Transport of selenium in groundwater at the Kesterson Reservoir in the Central Valley of California was strongly retarded because of chemical reduction and precipitation mediated by microbial activity. Under such conditions, negative correlations were documented between aqueous Se and Fe 2+ , Mn, and HzS. Locally, the presence of oxidizing species, notably O2 and NO3, suppressed this reduction, permitting Se mobilization in the shallow aquifer. Selenate, the dominant and most oxidized form of Se, was in electrochemical disequilibrium with subordinate concentrations of selenite. Normally slow inorganic reduction rates were accelerated by microbial activity which utilizes oxidized chemical species including selenate as electron donors during the oxidation of organic matter. Two stratified redox barriers to selenium migration were documented beneath Kesterson: an underlying shallow anoxic zone underlying most of the pond bottom, characterized by high organic content and sulfate reduction, and a deeper dynamic front established by localized Oz infiltration from the overlying ponds and Fe 2+ release from aquifer materials. The reducing nature of this deeper aquifer ultimately precludes Se transport to regional groundwater. [Long et al., 1990]. The U.S. Bureau of Reclamation and the Lawrence Berkeley Laboratory, University of California, have conducted an extensive groundwater chemical monitoring program over the last 6 years at the Kesterson Reservoir. Detailed data tabulations and programmatic reports have been issued on an ongoing basis [Lawrence Berkeley Laboratory, 1987, 1988]. The present paper only attempts to summarize the chemical data in these studies and focuses primarily on determining significant geochemical processes controlling Se mobility. The paper is presented in the context of a preceding paper in this series which discusses the hydrologic and geologic framework at Kesterson and conservative solute transport [Benson et al., this issue]. The conclusions reached in this study are important for designing remedial A total of over 150 monitoring wells were chemically sampled either on a one-time or periodic basis at Kesterson. The wells, situated principally along berms within the reservoir area and also at adjacent offsite locations (Figure 1), varied in total depth between 3 and 65 m. The monitoring well completion methods have been previously described [Benson et al., this issue]. Interstitial water in the anoxic bottom sediments of the ponds were also sampled in pits dug adjacent to the pond margins. Prior to sampling, approximately three well bore volumes of water were removed from a well using high-capacity vacuum and centrifugal pumps. Final sample collection was made using a peristaltic pump connected to a 0.22-/am acetate filter. Chemical parameters including p H, Eh, alkalinity, dissolved oxygen (DO), sulfide, Fe z+, and total Fe were determined in the field. Eh measurements were made in a flow-through cell using a platinum electrode which was checked against a Zobell solution and polished immed...

Heat flow and surface radioactivity were determined at two sites in the Precambrian shield of south Greenland. At Ivigtut heat flow q from two holes in Ketilidian (•1700-1500 m.y.) gneisses averaged 1.0 ñ 0.1 HFU (•zcal cm -• sec-•), and the radiogenic heat production Ao averaged 5.5 ñ 1.5 HGU (10 -•8 cal cm -3 sec-•). These data are consistent with other pairs of q and Ao values from the Canadian and Australian shields. In four holes near the northwestern edge of the Ilimaussaq alkali intrusion of Gardar (•1000 m.y.) age, however, q averages only 0.9 ñ 0.1 HFU, whereas Ao averages •20 HGU. This observation implies that, unless heat flow from the mantle is unexpectedly low in this part of Greenland, the highly radioactive part of the intrusion is quite thin (•1 km) and the underlying crustal rocks contain very little radioactivity.

A survey of the geochemical literature and unpublished data has resulted in the classification of U, Th, and K concentrations by rock type. Over 2500 data entries have been compiled, permitting calculation of their radiogenic heat production. In the igneous rocks mean heat production ranges from highs of 12‐20 heat production units (HPU: µWm−3) in some peralkaline intrusives, through ∼ 4 HPU in acidic, ∼ 2 in intermediate, and ∼ 1 in basic rocks, to a low of 0.3 HPU in ultramafic rocks. Siliceous clastic rocks generally have greater heat production (2 to 4 HPU) than do chemical sedimentary rocks, including the carbonates (0.4 to 2 HPU). The heat production of metamorphic rocks generally depends on the radioelement contents of their igneous and sedimentary predecessors, modified by metamorphic processes. Based on estimates of the proportion of the continental crust that specific rock types occupy, the weighted mean radiogenic heat production of the upper continental crust estimated from this data base is ∼ 3 HPU.

No abstract

Radioactivity in granitic rocks of the central Sierra Nevada varies both regionally and with rock type. It is lowest in the western foothills and increases eastward to the crest of the range. East of the crest radioactivity is generally lower than in the crestal region, but south of Big Pine Creek it increases eastward from low values in the Inconsolable granodiorite to high values (similar to those along the Sierra crest) in the Tinemaha granodiorite of the Poverty Hills. Within any given area radioactivity varies with silica and the alkalis; it is lowest in diorite and gabbro and progressively higher in quartz diorite, granodiorite, and quartz monzonite. On the west slope of the Sierra Nevada, the isotopic ages of the rocks increase westward, opposite to the direction of increase of radioactivity. This relation and preliminary heat flow values (which indicate a westward decrease in heat flow) are consistent with the concept of a vertically fractionated batholith in which the heat sources were concentrated in the upper parts. According to this concept, the oldest rocks have the lowest heat production because they have been eroded to the deepest levels. This concept fits less well with the relations east of the crest, where the isotopic ages of the granitic rocks are even older than those in the western foothills. The picture may be complicated by an original inhomogeneous distribution of the radioactive minerals in the source rocks from which the granitic magmas were derived. The proportions of U, Th, and K are generally constant within any given pluton and in plutons that are compositionally similar and of the same age, but they may be significantly different in plutons that are compositionally and temporally unrelated. Different proportions of the radioactive elements in three pairs of plutons that had been correlated on the basis of petrographic similarity suggested faulty correlations. Recent geologic work and isotopic age dating have shown that the correlations were incorrect in two of the cases. A limited study in fission‐track autoradiography suggests the U is mainly contained in biotite in rocks rich in biotite and in the non‐magnetic accessory minerals in rocks that contain little or no biotite.