Measurements of radar echo strength have been carried out on Ronne and George VI Ice Shelves, Antarctica. The differences in ice-column absorption between the two sites cannot be resolved by considering ice temperatures alone. We present an absorption model that resolves the discrepancy by considering both the ice temperature and the ice impurity concentrations. In Antarctic coastal sites the concentrations of acid and sea salt impurities, both of which affect absorption, have a marked spatial variability and can dominate the absorption budget. Using the absorption model, impurity levels in the ice column at the George VI site are deduced from the radar strength measurements and found to be typical of levels measured in Antarctic ice at similar elevations and distances from the sea. In particular the measurements suggest that the concentrations of biogenically-derived acids are much lower than the unusually high levels found on the east coast of the Antarctic Peninsula.