Most water flows beneath the ice surface a few 10's of cm down into the glacier. In addition to measuring the chemistry of the flowing water, we are also measuring the chemistry of isolated water pockets that are frequently found just beneath the ice surface. They are formed by rock dust absorbing solar radiation, warming, and melting into the ice. Although such cryoconite holes are common to most glaciers world wide, here in the dry valleys they seem to be perennially ice-covered. A cross-section of such a hole is shown in the following figure.
Cryoconite holes appear as clear-ish or dark colored patches on the ice surface as the figure shows.
Measuring the chemistry of cryoconite holes is useful for two reasons. Second, the walls of the melt pools melt back and intersect cryoconite holes. The contents of the holes, both water and sediment, drain into the large melt pools. The attached picture shows Martyn Tranter sampling one of the holes.