Three-Point Problem at a Landfill
A landfill sits on 25 m of unconsolidated sand and gravel over granite. A consultant installs three monitoring wells screened in the same unconfined aquifer: MW-1 at the northwest corner of the site, MW-2 600 m due east of MW-1, and MW-3 500 m due south of MW-1. Water-table elevations surveyed on the same day in a dry month are MW-1 = 43.4 m, MW-2 = 41.2 m, and MW-3 = 42.6 m. The unlined waste cell lies about 400 m southeast of MW-1. A stream is 1.2 km east of MW-1 and flows south. The town proposes an irrigation supply well screened in the same unit, 300 m due east of MW-1, pumping about 1,500 cubic meters per day, and asks the consultant whether the well will affect the landfill plume. The consultant's one-page memo states that the plume moves due south because MW-2 has the lowest water level. The town's peer reviewer disagrees.
1. Determine the direction and magnitude of the horizontal hydraulic gradient and the direction of groundwater flow. Show your components and state your assumptions.
Read the worked answer
Set x positive east and y positive north, with MW-1 at the origin.
East-west component: (41.2 - 43.4) / 600 m = -3.67 x 10^-3 m/m. Head falls to the east. North-south component: (43.4 - 42.6) / 500 m = +1.60 x 10^-3 m/m. Head falls to the south. Magnitude: square root of (3.67^2 + 1.60^2) x 10^-3 = 4.0 x 10^-3 m/m, that is 4.0 m of head per km. That is a steep gradient, plausible for a shallow sandy aquifer.
Groundwater moves opposite the gradient, so flow is east and south. The angle south of east is arctan(1.60/3.67) = 23.6 degrees, giving an azimuth of about 114 degrees - east-southeast.
Assumptions: steady state, the same measurement date, a homogeneous isotropic aquifer, negligible vertical flow, and a water table that is a subdued copy of the potentiometric surface. The consultant is wrong: one low well does not define a flow direction, because two independent components are needed. Flow is southeast, not due south, and the landfill lies downgradient.
2. Predict how the proposed pumping well would change the flow field, and assess whether it could draw leachate toward itself. What data are missing?
Read the worked answer
Pumping at 1,500 cubic meters per day will create a cone of depression around the new well and locally steepen and redirect the gradient toward it. Because the well sits 300 m east of MW-1 and the landfill lies southeast of MW-1, the well is roughly downgradient of the waste cell under the current flow field - the worst place for a supply well.
Whether it actually captures leachate depends on the size of its capture zone relative to the distance to the plume, on the pumping rate and duration, and on aquifer properties. The missing data are hydraulic conductivity and saturated thickness, which are needed to compute the contributing area, plus the plume's current extent and the depth of screening relative to any vertical flow.
The recommendations follow: run a pumping test to get conductivity and storativity, then build a simple flow or particle-tracking model; install a sentinel well between the landfill and the proposed well; monitor specific conductance, chloride and volatile organic compounds; and shift the well farther east or north, or reduce the pumping rate, unless the modeling shows the capture zone stays clear of the plume.