What is it that some of the most far-reaching groundwater on the Northeast does not lie beneath the land, but beneath the ocean floor, as well?
In the open ocean, out of the U.S. East Coast, scientists have been sampling an extensive expanse of low-salinity water locked under the ocean floor- an underground reservoir that will possibly reach all the way out to offshore New Jersey to Maine. The water lies beneath deposits of marine sediment, and the evidence so far obtained shows that it was deposited under glacial conditions about 20,000 years ago. Assuming that, provided it were as voluminous as estimated in the early calculations, it would be large enough to serve a metropolis need over centuries, a size which compels hydrogeologists to regard the continental shelf as part and parcel of the groundwater narrative, not as a limit to it.
The contemporary emphasis on defining this so-called offshore aquifer is a combination of the old hints and the new instruments. Federal offshore resource surveys that were conducted in the late 1960s and early 1970s accidentally hit fresher water under the seabed by boreholes. The discoveries remained decades long as an anomaly, until scientists started integrating localized drilling with local imaging techniques capable of peering through the walls of individual holes. An example of this is controlled-source electromagnetic sensing, which is based on the difference in conductivity of salt and fresh water; by comparing the conductivity with control values, scientists can map the probable locations of freshwater-bearingediments on a large scale without necessarily drilling everywhere. In one attempt, a towed transmitter-and-receiver was employed that could trace long conductivity profiles to support the existence of submarine freshwater aquifers off New Jersey and off of the Martha’s Vineyard and to demonstrate how difficult it is to estimate total volume without calibration of the signals with physical samples.
The immediate sampling was made the most direct with the three month drilling campaign referred to as the Expedition 501. Scientists working approximately 20 to 30 miles (30 to 50 kilometers) offshore of Massachusetts drilled as deep as approximately 1,300 feet (400 meters) beneath the ocean floor and drilled up approximately 13,200 gallons (50,000 liters) of water in numerous locations. These cores disclosed one important geologic aspect, namely, fresher and more water-laden sediment beneath a saltier period and topped by a relatively waterproof layer of clay and silt. That seal also serves to preserve the seawater on the surface, so that it does not easily intermix with other waters in a downward direction but it also suggests an essential geologic question of how so much fresh water could have been pressed into position in the first instance.
There is a preliminary chemical effort in the direction of an ice-age mechanism. As ice sheets were growing, the sea level fell and the pressure conditions at the ground changed due to the weight and motion of the ice. The water that melts at the base of an ice sheet can be forced down through the sediments with sufficient force to circumvent or exploit the frailty of the superimposed layers and fresh pore water is left confined in the entrapment of the ice when conditions altered. The other possible options considered by researchers are lengthy intervals of low sea level when rainwater can percolate through the exposed shelf sediments, but a glacial origin with a possible rainfall element at the ice front is current.
The indicators of salinity help in emphasizing the fact that the reservoir is not entirely fresh. The salt content at the nearest location of the drills was 1 part to 1,000 and as the water was drilled further, the salt levels increased gradually to the point where they were very low relative the normal seawater. The significance of this gradient is that it foreshadows mixing histories, geometry of flow pathways, and what the system may do in case somebody ever thought of using it as a resource.
The same gradient is also what indicates the inadequacy of mapping. Conductivity-based imaging is susceptible to salinity and porosity, i.e., fresher water in tighter sediments can be compared to saltier water in more open pore structures. The following move would then be less spectacle-oriented and more concerned with limitations: improving the estimates of the pore-space, refining the age models, and studying what the water is dissolving or containing in those other forms of movement. The coastal aquifers are also active chemical reactors, being mixed zones of some locations and sometimes referred to as subterranean estuaries where microbes can dramatically impact the chemistry of the ground water prior to the water ever encountering the sea.
To the engineering-minded, the new lesson is structural, the continent is not terminated by the shore-line. The Northeast shelf seems to hold the water in stratified sediments similarly to onshore aquifers, and the actual issue is to learn the location of that water, its isolation, and the degree to which it is interrelated with terrestrial groundwater systems–all previous to any hidden reservoir being dealt with as anything other than an extraordinary portion of underground structure.
