Abstract We investigated the viability of ground-penetrating radar (GPR) as a method to estimate the quantity of sediment stored behind the Merrimack Village Dam on the Souhegan River in southern New Hampshire. If the predam riverbed can be imaged, the thickness and volume of the reservoir deposit can be calculated without sampling. Such estimates are necessary to plan sediment management after dam deconstruction. In May 2008, we surveyed six cross sections with a Mala Geosciences ProEx 100 MHz GPR. In a related study, topographic surveys were conducted in 2008–2009 to monitor the sediment flux associated with the removal of the Merrimack Village Dam in August 2008. Within a month of the removal, these surveys mapped the predam riverbed in the uppermost cross sections in the former impoundment. We compared these surveys to our interpreted GPR images for one cross section to determine a calibrated velocity for the impounded sand of 0.043 ± 0.020 m/ns. We also estimated the radar velocity of the deposit by analyzing hyperbolic reflections in the GPR images, and found a similar result (0.039 m/ns). Using the calibrated velocity, we estimated a total volume of sediment stored behind the Merrimack Village Dam of 66,900 ± 9900 m 3 , which compares well to a previous estimate (62,000 m3) based on a depth-to-refusal survey. Our findings indicate that GPR is a useful technique for quantifying impounded sediment prior to dam removal in reservoirs containing 1–10 m of sand overlying a coarser predam riverbed, but it may be less effective in settings with finer and/or thicker impounded sediment.

ABSTRACT Sediment dynamics for the Louisiana coast are reviewed and compared to typical water management schedules for manipulated marsh impoundments to determine the potential for sediment deposition in managed marshes. The opportunity for sediment deposition and the availability of sediment for deposition along the Louisiana coast is greatest for sediment-poor regions during the passage of storms (i.e. , cold fronts and tropical storms) and for areas near rivers during spring floods. Manipulated impoundments have been constructed in order to reduce the rate of water exchange between the marsh and adjoining canals as a means to combat the impacts of saltwater intrusion and increased tidal amplitude caused by the extensive network of canals in the coastal zone. The water management schedule (i.e. , water flows and water levels) of these impoundments is usually based on the life cycle requirements of those plant and animal species targeted for management and does not necessarily coincide with the times of peak opportunity and availability of sediment for deposition. Hence, the achievement of management goals (e.g. , the improvement of waterfowl and wildlife habitat) requires that wetland managers balance the short-term benefits of secondary production and the long-term benefits of sediment deposition in a rapidly subsiding environment. This may result in the manager consciously trading one resource for another, such as sediment and freshwater for low water levels and improved habitat for secondary producers, even in the sediment-rich environs of the Atchafalaya delta region. An evaluation of the trade-offs required by impoundment management and recommendations for maximizing sediment capture in the impoundments are presented.

Abstract Groundwater in Hawaii is present as basal water, as dike-impounded water, and as perched water. Basal water, the fresher part of it forming a lens-shaped body floating on saline groundwater, is in dike-free volcanic rocks and in sedimentary rocks. Dike-impounded water is confined to volcanic rocks in eruptive zones. Perched water is present in all rocks and at all altitudes. Most recharge to groundwater is in the wet interior mountains. Therefore, the main areas of recharge are upslope from developed areas where most wastes are disposed of in the subsurface. This natural deterrent, by position of the recharge area, so far has kept much of the groundwater in its pristine state. Deterioration of the groundwater will increase as land development encroaches toward the recharge areas—the degree will depend greatly on the waste-disposal practices. Most natural discharge of groundwater is along the shore, downslope from disposal wells. Contamination of the beaches, rather than the water supply, is the main concern regarding subsurface disposal of wastes in the low areas under present conditions. The success of injection operations depends primarily on injection capacity and fate of the injected waste. To evaluate these factors properly, an understanding must be obtained of local hydrogeologic conditions, hydrodynamics of injection under Ghyben-Herzberg lens conditions, and possible chemical and biologic effects. The hydrogeology of Hawaii is understood fairly well and, where adequate information is not available, it usually is possible to collect these data by careful field investigation. Considerable information is available from other parts of the world on the hydrodynamics of wastewater injection. However, much of this information is not directly applicable to injection in the Hawaiian environment. Particularly troublesome are the complications caused by the extreme heterogeneity of receiving formations and Ghyben-Herzberg lens effects. Likewise, because chemical and geologic reactions depend on the nature of the injected waste, the receiving waters, and the receiving formations, many of the data collected elsewhere are not applicable in Hawaii.