The concentration s(x, t) of pollutants, in a one-dimensional reservoir at position x and time t satisfies the diffusion equation

$${{\partial s(x,t)} \over {\partial t}} = D{{{\partial ^2}s(x,t)} \over {\partial {x^2}}}$$

on the domain 0 $$\le$$ x $$\le$$ L, where D is the diffusion coefficient of the pollutants. The initial condition s(x, 0) is defined by the step-function shown in the figure.

The boundary conditions of the problem are given by $${{\partial s(x,t)} \over {\partial x}}$$ = 0 at the boundary points x = 0 and x = L at all times. Consider D = 0.1 m²/s, s₀ = 5 $$\mu$$mol/m and L = 10 m. The steady-state concentration $$\overline s \left( {{L \over 2}} \right) = s\left( {{L \over 2},\infty } \right)$$ at the center x = $${{L \over 2}}$$ of the reservoir (in $$\mu$$mol/m) is __________. (in integer)