Relation of hydrogeologic characteristics to distribution of radioactivity in ground water, Newark Basin, New Jersey

Abstract

The distribution of radioactivity in ground water in the Newark Basin is controlled by the lithology of the aquifer and the degree of contact between the water that flows through the fractured strata and the radioactive lithologic units. The primary water-bearing lithologic units of the Newark Basin that contain elevated levels of radioactivity are the arkosic sandstones of the Stockton Formation and the black mudstones of the lower part of the Passaic Formation. Lithologic and hydrogeologic characteristics of these rock aquifers that affect the spatial distribution of radionuclides in the water include the concentration of radioactive minerals in the strata; the continuity of radioactive strata; the orientation, depth, interconnectedness, and continuity of regional and local fracture patterns; and the geochemical environment, which affects radionuclide solubility. The stratigraphic zones of uranium enrichment and the continuity of the radioactive strata differ between the Stockton Formation and the lower part of the Passaic Formation. Uranium enrichment in both formations, however, is identified in permeable sections of strata that have sharp contacts with, or that are enclosed by, less permeable zones. In the Stockton Formation, basal conglomerates overlying fine-grained mudstones are the most radioactive lithology. In interbedded red and black mudstones of the lower part of the Passaic Formation, the uranium-rich zones are the black and white carbonate-rich siltstone and mudstone laminae overlying massive fine-grained mudstones. Most samples of the arkosic sandstones of the Stockton Formation tended to be slightly enriched in uranium relative to thorium, whereas all samples of the black mudstones of the lower part of the Passaic Formation were heavily enriched in uranium with respect to thorium, with a maximum concentration of uranium of 6,200 parts per million (ppm). Spatial orientation of radioactive zones is controlled by the depositional environment of the host lithology. Radioactive strata in the arkosic sandstones of the Stockton Formation, which is deltaic or fluvial/deltaic in origin, arc stratigraphically discontinuous and nonplanar. In contrast, the radioactive lacustrine black mudstones of the lower part of the Passaic Formation are laterally continuous and planar. Geophysical logs of boreholes indicate that the number and depths of probable waterbearing fractures are greater in the arkoses of the Stockton Formation and red mudstones of the lower part of the Passaic Formation than in the interbedded red and black mudstones of the lower part of the Passaic Formation, despite the fact that fracture-density counts in cores indicate that black mudstones are more highly fractured than the other rock types. The total number of fractures is greater and bedding-plane fractures are generally more common in the lower part of the Passaic Formation than in the Stockton Formation. Fractures in the Stockton Formation form a well-connected network consisting of both bedding-plane and several sets of high-angle fractures present on a regional scale. The large-scale continuity of interconnected bedding-plane fractures and high-angle regional fractures may be partly responsible for the higher yields observed for wells in the Stockton Formation than for wells in the Passaic Formation and may facilitate migration of dissolved radionuclides far from their source. The well-connected fracture network may increase the likelihood that circulating water comes in contact with radioactive strata. Water in the black mudstones of the lower part of the Passaic Formation may also contain high levels of radioactivity, primarily as a result of the high frequency of radioactive strata and of fracturing, rather than the presence of a well-connected regional fracture system. Ground-water chemistry is dominated by the calcium cation and the bicarbonate anion, both derived from abundant secondary calcite in the fractures. The concentrations of uranium and radium in water that comes into contact with radioactive rock are controlled by the water chemistry. Uranium is the dominant alpha emitter in the oxidizing waters in the red mudstones of the Passaic Formation, whereas radium-226 (226Ra) is the dominant alpha emitter in the suboxic (reducing) water in the black mudstones of the lower part of the Passaic Formation. Because the lithology of the Stockton Formation and, in places, the lower part of the Passaic Formation varies greatly, wells can intercept multiple fracture zones that can contain oxic or suboxic waters, thereby resulting in the presence of both uranium and radium in detectable quantities in the same water sample.