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This study used dissolved organic carbon (DOC) carbon-14 (14C) to determine groundwater ages that were then used to calculate groundwater travel times in southern Nevada aquifers that have low organic content. These travel times are compared with the more standard dissolved inorganic (DIC) 14C method for determining groundwater ages and travel times. Groundwater travel times for aquifers in southern Nevada using DOC 14C are thousands of years shorter than DIC 14C travel times for groundwater flow outside of recharge areas along four of the five flow paths evaluated for this study. The DOC 14C travel times range from 2,300 to 2,900 years (yrs) as compared to DIC 14C travel times that range from 8,200 to 22,000 yrs (uncorrected ages) and 4,700 to 19,000 yrs (corrected ages). The DOC 14C groundwater travel times in carbonate-rock and volcanic-rock aquifers of southern Nevada are similar to travel times determined from: (1) hydrogeologic data; (2) observations of rapid high-level tritium transport at the Nevada National Security Site; and (3) 550,000-yr δ18O and δ13C global climate records for calcite precipitated in Devils Hole, Nevada at the end of one of the flow paths. DOC 14C travel time calculations need to account for fewer processes than DIC 14C for aquifers that contain little organic matter and do not have redox reactions. DOC 14C travel-times can be calculated directly from the DOC 14C data in these aquifers without corrections if dissolution of organic carbon and sorption and matrix diffusion of DOC 14C onto or into the aquifer matrix is minimal. Laboratory experiments showed that little organic carbon was being leached from aquifer rocks, sorption of organic carbon ranged from 4.3% sorbed in carbonate rocks to 0.5% sorbed in volcanic rocks, and matrix diffusion coefficients were slower in lower porosity carbonate rocks (1.7 × 10−7 cm2/s) than in higher porosity volcanic rocks (2.9 × 10−7 cm2/s). The lack of dissolution of organic carbon in study area aquifers is also supported by the decrease in DOC along flow paths and the DOC composition of groundwater changing little as groundwater flows from recharge areas into the adjacent valleys. In contrast, DIC 14C groundwater travel-time calculations must be corrected for complex chemical reactions and physical processes, including mineral/gas dissolution, mineral/gas precipitation/exsolution, cation exchange, and carbon isotopic exchange that can significantly change the amount of DIC 14C in groundwater along flow paths by processes other than radioactive decay. DIC 14C can also be affected by sorption and matrix diffusion, but these processes may, or may not, be captured in geochemical modeling of precipitation/dissolution and carbon isotopic exchange reactions. Corrected DIC 14C ages, therefore, represent maximum ages.

The Quaternary coastal plain aquifer down gradient of the Wadi Watir catchment is the main source of potable groundwater in the arid region of south Sinai, Egypt. The scarcity of rainfall over the last decade, combined with high groundwater pumping rates, have resulted in water-quality degradation in the main well field and in wells along the coast. Understanding the sources of groundwater salinization and amount of average annual recharge is critical for developing sustainable groundwater management strategies for the long-term prevention of groundwater quality deterioration. A combination of geochemistry, conservative ions (Cl and Br), and isotopic tracers ( 87/86 Sr, δ 81 Br, δ 37 Cl), in conjunction with groundwater modeling, is an effective method to assess and manage groundwater resources in the Wadi Watir delta aquifers. High groundwater salinity, including high Cl and Br concentrations, is recorded inland in the deep drilled wells located in the main well field and in wells along the coast. The range of Cl/Br ratios for shallow and deep groundwaters in the delta (∼50–97) fall between the end member values of the recharge water that comes from the up gradient watershed, and evaporated seawater of marine origin, which is significantly different than the ratio in modern seawater (228). The 87/86 Sr and δ 81 Br isotopic values were higher in the recharge water (0.70,723 87/86 Sr 81 Br 87/86 Sr 81 Br 37 Cl isotopic values were lower in the recharge water (−0.48 37 Cl 37 Cl A three-dimensional, variable-density, flow-and-transport SEAWAT model was developed using groundwater isotopes ( 87 Sr/ 86 Sr, δ 37 Cl and δ 81 Br) and calibrated using historical records of groundwater level and salinity. δ 18 O was used to normalize the evaporative effect on shallow groundwater salinity for model calibration. The model shows how groundwater salinity and hydrologic data can be used in SEAWAT to understand recharge mechanisms, estimate groundwater recharge rates, and simulate the upwelling of deep saline groundwater and seawater intrusion. The model indicates that most of the groundwater recharge occurs near the outlet of the main channel. Average annual recharge to delta alluvial aquifers for 1982 to 2009 is estimated to be 2.16 × 10 6 m 3 /yr. The main factors that control groundwater salinity are overpumping and recharge availability.

Fluoride is considered beneficial to teeth and bones when consumed in low concentrations, but at elevated concentrations it can cause dental and skeletal fluorosis. Most fluoride-related health problems occur in poor, rural communities of the developing world where groundwater fluoride concentrations are high and the primary sources of drinking water are from community hand-pump borehole drilled wells. One solution to drinking high fluoride water is to attach a simple de-fluoridation filter to the hand-pump; and indigenous materials have been recommended as low-cost sorbents for use in these filters. In an effort to develop an effective, inexpensive, and low-maintenance de-fluoridation filter for a high fluoride region in rural northern Ghana, this study conducted batch fluoride adsorption experiments and potentiometric titrations to investigate the effectiveness of indigenous laterite and bauxite as sorbents for fluoride removal. It also determined the physical and chemical properties of each sorbent. Their properties and the experimental results, including fluoride adsorption capacity, were then compared to those of activated alumina, which has been identified as a good sorbent for removing fluoride from drinking water. The results indicate that, of the three sorbents, bauxite has the highest fluoride adsorption capacity per unit area, but is limited by a low specific surface area. When considering fluoride adsorption per unit weight, activated alumina has the highest fluoride adsorption capacity because of its high specific surface area. Activated alumina also adsorbs fluoride well in a wider pH range than bauxite, and particularly laterite. The differences in adsorption capacity are largely due to surface area, pore size, and mineralogy of the sorbent.

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.

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.