Pesticide is applied to a 10 ha (hectare) field at an average rate of 1 kg/ha every month. The soil is regarded as 20 cm deep and well-mixed. The pesticide evaporates at a rate of 2% of the amount present per day and it degrades microbially with a rate constant of 0.05 day-1. What will be the steady state concentration of pesticide in g/m3 and in unit of g/g, assuming the soil density as 2500 kg/m3.

Question

Pesticide is applied to a 10 ha (hectare) field at an average rate of 1 kg/ha every month. The soil is regarded as 20 cm deep and well-mixed. The pesticide evaporates at a rate of 2% of the amount present per day and it degrades microbially with a rate constant of 0.05 day-1. What will be the steady state concentration of pesticide in g/m3 and in unit of g/g, assuming the soil density as 2500 kg/m3.

Answer 1

To find the steady-state concentration of the pesticide in g/m3 and in units of μg/g, we need to calculate the pesticide concentration in the soil after it reaches equilibrium between application, evaporation, and degradation.

First, let's convert the area from hectares to square meters:
10 hectares = 10 * 10,000 m2 = 100,000 m2

Next, let's convert the depth from centimeters to meters:
20 cm = 20 / 100 m = 0.2 m

Now, we can calculate the total volume of soil in the field:
Volume = area * depth = 100,000 m2 * 0.2 m = 20,000 m3

To determine the amount of pesticide applied each month, we multiply the average rate by the area:
Amount of pesticide applied = average rate * area = 1 kg/ha * 10 ha = 10 kg/month

Since the evaporation rate is given as a percentage per day, we need to convert it to a decimal:
Evaporation rate = 2% = 2/100 = 0.02

Similarly, the microbial degradation rate constant is given as a rate per day.

Now, let's calculate the steady-state concentration of the pesticide.

Step 1: Calculate the amount of pesticide that remains in the soil after each month:
Remaining pesticide = Amount of pesticide applied - Amount evaporated - Amount degraded
Amount evaporated = amount present * evaporation rate
Amount degraded = amount present * degradation rate constant

Step 2: Convert the remaining pesticide amount to concentration in g/m3:
Pesticide concentration = Remaining pesticide / Volume of soil

Lastly, convert the concentration to μg/g by using the given soil density:
μg/g = Pesticide concentration in g/m3 * 1,000,000 μg/g / soil density in g/m3

By using these calculations, you can determine the steady-state concentration of pesticide in g/m3 and in units of μg/g.