Geochemical and groundwater flow modeling of multiport-instrumented coreholes (GW-131 through GW-135)

Technical Report · Wed Jul 01 04:00:00 EDT 1992

OSTI ID:10169783

Toran, L E ^[1]; Saunders, J A ^[2]

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1. Oak Ridge National Lab., TN (US)
2. Auburn Univ., AL (US)

Evolution of groundwater from Ca-HCO{sub 3}-type in recharge areas to Na-HCO{sub 3}-type further along flow paths is typically attributed to exchange of Na for Ca on exchange sites of clays. Geochemical modeling indicates that Na-HCO{sub 3} waters can also be produced by aluminosilicate alteration and precipitation of secondary minerals. To test the feasibility of Na-HCO{sub 3} production by silicate alteration on the Oak Ridge Reservation, groundwater samples from multiport samplers at depths of 200 to 1,000 ft were modeled with a reaction path geochemical model. Observed groundwater compositions were reproduced by the model without invoking cation exchange. Secondary minerals precipitated in the model are similar to those present along fractures in the shale host rocks. Residence times of the Na-HCO{sub 3} zone calculated from a particle tracking groundwater flow model indicated this zone is stable after flushing thousands of pore volumes. This flow modeling provides further evidence that the proposed aluminosilicate alteration process is at least in part responsible for the observed groundwater geochemistry because if Na-HCO{sub 3} groundwater was produced by cation exchange, then it should be flushed out by the Ca-Mg-HCO{sub 3} recharge water when exchange sites are filled. Thus, the lower boundary of the Ca-Mg-HCO{sub 3} groundwater may be an indicator of active circulation and important in determining depth of monitoring for contaminants. Evolution of Ca-Mg-SO{sub 4}-type water has been influenced by dedolomitization driven by gypsum dissolution. Secondary barite dissolution may also occur. Deeper Na-Ca-Cl-SO{sub 4} type water may be the result of advective mixing, matrix diffusion, or upward diffusion, or some combination of these in a transition between fresh waters and brine.

• Research Organization: Oak Ridge Y-12 Plant, TN (United States); Oak Ridge National Lab., TN (United States)
• Sponsoring Organization: USDOE, Washington, DC (United States)
• DOE Contract Number: AC05-84OS21400
• OSTI ID: 10169783
• Report Number(s): Y/TS--875; ORNL/GWPO--0004; ON: DE93017259
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
540220
58 GEOSCIENCES
580000
CALCIUM CARBONATES
CHEMICALS MONITORING AND TRANSPORT
CLAYS
DISSOLUTION
DRILL CORES
ENVIRONMENTAL TRANSPORT
FEASIBILITY STUDIES
FLOW MODELS
GEOCHEMISTRY
GEOLOGY
GEOSCIENCES
GROUND WATER
HYDRAULIC CONDUCTIVITY
HYDROLOGY
ION EXCHANGE
SITE CHARACTERIZATION
SODIUM CARBONATES
WATER CHEMISTRY