Validation of a multilevel sampling device to determine the vertical variability of chlorinated solvent in a contaminated aquifer

Abstract

The vertical heterogeneity of contaminant concentrations in aquifers is well known, but obtaining representative samples is still a subject of debate. In this paper, the question arises from sites where numerous fully screened wells exist and there is a need to define the vertical distribution of contaminants. For this purpose, several wells were investigated with different techniques on a site contaminated with chlorinated solvents. A core-bored well shows that a tetrachloroethene (PCE) phase is sitting on and infiltrating a less permeable layer. Downstream of the cored well, the following sampling techniques were compared on fully screened wells: low flow pumping at several depths, pumping between packers and a new multilevel sampler for fully screened wells. Concerning low flow rate pumping, very low gradients were found, which may be due to the existence of vertical flow inside the well or in the gravel pack. Sampling between packers gave results comparable with the cores, separating a layer with PCE and trichloroethene from another one with cis 1,2-dichloroethene and vinyl chloride as major compounds. Detailed sampling according to pumped volume shows that even between packers, cleaning of the inter-packer volume is necessary before each sampling. Lastly, the proposed new multilevel sampler gives results similar to the packers but has the advantages of much faster sampling and a constant vertical positioning, which is fairly important for long-term monitoring in highly stratified aquifers.

Field Evidence of Dissolution and Degradation Rates Enhancement During ISCR and ENA Treatments of Chlorinated Solvents

“…A large vertical gradient of contaminant concentrations was observed (Exhibit 2). These variations would be even higher with smaller interpacker space because of the small thickness of the highly contaminated layer (Barnier et al, 2012). In the two pilot zones, similar profiles are observed.…”

“…Multilevel samplers were installed to sample water at the following depths: 3 to 4, 4 to 6, and 6 to 8 meters. These multilevel samplers were developed during the project and were thoroughly tested before use (Barnier, Palmier, & Atteia, 2012). Sampling between 3 and 4 meters depth has been impossible in ENA monitoring wells because of the presence of oil.…”