55 ft (17 m) thick, ½ mile (0.8 km) wide, and up to 2 mi (3.2 km) long that extends northwestward into Gable Gap from the northern 200 East Area (see Figures 4.7, B.5, and C.6).…”

“…Groundwater beneath the 200 Areas flows east to southeast; however, some groundwater also appears to flow northwestward through Gable Gap (DOE/RL 2010). This pathway is indicated by groundwater-contaminant plumes (uranium and technetium-99) that extend northwestward from the presumed source in the 200 East Area (Serne et al 2010). As such, Gable Gap may represent a potential flow path to the Columbia River for some groundwater contaminants on the Hanford Site.…”

“…• Appendix E -Borehole Information and Tops of Stratigraphic Unit Contacts Numerous studies describing the hydrogeology of the area create the foundation from which the conceptual model has evolved (e.g., Tallman et al 1979;DOE 1988;Last et al 1989;Hoffman et al 1992;WHC 1992;Lindsey 1995;Williams et al 2000;Wood et al 2000;Reidel and Chamness 2007;and Serne et al 2010). Previous investigations focusing more specifically on Gable Gap include surface geologic mapping (Fecht 1978;Myers et al 1979), subsurface geophysical investigations (Holmes and Mitchell 1981;Ault 1981, Repasky et al 2009, and aquifer intercommunication studies between the unconfined aquifer and the Rattlesnake Ridge interbed (Strait and Moore 1982;Graham et al 1984;Jensen 1987;Spane and Webber 1995).…”