Groundwater and Wells

A confined aquifer 40 m thick has a specific storage of 1.0 × 10⁻⁵ per meter and a porosity of 0.25. Pumping lowers the potentiometric surface by an average of 5 m over 10 km² while the aquifer stays fully saturated. The volume of water released from storage is most nearly:

A confined aquifer is pumped at 350 gpm. Drawdown at an observation well 150 ft away plots as a straight line on semilog paper with a slope of 4.2 ft per log cycle, and the line crosses zero drawdown at 1.5 minutes. Use T = 2.3 Q / (4 pi x slope), S = 2.25 T t0 / r² and 7.48 gal per ft3. The storativity is most nearly:

A sand aquifer 150 m wide and 12 m thick has a hydraulic conductivity of 20 m/day and an effective porosity of 0.30. Hydraulic heads measured 700 m apart along the flow direction are 48.6 m and 46.5 m. The groundwater flow through the section is most nearly:

Two observation wells 800 ft apart along the flow direction in a confined aquifer show heads of 352.4 ft and 348.8 ft. The aquifer is 35 ft thick and 1,200 ft wide, with a hydraulic conductivity of 600 gpd/ft2. The flow through the aquifer is most nearly:

Darcy's law assumes laminar flow with velocity proportional to the hydraulic gradient. In which setting is the law least likely to describe the flow accurately?

For seepage under a sheet-pile cutoff, the head loss is 18 ft and the flow net has 12 equipotential drops of equal head loss. The last flow net square at the downstream exit has a length of 2.0 ft along the flow line. The sand has a saturated unit weight of 125 lb/ft3; water weighs 62.4 lb/ft3. The flow net has 4 flow channels. The factor of safety against heave (critical gradient over exit gradient) is most nearly:

A flow net drawn for seepage under a 250 ft long concrete dam on a sand foundation has 4 flow channels and 12 equipotential drops. The headwater is 30 ft above the base and the tailwater is 12 ft above the base. The foundation sand has a hydraulic conductivity of 2.5 × 10⁻⁵ ft/s. The total seepage under the dam is most nearly:

A plume front is 500 m upgradient of a supply well. The aquifer has a hydraulic conductivity of 15 m/day, a hydraulic gradient of 0.003, a total porosity of 0.35 and an effective porosity of 0.25. Ignoring dispersion and retardation and using 365 days per year, the time for the front to reach the well by advection is most nearly:

A production well has a static water level 85 ft below the ground surface. After pumping at 420 gpm long enough for the level to stabilize, the pumping water level is 117 ft below the surface. The specific capacity of the well is most nearly:

Two wells pump at the same rate for the same time from aquifers with the same transmissivity. One aquifer is confined and the other is unconfined. The cone of depression around the confined well extends much farther. The main reason is that the confined aquifer:

A well fully penetrates an unconfined aquifer with a saturated thickness of 30 m (measured from the horizontal impermeable base) and a hydraulic conductivity of 25 m/day. At steady state, observation wells 20 m and 80 m away show drawdowns of 2.8 m and 1.2 m. The pumping rate is most nearly:

A confined sand aquifer is 60 ft thick and has a hydraulic conductivity of 45 ft/day. Use 7.48 gal per ft3. Its transmissivity is most nearly:

A step test shows that a well pumping 600 gpm has 24.0 ft of drawdown inside the casing. The aquifer properties from an observation well predict 18.0 ft of drawdown at the well's outer radius from aquifer loss alone. Taking well efficiency as the aquifer-loss drawdown over the measured drawdown, the efficiency is most nearly:

Two wells 400 ft apart each pump 300 gpm from a confined aquifer with a transmissivity of 2,000 ft2/day. Apply the steady Thiem relation s = Q ln(R/r) / (2 pi T) for each well, with a radius of influence R = 1,500 ft, and 7.48 gal per ft3. By superposition, the total drawdown at the point midway between the wells is most nearly:

Over six years, a municipal well's specific capacity has fallen from 25 to 14 gpm/ft. The static water level and the levels in nearby observation wells have not changed, and the pump delivers its rated flow. The most likely cause is: