Ca2+ and/or Mg2+ reduce the negativity of the cell surface charge,
resulting in a decrease in the activity of UO2
2+ (and/or UO2OH+) at
the plant/water interface (boundary layer), and consequently, less U
bound to physiologically active cell surface sites. In the absence of a
BLM for U, the results of this study (together with previous work)
reinforce the need for a more flexible, hardness-dependent, U guideline
for the protection of selected freshwater biota (such as plants and
invertebrates, where water hardness is known to decrease U toxicity).
Perhaps there is scope to combine this with the recently developed
DOC-dependent algorithm for U.