HIGHLIGHTS OF 2015 BIRDSALL-DREISS LECTURES: SIMPLE MODELING OF LARGE AQUIFERS, VARIABLE-DENSITY GROUNDWATER-FLOW, AND PERMAFROST INTERACTION WITH GROUNDWATER

Some highlights of each of the three 2015 Birdsall-Dreiss lectures are presented.

(1) Simply-structured models are the most-effective means of dealing with uncertainties in large hydrogeologic systems with sparse data. An example is the Nubian aquifer, the world's largest non-renewable groundwater resource, a transboundary aquifer located in Chad, Egypt, Libya, and Sudan. Questions regarding resource fate, equitable use, and adverse impacts of pumping on oases and shallow cross-border wells inspired this Global Environmental Facility (GEF) project led by the International Atomic Energy Agency (IAEA). Simply-structured modeling provides robust answers to these questions, and provides a relatively simple tool that could be adopted, modified and used by water managers in each country.

(2) Small variations in groundwater density can lead to interesting, unexpected flow patterns that depend on whether the density distribution is stable (freshwater over brine), side-by-side (coastal aquifers), or inverted. Inverted density occurs in geothermal areas, sabkhas and salt ponds, as well as in areas of coastal storm-surge or tsunamis. A practical example of seawater overwash on an atoll island demonstrates how model analysis might deal with inherent difficulties in inverted-density systems so that these can be more effectively managed.

(3) There is limited knowledge about the hydrogeology of cold regions because most human population lives in temperate-climate areas, yet 25% of Earth’s land area is underlain by permafrost. Subsurface ice is a flow barrier, so the frozen ground pattern controls surface and subsurface water flows in cold regions, in turn, affecting geochemistry and ecology. Recent work by the U.S. Geological Survey and partners focused on a permafrost region of interior Alaska. Efforts were made to understand ground ice and water flow interactions and to predict hydrologic changes resulting from climate evolution. Permafrost distribution was mapped by airborne geophysical surveys at an unprecedented large scale. A new computer code simulating groundwater flow with heat transport and groundwater freeze/thaw has allowed assessment of subsurface ice-flow interactions for paleoclimate, current day, and future climate-change scenarios.