Alan H. Welch

Geochemical evolution of ground water in Smith Creek Valley—a hydrologically closed basin in central Nevada, U.S.A.

Abstract Smith Creek Valley is a hydrologically closed basin in which ground water is recharged by subsurface inflow from sorrounding mountains and infiltration of streamflow into alluvial-fan deposits near the mountains. Ground water is discharged by evapotranspiration from shallow ground-water areas in the central part of the basin. Dominant ions in the dilute recharge water are Na, Ca and HCO 3 . Dissolved solids concentration increases during flow through the basin-fill sediments, with Na becoming increasingly dominant. In the discharge area, a bare-soil playa sorrounded by phreatophytic vegetation, ground-water salinity, dominated by Na and Cl, increases markedly. The main processe controlling geochemical evolution of ground water in the basin-fill aquifer were identified using major-ion chemistry, mass-balance calculations, thermodynamic calculations, stable isotopes, and mineral identification. These processes are: (1) dissolution of volcanic tuff and tuff-derived basin-fill deposits; (2) cation exchange of Ca and Mg in the water for Na in clay minerals; (3) weathering of plagioclase to montmorillonite; (4) precipitation of zeolite minerals; (2) concentration of dissolved constituents by evapotranspiration; (6) dissolution of Cl and SO 4 evaporative salts; (7) precipitation of calcite.

Arsenic in Ground Water Supplies of the United States

Publisher Summary High arsenic (As) concentrations in ground water have been documented in many areas of the United States. Within the last decade, parts of Maine, Michigan, Minnesota, South Dakota, Oklahoma, and Wisconsin have been found to have widespread As concentrations exceeding 10 μg/L. These high concentrations most commonly result from the upflow of geothermal water, dissolution of, or desorption from, iron-oxide, and dissolution of sulfide minerals. Because the MCL for As is currently being evaluated, estimating the exceedance frequency for different As concentrations in regulated water supplies is particularly timely. The estimates of the frequency of exceedance, which are based on analyses of about 17,000 ground water samples, suggest that about 40% of both large and small regulated water supplies have As concentration greater than 1 μg/L. The frequency of exceedance decreases for greater As concentrations—about 5% of systems are estimated to have As concentration greater than 20 μg/L. Comparison of these estimates with previously published work, based on 275 samples collected from regulated water supplies, shows very good agreement for the United States as a whole, although the two approaches yield somewhat different results for some parts of the nation.

Radionuclides in ground water of the Carson River Basin, western Nevada and eastern California, U.S.A.

Abstract Ground water is the main source of domestic and public supply in the Carson River Basin. Ground water originates as precipitation primarily in the Sierra Nevada in the western part of Carson and Eagle Valleys, and flows down gradient in the direction of the Carson River through Dayton and Churchill Valleys to a terminal sink in the Carson Desert. Because radionuclides dissolved in ground water can pose a threat to human health, the distribution and sources of several naturally occurring radionuclides that contribute to gross-alpha and gross-beta activities in the study area were investigated. Generally, alpha and beta activities and U concentration increase from the up-gradient to down-gradient hydrographic areas of the Carson River Basin, whereas222Rn concentration decreases. Both226Ra and228Ra concentrations are similar throughout the study area. Alpha and beta activities and U concentration commonly exceed 100 pCi/l in the Carson Desert at the distal end of the flow system. Radon-222 commonly exceeds 2,000 pCi/l in the western part of Carson and Eagle Valleys adjacent to the Sierra Nevada. Radium-226 and228Ra concentrations are Alpha-emitting radionuclides in the ground water originated from the dissolution of U-rich granitic rocks in the Sierra Nevada by CO2, oxygenated water. Dissolution of primary minerals, mainly titanite (sphene) in the granitic rocks, releases U to the water. Dissolved U is probably removed from the water by adsorption on Fe- and Mn-oxide coatings on fracture surfaces and fine-grained sediment, by adsorption on organic matter, and by coprecipitation with Fe and Mn oxides. These coated sediments are transported throughout the basin by fluvial processes. Thus, U is transported as dissolved and adsorbed species. A rise in the water table in the Carson Desert because of irrigation has resulted in the oxidation of U-rich organic matter and dissolution of U-bearing coatings on sediments, producing unusually high U concentration in the ground water. Alpha activity in the ground water is almost entirely from the decay of U dissolved in the water. Beta activity in ground water samples is primarily from the decay of40K dissolved in the water and ingrowth of238U progeny in the sample before analysis. Approximately one-half of the measured beta activity may not be present in ground water in the aquifer, but instead is produced in the sample after collection and before analysis. Potassium-40 is primarily from the dissolution of K-containing minerals, probably K-feldspar and biotite. Radon-222 is primarily from the decay of226Ra in the aquifer materials. Radium in the ground water is thought to be mainly from alpha recoil associated with the decay of Th in the aquifer material. Some Ra may be from dissolution (or desorption) or Ra-rich coatings on sediments.

