Assignment # 2

The research paper reviewed here is titled "Hydraulic Gradient Control for Groundwater Contaminant Removal" by Dorothy F. Atwood and Steven M. Gorelick in the Journal of Hydrology, 1984.

In this study, an aquifer recovery methodology is proposed using linear programming for gradient control wells placement and a solute transport finite difference model to evaluate the contaminant plume characteristics and performance of the LP based well scheme.

The model area is the Rocky Mountain Arsenal near Denver, Colorado which used to be a site for toxic chemical processing and manufacture and has a history of bad disposal practices and groundwater contamination. The geology and geophysics of the area is also well known. The specific area considered here consists of the north boundary of the Rocky Mountain Arsenal where preventing the contaminant migration has been crucial. Flow in the unconfined aquifer is generally from south to north. The goal of the restoration procedure is to prevent further contaminant migration during aquifer cleanup especially beyond the northern boundary of the arsenal. The area is approximated and modeled in 702 finite difference nodes each of 76.2 m sides.

The hydraulic gradient control procedure requires the use of models of groundwater flow and solute transport in groundwater. The process is a two stage:

Stage 1: Simulating Contaminating distribution.

In stage one, contaminant transport is simulated through time using an assumed velocity field (corresponding to successful hydraulic screening) to determine the location of the plume boundary relative to the potential gradient control wells. Three steps are necessary here:
1) an approximate velocity filed assumed.
2) the central contaminant removal system design and
3) the contaminant distribution simulated.

Stage 2: Select best wells and best rates

Linear programming combined with groundwater flow simulation is used to select the wells and determine the pumping/recharge schedules that most effectively stop the migration of the plume by controlling the hydraulic gradient during cleanup. This is accomplished with a single global optimisation which minimises the total pumping/recharge over all time subject to a series of hydraulic constraints that force the gradient to be towards the center contaminant removal well.

The study in conclusion states that, this LP approach to optimize pumping rates not only saves cost but also reduces the time of effective removal from 22 years if no such scheme is applied to a little over 12 years. Although, the author do agree that there is a lot more which can be done as in using a mixed-integer programming technique to optimize the number of wells for gradient control.

My comments:

I think the idea is really good and effective. As the results suggest there is a scope of tremendous improvement in the contaminant removal and its quicker incurring lesser costs etc. I would like to suggest the following:

1) The gradient control should be revisited in order to be able to determine the optimum concentrations of contaminant due to movement of groundwater which can be removed. This can be done with an advanced groundwater modeling s/w like MODFLOW and I think can be used to use gradient control to be more effective.

2) The non-linearities in the management options can be modelled using other optimisation approaches to give an accurate model. Mixed-integer programming should be included for optimum number of wells and their placements.