Solution Exercise 1

William Robertson

Solution Exercise 1

Answer: yes

Explanation:

Drainfield loading:

q = wastewater loading + net recharge (i.e., precipitation – evapotranspiration)

$\displaystyle q=3.65\frac{m}{yr}+\left ( 0.9\frac{m}{yr}-0.6\frac{m}{yr} \right )=3.95\frac{m}{yr}$

Groundwater will reside under the drainfield for:

$\displaystyle residence\ time\ under\ drainfield=\frac{width\ field}{average\ linear\ velocity}$

$\displaystyle =\frac{30\ m}{100\ \frac{m}{yr}}=0.3\ yr$

Plume thickness at the downgradient edge of the drainfield will be:

$\displaystyle b=\frac{q\ast t}{porosity}$

$\displaystyle b=\frac{3.95\frac{m}{yr}\ 0.3\ yr}{0.3}=3.95\ m \sim 4\ m$

Downgradient from the drainfield the average linear vertical velocity will derive from precipitation recharge:

$\displaystyle average\ linear\ vertical\ velocity=\frac{net\ recharge\ rate}{porosity}$

$\displaystyle =\frac{0.9\frac{m}{yr}-0.6\frac{m}{yr}}{0.3}=1\frac{m}{yr}$

Given the average linear lateral velocity of 100 m/yr, the septic system plume will take 3 years to reach the well.

$\displaystyle time\ to\ reach\ well=\frac{300\ m}{100\frac{m}{yr}}=3\ yr$

Ignoring dispersion, during those three years, the plume would be driven vertically downward by the net recharge. With the average linear vertical velocity of 1 m/yr, the 4-m thick plume from the septic field would occur at a depth of 3 to 7 m below the water table at the well location, and could thus pass through the well screen that spans from 5 to 7 m below the water table.

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