absence of viable microbes cannot be excluded. Extreme oligotrophic environments are themselves unusual and scientifically interesting.

Ice at the base of the Antarctic ice sheet is not older than 1-2 million years. Thus the maximum separation time between ice deposited at the surface as snowfall and ice melting at the base of the ice sheet is 2 million years. Microbes deposited at the surface of the ice sheet 1-2 million years in the past are, at present, inoculating the subglacial aquatic environment. These aquatic systems are likely to contain materials, derived from the ice and possibly from bedrock and preglacial sediments that, despite the high pressure and low temperature, would allow slow growth of some microbes. A full discussion of the potential biological environment is presented in Chapter 3.

There is considerable uncertainty in the values of the coefficients that govern the partitioning of chemical and biological constituents of subglacial aquatic environments during freeze-on of ice accreting at the bottom of the ice sheet. This uncertainty makes it challenging to determine the chemical and microbial concentrations of water by analyzing chemical and microbial concentrations in ice accreted at the lake surface.

It is possible that contaminants will be introduced into subglacial aquatic environments during exploration and sampling. If these environments constitute a subglacial drainage system that is organized into hydrologic catchments, then potential downstream impacts must be taken into account; contamination introduced near the headwaters of a drainage system will have greater impact than the same level of contamination at sites farther downstream. However, the volume associated with each subglacial environment and the decreased residence times that result from subglacial connectivity may help reduce the impact of contamination.