Polak et al find an average threshold of 9.5 ppt for the negative enantiomer of Geosmin, for the positive enantiomer it's 78 ppt. The range for both enantiomers is between 4 ppt to > 100 ppt across 50 human subjects. Polak, E.H.; Provasi, J. (1992). "Odor sensitivity to geosmin enantiomers". Chemical Senses. 17 (1): 23–26. doi:10.1093/chemse/17.1.23.
H.H. Baek, K.R. Cadwallader (1999) Contribution of Free and Glycosidically Bound Volatile Compounds to the Aroma of Muscadine Grape Juice Journal of Food Science 64 (3), 441–444 doi:10.1111/j.1365-2621.1999.tb15059.x