Calculation of true thickness of the bed

35. The total amount of an element dissolved in the Ocean is $10^7$ kg and its removal rate from seawater is $10^5$ g/y. Under steady-state condition, the residence time of the element is

(1) 10^15 years

(2) 10^5 years

(3) 10^2 years

(4) 10^0.2 years

Solution:

Residence time is the average time that a substance remains in solution in seawater.

Residence Time (yrs.) = Total amount of ion in seawater (kg) / Input rate (kg/yr)

where,

Input rate = Avg. ion con. in rivers (kg/km³) x River discharge (km³/yr)

 So, Total amount of ion dissolved in sea water is =$10^7$kg

= $10^{10}$ g
 
Removal rate of sea water is =$10^5$ g/y


Residence Time (yrs.) = $\frac{10^{10}}{10^5}$ g/y


                                      = $10^5$ grams

 
Reference:https://www.e-education.psu.edu/earth540/content/c3_p7.html