In situ Arsenic Remediation in a fractured, alkaline aquifer

In situ removal of arsenic from ground water used for water supply has been accomplished in circum-neutral ground water containing high dissolved iron concentrations. In contrast, the ground water at our study site is alkaline, contains measurable dissolved oxygen and little dissolvediron. Because the dissolved iron concentration is low in the basalt aquifer, the iron oxide content of the aquifer would not increase with successive pumping cycles unless iron is added to the injected water. Additionally, the high pH limits adsorption onto iron oxide present in the aquifer. Having the ability to lower arsenic concentrations in high-pH, oxic ground water could have wide application because similar high arsenic ground water is present in many parts of the world.

Gross-beta activity in ground water : natural sources and artifacts of sampling and laboratory analysis

Abstract Gross-beta activity has been used as an indicator of beta-emitting isotopes in water since at least the early 1950s. Originally designed for detection of radioactive releases from nuclear facilities and weapons tests, analysis of gross-beta activity is widely used in studies of naturally occurring radioactivity in ground water. Analyses of about 800 samples from 5 ground-water regions of the United States provide a basis for evaluating the utility of this measurement. The data suggest that measured gross-beta activities are due to (1) long-lived radionuclides in ground water, and (2) ingrowth of beta-emitting radionuclides during holding times between collection of samples and laboratory measurements. Although40K and228Ra appear to be the primary sources of beta activity in ground water, the sum of40K plus228Ra appears to be less than the measured gross-beta activity in most ground-water samples. The difference between the contribution from these radionuclides and gross-beta activity is most pronounced in ground water with gross-beta activities > 10 pCi/L, where these 2 radionuclides account for less than one-half the measured ross-beta activity. One exception is groundwater from the Coastal Plain of New Jersey, where40K plus228Ra generally contribute most of the gross-beta activity. In contrast,40K and228Ra generally contribute most of beta activity in ground water with gross-beta activities The gross-beta technique does not measure all beta activity in ground water. Although3H contributes beta activity to some ground water, it is driven from the sample before counting and therefore is not detected by gross-beta measurements. Beta-emitting radionuclides with half-lives shorter than a few days can decay to low values between sampling and counting. Although little is known about concentrations of most short-lived beta-emitting radionuclides in environmental ground water (water unaffected by direct releases from nuclear facilities and weapons tests), their activities are expected to be low. Ingrowth of beta-emitting radionuclides during sample holding times can contribute to gross-beta activity, particularly in ground water with gross-beta activities > 10 pCi/L. Ingrowth of beta-emitting progeny of238U, specifically234Pa and234Th, contributes much of the measured gross-beta activity in ground water from 4 of the 5 areas studied. Consequently, gross-beta activity measurements commonly overestimate the abundance of beta-emitting radionuclides actually present in ground water. Differing sample holding times before analysis lead to differing amounts of ingrowth of the two progeny. Therefore, holding times can affect observed gross-beta measurements, particularly in ground water with238U activities that are moderate to high compared with the activity of40K plus228Ra. Uncertainties associated with counting efficiencies for beta particles with different energies further complicate the interpretation of gross-beta measurements.

Shallow subsurface temperature surveys in the basin and range province, U.S.A.—I. Review and evaluation

Abstract Temperature surveys at depths of 1–2 m have had varying success in geothermal exploration in the Basin and Range province. The most successful surveys have identified patterns of near-surface thermal-fluid flow within areas of less than 2 km 2 . Results have been less consistent in larger areas where zones of hydrothermal upflow are less well known, nongeothermal perturbing factors are significant and lateral variations in shallow subsurface temperature are small. Nongeothermal perturbations can be minimized by use of mean annual temperatures instead of synoptic temperatures, by physically based simulation of ground temperatures or by statistical modeling.