Final answer:
Cadmium and nitrates are two substances likely to accumulate in the subsoils of Central Otago, resulting from agricultural practices and environmental contaminants.
Step-by-step explanation:
Substances that are likely to accumulate in subsoils of Central Otago soils include cadmium and nitrates. Cadmium can be found in soil due to the use of phosphate fertilizers or from industrial contamination, leading to its accumulation in the subsoil layers. Nitrates often increase in soil from agricultural runoff, particularly from the use of fertilizers. These nitrates can leach into subsoil and accumulate, potentially leading to groundwater contamination. Both of these substances are of concern due to their environmental impacts and the potential risk to human health.