Jerry F. Ayers

Hydrology of small oceanic islands — Utility of an estimate of recharge inferred from the chloride concentration of the freshwater lenses

Abstract In Bermuda, as in other coastal and oceanic-island environments, rainfall has a significant chloride concentration. As a result, there is a relatively straightforward way of estimating groundwater recharge by considering the Cl− ion as a tracer which is concentrated by evapotranspiration. The Cl− concentration of rainfall in Bermuda is about 15 ppm. That of the freshest part of the largest Ghyben-Herzberg lens is about 60 ppm. Taking the 60 ppm value as an indicator of the Cl− concentration of recharge, the average recharge rate is estimated at 0.25 of the 147-cm/yr. average rainfall, or about 37 cm/yr. This estimate is similar to two other estimates of recharge in Bermuda, each derived from hydrogeologic field data: 1. (1) A 33-cm/yr. estimate inferred from a 2·106-m2 area of diversion in which: (a) outflows (extractions) are 2870 m3/day; and (b) the change in storage is estimated at 1100 m3/day, from the rate of lowering of the water table. 2. (2) An estimate of 35 to 45 cm/yr. resulting from combination of: (a) the ratio of recharge to hydraulic conductivity of the Paget Formation (12·10−6), determined from the configuration of the Ghyben-Herzberg lenses; and (b) the hydraulic conductivity of the Paget Formation (85–100 m/day), estimated from the behavior of the water table. The agreement between the three estimates of recharge indicates that the rather simple and inexpensive technique of calculating recharge from Cl− content of rainfall and fresh groundwater can be a useful addition to the arsenal of techniques by which recharge of small oceanic islands can be evaluated.

Geochemistry of K-rich alkaline lakes, Western Sandhills, Nebraska, USA

Abstract The Alkali Lakes region of the western Sandhills, Nebraska, has lakes that range in composition from freshwater to brine with TDS exceeding 250,000 mg/1. An unusual geochemical feature of these lakes is the conservative behavior of K with concentrations exceeding 1,900 mmoles/kg (86,000 mg/1). The lakes are dominantly Na-K-CO3-(SO4)-(Cl) and Na-K-SO4-CO3-(Cl) waters. Lakes occupy interdunal areas where there is little or no surface runoff. Groundwater primarily from locally derived precipitation is the principle source of water and solutes. This origin for the source water contrasts with closed-basin saline lake complexes, where surface water from the adjacent areas flows directly into the lakes or recharges the groundwater system. The principle geochemical process controlling lake chemistry is evaporative concentration. Other processes are operating, but to a lesser extent; these include mineral precipitation and dissolution and organically mediated sulfate reduction. Geochemical mass-balance modeling indicates distinct differences in the amount of water that is required to be evaporated to produce the observed lake compositions. These differences are related to the groundwater inflow to outflow ratio for individual lakes. This emphasizes that, although evaporation and related processes control the geochemical evolution, the local hydrology of individual lakes regulates the extent to which these processes will proceed.

Shallow seismic refraction used to map the hydrostratigraphy of Nukuoro Atoll, Micronesia

Abstract Results from a shallow seismic-refraction survey on Nukuoro indicate that the distribution of fresh and brackish groundwater on that atoll island is controlled by a three-dimensional mosaic of carbonate facies. Each facies is characterized by a distinct seismic velocity which is dependent on the grain size, composition, and abundance of carbonate cement. The refraction survey further indicates that the upper saturated zone is composed of mostly unconsolidated sediments underlain by highly permeable, well-indurated limestone. The fresh-water lens and associated transition zone of fresh to saline groundwater occur within the upper unconsolidated sediments. Measurements of chloride-ion concentrations in water samples collected from sites across the island indicate an asymmetric fresh-water lens with the thickest part of the lens located near the lagoon shoreline. During the 1983 drought, saline water intruded into the islands central topographical depression where taro is cultivated. The intrusion was caused by tidal pumping (upward movement) of brackish water underlying a reef-flat plate, which forms a confining layer over a significant part of the island.