George H. Davis

Application of environmental isotope tracers to modeling in hydrology

Advances in nuclear science and computer development have made practicable chemical-transport models employing the environmental isotopes found in natural waters. The heavy isotopes of hydrogen, deuterium (stable) and tritium (radioactive), forming part of the water molecule, are the most conservative tracers of water known. The heavy stable oxygen isotope, 18O, circulates in tandem with deuterium in the hydrologic cycle. Tritium fallout varies seasonally and annually in a systematic fashion, and due to its property of radioactive decay is highly useful for dating time of recharge of soil moisture and groundwater. 14C is valuable for tracing old groundwater owing to its relatively long half-life of 5568 yr., but detailed knowledge of the hydrochemistry of the system is required for successful modeling. Deuterium and 18O are of principal value because they reflect the temperature at which precipitation condensed. Thus, they show pronounced seasonal variability and also the effects of altitude, latitude, distance from the sea, amount of precipitation, history of evaporation, and long-period climatic cycles. Principal areas of application of environmental isotopes to hydrologic modeling have been in soil-moisture transport, modeling of mixing and evapotranspiration in large lakes and swamps, glacial-ice movement, groundwater flow, and transport of contaminants in groundwater.

Land subsidence and sea level rise on the Atlantic Coastal Plain of the United States

Land subsidence due to decline in head in confined aquifers, related to municipal and industrial water pumpage, is widespread in the Atlantic Coastal Plain. Although not a major engineering problem, subsidence greatly complicates adjustment of precise leveling and distorts prediction of future sea-level rise. When preconsolidation stress equivalent to about 20 m of head decline is exceeded compaction of fine-grained sediments of the aquifer system begins, and continues until a new head equilibrium is attained between fine and coarse units. The ratio subsidence/head decline is quite consistent, ranging from 0.0064 in southeastern Virginia to 0.0018 at Dover, Delaware and Atlantic City, New Jersey. Higher values are related to the occurrence of montmorillonite as the predominant clay mineral present. Review of tide gauge records indicates that gauges not affected by land subsidence or other local secular effects have been sinking relative to sea level since 1940 at rates averaging about 2.5 mm/yr, of which 0.6 mm/yr is ascribed to glacio-isostatic adjustment to unloading of North America resulting from melting of late Pleistocene glaciers, and about 0.9 mm/yr is ascribed to steric sea-level rise related to ocean warming. The residual 1 mm/yr of relative sea-level rise is not well understood, but may be related to regional tectonic subsidence of the Atlantic coast.