Question

How do differences in ax  affect the arrival time of the leading edge of the contaminant plume

Answer 1

This type of water pollution can also occur naturally due to the presence of a minor and unwanted constituent, contaminant or impurity in the groundwater, in which case it is more likely referred to as contamination rather than pollution. The pollutant often creates a contaminant plume within an aquifer.

To simplify the search in Locate the Plume, it is assumed that the plume, which has well-defined
leading and trailing edges, is moving advectively with the groundwater flow. The student must
know the groundwater flow direction from the spill site, the average groundwater velocity, and
the elapsed time that the plume has been in transit to determine where it is with respect to the
spill site. The approximate groundwater flow path from the spill site to the river is provided in
the Map View interface to limit the search area for the moving target. The student uses Darcy’s
law and aquifer porosity to estimate average groundwater velocity. The student must also
estimate how long the leading and trailing edges of the plume have been moving from the spill
site toward the river. Monitoring wells are then installed within a target area defined by the
approximate groundwater flow path and the estimated distance traveled by the leading and
trailing edges of the plume from the spill site.
To make the initial estimate of average groundwater velocity, aquifer porosity must be derived
from drillers’ logs of water wells and a table of representative porosity values from all types of
alluvial materials. Because the “Main Aquifer” consists of several different lithologies, the
average porosity of the “Main Aquifer” must be estimated by weighting the representative
porosities of each lithology relative to total thickness of the “Main Aquifer”. Once the weighted
average porosity values have been calculated for each of the 6 well sites, the porosity of the
alluvial aquifer at the spill site is calculated as the simple arithmetic average of the weighted
porosity values.
In the Remediate the Plume simulation the student develops on-site designs for the pump-and-
treat (P&T), permeable reactive barrier (PRB), and in situ redox manipulation (ISRM)
technologies and selects from these the single most cost-effective design that will eliminate the
contamination threat to the Buffalo River. P&T consists of a production-injection well couplet,
and a treatment plant. The production-injection well couplet is situated such that the plume is
contained entirely within the couplet’s capture zone (Fetter, 1993). Ground water is pumped
from the production well installed down gradient of the plume and piped to a treatment plant for
removal of contaminants. The treated water is piped to an injection well where it is put back into
the aquifer upradient of the plume. The couplet’s capture zone develops by superposition of the
cone of depression created by withdrawing water from the aquifer by pumping and the cone of
impression created by the addition of water to the aquifer by injection. The resulting capturezone is elliptical in shape and oriented such that the long axis of the ellipsoid connects the
production and injection wells and parallel to the direction of groundwater flow.
The treatment method in this simulation uses the Forager™ technology, which adsorbs dissolved
ions, including chromium onto a sponge or granular surface coated with a polymer (US EPA,
1995). The order of the polymer’s affinity for metallic and non-metallic ions in solution is:
(Greatest affinity) Au+3 > Cu+2 > Cd+2 > Hg+2 > Pb+2 > Ni+2 > Mn+2 > Fe+3 > Co+2 > Zn+2 >
Au(CN)-2 > SeO4
-2 > AsO4
-3 > CrO4
-2 > UO4
-2 > Ag+1 >> Al+3 > Mg+2 > Na+1, K+1, Ca+2, Cl-1 and
SO4
-2 (Least affinity).
Water to be treated is passed through columns or beds filled with the Forager™ polymer-coated
sponges or granules to produce the desired treatment result.
Unlike the P&T technology, the PRB relies on the passive flow of ground water through a
constructed reactive zone containing granular elemental iron to remove contaminants by means
of redox chemical reactions (Powell et al., 1998). Elemental iron (Fe0
) removes the highly
mobile Cr+6 from ground water and transforms it to the immobile Cr+3 by donating electrons to it.
In the process, elemental iron is oxidized to Fe+3 and the Cr+6 is reduced to Cr+3. The net
chemical reaction is:
Fe0
+ CrO4
-2 + 4H2O => Fe(OH)3 + Cr(OH)3 + 2OH-1.
Iron and chromium hydroxides [Fe(OH)3 and Cr(OH)3] are precipitated onto the granular matrix
materials that fill the reactive zone and the pH of the water downgradient of the barrier is
increased due to the addition of hydroxyl (OH) ions.
ISRM technology also relies on a similar chemical mechanism to remove the dissolved
chromium contamination (Cr+6) from solution (Vermeul et al., 2004). ISRM creates a treatment
zone within the aquifer by injecting a buffered sodium dithionite (Na2S2O4) solution into the
aquifer. Sodium dithionite dissolves in water to eventually produce sodium (Na+1) and sulfoxyl
(SO2
-1) ions:
Na2S2O4 => 2Na+1 + 2SO2
-1.
The injected sulfoxyl ions reduce Fe+3 to Fe+2 in the aquifer materials:
SO2
-1 + Fe+3 + H2O => SO3
-2 + Fe+2 + 2H+1.
After a predetermined reaction time, the injected solution is recovered from the aquifer by
pumping the injection wells. Later, as the Cr+6 -contaminated groundwater flows through the
treatment zone, the chromium is precipitated as a byproduct of redox chemical reactions with
iron-bearing minerals on the aquifer matrix. Fe+2 ions donate electrons to Cr+6 ions, which causes
Cr+6 to be reduced to the relatively immobile Cr+3. In the process Fe+2 is oxidized to Fe+3.