WARTS AND ALL: ANOMALOUS PLUME BEHAVIOR IN A GLACIAL AQUIFER SYSTEM

Groundwater remediation projects are often hampered by insufficient information to fully constrain the subsurface distribution of aquifer properties and contaminant concentrations. Consequently, practicing hydrogeologists rely upon insights drawn from massively instrumented contaminant transport experiments or apply idealized (e.g., homogenous, uniform, infinite) models of aquifers to make predictions and guide remedial decision-making. Occasionally, abundant data from sites with extensive remedial operations provide an opportunity to test the veracity of simple models and widely held generalizations. One such case is the Pall Life Sciences site west of Ann Arbor, Michigan, where more than 170 monitoring wells and 20 extraction wells have been installed to investigate and remediate 1,4-dioxane in a 8 km² area. Over the past 40 years, plumes of 1,4-dioxane (a miscible organic solvent that does not readily degrade or adsorb to soil particles) have migrated several kilometers in several different directions from the site through approximately 80 m of surficial glacial drift.

Numerous examples of transport behavior that is inconsistent with simplified conceptual models of contaminant transport have been recognized at the site. These include: i) non-detect values in monitoring wells located in the direct path of a plume, ii) unexpected contaminant detection in distal monitoring wells; iii) apparent upgradient migration; iv) branching and lateral fingering of plumes; v) sporadic vertical and horizontal concentration gradients; and vi) temporal contaminant concentration fluctuations. Many of these anomalies are difficult to explain with a conventional advection-dispersion transport model, even when gross heterogeneities in aquifer/aquitard distribution are taken into account. Potential explanations for the observed behavior are: i) insufficient monitoring well spacing (horizontal and vertical); ii) large monitoring well screen intervals; iii) unrecognized preferential flow paths; iv) unrecognized barriers to flow; v) temporal variation in magnitude and direction of hydraulic gradients in response to seasonal recharge and remedial pumping; and vi) a complex loading history. This presentation will illustrate anomalous transport behavior and discuss potential explanations at the site.