Probably soil. And from soil- hay and other animal feeds and from meat/dairy products - ingestion.
From Wikipedia:
Together with caesium isotopes 134Cs, 137Cs, and iodine isotope 131I it was among the most important isotopes regarding health impacts after the Chernobyl disaster.
Strontium-90 is a “bone seeker” that exhibits biochemical behavior similar to calcium, the next lighter Group 2 element. After entering the organism, most often by ingestion with contaminated food or water, about 70–80% of the dose gets excreted. Virtually all remaining strontium-90 is deposited in bones and bone marrow, with the remaining 1% remaining in blood and soft tissues. Its presence in bones can cause bone cancer, cancer of nearby tissues, and leukemia.
OK, you should be aware that there isn’t good evidence linking Chernobyl with Leukemia increase!
This is why many people think danger from Strontium 90 is badly overrated, while perhaps danger radioactive iodine was underestimated.
http://www.ncbi.nlm.nih.gov/pubmed/18049227
The accident at the Chernobyl Nuclear Power Plant in Ukraine in 1986 led to a substantial increase of thyroid cancer among those exposed as children. The other cancer that is the most sensitive to the effects of ionizing radiation is leukemia, and this paper evaluates the evidence relating exposure to Chernobyl radioactivity and leukemia risk. Two types of objectives are identified, namely, scientific evidence and public health, and two approaches to addressing such objectives are discussed. Empirical studies in affected populations are summarized, and it is concluded that, possibly apart from Russian cleanup workers, no meaningful evidence of any statistical association between exposure and leukemia risk as yet exists.