Search for: All records

Recent advances in marine electromagnetic surveys have allowed geophysicists to interpret and map offshore freshwater resources with unprecedented resolution and to test inferences regarding onshore-offshore hydrologic connections. To date, however, little is known about the timing or isotopic composition of this unconventional water resource. Here, we reconstructed the Pleistocene hydrogeology of the U.S. Atlantic continental shelf using a cross-sectional paleo-hydrogeologic model to explore possible mechanisms and timing of freshwater emplacement offshore Martha’s Vineyard, Massachusetts. We considered two scenarios in which the Laurentide ice sheet extended different distances offshore, and a third scenario without any ice sheet. The hydrostratigraphic framework was constructed by integrating borehole lithology data, seismic data, and formation resistivity data. Model results were compared to formation resistivity data as well as borehole salinity, groundwater residence time, and stable isotope profiles. Neither of the ice-sheet scenarios provided a significantly better fit to the onshore isotopic and offshore salinity observations than the third scenario. All three model scenarios predicted freshwater emplacement within Tertiary and Cretaceous units. Pleistocene deposits were largely devoid of freshened groundwater. Simulated groundwater residence times for the midshelf region ranged between 104 and 106 yr at depths of <500 m. Simulated groundwater ages from wells completed within Pleistocene confined aquifers are consistent with measured groundwater ages within confined aquifers of Martha’s Vineyard and Nantucket Island (2750−5900 yr). Analysis of onshore 3H/3He dating data indicates that some wells contain a mixture of old and modern (<60 yr) groundwater. Calculated fossil groundwater in the midshelf region that included ice-sheet loading retained relatively low δ18O values, consistent with glacial meltwater recharge. Model results suggest that much of the freshwater emplacement occurred within the last glacial cycle and that the island and offshore hydrogeologic systems appear to be connected. 

Abstract We describe a new automatic seepage meter for use in soft bottom streams and lakes. The meter utilizes a thin‐walled tube that is inserted into the streambed or lakebed. A hole in the side of the tube is fitted with an electric valve. Prior to the test, the valve is open and the water level inside the tube is the same as the water level outside the tube. The test starts with closure of the valve, and the water level inside the tube changes as it moves toward the equilibrium hydraulic head that exists at the bottom of the tube. The time rate of change of the water level immediately after the valve closes is a direct measure of the seepage rate (q). The meter utilizes a precision linear actuator and a conductance circuit to sense the water level to a precision of about ±0.1 mm. The meter can also provide an estimate of vertical hydraulic conductivity (K_v) if data are collected for a characteristic time. The detection limit forqdepends on the vertical hydraulic head gradient. ForK_v = 1 m/day,qof about 2 mm/day can be measured. Results from a laboratory sand tank show excellent agreement between measured and trueq, and results from a field site are similar to values from calculations based on independent measurements ofK_vand vertical head gradients. The meter can provide rapid (30 min)qmeasurements for both gaining and losing systems and complements other methods for quantifying surface water groundwater interactions. 

Abstract We utilized 251 measurements from a recently developed automated seepage meter (ASM) in streambeds in the Nebraska Sand Hills, USA to investigate the small‐scale spatial variability of groundwater seepage flux (q) and the ability of the ASM to estimate mean q at larger scales. Small‐scale spatial variability of q was analyzed in five dense arrays, each covering an area of 13.5–28.0 m²(169 total point measurements). Streambed vertical hydraulic conductivity (K) was also measured. Results provided: (a) high‐resolution contour plots of q and K, (b) anisotropic semi‐variograms demonstrating greater correlation scales of q and K along the stream length than across the stream width, and (c) the number of rows of points (perpendicular to streamflow) needed to represent the groundwater flux of areas up to 28.0 m². The findings suggest that representative streambed measurements are best conducted perpendicular to streamflow to accommodate larger seepage flux heterogeneity in this direction and minimize sampling redundancy. To investigate the ASM's ability to produce accurate mean q at larger scales, seepage meters were deployed in four stream reaches (170–890 m), arranged in three to six transects (three to eight points each) per reach across the channel. In each reach, the mean seepage flux from ASMs was compared to the seepage flux from bromide tracer dilution. Agreement between the two methods indicates the viability of a modest number of seepage meter measurements to determine the overall groundwater flux to the stream and can guide sampling for solutes and environmental tracers.